WEBVTT

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Quantum mechanics is perhaps the most successful 
theory in all of science. It predicts the behavior  

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of the invisible subatomic world with astonishing 
precision. It allows us to explain everything from  

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chemistry to stars and our quantum technology 
underlies the digital age. The catch is that  

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the reality that it describes is completely 
insane. Particles are waves of possibility,  

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alternate realities that split and merge 
between measurements. And absent measurement,  

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reality may not exist as we know it. A century 
after the discovery of quantum mechanics,  

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we still don't know what this incredibly 
powerful theory means. This longplay episode  

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brings together the quantum journey that we've 
been on for the past decade of spaceime. the  

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foundational ideas, experiments, and even the 
crazier speculations of quantum mechanics.

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foundational ideas, experiments, and even the 
crazier speculations of quantum mechanics.

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Wouldn't it be nice to be everywhere at 
once? According to quantum mechanics,  

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you are at least a little bit. Quantum mechanics 
is a spectacularly weird theory. One of its  

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basic tenets is that certain properties 
of an object are fundamentally uncertain.  

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They don't hold well- definfined values but 
instead must be described as a distribution  

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of possible states of being. Each specific state 
has a certain probability of being true when the  

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object is observed or more accurately until 
a quantum object interacts with something.  

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All possible states are just as real as each 
other. Although not necessarily equally likely,  

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there's a distribution of probabilities for 
each of an object's quantum properties. That  

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distribution and the way it changes over time is 
coded in the object's wave function. The reduction  

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of a fuzzy possibility space into a specific 
measurable property is sometimes referred to  

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as the collapse of the wave function. We'll 
talk a lot more about this awesome weirdness  

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in the future. For now, let's just look at the 
strange consequences of quantum uncertainty in  

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an object's location. This is one of the early 
realizations in the development of quantum theory.  

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French mathematician and physicist Louie de Bruyer 
figured out that any material object is really a  

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matter wave. It can be described as a wave packet 
of position probability and that wave packet has  

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a wavelength. This de Bruyer wavelength defines 
how well determined an object position is. A large  

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wavelength means a highly uncertain position. A 
small wavelength means a well-defined position.  

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That's true of subatomic particles and it's sort 
of true of anything. Right now, I'm mostly right  

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here, but there's also a smaller chance that 
I'm here, here, or here. There's an infinite  

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decimal chance that I'm on the moon. Observe me, 
and you'll collapse my wave function and probably  

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find me pretty much exactly where you expect 
to. See, an object's debroy wavelength depends  

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on its momentum. So, mass times velocity. Higher 
momentum means a smaller wavelength. In fact, it's  

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the minuscule plank constant divided by momentum. 
Humans are made up of several tens of kilograms of  

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thermally moving particles and have de Bruy 
wavelengths a couple of orders of magnitude  

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smaller than the plank length. You're everywhere 
in the universe, but not very much. You're as  

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right there as it's possible to be. But what about 
something much smaller? say a tightly bound bundle  

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with two protons and two neutrons that we call an 
alpha particle. On its own, this would be a helium  

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nucleus. But these bundles also exist as parts 
of heavier atomic nuclei. There an alpha particle  

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is snugly bound into the nucleus by the strong 
nuclear force. We can imagine an alpha particle  

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as being like a ball trapped in a steep valley 
of potential energy. It can roll around inside,  

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but unless it has a very large kinetic energy, 
it will never roll over the edges. But quantum  

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objects aren't at all like balls. Their positions 
are not well defined. As an alpha particle  

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approaches the force barrier of the nucleus, its 
wave packet is reflected backwards. Usually, see,  

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that wave packet describes a range of possible 
locations for the approaching particle. But that  

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possibility space does not end suddenly at the 
force barrier. Instead, it drops off quickly  

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exponentially through the steep walls. However, 
it never quite reaches zero. There remains a  

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tiny tail of probability outside the nucleus 
beyond the reach of the strong nuclear force.  

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That means there's a very tiny chance that instead 
of bouncing off the wall, the particle will at the  

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last minute resolve its position in that unlikely 
outside bit of its possibility space. That looks  

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like the particle teleporting out of the nucleus. 
This process is called quantum tunneling.  

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When it's an alpha particle escaping a nucleus, 
this is one of the most important mechanisms for  

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radioactive decay. Quantum tunneling also goes in 
the other direction. Protons, neutrons, electrons,  

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and alpha particles can quantum tunnel into nuclei 
in various types of fusion and particle capture  

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phenomena. In fact, without quantum tunneling, 
stars could not fuse hydrogen into heavier nuclei.  

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A variety of modern electronics also rely on the 
tunneling phenomenon, including the transistor.  

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But how quickly does the alpha particle move 
through this barrier? Well, as far as we know,  

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it's instantaneous. That suggests a velocity 
faster than light, which sounds problematic.  

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It's actually extremely hard to test this 
because we can't make clocks accurate enough  

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to time such a ridiculously quick event. Here's 
one way you might test this. Remember the LIGO  

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interpherometer that discovered gravitational 
waves? Laser beams are sent down paths at right  

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angles and then brought back together. The photon 
wave packets interact with each other and produce  

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an interference pattern that is incredibly 
sensitive to differences in path lengths. Well,  

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that's a Michaelelsson interferometer, 
but let's change the arrangement a bit.  

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We want to send individual photons instead of 
lasers. and we want to block one of the paths  

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with a very thin reflective barrier like 
this. In the absence of quantum tunneling,  

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that barrier should reflect its photon 100% of 
the time. But just like with the alpha particle,  

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as the photon approaches the barrier, the wave 
packet defining its possible location extends  

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weekly beyond the barrier. About 99% of the time 
the photon is reflected, but 1% of the time it  

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will resolve itself beyond the barrier and it'll 
continue on its path. If those rare tunneling  

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photons really do travel instantaneously through 
the width of the barrier, then they should arrive  

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at the detector slightly ahead of the photon that 
travels the unimpeded path. That will be apparent  

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when their wave packets don't line up perfectly at 
the other end. But for this to work, the original  

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path lengths of the intererometer need to be very 
precisely equal. To make that work, you need to  

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use a second and perhaps even weirder feature of 
quantum mechanics. Quantum entanglements. That is  

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a huge topic for another video. It offers another 
tantalizing hint at faster than light influences.  

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For now, let me just say that in order to produce 
these entangled states, the path length of the  

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intererometer needs to be identical to very 
high precision. Tune the path lengths until  

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the weird effects of entanglement emerge, and 
you know that they're equal. At that point,  

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you can get an incredibly precise measurement of 
any differences in photon travel time. A team of  

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very clever physicists have successfully performed 
this experiment. Reference link below. What did  

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they find? The tunneling photon does arrive a 
tiny bit earlier than its partner. It appears to  

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teleport through the barrier and so travel faster 
than light. Sweet. So scale up from photons to  

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people and we have transporter beams, right? Well, 
this apparent violation of relativity only occurs  

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deep within the quantum realm. A particle resolves 
its location anywhere within the vicinity of its  

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de Bruy wavelength. That uncertainty in location 
allows tunneling. But even without a barrier,  

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this location fuzziness leads to uncertainty 
in the arrival time of the photon. An unimpeded  

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photon could arrive at the earlier time of the 
tunneling photon because its wave packet includes  

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that in its range of possible positions. When 
you add the barrier, all you're really doing is  

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reshaping the wave packet, selecting only the 
possibility space of early arrival. This can  

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look like an increase in the speed of light, but 
only within the uncertainty range defined by the  

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de Bruy wavelength. More accurately, it's within 
the uncertainty range defined by the Heisenberg  

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uncertainty principle, which is perhaps the deeper 
principle from which the de bruy wavelength comes.  

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This is something we'll also get back to in 
detail. Any macroscopic object is subject to  

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a very well-defined speed limit. But in the 
quantum realm, Heisenberg uncertainty does  

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seem to allow instantaneous motion and even 
perhaps causality violation within quantum  

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limits. Stay tuned for the implications of this 
on both quantum and cosmic scales of spaceime.

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The plank constant defines the size scale at 
which the familiar physics of our macroscopic  

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reality gives way to the weirdness of the quantum 
world. You might not expect the quantum behavior  

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of the microscopic to be observable on all scales 
of the universe. But it is in fact you can see the  

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effect of this quantum behavior and even measure 
the plank constant just by observing the color of  

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sunlight. Zeno's famous paradox tells us that it's 
impossible to overtake a tortoise. Run as fast  

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as you like. By the time you reach the tortoise's 
initial position, it'll have moved forward. Reach  

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that second position, it'll have moved forward 
again. To overtake a tortoise, you need to  

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travel to its previous position infinite times. 
Each of those steps is shorter than the last,  

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but there are still an infinite number of them. 
There are a few problems with this paradox, but  

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one is that it assumes that space is infinitely 
divisible. This is not true. As your distance to  

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the tortoise becomes unthinkably small, there 
arises a quantum uncertainty in your location.  

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Get close enough to it and this quantum blurriness 
means it's impossible to say whether your location  

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is really behind or in front of the tortoise. We 
talked about this recently when we discussed the  

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de bruy wavelength. This and more formally the 
Heisenberg uncertainty principle describes the  

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smallest distance for which an object's location 
can be meaningfully defined. The tiny plank  

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constant at 6.63x 10us 34 JW seconds sets the 
scale of this quantum blurriness. So it also sort  

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of defines a pixel scale to reality. In many ways, 
it defines the divisibility of the quantum world.  

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In fact, the plank constant appears in essentially 
all equations that describe quantum phenomena. the  

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Heisenberg uncertainty principle and the de bruy 
wavelength but also the Schrodinger equation the  

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energy levels of electron orbits and importantly 
the relationship between the energy and frequency  

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of a photon. It also sets the size of the plank 
length which is hypothetically the length below  

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which the concept of length loses meaning. 
It may define the scale of quantum reality,  

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but the influence of the plank constant can 
be seen even on our scale. For example, along  

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with the sun's temperature, it sets the color of 
sunlight. If the plank constant were 25% smaller,  

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the sun would be violet, all else being equal. In 
fact, the mystery of why hot things glow the color  

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that they do led us to discover the quantum 
universe in the first place. Science fact.  

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Everything in the universe glows with the 
light of its own internal heat. Heat is  

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just the energy in the random motion of particles 
comprising an object. Accelerated charges produce  

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electromagnetic radiation light. And so an object 
made of jiggling charged particles like electrons  

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and protons glows. The hotter an object is, the 
faster its particles jiggle. And so the average  

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frequency of the resulting particles of light of 
photons increases with temperature. This average  

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frequency defines the color that we see. The 
sun is yellow because its 6,000 Kelvin surface  

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produces more photons in the greeny yellow part of 
the electromagnetic spectrum than anywhere else.  

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The blue super giant star Ryel is 12,000 Kelvin 
and so it pumps out lots of highfrequency blue  

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light and even more ultraviolet. Your temperature 
is around 310 Kelvin. So your heat glow is mostly  

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in low frequency infrared photons. It was Sir 
Isaac Newton who first analyzed this heat glow  

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in the 1660s when he used a prism to split 
sunlight into its component colors. But he  

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didn't realize that the relative brightnesses 
of those colors held the key to the quantum  

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world. By the late 1800s, the distribution of 
brightnesses with frequency produced by hot  

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objects had been mapped in careful experiments 
that blacked out anything but the glow of heat.  

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The resulting black body spectrum looks like a 
lopsided bell curve. However, the deep physics  

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behind this shape remained a mystery. The 
key to unlocking the mystery lay in finding  

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a mathematical description for the black body 
spectrum. At the beginning of the 20th century,  

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a couple more Brits, Lordley and Sir James Jeans, 
attacked the problem with a relatively new idea  

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called the equipartition theorem. It states that 
an object's heat energy will end up jiggling all  

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of its particles in all the ways that they can 
be jiggled. In physics speak, at equilibrium,  

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energy is evenly spread between all possible 
energy states. This simple idea allowed our good  

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Englishman to figure out the frequencies of the 
photons produced by all of this thermal motion.  

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The resulting jeans law described the black body 
spectrum perfectly for low frequency infrared  

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light. But for high frequencies like visible 
and ultraviolet, it predicted brightnesses that  

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were way too high. Worse, it predicted that the 
intensity should eventually approach infinity as  

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frequency increased. It paints the crazy picture 
of a universe full of infinite extreme energy  

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gamma radiation. This appears not to be the case. 
This was called the ultraviolet catastrophe. It  

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was catastrophic because it meant that something 
was fundamentally wrong with the classical physics  

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that went into the genans law. The problem turned 
out to be that in classical physics everything  

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can be infinitely divided. There's no smallest 
anything. The genes calculation allows particles  

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to vibrate with any amount of energy all the way 
down to infinite decimally tiny wiggles. When they  

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tried to mathematically distribute heat energy to 
equipartition it across possible energy states,  

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way too much energy got packed into the countless 
very tiny energy states at high frequencies.  

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Mathematically, Raian genes were chasing 
Zeno's tortoise, infinitely dividing the  

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smallest remaining energy states and so never ever 
finding an end to them. German physicist Max Carl  

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Ernst Ludwig plank resolved the catastrophe 
almost by accident. While searching for a new  

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mathematical approach to deriving the black body 
spectrum, he needed some sort of math trick to  

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count those supposedly infinite energy states. 
In what he described as a moment of desperation,  

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he tried something pretty silly. He decided that 
those particles could only vibrate with energies  

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that were a multiple of some minimum energy. He 
quantized the energy states. He set this minimum  

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energy to be the frequency of a particle's 
vibration times a very very small number,  

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a number that had yet to be measured. That number 
became the plank constant. It was a move similar  

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to smooshing your hands on the keyboard when your 
computer freezes. But it worked. It allowed plank  

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to equipartition energy in a way that solved the 
ultraviolet catastrophe. Why? Because it limited  

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how much energy those high frequency vibrations 
could hold. Plank's new equation described the  

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shape of the black body spectrum exactly across 
all frequencies of light. It became plank's law.  

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Now, Max Planck didn't originally think that 
these quantized energy levels were real. It  

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was supposed to be a math trick. He expected 
his new constant to turn out to be zero, which  

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would mean no interval between energy states, no 
quantization, no minimum energy. In that case,  

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the plank constant would have canceled out in 
the final equation, but it didn't. The plank  

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constant is firmly entrenched in the plank black 
body law. Energy quantization is real. Now the  

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actual value of the plank constant still had 
to be measured. But given a mathematical form  

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for the black body spectrum, it was possible to do 
this just by varying the constant until the plank  

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law matched the observed spectrum. But once you 
know the plank law and an object's temperature,  

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you can calculate the plank constant just by 
finding the brightest part of an object's heat  

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glow. For example, by observing the color of the 
sun. As usual, it took Albert Einstein to fully  

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understand the physics behind Planck's strange 
quantized vibrations. Einstein realized that it's  

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actually light that is quantized. Those little 
vibrating particles do have quantized energies,  

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but it's because they can only gain or lose energy 
by absorbing or emitting one particle of light at  

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a time. And that light comes in indivisible energy 
packets. Plank's discovery was the clue Einstein  

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needed to hypothesize the existence of the photon. 
Part wave, part particle, carrying a quantum of  

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energy equal to the now familiar frequency of 
the wave times the plank constant. Einstein  

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proved this through the photoelectric effect. 
It got him the 1921 Nobel Prize in physics.  

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Shortly following Planck's Nobel of 1918, these 
discoveries sparked a frenzy of sciencing that  

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led to the quantum revolution of the 1920s. And 
that little number that Maxplank came up with in  

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his moment of desperation, the plank constant, 
remains at the heart of all things quantum. But  

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not just quantum. By defining the shape of the 
black body spectrum, the plank constant can be  

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red in the color of the sun and the stars in 
the brightness of the different colors of the  

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rainbow and combined with a small handful of 
other fundamental constants. It governs the  

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behavior of everything in this spaceime. One of 
the strangest experimental results ever observed  

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has got to be that of the single particle double 
slit experiment. It's one of the most stunning  

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illustrations of how the quantum world is very 
very different from the large scale world of  

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our physical intuition. In fact, it hints that the 
fundamental nature of reality may not be physical  

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at all, at least in any sense that we're familiar 
with. Let's start with the familiar. In fact,  

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let's start with a rubber ducky. It bobs up and 
down in a pool, causing periodic ripples to spread  

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out. Some distance away, those waves encounter a 
barrier with two gaps cut in it. Most of the wave  

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is blocked, but ripples pass through the gaps. 
When the new ripples start to overlap each other,  

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they produce this really cool pattern. It's called 
an interference pattern. It's due to the fact that  

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in some places the peak of the ripple from one 
gap stacks on top of the peak from the other gap,  

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making a more extreme peak. You also get more 
extreme dips when two troughs overlap. We call  

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this constructive interference. But when the peak 
from one wave encounters the trough from another,  

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they cancel out, leaving nothing. Destructive 
interference. So we have these alternating  

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tracks of wavy and flat water. Any type of wave 
should make an interference pattern like this.  

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for example, water waves and sound waves, but 
also light waves. This double slit interference  

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of light was first observed by Thomas Young back 
in 1801. A source of light passing through two  

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very thin slits produces bands of light 
and dark stripes, alternating regions of  

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constructive and destructive interference on a 
screen. Of course, we now know that light is a  

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wave in the electromagnetic field thanks to the 
work of James Clark Maxwell a century later. So,  

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it makes perfect sense that it should produce 
an interference pattern, right? But wait,  

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we also know that light comes in indivisible 
little bundles of electromagnetic energy called  

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photons. Einstein demonstrated this through the 
photoelectric effect, but his clue came from the  

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quantized energy levels of Maxplank's black body 
radiation law. Check out our episode on this for  

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the details. Okay, so each photon is a little 
bundle of waves, waves of electromagnetic field,  

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and each bundle can't be broken into smaller 
parts. That means that each photon should have  

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to decide whether it's going to go through one 
slit or the other. It can't split in half and then  

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recombine on the other side. That shouldn't be a 
problem as long as you have at least two photons.  

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One photon passes through each slit and then the 
two photons interact with each other on the other  

24:12.915 --> 24:18.595
side and produce our interference pattern. But 
here we get to one of the craziest experimental  

24:18.595 --> 24:25.795
results in all of physics. The interference 
pattern is seen even if you fire those photons one  

24:25.795 --> 24:32.115
at a time. Well, let me back up a bit. The first 
photon is detected as having arrived at a very  

24:32.115 --> 24:38.675
particular location on the screen. The second, 
third, and fourth photons also, they deliver their  

24:38.675 --> 24:45.875
energy at a single spot. And so, they appear to be 
acting like particles of well-determined position.  

24:45.875 --> 24:52.275
But check it out. If you keep firing those 
single photons, you start to see our interference  

24:52.275 --> 24:59.635
pattern emerge once again. By the way, Veritassium 
actually conducts this experiment in his excellent  

24:59.635 --> 25:07.235
series on the double slit experiment. Really 
worth a look. This is so bizarre. This pattern has  

25:07.235 --> 25:12.675
nothing to do with how each photon's energy gets 
spread out. As was the case with the water wave,  

25:12.675 --> 25:18.995
each photon dumps all of its energy at a single 
point. No, the pattern emerges in the distribution  

25:18.995 --> 25:27.395
of final positions of many completely unrelated 
photons. How can that be? Each photon has no idea  

25:27.395 --> 25:33.715
where previous photons landed or where future 
photons will land. Yet, each photon reaches  

25:33.715 --> 25:39.715
the screen knowing which regions are the most 
likely landing spots and which are the least  

25:39.715 --> 25:47.315
likely. It knows the interference pattern of a 
pure wave that passed through both slits equally,  

25:47.315 --> 25:52.435
and it chooses its landing point based on that. 
It turns out that the photon isn't the only  

25:52.435 --> 25:57.715
thing that does this. Shoot a single electron 
through a pair of slits and it'll also appear  

25:57.715 --> 26:02.755
to land at a single spot on the screen. But 
fire many electrons and they slowly build up  

26:02.755 --> 26:09.795
the same sort of interference pattern. This crazy 
effect has even been observed with whole atoms,  

26:09.795 --> 26:17.075
even whole molecules. Buckminister ferine. 
Bucky balls are gigantic spherical molecules of  

26:17.075 --> 26:23.795
60 carbon atoms and have been observed to produce 
double slit interference under special conditions.  

26:23.795 --> 26:30.995
We have to conclude that each individual photon 
electron or baky ball travels through both slits  

26:30.995 --> 26:39.315
as some sort of wave. That wave then interacts 
with itself to produce an interference pattern.  

26:39.315 --> 26:44.995
except that here the peaks of that pattern 
are regions where there's more chance that the  

26:44.995 --> 26:51.875
particle will find itself. It looks like a wave 
of possible undefined positions that at some point  

26:51.875 --> 26:58.515
for some reason resolves itself into a single 
certain position. We also saw this winess in  

26:58.515 --> 27:04.675
position when we talked about quantum tunneling. 
In fact, several quantum properties like momentum,  

27:04.675 --> 27:11.235
energy, and spin all display similar winess in 
different situations. We call the mathematical  

27:11.235 --> 27:16.755
description of this wavelike distribution 
of properties a wave function, describing  

27:16.755 --> 27:23.075
the behavior of the wave function is the heart of 
quantum mechanics. But what does the wave function  

27:23.075 --> 27:29.635
represent? What are these waves of or waves in? 
Let's start with what we do know about the double  

27:29.635 --> 27:36.595
slit result. We know where the particle is at 
both ends. It starts its journey wherever we put  

27:36.595 --> 27:44.515
the laser or electron gun or bucky ball trebushe 
and it releases its energy at a well- definfined  

27:44.515 --> 27:52.195
spot on the screen. So the particle seems to be 
more particle like at either end but wavelike  

27:52.195 --> 27:59.955
in between. That wave holds the information about 
all the possible final positions of the particle,  

27:59.955 --> 28:06.995
but also about its possible positions at every 
stage in the journey. In fact, the wave must  

28:06.995 --> 28:16.195
map out all possible paths that the particle could 
take. We have this family of could be trajectories  

28:16.195 --> 28:22.755
from start to finish. And for some reason when 
the wave reaches the screen it chooses a final  

28:22.755 --> 28:30.355
location that implies choosing from these possible 
paths. So what causes this transition between a  

28:30.355 --> 28:36.835
wave of many possibilities and a well- definfined 
thing at a particular spot within that mysterious  

28:36.835 --> 28:43.715
span between the creation and the detection. 
Is the particle anything more than a space of  

28:43.715 --> 28:49.475
possibility? Okay, we're adding more questions 
than we're answering. We still didn't figure out  

28:49.475 --> 28:56.355
what the wave is made of. In fact, the answers 
aren't known, but the various interpretations  

28:56.355 --> 29:03.395
of quantum mechanics do try. Let's talk about the 
view favored by Verer Heisenberg and Neils Boore,  

29:03.395 --> 29:09.635
who pioneered quantum mechanics at the University 
of Copenhagen in the 1920s. The Copenhagen  

29:09.635 --> 29:16.595
interpretation says that the wave function doesn't 
have a physical nature. Instead, it's comprised  

29:16.595 --> 29:23.955
of pure possibility. It suggests that a particle 
traversing the double slit experiment exists only  

29:23.955 --> 29:31.075
as a wave of possible locations that ultimately 
encompasses all possible paths. It's only when the  

29:31.075 --> 29:37.555
particle is detected that a location and the path 
it took to get there are decided. The Copenhagen  

29:37.555 --> 29:44.355
interpretation calls this transition from a 
possibility space to a defined set of properties  

29:44.355 --> 29:51.475
the collapse of the wave function. It tells us 
that prior to the collapse, it's meaningless to  

29:51.475 --> 29:57.475
try to define a particle's properties. It's almost 
like the universe is allowing all possibilities  

29:57.475 --> 30:04.515
to exist simultaneously, but holds off choosing 
which actually happened until the last instant.  

30:04.515 --> 30:10.595
weirder. Those different possible paths, those 
different possible realities interact with each  

30:10.595 --> 30:16.835
other. That interaction increases the chance that 
some paths become real and decreases the chance of  

30:16.835 --> 30:22.675
others. There's an interaction between possible 
realities that is seen in the distribution of  

30:22.675 --> 30:29.395
final positions in the interference pattern. That 
pattern is real even though the vast majority of  

30:29.395 --> 30:37.235
paths involved in producing the interference never 
attain reality. In the Copenhagen interpretation,  

30:37.235 --> 30:43.635
that final choice of the experiment of the 
universe is fundamentally random within the  

30:43.635 --> 30:49.235
constraints of the final wave function. The theory 
of quantum mechanics produces stunningly accurate  

30:49.235 --> 30:55.475
predictions of reality and it's completely 
consistent with the Copenhagen interpretation.  

30:55.475 --> 31:01.235
But this is not the only interpretation that 
works. There are interpretations that give the  

31:01.235 --> 31:08.355
wave function a physical reality. Remember, we 
know that light is a wave in the electromagnetic  

31:08.355 --> 31:14.675
field. And quantum field theory tells us that 
all fundamental particles are waves in their  

31:14.675 --> 31:22.515
own fields. This may give us a more physical 
medium that drives these waves of possibility.  

31:22.515 --> 31:27.635
And if you thought the Copenhagen interpretation 
was freaky, wait till we get to the many worlds  

31:27.635 --> 31:36.115
interpretation, which we will right here on 
Spacetime. Can reality be adjusted after events  

31:36.115 --> 31:42.355
have occurred? That's the unsettling implication 
of the delayed choice quantum eraser experiment.  

31:42.355 --> 31:48.675
We recently talked about the weird results of the 
single particle double slit experiment. They imply  

31:48.675 --> 31:56.115
some startling things about the nature of reality. 
Today I want to talk about an additional possibly  

31:56.115 --> 32:02.435
even stranger version of this experiment whose 
results force us to reconsider the nature of  

32:02.435 --> 32:07.875
causality itself. Now this episode isn't going 
to make a lot of sense unless you've seen the  

32:07.875 --> 32:15.075
first one. So go ahead and watch it if you haven't 
already. I'll wait right here. Okay. So the single  

32:15.075 --> 32:21.395
particle double slit experiment suggests that 
things may not exist as well- definfined even real  

32:21.395 --> 32:28.195
particles in that strange interim between creation 
and detection. There's a fuzzy space in which we  

32:28.195 --> 32:34.275
don't know the particle's location or path. The 
Copenhagen interpretation would tell us that in  

32:34.275 --> 32:42.595
this space a particle is only its wave function, 
a distribution of possible properties. It's a  

32:42.595 --> 32:49.555
probability wave that does all the usual wavelike 
stuff like making interference patterns until  

32:49.555 --> 32:55.635
something happens to collapse it. At that point, 
the Copenhagen interpretation tells us that a  

32:55.635 --> 33:03.315
true transition happens between wave and particle. 
But is that right? And if so, what really causes  

33:03.315 --> 33:10.915
this transition? Does observation of the particles 
location force the universe into settling down and  

33:10.915 --> 33:17.315
deciding which particular reality we happen to be 
in? To head off any wild metaphysical giddiness,  

33:17.315 --> 33:22.915
I want to say right now that there's absolutely no 
reason to believe that observation by a physicist  

33:22.915 --> 33:29.155
is better at collapsing wave functions than 
observation by an electronic detector or a  

33:29.155 --> 33:34.355
house plant for that matter. We'll talk about 
what observation really means at a later point,  

33:34.355 --> 33:39.155
but it's still pretty interesting to see what 
happens if we try to observe the wave function  

33:39.155 --> 33:45.155
at different points in the double slit experiment. 
The great mystery of the experiment is that very  

33:45.155 --> 33:52.435
particle-like things appear to traverse both slits 
simultaneously, like you might expect of a wave.  

33:52.435 --> 33:59.075
Physicists love a good mystery and so have tried 
very very hard to peak to see which slit these  

33:59.075 --> 34:04.915
particles actually travel through before they 
produce the famous interference pattern. Turns  

34:04.915 --> 34:11.235
out the universe is on to us. Any experiment that 
determines unambiguously which slit the particle  

34:11.235 --> 34:18.755
traverses destroys the interference pattern. 
Instead, particles land in simple clumps,  

34:18.755 --> 34:24.755
one for each slit, as though they were traveling 
as simple particles the whole time. This is even  

34:24.755 --> 34:31.315
true if you place detectors on the far side of the 
slits after the wave particle thing should have  

34:31.315 --> 34:37.395
already been interfering with itself. It's like 
the wave function is collapsing retroactively,  

34:37.395 --> 34:42.035
as if the universe is saying, "Okay, guys, the 
human figured out which way you went. No more  

34:42.035 --> 34:47.155
funny stuff. better pretend like you are particles 
that whole time. But there's a huge problem with  

34:47.155 --> 34:53.395
this interpretation. It's impossible to make 
these measurements without messing up the wave.  

34:53.395 --> 34:59.315
The interference pattern happens because the waves 
emerging from each slit are what we call coherent,  

34:59.315 --> 35:02.755
which is a fancy way of saying that the 
relationship between the waveforms emerging  

35:02.755 --> 35:09.875
from the two slits. So the locations of peaks and 
valleys is predictable and stays consistent as the  

35:09.875 --> 35:16.275
waves move forward. This coherence is what allows 
the waves to produce the interference pattern in  

35:16.275 --> 35:21.955
the first place. But when you place some device 
in the path of either wave, you mess with this  

35:21.955 --> 35:28.275
coherence and so affect the pattern that reaches 
the screen. By the way, the double slit experiment  

35:28.275 --> 35:34.595
where you try to determine which slit is traversed 
is called a which way experiment. And if the test  

35:34.595 --> 35:41.475
is done on the far side of the slits, it's called 
a delayed choice experiment. Physicists hate being  

35:41.475 --> 35:46.915
outsmarted by the universe. So, they've come up 
with clever ways to measure which way the particle  

35:46.915 --> 35:53.235
traveled while still preserving coherence. I'm 
going to talk about the most famous performed in  

35:53.235 --> 36:00.595
1999. This experiment made use of a very special 
type of crystal that absorbs an incoming photon  

36:00.595 --> 36:07.155
and creates two new photons, each with half 
the energy of the original. These new photons  

36:07.155 --> 36:13.635
are twins of each other. In fact, they're an 
entangled pair. So, fundamentally connected in  

36:13.635 --> 36:19.555
strange ways that we'll come back to. So, place 
this crystal in front of the double slit to make  

36:19.555 --> 36:27.075
coherent entangle pairs of any photons passing 
through. Send one of each pair off to the screen  

36:27.075 --> 36:32.675
to produce our interference pattern and use the 
other to figure out which slit the original photon  

36:32.675 --> 36:40.035
passed through. Let's focus on detectors A and B. 
Here, detector A lights up if the original photon  

36:40.035 --> 36:45.635
passed through slit A and detector B lights up 
for slit B. If we run this for a bunch of photons,  

36:45.635 --> 36:53.715
we see that whenever detectors A or B light up, we 
get a simple pile of photons here at the screen.  

36:53.715 --> 36:58.755
No interference pattern at all. It's as though 
any knowledge of which way the original photon  

36:58.755 --> 37:05.555
traveled stops it from acting like a wave during 
its passage through the slits. and crazier. This  

37:05.555 --> 37:12.595
experiment was set up so that photons reach 
A or B after their twins reach the screen. So  

37:12.595 --> 37:17.475
a photon lands on the screen according to the 
pattern defined by its wave function and then  

37:17.475 --> 37:24.355
later its entangled partner reaches detector A 
or B and somehow retroactively influences the  

37:24.355 --> 37:29.395
previous landing position. It's like the second 
photon is saying, "Whoa, whoa, whoa. Someone  

37:29.395 --> 37:35.395
figured out which slit I came through. You better 
look like you came through that one, too. This is  

37:35.395 --> 37:41.315
amazing. And given that we only interact with one 
of the entangled pair, surely that means we aren't  

37:41.315 --> 37:46.995
messing with the other. So, we aren't ruining the 
interference pattern with decoherence. Could it  

37:46.995 --> 37:55.235
get any weirder? This is quantum mechanics. So, 
yeah, this extra stuff is the quantum eraser. Its  

37:55.235 --> 38:02.515
job is to destroy any information about the path 
of the photons. These devices are beam splitters,  

38:02.515 --> 38:08.675
just half silvered mirrors. They work by allowing 
50% of the photons through while reflecting the  

38:08.675 --> 38:15.715
other 50%. Now you have a new possible outcome. 
Instead of being reflected to detectors A or B,  

38:15.715 --> 38:21.715
half of the photons end up in detectors C 
or D. But this clever arrangement ensures  

38:21.715 --> 38:29.875
that if C or D light up, we have no idea which 
slit that photon came from. If we only look at  

38:29.875 --> 38:38.035
the photons whose twins end up at detector C or 
D, we do see an interference pattern. It looks  

38:38.035 --> 38:44.275
like the simple act of scrambling the which way 
information retroactively sends the message, okay,  

38:44.275 --> 38:49.555
chill. The observer lost the info of which slit 
we went through. It's safe to have gone through  

38:49.555 --> 38:56.915
both again. Are we forced to double down on the 
interpretation that observation of the path causes  

38:56.915 --> 39:02.595
the collapse of the wave function and that the 
wave function can collapse all the way back to  

39:02.595 --> 39:09.075
wherever our new knowledge extends to in the 
past some sort of retroactive reality cascade?  

39:09.075 --> 39:15.395
This is a pretty wild story. For that reason, it 
makes sense to be cautious even of the Copenhagen  

39:15.395 --> 39:20.595
interpretation. Part of the appeal of the 
Copenhagen interpretation is that it avoids any  

39:20.595 --> 39:27.395
physical interaction that moves faster than light. 
See, when a spread out wave function resolves  

39:27.395 --> 39:34.755
itself into a set of known properties, say the 
location of a particle on the double slit screen,  

39:34.755 --> 39:40.355
somehow the entire wave function knows to 
do this to collapse at the same instant.  

39:40.915 --> 39:46.275
But if these wave functions aren't physical, 
as the Copenhagen interpretation would tell us,  

39:46.275 --> 39:52.195
then there's no real instantaneous physical 
interaction. By contrast, a physical  

39:52.195 --> 39:58.835
interpretation of the wave function like the de 
Bruy bone pilot wave theory requires an underlying  

39:58.835 --> 40:05.075
physicality, a set of defined properties that 
evolves with the wave function, so-called  

40:05.075 --> 40:10.595
hidden variables. That's a bit uncomfortable 
because to explain experimental results,  

40:10.595 --> 40:16.835
those physical properties need to act and change 
instantly at any distance. They need to have what  

40:16.835 --> 40:23.635
we call non-locality. Now the delayed choice 
quantum eraser double slit experiment doesn't  

40:23.635 --> 40:29.715
tell us whether the wave function is physical or 
not but it tells us that the Copenhagen or any  

40:29.715 --> 40:35.955
other metaphysical interpretation of the wave 
function is no less well crazy sounding than a  

40:35.955 --> 40:41.875
physical interpretation. In fact the solution 
may lie in this fascinating phenomenon of  

40:41.875 --> 40:48.355
quantum entanglement. As we'll see in the future, 
entangled particles really are able to influence  

40:48.355 --> 40:55.795
each other instantaneously. And we'll see that 
their non-locality doesn't violate causality.  

40:55.795 --> 41:03.235
So perhaps they can even affect coherence and 
decoherence retroactively and physically without  

41:03.235 --> 41:10.035
making a causal mess. Perhaps this thing we call 
observation is just entanglement between the  

41:10.035 --> 41:17.955
observer and the experiment. Perhaps the evolving 
tapestry of entanglement in all its impossible  

41:17.955 --> 41:26.595
complexity is what really defines reality in 
this spaceime. The quantum eraser experiment  

41:26.595 --> 41:32.995
tantalizes us with the apparent instantaneous flow 
of information between entangled photon pairs.  

41:32.995 --> 41:39.795
In fact, the delayed choice quantum eraser seems 
to show that information can travel backwards in  

41:39.795 --> 41:46.995
time. For today's challenge question, I'm going to 
propose a way to use it to create a timetraveling  

41:46.995 --> 41:54.115
communication device and use it to cheat on the 
lottery. I want your honest opinion. First though,  

41:54.115 --> 42:01.315
you really should go back and watch that quantum 
eraser episode unless your memory is incredible.  

42:01.315 --> 42:07.715
Good. Let me review the important bits. Anyway, 
we start with a standard double slit apparatus.  

42:07.715 --> 42:11.955
Photons are fired one at a time through 
the two slits. On the opposite side,  

42:11.955 --> 42:16.835
each photon is split into an entangled pair of 
photons. One of these is sent off to a screen  

42:16.835 --> 42:22.835
where its location is recorded, while the other 
is used to identify which slit the original photon  

42:22.835 --> 42:27.955
passed through. The setup looks like this. In 
the interest of accuracy, let me add a little  

42:27.955 --> 42:32.515
detail that I skipped in the previous video. 
When the researchers conducted this experiment,  

42:32.515 --> 42:38.675
they didn't bother with detector B. There were 
really only two sets of possible outcomes. One,  

42:38.675 --> 42:44.595
a photon finds its way to detector A. In that 
case, we know the original photon must have passed  

42:44.595 --> 42:52.995
through slit A. Or two, either detector C or D 
fire, in which case we have no idea which slit was  

42:52.995 --> 42:59.475
traversed. And the result is that photons whose 
entangled twins land at A produce no interference  

42:59.475 --> 43:04.195
pattern. They just land in a single pile as though 
they had traveled as particles through the entire  

43:04.195 --> 43:10.835
experiment. But photons landing at C or D have 
twins that do produce an interference pattern  

43:10.835 --> 43:16.435
at the screen. One important extra piece of 
information, that interference pattern is slightly  

43:16.435 --> 43:23.955
different depending on whether C or D fire. 
The pattern for C has peaks that line up with  

43:23.955 --> 43:30.835
the valleys of the interference pattern from D. 
But you see the clear patterns when you separate  

43:30.835 --> 43:36.435
those C and D photons. Okay. So the pattern that 
forms at the screen depends on whether we gain  

43:36.435 --> 43:43.155
knowledge of the path of the original photon. But 
crazily all of this detecting or erasing of path  

43:43.155 --> 43:49.955
information happens after each photon lands at 
the screen. After those photons land according  

43:49.955 --> 43:55.955
to an interference distribution or a single pile 
distribution. The information regarding whether  

43:55.955 --> 44:02.595
the observer knows the path appears to travel 
back in time even if it was only a tiny fraction  

44:02.595 --> 44:07.955
of a second in the experiment that was performed. 
Okay, so with a couple of slight modifications,  

44:07.955 --> 44:12.835
I think we can turn this into a communication 
device that can not only send information  

44:12.835 --> 44:18.035
instantaneously between two points, but 
can also send information back in time.  

44:18.595 --> 44:23.235
So, my evil genius plan. I'm going to build 
a version of this experiment that will let me  

44:23.235 --> 44:28.675
send tomorrow's winning lottery numbers back to 
myself. Here's how it works. First, let's bring  

44:28.675 --> 44:33.875
back detector B. We don't really need it, but it 
makes the explanation simpler. As it was executed,  

44:33.875 --> 44:41.555
photons can travel to the which way section, so 
detectors A and B, or to the eraser section, so C  

44:41.555 --> 44:48.835
and D. That choice is made by these beam splitters 
which randomly reflect 50% of photons and let 50%  

44:48.835 --> 44:54.595
pass through. But what if we could make this 
choice? Let's replace these beam splitters with  

44:54.595 --> 45:00.195
mirrors attached to a switch. With the mirrors 
in place, photons are reflected to the which way  

45:00.195 --> 45:05.715
detectors and no interference pattern is formed. 
But activate the switch and the mirrors move out  

45:05.715 --> 45:11.395
of the way. Photons travel through to the eraser 
section resulting in an interference pattern at  

45:11.395 --> 45:17.635
the screen. We make the decision of whether to 
know the path of the original photon or whether to  

45:17.635 --> 45:23.075
erase that knowledge. This conscious choice should 
be no different to the random decision of the beam  

45:23.075 --> 45:29.715
splitter. Now we have a way to send a binary code 
from the switch which is located at the which way/  

45:29.715 --> 45:35.395
eraser end of the experiment to the screen which 
could theoretically be very distant in both space  

45:35.395 --> 45:42.115
and time. Okay, we need one more alteration. 
Before the photons get to the which way end,  

45:42.115 --> 45:47.555
we freeze them for a day. That's long enough 
to buy lots of lottery tickets before the draw.  

45:47.555 --> 45:52.195
Actually, you can't really freeze photons. So, 
maybe we bounce them between Earth and the Moon  

45:52.195 --> 45:57.635
like 8,000 times. Let's just assume we can do 
that part perfectly. So, we turn on our device.  

45:57.635 --> 46:02.835
Photons start hitting the screen, building 
up some pattern. Once the lottery is drawn,  

46:02.835 --> 46:09.155
we use our switch to tap out the lottery numbers 
in binary code. Those numbers will emerge in the  

46:09.155 --> 46:14.435
interference patterns that were recorded a 
day ago. Here's my question. Why am I going  

46:14.435 --> 46:20.675
to be sadly disappointed after I turn on my 
invention? Oh, I do get a signal. Nice work,  

46:20.675 --> 46:26.995
future me. And that signal does contain the 
interference patterns from my future choices  

46:26.995 --> 46:35.315
with perfect fidelity. But for some reason, I 
can't read the numbers. What went wrong? Remember,  

46:35.315 --> 46:39.715
the landing location of each individual photon 
passing through to the interference screen of  

46:39.715 --> 46:45.795
this experiment does seem to be influenced by 
a decision that is made regarding each of those  

46:45.795 --> 46:52.515
photons entangled partners in the future. That 
decision was whether we would know the path of the  

46:52.515 --> 46:59.395
original photon, thus eliminating an interference 
pattern or to erase our knowledge of that path  

46:59.395 --> 47:04.435
which brings the interference pattern back. That 
decision is made randomly by a beam splitter in  

47:04.435 --> 47:09.235
the original experiment. But it's conceivable 
that the experiment could be adjusted so that  

47:09.235 --> 47:15.875
a person could make the decision. So why can't 
I send winning lottery numbers back to myself?  

47:15.875 --> 47:20.675
The reason is that there's absolutely no way 
to tell if any given photon at the interference  

47:20.675 --> 47:26.115
screen has a known path until you compare the 
results of the screen with the results at the  

47:26.115 --> 47:32.195
detector. In fact, the distribution of photons 
at the screen always looks like a single blurred  

47:32.195 --> 47:37.715
distribution. There's no visible interference 
pattern at all. It's only when you flag which  

47:37.715 --> 47:45.315
photons had twins arriving at detectors A, B, 
C, or D that you see patterns arise. In fact,  

47:45.315 --> 47:51.235
even if you remove all of the A and B photons, you 
still don't see an interference pattern until you  

47:51.235 --> 47:59.475
distinguish C versus D. And this is because those 
photons have interference bands that are exactly  

47:59.475 --> 48:05.315
out of phase. The peaks of C line up with the 
troughs of D, and together they look like the  

48:05.315 --> 48:11.795
same sort of blur you get if you combine A and B. 
This is pretty insane. The photon positions are  

48:11.795 --> 48:18.435
decided and presumably those patterns are embedded 
in the distribution. Those embedded patterns are  

48:18.435 --> 48:25.475
set by the eventual destination of the entangled 
partners of those photons. But the distribution  

48:25.475 --> 48:31.715
may be in place long before those twins finish 
their journey. Yet we can't extract the patterns  

48:31.715 --> 48:36.995
of the photons that landed at the screen until 
we get the information of which detectors their  

48:36.995 --> 48:46.035
entangled twins hit. That information can't 
travel backwards in time or faster than light.

48:46.035 --> 48:49.715
Is there a hidden physical reality that 
underlies the strange behavior of the  

48:49.715 --> 48:56.675
quantum world? Or is that reality an illusion in 
the eye of the observer? The weird phenomenon of  

48:56.675 --> 49:02.835
quantum entanglement gives us quite startling 
clues to the answer. Babies may suck at math,  

49:02.835 --> 49:08.035
but they're actually surprisingly good at quantum 
mechanics. Well, one fundamental aspect of it  

49:08.035 --> 49:14.515
anyway, the Peekaboo game is thought to be so 
hilarious to babies because they lack object  

49:14.515 --> 49:18.595
permanence. Hide your face with your hands and 
a baby won't automatically assume that you kept  

49:18.595 --> 49:23.715
existing. Reveal your face again and it's as 
though you popped into existence from nothing,  

49:23.715 --> 49:28.595
which is, of course, a hilarious thing to do. 
Kids get over this pretty quickly as they learn  

49:28.595 --> 49:33.555
that things don't magically pop into and out 
of existence for no reason. By the time they  

49:33.555 --> 49:38.115
grow up and go to college to study physics, 
the notion of object permanence is so deeply  

49:38.115 --> 49:43.715
embedded that we don't even bother teaching it in 
physics 101. And yet, the idea that the universe  

49:43.715 --> 49:49.155
keeps existing when we're not looking at it is 
a pretty fundamental implied assumption behind  

49:49.155 --> 49:54.835
all of classical physics. Indeed, most of science 
takes it for granted that the universe is real,  

49:54.835 --> 50:00.675
whether or not we're looking at it. This notion 
that the universe exists independent of the mind  

50:00.675 --> 50:08.515
of the observer is called realism in physics. But 
quantum mechanics is so bizarre that it still has  

50:08.515 --> 50:13.715
scientists wondering if we need to reject even 
this basic premise. This was the source of one of  

50:13.715 --> 50:20.195
the most heated debates at the advent of quantum 
mechanics. On the one hand, Neils Boore insisted  

50:20.195 --> 50:25.795
that it was meaningless to assign reality to 
the universe in the absence of observation.  

50:26.515 --> 50:31.875
In the intervals between measurements, 
quantum systems truly exist as a fuzzy  

50:31.875 --> 50:39.875
mixture of all possible properties. What we call 
a superposition of states in between observations.  

50:39.875 --> 50:46.355
The wave function describing this superposition 
is a complete description of reality. And our  

50:46.355 --> 50:53.315
experience of a well-defined material universe 
only has meaning at the moment of measurement.  

50:53.315 --> 51:00.755
This peekab-boo universe is the heart of Bor's 
Copenhagen interpretation. On the other hand,  

51:00.755 --> 51:07.555
Albert Einstein insisted on an objective reality, 
a reality independent of our observation of it. He  

51:07.555 --> 51:13.875
insisted that the wave function and by extension 
quantum mechanics is incomplete. There must exist  

51:13.875 --> 51:19.075
what we call hidden variables that reflect a 
more physical underlying reality. In an effort  

51:19.075 --> 51:25.555
to demonstrate the silliness of Bor's idea, 
Einstein along with Boris Podolski and Nathan  

51:25.555 --> 51:32.035
Rosen proposed a quantum scenario that showed that 
in order to abandon the assumption of realism,  

51:32.035 --> 51:39.155
you also had to abandon a concept almost as 
sacred. Locality. Locality is the idea that each  

51:39.155 --> 51:44.915
bit of the universe only acts on its immediate 
surroundings. This is fundamental to Einstein's  

51:44.915 --> 51:50.275
relativity which tells us that the chain of 
cause and effect can't propagate any faster  

51:50.275 --> 51:56.355
than the speed of light. The Einstein Podilski 
Rosen or EPR paradox introduces one of the most  

51:56.355 --> 52:03.395
mysterious ideas in quantum mechanics. Quantum 
entanglement. Here's the idea. Two particles  

52:03.395 --> 52:08.515
interact briefly. They influence each other so 
that their properties are somehow connected.  

52:08.515 --> 52:14.835
And yet we refrain from measuring these properties 
to preserve quantum uncertainty. Quantum mechanics  

52:14.835 --> 52:20.755
requires that we describe the particle pair with 
a single combined wave function that encompasses  

52:20.755 --> 52:27.555
all possible states for both particles. We call 
such particles an entangled pair. Now, according  

52:27.555 --> 52:33.395
to the Copenhagen interpretation, any measurement 
of one particle automatically collapses the entire  

52:33.395 --> 52:38.835
entangled wave function and so affects the 
results of measurements of the other particle.  

52:38.835 --> 52:45.635
That's an influence that could theoretically be 
transmitted instantly across any distance and  

52:45.635 --> 52:53.955
even back in time, violating locality and possibly 
violating causality. Einstein at Al thought this  

52:53.955 --> 52:59.555
was very silly. They thought that every spatial 
point in the universe must be real and physical  

52:59.555 --> 53:07.315
and defined by knowable quantities, local hidden 
variables that could affect each other no faster  

53:07.315 --> 53:13.315
than the speed of light. The argument between Bor 
and Einstein felt a bit philosophical at the time,  

53:13.315 --> 53:19.795
but in 1964, Irish physicist John Stewart Bell 
proposed an experiment to resolve the debate. It  

53:19.795 --> 53:26.275
involves entangled electron and posetron pairs. 
When spontaneously created from a photon, these  

53:26.275 --> 53:32.835
particles will always be spinning in opposite 
directions to each other. However, until measured,  

53:32.835 --> 53:38.435
we can't know which direction either is spinning. 
We just know that they're opposite each other.  

53:38.435 --> 53:44.355
Their wave functions are therefore entangled. 
Measurement of the spin of one of these particles  

53:44.355 --> 53:49.795
tells us the spin of the other, no matter how 
large the distance between them. But in quantum  

53:49.795 --> 53:54.835
mechanics, measurement actually affects the thing 
you're measuring. In the case of quantum spin,  

53:54.835 --> 54:00.195
that measurement effect is especially weird. 
We define spin direction according to the spin  

54:00.195 --> 54:06.515
axis. That axis can point in any direction. But 
to measure spin direction, we need to choose  

54:06.515 --> 54:13.235
an axis to align our measuring device. We always 
find that the observed quantum spin aligns itself  

54:13.235 --> 54:19.075
with our chosen measurement axis. If we choose 
to measure vertically, the spin will turn out  

54:19.075 --> 54:26.995
to be up or down. If we measure horizontally, 
it'll be left or right. Measurement forces the  

54:26.995 --> 54:33.395
alignment of the measured particle. But how does 
this affect the spin of its entangled partner? The  

54:33.395 --> 54:41.875
answer would settle the bore Einstein debate. Let 
me explain. Scenario one. If Einstein was right,  

54:41.875 --> 54:47.635
imagine the response of each particle to all 
possible spin measurements is encoded in each  

54:47.635 --> 54:54.995
particle at the moment of their creation as hidden 
variables local to each particle. Nothing we do  

54:54.995 --> 55:01.635
later to one particle will then affect the other. 
When we later measure the spins of both particles,  

55:01.635 --> 55:06.195
there will be a correlation in the results 
because the particles were once connected.  

55:06.195 --> 55:11.795
But there'll be no correlation due to our 
choice of measurement axis. Scenario two,  

55:11.795 --> 55:18.035
B was right. What if between creation and 
measurement, the electron and posetron only  

55:18.035 --> 55:23.955
exist as a wave function of all possible states. 
In that case, measurement of one particle spin  

55:23.955 --> 55:30.915
should cause the entire wave function to collapse 
to take on defined values. Both particles should  

55:30.915 --> 55:36.995
then manifest opposite spins along whichever 
axis we choose for one of the particles. That  

55:36.995 --> 55:41.875
should lead to a correlation between our choice 
of measurement axis for the first particle and the  

55:41.875 --> 55:48.515
spin direction then measured for the second. This 
is exactly the spooky action at a distance that  

55:48.515 --> 55:54.195
made Einstein so uncomfortable. So John Stewart 
Bell figured out a set of observable results,  

55:54.195 --> 56:01.475
the so-called Bell inequalities that we'd expect 
to see in the case that Einstein was right and  

56:01.475 --> 56:07.955
quantum mechanics needs local hidden variables. 
But if an entanglement experiment violates the  

56:07.955 --> 56:15.235
Bell inequalities, then local realism is also 
violated. By the way, Veritassium describes this  

56:15.235 --> 56:20.995
experiment in much more detail in this video. 
I recommend you check it out. It's a tricky  

56:20.995 --> 56:27.555
experiment because entangled quantum states are 
hard to produce but even harder to sustain. Any  

56:27.555 --> 56:33.475
interaction can destroy the entanglement. But 
in the early 80s, French physicist Alana Spect  

56:33.475 --> 56:40.035
succeeded. Instead of looking at the entangled 
spins of an electron posetron pair, he used photon  

56:40.035 --> 56:45.715
pairs with entangled polarizations. Polarization 
is just the alignment of a photon's electric and  

56:45.715 --> 56:51.635
magnetic fields. But the principle is the same and 
aspect found that there was a correlation between  

56:51.635 --> 56:57.395
the choice of polarization measurement axis for 
one photon and the final polarization direction of  

56:57.395 --> 57:04.275
its entangled partner. The bell inequalities were 
violated. The experiment was even set up so that  

57:04.275 --> 57:09.795
the influence had to travel between the photons 
at faster than the speed of light. Since then,  

57:09.795 --> 57:15.075
many experiments have verified this result over 
larger and larger distances. The instantaneous  

57:15.075 --> 57:20.675
influence has been observed over many kilometers 
at this point. The delayed choice quantum eraser  

57:20.675 --> 57:26.435
which we already covered is yet another example 
of this strange result. It's now been thoroughly  

57:26.435 --> 57:32.275
confirmed that the bell inequalities are violated 
suggesting that the wave function cannot have  

57:32.275 --> 57:38.515
local hidden variables. So does this confirm 
the Copenhagen interpretation and kill both  

57:38.515 --> 57:44.915
locality and realism? Do we live in a peekab-boo 
universe that vanishes into quantum abstraction  

57:44.915 --> 57:51.155
when we aren't looking at it? Are babies really 
better at quantum mechanics than Einstein? Well,  

57:51.155 --> 57:58.035
not so fast. The results of these entanglement 
experiments do seem to violate local realism,  

57:58.035 --> 58:04.915
but that may mean a violation of realism or just 
of locality. In fact, it was Dr. Bell's opinion  

58:04.915 --> 58:12.195
that the violation of his inequalities disproved 
only locality. Realism could be salvaged.  

58:12.195 --> 58:18.595
Non-locality requires that entangled particles 
affect each other instantaneously. That sounds  

58:18.595 --> 58:24.595
blasphemous to anyone who accepts Einstein's 
theory of relativity. However, non-locality and  

58:24.595 --> 58:31.395
relativity can actually be perfectly consistent. 
Relativity requires that causality is preserved.  

58:31.395 --> 58:38.195
So no faster than light information flow. But none 
of these entanglement experiments allow any real  

58:38.195 --> 58:44.515
information to be transmitted between particles. 
It's only possible to see the influence between  

58:44.515 --> 58:50.195
the entangled partners after measurements have 
been made and those measurements are compared.  

58:50.195 --> 58:56.195
Just as we saw with the delayed choice quantum 
eraser, the universe seems to conspire to avoid  

58:56.195 --> 59:02.035
the paradox of information traveling faster 
than light or backwards in time. The Copenhagen  

59:02.035 --> 59:09.555
interpretation remains consistent with all quantum 
observations. Neils Bore's peekaboo universe may  

59:09.555 --> 59:15.315
be the universe we live in. However, realist 
and hidden variable interpretations are also  

59:15.315 --> 59:21.795
consistent as long as they abandon locality. For 
example, entangled particles may be dimensionally  

59:21.795 --> 59:28.915
connected by Einstein Rosen bridges, wormholes 
that allow instantaneous contact even between  

59:28.915 --> 59:36.915
great spatial separations. Also, the De Bruy Bone 
pilot wave theory works by assuming realism and  

59:36.915 --> 59:44.515
non-local hidden variables. There's even a way 
around all of this without sacrificing realism or  

59:44.515 --> 59:51.475
locality. That's the many worlds interpretation. 
We'll get back to these in an upcoming episode of  

59:51.475 --> 59:57.635
Spacetime. The weird rules of the subatomic world 
are very, very different to those of the familiar  

59:57.635 --> 01:00:05.235
large scale universe. A huge outstanding question 
is when and why does the weirdness of quantum  

01:00:05.235 --> 01:00:11.475
mechanics give way to classical physics? One 
answer to this question suggests that the entire  

01:00:11.475 --> 01:00:18.755
universe is so much weirder than we imagined. or 
should I say the multiverse. One of the strangest  

01:00:18.755 --> 01:00:24.915
features of the quantum description of reality is 
the idea of superposition. We can't describe the  

01:00:24.915 --> 01:00:30.035
most fundamental building blocks of our universe 
with defined singular properties. Instead,  

01:00:30.035 --> 01:00:36.755
they seem to behave as probability clouds 
of all properties they might have were we  

01:00:36.755 --> 01:00:42.675
to try to measure them. Mathematically, this 
is encapsulated in the wave function of a  

01:00:42.675 --> 01:00:47.875
quantum particle or system of particles. The 
best illustration of why we need to describe  

01:00:47.875 --> 01:00:53.075
the quantum world this way is the famous double 
slit experiment. We did an episode on it. Check  

01:00:53.075 --> 01:00:58.595
it out if you aren't familiar. But to summarize, 
a stream of photons or electrons or even molecules  

01:00:58.595 --> 01:01:03.635
travels from some point to a detector screen 
via a pair of slits. These particles arrive at  

01:01:03.635 --> 01:01:08.915
the screen distributed like the interference 
pattern you would expect from a simple wave.  

01:01:08.915 --> 01:01:15.075
Quantum mechanics very successfully predicts this 
result by describing each particle's journey as a  

01:01:15.075 --> 01:01:21.555
superp position of all possible trajectories. In 
other words, the particle simultaneously takes  

01:01:21.555 --> 01:01:28.835
all possible paths which means it passes through 
both slits. It tries out all histories between  

01:01:28.835 --> 01:01:35.155
launch and landing and those many maybe histories 
somehow interact with each other to determine the  

01:01:35.155 --> 01:01:42.115
most likely final destination when a measurement 
is made. In a sense, different possible superposed  

01:01:42.115 --> 01:01:50.035
histories appear to converge on one final outcome. 
But what causes that convergence? In the original  

01:01:50.035 --> 01:01:55.635
Copenhagen interpretation of quantum mechanics, 
the act of measurement was thought to collapse  

01:01:55.635 --> 01:02:01.795
possibility space into a single reality, at 
least with respect to the measured property.  

01:02:01.795 --> 01:02:07.715
It collapses the wave function. That collapse 
signifies the transition between the quantum and  

01:02:07.715 --> 01:02:14.995
classical realms. One of the founders of quantum 
mechanics, Owen Schrodinger, found this ridiculous  

01:02:14.995 --> 01:02:20.275
and he proposed his famous Schrodinger's cat 
thought experiment to highlight the absurdity. It  

01:02:20.275 --> 01:02:27.795
goes like this. A cat is in a box with a flask of 
poison. A machine containing a radioactive element  

01:02:27.795 --> 01:02:34.595
is set to shatter the flask in the event that 
the radioactive element decays. If that happens,  

01:02:34.595 --> 01:02:42.275
the cat dies. that radioactive decay is a purely 
quantum process. And so until it's observed,  

01:02:42.275 --> 01:02:49.555
it exists in a superp position of states. It has 
both decayed and not decayed. But doesn't that  

01:02:49.555 --> 01:02:55.475
mean that the entire macroscopic system attached 
to that quantum event is also in superp position?  

01:02:55.475 --> 01:03:03.795
If so, then the cat should be simultaneously alive 
and dead until we open the box. But why can't the  

01:03:03.795 --> 01:03:10.995
cat collapse its own wave function? And from its 
point of view, is the physicist outside also a  

01:03:10.995 --> 01:03:17.075
quantum blur until the box is opened? And what 
about the entire rest of the universe that's not  

01:03:17.075 --> 01:03:24.035
currently being observed by physicists or cats? 
Many adherence to Copenhagen now have a more  

01:03:24.035 --> 01:03:30.515
sensible resolution to the paradox of Schroinger's 
cat. It's that quantum superposition doesn't  

01:03:30.515 --> 01:03:38.275
extend to macroscopic scales. It disappears when 
different quantum scale histories diverge. This  

01:03:38.275 --> 01:03:44.675
is called decoherence. When the wave functions 
describing quantum systems overlap sufficiently,  

01:03:44.675 --> 01:03:50.195
in other words, they are coherent. It's possible 
to get interference in the double slit experiment  

01:03:50.195 --> 01:03:55.235
and spookily correlated quantum entanglement 
measurements. But when these systems interact  

01:03:55.235 --> 01:04:01.875
with their environment, coherence is lost and 
parallel histories fall out of alignment. They  

01:04:01.875 --> 01:04:06.035
can no longer interact with each other. By the 
Copenhagen interpretation, we might say that  

01:04:06.035 --> 01:04:11.475
the universe chooses the final outcome of all 
of those histories. It doesn't exactly choose  

01:04:11.475 --> 01:04:18.595
a single history. Instead, it chooses an end 
result, say particle location on a screen or  

01:04:18.595 --> 01:04:25.395
cat alive or deadness based on those histories. 
If a larger number of possible histories lead  

01:04:25.395 --> 01:04:31.315
to a given result, then it's more likely that the 
universe will select that outcome. The Copenhagen  

01:04:31.315 --> 01:04:38.035
interpretation says that this selection happens 
in a fundamentally random way. The universe plays  

01:04:38.035 --> 01:04:44.595
dice even if the dice are weighted towards 
certain results. It is what we would call a  

01:04:44.595 --> 01:04:51.235
non-deterministic interpretation because there's 
no underlying predictability behind the selection.  

01:04:51.235 --> 01:04:56.355
However, there is another way to interpret the 
transition between the quantum and classical  

01:04:56.355 --> 01:05:04.755
worlds. What if the wave function never collapses? 
If we can imagine a cat in a superp position of  

01:05:04.755 --> 01:05:10.755
states, alive and dead, why stop at the cat? 
What if the family of possible states extends  

01:05:10.755 --> 01:05:16.595
beyond the radioactive decay, beyond the cat, 
and includes the observer and indeed the entire  

01:05:16.595 --> 01:05:23.155
universe, too? If we open the box and find that 
the cat is alive, it's because we are part of an  

01:05:23.155 --> 01:05:30.755
entire quantum timeline in which the radioactive 
decay and subsequent poisoning never happened.  

01:05:30.755 --> 01:05:36.915
But there's an equally valid timeline in which 
it did and another version of us experiencing  

01:05:36.915 --> 01:05:43.395
that. This sounds outrageous, but it's a very 
serious interpretation of the mathematics of  

01:05:43.395 --> 01:05:52.115
quantum mechanics. It was proposed by Hugh Everett 
in his 1957 PhD thesis entitled the theory of the  

01:05:52.115 --> 01:05:58.915
universal wave function. It's come to be known 
as the many worlds interpretation. To outline  

01:05:58.915 --> 01:06:04.115
the idea without killing so many cats, let's talk 
about what this means in the context of the double  

01:06:04.115 --> 01:06:10.435
slit experiment. The Copenhagen interpretation 
tells us that the superp position of particle  

01:06:10.435 --> 01:06:18.435
trajectories of histories merges into the 
single timeline of the observer's reality.  

01:06:18.435 --> 01:06:26.115
Many worlds says this merging never happens. 
Those alternative histories continue and we  

01:06:26.115 --> 01:06:33.155
find ourselves in just one of those timelines. 
Which one? Well, they're all equally likely,  

01:06:33.155 --> 01:06:39.235
but some look very similar to each other. 
For example, many histories lead to photons  

01:06:39.235 --> 01:06:45.795
landing on the bright bands of the interference 
pattern and very few to the dark bands. We tend  

01:06:45.795 --> 01:06:51.955
to find ourselves in the more common families 
of histories. This is a pretty crazy notion.  

01:06:51.955 --> 01:06:57.555
The many worlds interpretation invites the idea 
that reality splits into different branches every  

01:06:57.555 --> 01:07:03.555
time quantum states diverge into different 
possibilities. For example, at every particle  

01:07:03.555 --> 01:07:09.475
interaction everywhere in the universe, this would 
lead to an unthinkably large number of alternate  

01:07:09.475 --> 01:07:16.355
timelines or worlds that contain all possible 
realizations of this universe since the big bang.  

01:07:16.355 --> 01:07:23.315
It seems extravagant to propose uncountable 
eternally branching universes just to get out  

01:07:23.315 --> 01:07:28.915
of collapsing away function. It's like building 
an entirely new house to avoid doing the dishes.  

01:07:28.915 --> 01:07:35.075
But remember, the Copenhagen interpretation itself 
proposes multiple worlds in the superposition of  

01:07:35.075 --> 01:07:42.755
paths or properties of a quantum system. Both many 
worlds and Copenhagen create alternate realities.  

01:07:42.755 --> 01:07:47.795
It's just that Copenhagen merges them into 
a single timeline with its wave function  

01:07:47.795 --> 01:07:54.755
collapse. The superposition of states of many 
worlds can be thought of as overlaid histories,  

01:07:54.755 --> 01:08:01.795
slices of a universal wave function that diverge 
from each other as the universe evolves, but none  

01:08:01.795 --> 01:08:07.955
ever vanish. Many worlds may in fact be the more 
pure interpretation of the mathematics of quantum  

01:08:07.955 --> 01:08:14.035
mechanics because there's nothing in that math 
that requires the collapse of the wave function.  

01:08:14.035 --> 01:08:20.675
So many worlds is more economical in the number of 
unsupported concepts it adds to quantum mechanics,  

01:08:20.675 --> 01:08:26.275
even if it isn't particularly economical in 
the number of universes it predicts. Now,  

01:08:26.275 --> 01:08:31.715
Everett's idea wasn't taken too seriously when 
it was first proposed. That may have been in part  

01:08:31.715 --> 01:08:37.315
because he wasn't a well-known physicist. He was 
just a graduate student who all but disappeared  

01:08:37.315 --> 01:08:44.195
into military research at the Pentagon right after 
graduation. But another point of resistance must  

01:08:44.195 --> 01:08:51.315
be the overwhelming existential crisis induced 
by the idea of near infinite versions of oneself.  

01:08:51.315 --> 01:08:57.395
Many worlds may imply that every possible version 
of you exists out there. uh you're just the one  

01:08:57.395 --> 01:09:04.195
who happens to be experiencing this branch of 
reality. Every other possible life path, including  

01:09:04.195 --> 01:09:10.675
those branching in different directions from 
every decision you ever made, may be just as real.  

01:09:10.675 --> 01:09:17.235
In fact, each may be real in vast multitudes. 
There's no more evidence for many worlds than  

01:09:17.235 --> 01:09:22.915
there is for other mainstream interpretations of 
quantum mechanics. And it is somewhat mainstream  

01:09:22.915 --> 01:09:30.915
these days with many noted physicists being 
swayed by its parimony, its economy of ideas,  

01:09:30.915 --> 01:09:35.715
but it remains an interpretation. And so 
although it is supported by the incredibly  

01:09:35.715 --> 01:09:40.675
successful mathematics of quantum mechanics, it is 
not yet added a prediction that might distinguish  

01:09:40.675 --> 01:09:47.555
it from other equally supported interpretations. 
Nor is it complete in its explanation. There are  

01:09:47.555 --> 01:09:51.635
some ideas about what's really happening 
when these neighboring coherent histories  

01:09:51.635 --> 01:09:57.235
interact or why the wave function translates 
to probabilities the way it does, but they are  

01:09:57.235 --> 01:10:04.275
far from generally accepted. Unlike Copenhagen, 
many worlds is a deterministic interpretation.  

01:10:04.275 --> 01:10:09.955
Any given timeline is a predictable chain of 
cause and effect. It explains the apparent  

01:10:09.955 --> 01:10:16.595
randomness of quantum mechanics with a sort of 
observer bias. All possibilities are chosen at  

01:10:16.595 --> 01:10:21.795
every junction and we just happen to be seeing 
the one that happened in the branch that we  

01:10:21.795 --> 01:10:30.115
occupy. That adds a second possible cause for 
philosophical unease in a purely deterministic  

01:10:30.115 --> 01:10:35.715
universe. What happens to free will? If we're 
going to make all possible decisions anyway,  

01:10:35.715 --> 01:10:42.835
why sweat any given choice? Well, sure. But of 
all those countless future branches of reality,  

01:10:42.835 --> 01:10:48.115
some are going to be pretty amazing. Think 
of it as a choose your own adventure and  

01:10:48.115 --> 01:10:56.435
steer this version of you towards one of the 
more awesome many world branches of spaceime.

01:10:57.955 --> 01:11:03.795
There is one interpretation of the meaning of 
quantum mechanics that somehow manages to skip  

01:11:03.795 --> 01:11:10.755
a lot of the wildly extravagant or near mystical 
ideas of the mainstream interpretations. It's the  

01:11:10.755 --> 01:11:18.675
deb pilot wave theory and despite its alluringly 
intuitive nature for some reason it remains a  

01:11:18.675 --> 01:11:24.115
fringe theory. Misinterpretation of the ideas 
of quantum mechanics has spawned some of the  

01:11:24.115 --> 01:11:33.635
worst quackery pseudocience hoo-ha and unfounded 
mystical storytelling of any scientific theory.  

01:11:33.635 --> 01:11:39.715
It's easy to see why. There are some pretty out 
there explanations for the processes at work  

01:11:39.715 --> 01:11:46.115
behind the incredibly successful mathematics of 
quantum mechanics. These explanations claim stuff  

01:11:46.115 --> 01:11:53.075
like things are both waves and particles at the 
same time. The act of observation defines reality,  

01:11:53.075 --> 01:11:59.235
cats are both alive and dead, or even that the 
universe is constantly splitting into infinite  

01:11:59.235 --> 01:12:04.435
alternate realities. The weird results of 
quantum experiments seem to demand weird  

01:12:04.435 --> 01:12:10.035
explanations of the nature of reality. But there 
is one interpretation of quantum mechanics that  

01:12:10.035 --> 01:12:18.835
remains comfortably almost stodgily physical. 
That's de Bruy Bow pilot wave theory. Pilot  

01:12:18.835 --> 01:12:25.395
wave theory also known as bomium mechanics stands 
in striking contrast to the much more mainstream  

01:12:25.395 --> 01:12:31.875
ideas of for example the Copenhagen and many 
worlds interpretations. Now we've covered both of  

01:12:31.875 --> 01:12:39.235
those before and those episodes are really worth 
a look. Pilot wave theory is perhaps the most  

01:12:39.235 --> 01:12:45.555
solidly physical, even mundane of the complete 
and self-consistent interpretations of quantum  

01:12:45.555 --> 01:12:52.675
mechanics. But at the same time, it's considered 
one of the least orthodox. Why so? Because  

01:12:52.675 --> 01:12:58.755
orthodoxy equals radicalism plus time. And the 
founding fathers of the Copenhagen interpretation  

01:12:58.755 --> 01:13:05.395
of quantum mechanics, Neils Boore and Vera 
Heisenberg were radicals. When quantum theory  

01:13:05.395 --> 01:13:11.075
was coming together in the 20s, they were fervent 
about the need to reject all classical thinking  

01:13:11.075 --> 01:13:16.835
in interpreting the strange results of early 
quantum experiments. One aspect of that radical  

01:13:16.835 --> 01:13:23.235
thinking was that the wave function is not a wave 
in anything physical but an abstract distribution  

01:13:23.235 --> 01:13:29.395
of probabilities. Borne Heisenberg insisted that 
in the absence of measurement, the unobserved  

01:13:29.395 --> 01:13:35.395
universe is only a suite of possibilities of the 
various states it could take were a measurement to  

01:13:35.395 --> 01:13:42.995
be made. And that upon measurement, fundamental 
randomness determines the properties of say the  

01:13:42.995 --> 01:13:50.355
particle that would emerge from its wave function. 
This required an almost mystical duality between  

01:13:50.355 --> 01:13:57.075
the wave and particle-like nature of matter. 
Not everyone was so sure. Einstein famously  

01:13:57.075 --> 01:14:03.235
hated the idea of fundamental randomness. But to 
counter Bor and Heisenberg, there needed to be a  

01:14:03.235 --> 01:14:08.595
full theory that described how a quantum object 
could show both wave and particle-like behavior  

01:14:08.595 --> 01:14:16.035
at the same time without being fundamentally 
probabilistic. That theory came from Louie  

01:14:16.035 --> 01:14:21.555
De Bruy, the guy who originally proposed the 
idea that matter could be described as waves  

01:14:21.555 --> 01:14:28.035
right at the beginning of the quantum revolution. 
Droy's theory reasoned that there was no need for  

01:14:28.035 --> 01:14:34.835
quantum objects to transition in a mystical way 
between non-real waves and real particles. Why  

01:14:34.835 --> 01:14:42.835
not just have real waves that push around real 
particles? This is pilot wave theory. In it,  

01:14:42.835 --> 01:14:49.875
the wave function describes a real wave of some 
stuff. This wave guides the motion of a real  

01:14:49.875 --> 01:14:55.955
pointlike particle that has a definite location 
at all times. Importantly, the wave function in  

01:14:55.955 --> 01:15:02.675
pilot wave theory evolves exactly according to 
the Schrodinger equation. That's the equation at  

01:15:02.675 --> 01:15:09.395
the heart of all quantum mechanics that tells the 
wave function how to change across space and time.  

01:15:09.395 --> 01:15:14.995
This means that pilot wave theory makes the 
same basic predictions as any other breed of  

01:15:14.995 --> 01:15:21.635
quantum mechanics. For example, this guiding 
wave does all the usual wavy stuff like form  

01:15:21.635 --> 01:15:27.635
an interference pattern when it passes through 
a pair of slits. Because particles follow the  

01:15:27.635 --> 01:15:33.155
paths etched out by the wave, it'll end 
up landing according to that pattern.  

01:15:33.155 --> 01:15:39.635
The wave defines a set of possible trajectories 
and the particle takes one of those trajectories,  

01:15:39.635 --> 01:15:45.315
but the choice of path isn't random. If you know 
the exact particle position and velocity at any  

01:15:45.315 --> 01:15:52.115
point, you could figure out its entire future 
trajectory. Apparent randomness arises because  

01:15:52.115 --> 01:15:58.275
we can't ever have a perfect measurement of 
initial position, velocity, or other properties.  

01:15:58.275 --> 01:16:06.675
But this hypothetical predictability means that a 
pilot wave universe is completely deterministic.  

01:16:06.675 --> 01:16:13.955
When De Bruyne presented his still incomplete 
theory at the famous Solve conference of 1927,  

01:16:13.955 --> 01:16:20.275
it didn't go down so well. Technical objections 
were raised and Neilsborg doubled down on the  

01:16:20.275 --> 01:16:27.395
probabilistic interpretation. De Bruyne was 
convinced and he dropped pilot waves altogether.  

01:16:27.395 --> 01:16:34.995
The idea was forgotten for decades and Copenhagen 
became the orthodoxy. It took until 1952 for  

01:16:34.995 --> 01:16:40.595
another physicist, David Bow, to feel very 
uncomfortable with some of the wackiness of  

01:16:40.595 --> 01:16:47.395
Copenhagen and to rediscover De Bruyy's old idea. 
Bow took up where De Bruy left off and completed  

01:16:47.395 --> 01:16:55.875
the theory. The result was bombium mechanics, 
also known as De Bruy Bow pilot wave theory.  

01:16:55.875 --> 01:17:02.275
These days, more and more serious physicists are 
favoring Bow's ideas. However, it's far from being  

01:17:02.275 --> 01:17:08.995
broadly accepted. De Bruyne himself remained 
firmly in the Copenhagen camp even after Bur's  

01:17:08.995 --> 01:17:15.075
efforts. See, although pilot wave theory makes 
all of the usual predictions of quantum mechanics,  

01:17:15.075 --> 01:17:20.355
it has some really fundamental differences. 
Those differences are in the sort of special  

01:17:20.355 --> 01:17:27.075
thinking you need to do in order to accept pilot 
waves over other interpretations. In fact, most  

01:17:27.075 --> 01:17:35.555
of the arguments for or against it are about this 
special thinking. Are you more or less comfortable  

01:17:35.555 --> 01:17:42.355
with the oddness of pilot waves versus the 
oddness, say, of Copenhagen or many worlds? So,  

01:17:42.355 --> 01:17:49.475
what uncomfortable thinking does pilot wave theory 
require? Well, for one thing, it needs a teensy  

01:17:49.475 --> 01:17:56.035
bit of extra math that mainstream interpretations 
don't, as well as the Schrodinger equation that  

01:17:56.035 --> 01:18:01.715
tells the wave function how to change. It also 
has a guiding equation that tells the particle  

01:18:01.715 --> 01:18:10.515
how to move within that wave function. That extra 
math is considered unparimious to some, a needless  

01:18:10.515 --> 01:18:17.075
added complexity. However, the guiding equation 
is derived directly from the wave function. So,  

01:18:17.075 --> 01:18:23.635
some would argue that it was there all along. A 
more troubling requirement of bomium mechanics is  

01:18:23.635 --> 01:18:28.675
that it does contain real complexity that is not 
encoded in the wave function. That's something  

01:18:28.675 --> 01:18:36.195
that Neil's bore was so fervently against. Burmium 
mechanics has so-called hidden variables, details  

01:18:36.195 --> 01:18:40.835
about the state of the particle that are not 
described by the wave function. According to pilot  

01:18:40.835 --> 01:18:45.795
wave theory, the wave function just describes 
the possible distribution of those variables  

01:18:45.795 --> 01:18:53.795
given our lack of perfect knowledge. But hidden 
variables have a bad rap in quantum mechanics.  

01:18:53.795 --> 01:18:59.795
Pretty soon after de Bruy first proposed pilot 
waves, the revered mathematician John von Noman  

01:18:59.795 --> 01:19:05.315
published a proof showing that hidden variable 
explanations for the wave function just couldn't  

01:19:05.315 --> 01:19:12.595
work. That proclamation contributed to the long 
shelving of pilot wave theory. But in fact,  

01:19:12.595 --> 01:19:19.075
vonoman didn't quite get the full answer. It turns 
out that the restriction against hidden variables  

01:19:19.075 --> 01:19:27.155
only applies to local hidden variables. So there 
can't be extra information about a specific region  

01:19:27.155 --> 01:19:33.155
of the wave function that the rest of the wave 
function doesn't know. This was figured out pretty  

01:19:33.155 --> 01:19:40.195
soon after vonoman's paper by German mathematician 
Greta Herman. Although her reputation wasn't  

01:19:40.195 --> 01:19:47.395
noticed until it was reerived by John Bell in the 
1960s. This helped the resuscitation of pilot wave  

01:19:47.395 --> 01:19:55.875
theory because bombium mechanics doesn't use local 
hidden variables. Its hidden variables are global.  

01:19:55.875 --> 01:20:01.635
The entire wave function knows the location, 
velocity, and spin of each particle. This  

01:20:01.635 --> 01:20:07.395
non-locality is another weird thing you have to 
believe in order to accept pilot waves. Not only  

01:20:07.395 --> 01:20:11.795
does the entire wave function know the properties 
of the particle, but the entire wave function can  

01:20:11.795 --> 01:20:17.235
be affected instantaneously. So a measurement 
at one point in the wave function will affect  

01:20:17.235 --> 01:20:23.075
its shape elsewhere. This can therefore affect 
the trajectories and properties of particles  

01:20:23.075 --> 01:20:30.835
carried by that wave potentially very far away. 
But quantum entanglement experiments show that  

01:20:30.835 --> 01:20:37.235
this sort of spooky action at a distance is a 
very real phenomenon. Again, we've gone into  

01:20:37.235 --> 01:20:43.875
the non-locality of entangled particles in detail 
before. Also worth a look. It's a tough idea to  

01:20:43.875 --> 01:20:49.555
swallow, but experiments indicate that some 
type of non-locality is real, whether or not  

01:20:49.555 --> 01:20:55.555
we accept pilot waves. It would be remiss of me 
to talk about pilot waves without mentioning the  

01:20:55.555 --> 01:21:01.795
amazing analogy that was discovered in bouncing 
droplets on a vibrating pool of oil. I'll let  

01:21:01.795 --> 01:21:08.275
Veritasium cover this one in detail. It's pretty 
amazing. But we see many of the familiar quantum  

01:21:08.275 --> 01:21:14.915
phenomena appear in this macroscopic system of 
a suspended oil droplet following its own pilot  

01:21:14.915 --> 01:21:23.155
wave. Now we shouldn't take a macroscopic analogy 
as proof of microscopic reality but it certainly  

01:21:23.155 --> 01:21:29.075
demonstrates that this sort of thing does happen 
in this universe at least on some scales. I should  

01:21:29.075 --> 01:21:35.955
probably also add that debone pilot wave theory 
is certainly wrong and I don't think anyone  

01:21:35.955 --> 01:21:42.275
could deny that because it doesn't account for 
relativity either special or general. That means  

01:21:42.275 --> 01:21:49.395
it's at best incomplete. While regular mechanics 
has quantum field theory as its relativistic  

01:21:49.395 --> 01:21:56.035
version, pilot wave theory hasn't quite got 
there yet. Quantum field theory pretty explicitly  

01:21:56.035 --> 01:22:03.315
requires that all possible particle trajectories 
be considered equally real. Pilot wave theory  

01:22:03.315 --> 01:22:10.675
postulates that the particle really takes a single 
actual trajectory, the bone trajectory. This is  

01:22:10.675 --> 01:22:17.075
not consistent with quantum field theory and so 
there isn't a complete relativistic formulation of  

01:22:17.075 --> 01:22:23.715
bombium mechanics yet. But there is good effort in 
that direction. Now, let's not even start talking  

01:22:23.715 --> 01:22:30.835
about gravity. No version of quantum mechanics 
has that sorted out. Also, we can't ignore  

01:22:30.835 --> 01:22:36.275
the fact that the initial motivation behind 
pilot wave theory was to preserve the idea of  

01:22:36.275 --> 01:22:44.515
real particles. And I think we need to be dubious 
about that sort of classical bias. All that said,  

01:22:44.515 --> 01:22:51.155
pilot wave theory does do something remarkable. It 
shows us that it's possible to have a consistent  

01:22:51.155 --> 01:22:57.555
interpretation of quantum mechanics that is 
both physical and deterministic. No hoo-ha  

01:22:57.555 --> 01:23:07.475
needed. Maybe something like pilot waves really 
do drive the microscopic mechanics of spaceime.

01:23:07.475 --> 01:23:14.275
It's 1928. Over the past quarter century, the 
greatest geniuses of the modern era discovered  

01:23:14.275 --> 01:23:19.635
the two keys to the fundamental nature of 
reality. Einstein's theory of special and  

01:23:19.635 --> 01:23:25.795
general relativity had changed forever the way 
we think about motion, space, and time. And the  

01:23:25.795 --> 01:23:30.515
emerging field of quantum mechanics had radically 
altered our understanding of the fundamental  

01:23:30.515 --> 01:23:37.795
building blocks of the universe. Yet this year 
1928, one brilliant insight would bring these  

01:23:37.795 --> 01:23:45.315
theories together and unveil the quantum fabric 
of reality. It would also predict the existence of  

01:23:45.315 --> 01:23:51.395
antimatter. By the late 1920s, Einstein and Plunk 
had already shown that light is a particle as well  

01:23:51.395 --> 01:23:57.955
as a wave. And Louis de Bruy had shown that all 
matter has this dual wave particle nature. Bore,  

01:23:57.955 --> 01:24:03.715
Heisenberg, Bourne and Pley and others pieced 
together a mathematical description for the  

01:24:03.715 --> 01:24:11.795
weird nature of subatomic particles. Then in 1926, 
Owen Schroinger wrote down his famous equation,  

01:24:11.795 --> 01:24:18.915
the Schrodinger equation, which breathed life into 
this emerging model. It describes how these matter  

01:24:18.915 --> 01:24:25.315
waves represented as wave functions change 
over time and allowed physicists to predict  

01:24:25.315 --> 01:24:31.875
the evolution of quantum systems such as the 
strange interference pattern in the famous double  

01:24:31.875 --> 01:24:38.595
slit experiment. Yet, everyone knew there was a 
problem. First and most obvious, the Schrodinger  

01:24:38.595 --> 01:24:45.555
equation is totally incompatible with Einstein's 
relativity. In relativity, the dimensions of space  

01:24:45.555 --> 01:24:52.035
and time are intrinsically connected and they flow 
into each other as frames of reference change. But  

01:24:52.035 --> 01:24:57.875
the Schroinger equation tracks the evolution of a 
particle's wave function according to one and only  

01:24:57.875 --> 01:25:04.035
one clock. Typically, the clock in the reference 
frame of the observer. Relativity tells us that  

01:25:04.035 --> 01:25:10.835
the passage of time depends on velocity. So, the 
Schroinger equation only works for slowmoving  

01:25:10.835 --> 01:25:17.235
objects. That's a problem. Subatomic particles 
are often moving at close to the speed of light.  

01:25:17.235 --> 01:25:22.195
The other problem with the Schrodinger equation 
is that it describes particles as simple wave  

01:25:22.195 --> 01:25:29.475
functions, distributions of possible positions 
and momenta that have no internal properties. Yet,  

01:25:29.475 --> 01:25:35.875
we now know that many elementary particles 
have an internal property called spin.  

01:25:35.875 --> 01:25:41.955
That doesn't mean that they're actually rotating, 
but spin does result in a sort of quantum angular  

01:25:41.955 --> 01:25:48.995
momentum. For example, electrons spin causes them 
to align themselves with magnetic fields just like  

01:25:48.995 --> 01:25:54.995
a rotating electric charge would. The axis of spin 
can point in different directions, for example,  

01:25:54.995 --> 01:26:02.435
up or down. The discovery of quantum spin starts 
with an Austrian physicist named Wolf Gang Pi. PI  

01:26:02.435 --> 01:26:07.955
realized that to explain electron energy levels 
in atoms, those electrons must obey a rule that  

01:26:07.955 --> 01:26:14.195
we call the pi exclusion principle. It states 
that no electron can occupy the same quantum  

01:26:14.195 --> 01:26:20.195
state as another electron. In fact, it applies 
to all particles called firmians. In the case of  

01:26:20.195 --> 01:26:27.155
electrons in atoms, it suggests that we should 
only find one electron per atomic orbital if  

01:26:27.155 --> 01:26:34.275
we count each orbital as a quantum state. However, 
we actually observe two electrons per orbital. And  

01:26:34.275 --> 01:26:41.475
so, PI realized there must exist a hidden quantum 
state. PI introduced what we call a new degree of  

01:26:41.475 --> 01:26:47.955
freedom internal to electrons. One that could take 
on one of two values. Let's call those values up  

01:26:47.955 --> 01:26:54.195
and down. that would allow two separate electrons, 
one up, one down, to occupy the same atomic energy  

01:26:54.195 --> 01:27:00.755
level without occupying the same quantum state and 
therefore violating the pi exclusion principle.  

01:27:00.755 --> 01:27:06.035
Other physicists soon figured out that this 
new quantum state represented spin and the up  

01:27:06.035 --> 01:27:12.435
and down degrees of freedom were the direction of 
pointing of the angular momentum axis. We now call  

01:27:12.435 --> 01:27:19.475
these two component wave functions spinors. Now 
it's okay to ignore spin in the old Schrodinger  

01:27:19.475 --> 01:27:25.395
equation and get approximate answers. But when a 
magnetic field is present, spin direction becomes  

01:27:25.395 --> 01:27:32.595
very important. So for fastoving electrons and 
for electrons in electromagnetic fields, the  

01:27:32.595 --> 01:27:39.235
Schrodinger equation gives the wrong answers. The 
problem consumed a brilliant British physicist,  

01:27:39.235 --> 01:27:45.955
Paul Drack. He wanted a fully relativistic version 
of the Schrodinger equation that worked for  

01:27:45.955 --> 01:27:54.755
electrons. In a way, he started with relativity. 
He wrote down Einstein's famous equation E= MC²,  

01:27:54.755 --> 01:28:01.315
but in its full form, including momentum. He then 
used quantum mechanical expressions for energy and  

01:28:01.315 --> 01:28:09.635
momentum. The result was a huge mess. But Drack 
stumbled upon a single simple idea that caused  

01:28:09.635 --> 01:28:16.755
the resulting horrendous mathematics to collapse 
into an incredibly simple beautiful equation. That  

01:28:16.755 --> 01:28:23.155
simplification required DAC to expand the internal 
workings of the electron even further. Instead of  

01:28:23.155 --> 01:28:30.675
having a two component spinner up and down as in 
power theory, he needed four components. Now, he  

01:28:30.675 --> 01:28:36.995
had no idea what those two additional mysterious 
components might mean. But the resulting equation  

01:28:36.995 --> 01:28:41.795
was so simple and elegant that somehow Direct 
knew that he was on to something. The resulting  

01:28:41.795 --> 01:28:48.675
DRA equation describes the space-time evolution of 
this weird four component particle wave function  

01:28:48.675 --> 01:28:54.755
represented by the symbol s. It contains the 
marks of both quantum mechanics in the plank  

01:28:54.755 --> 01:29:01.555
constant and relativity in the speed of light. The 
direct equation perfectly predicts the motion of  

01:29:01.555 --> 01:29:09.395
electrons at any speed, even in an electromagnetic 
field. It was a major victory. But it opened up  

01:29:09.395 --> 01:29:14.835
even more questions than it answered. To begin 
with, what on earth were those two extra degrees  

01:29:14.835 --> 01:29:20.675
of freedom in the four component electron? The 
answer came from trying to calculate the energy  

01:29:20.675 --> 01:29:27.475
of the electron using this equation. It predicted 
something totally bizarre. It allowed electrons to  

01:29:27.475 --> 01:29:34.435
exist in states of negative energy. If true, that 
would lead to some weird effects. For example,  

01:29:34.435 --> 01:29:41.795
a lone electron moving in an electromagnetic field 
could keep releasing energy as light infinitely  

01:29:41.795 --> 01:29:47.715
and sink lower and lower to infinite negative 
energy states. There was no bottom to the energy.  

01:29:47.715 --> 01:29:53.955
Well, we know perfectly well that this doesn't 
happen. Dur came up with an idea to explain this.  

01:29:53.955 --> 01:30:00.835
We call it the durac sea. Imagine an infinitely 
deep ocean of electrons that exists everywhere in  

01:30:00.835 --> 01:30:06.355
the universe. These electrons occupy all of 
the negative energy states all the way from  

01:30:06.355 --> 01:30:14.515
negative infinity up to zero. The only time we can 
actually interact with an electron is when one has  

01:30:14.515 --> 01:30:20.675
a positive energy which would leave it sitting on 
top of the sea. This is where the ply exclusion  

01:30:20.675 --> 01:30:26.675
principle comes back. If the energy states of 
this imaginary ocean are all completely full,  

01:30:26.675 --> 01:30:32.595
then that one extra electron can't lose any 
more energy. It just floats on top of the sea.  

01:30:32.595 --> 01:30:38.915
The idea of the Drax Sea leads to its own weird 
predictions. Remove one electron from the surface  

01:30:38.915 --> 01:30:45.315
and it leaves a hole. That hole should act like 
a particle all by itself. Bit like an eddy on the  

01:30:45.315 --> 01:30:51.155
surface of a pool of water. It would move around. 
It would have inertia acting like it had the mass  

01:30:51.155 --> 01:30:56.995
of the missing electron. It would also act like it 
had the opposite electric charge to the electron,  

01:30:56.995 --> 01:31:03.475
a positive charge. And if a positive energy 
electron found one of these holes, it would  

01:31:03.475 --> 01:31:10.835
fall in, annihilating both and releasing all of 
the energy bound up in their masses. Of course,  

01:31:10.835 --> 01:31:17.875
there is something in our universe that acts 
exactly like holes in the Durac Sea. is called  

01:31:17.875 --> 01:31:25.555
antimatter and DRA had just predicted its 
existence. Now the DRA sea itself doesn't  

01:31:25.555 --> 01:31:32.915
really exist but it was one of the first attempts 
to describe something very real. The idea of a  

01:31:32.915 --> 01:31:38.995
quantum field. We now know that every elementary 
particle has an associated field that fills all  

01:31:38.995 --> 01:31:46.435
of space. These fields are more like membranes 
than infinitely deep oceans. They have a very  

01:31:46.435 --> 01:31:52.435
definite energy, usually zero. And the elementary 
particles that we know and love are just regions  

01:31:52.435 --> 01:31:59.235
where a field has a bit more energy. That energy 
manifests as vibrations in the field. Now, quantum  

01:31:59.235 --> 01:32:06.195
field theory is a very deep topic and it'll be the 
subject of upcoming episodes. But for now, let's  

01:32:06.195 --> 01:32:12.995
get to the bottom of these holes. Paul Durac's 
negative energy solutions describe antimatter,  

01:32:12.995 --> 01:32:19.315
not holes in the Direct Sea. Only a few years 
after Durac wrote down his equation in 1928,  

01:32:19.315 --> 01:32:26.915
the posetron, the antimatter electron, was spotted 
in cosmic rays by Carl Anderson. Antimatter is  

01:32:26.915 --> 01:32:33.395
very real. But what is it? Well, it's a vibration 
in the same quantum field as its regular  

01:32:33.395 --> 01:32:40.035
matter counterpart. Antimatter's existence is 
fundamentally tied to these weird four component  

01:32:40.035 --> 01:32:47.155
electrons that Durk invented to make his equation 
work. Those two extra components correspond to  

01:32:47.155 --> 01:32:53.635
the up and down spins of the electron's antimatter 
counterpart. Two spin directions for the electron,  

01:32:53.635 --> 01:33:00.995
two for the posetron. A four component spinner. In 
fact, the electron and the posetron cannot exist  

01:33:00.995 --> 01:33:06.515
without each other. They are two sides of the same 
coin. Positive and negative energy solutions of  

01:33:06.515 --> 01:33:12.035
the same type of vibration in the electron field. 
It's actually a tiny bit more complicated than  

01:33:12.035 --> 01:33:18.115
that and way more awesome, but there will be time 
for all of that in the future. So, all elementary  

01:33:18.115 --> 01:33:24.115
particles have a quantum field and all have an 
antimatter counterpart. Just as with the holes in  

01:33:24.115 --> 01:33:30.835
the direct sea, antimatter particles have the same 
mass as their counterparts, but opposite charge.  

01:33:30.835 --> 01:33:36.915
That mass is very real. It's not negative mass 
despite this negative energy description. When  

01:33:36.915 --> 01:33:42.435
matter antimatter counterparts find each other, 
they annihilate, releasing an awful lot of very  

01:33:42.435 --> 01:33:48.675
real energy. A penny of antimatter could be 
used to launch a goodsized rocket into orbit.  

01:33:48.675 --> 01:33:55.235
Dur's incredible insight in combining quantum 
mechanics and relativity revealed an entire  

01:33:55.235 --> 01:34:01.315
flip side of our universe with his prediction 
of antimatter. It was also a key step in the  

01:34:01.315 --> 01:34:06.195
discovery of quantum fields and quantum field 
theory and the development of the standard model  

01:34:06.195 --> 01:34:11.315
of particle physics which have become our 
best description of the underlying workings  

01:34:11.315 --> 01:34:21.475
of reality. And that's a quantum rabbit hole that 
we'll jump into very soon right here on Spacetime.

01:34:21.475 --> 01:34:26.915
Sometimes intuitive large-scale phenomena can 
give us incredible insights into the extremely  

01:34:26.915 --> 01:34:34.435
unintuitive world of quantum mechanics. Today, 
the humble soundwave is going to open the door to  

01:34:34.435 --> 01:34:40.435
really understanding Heisenberg's uncertainty 
principle and ultimately quantum fields and  

01:34:40.435 --> 01:34:45.715
Hawking radiation. One of the most difficult 
ideas to swallow in quantum mechanics is Verer  

01:34:45.715 --> 01:34:50.355
Heisenberg's famous uncertainty principle. 
It expresses the fundamental limit on the  

01:34:50.355 --> 01:34:55.875
knowability of our universe. We've discussed it 
in earlier videos on quantum mechanics. But it's  

01:34:55.875 --> 01:35:01.875
time we looked a little deeper. See, the apparent 
weirdness of the uncertainty principle hints at  

01:35:01.875 --> 01:35:08.195
an even weirder underlying reality that gives 
rise to it. The universe we experience seems  

01:35:08.195 --> 01:35:14.355
to be constructed of singular particles with 
well- definfined properties. But this intuitive  

01:35:14.355 --> 01:35:21.075
mechanical reality is emergent from an underlying 
reality in which the particles that form matter  

01:35:21.075 --> 01:35:27.715
arise from the combination of an infinity 
of possible properties. And forget matter,  

01:35:27.715 --> 01:35:34.275
the vacuum itself can be thought of as constructed 
from the sum of infinite possible particles. If we  

01:35:34.275 --> 01:35:40.515
fully unravel this idea, we'll be on the verge 
of tackling things like Hawking radiation. But  

01:35:40.515 --> 01:35:48.275
as you'll see today in that unraveling, we are led 
unavoidably to Heisenberg's uncertainty principle.  

01:35:48.275 --> 01:35:53.795
The uncertainty principle is most often expressed 
in terms of position and momentum. We cannot  

01:35:53.795 --> 01:36:00.355
simultaneously know both position and momentum 
for a quantum system with absolute precision. Try  

01:36:00.355 --> 01:36:07.155
to perfectly nail down a particle's position and 
we have complete uncertainty about its momentum.  

01:36:07.155 --> 01:36:11.395
And it's not just because our measurement 
of position requires us to interact with the  

01:36:11.395 --> 01:36:17.075
particle, therefore changing its momentum. No, 
the uncertainty principle exists alongside this  

01:36:17.075 --> 01:36:23.555
observer effect. It's instead a statement about 
how much information we are ever able to extract  

01:36:23.555 --> 01:36:29.635
from a quantum system. To understand the origin 
of the uncertainty principle, we don't need to  

01:36:29.635 --> 01:36:35.555
know any quantum mechanics. At least not to start 
with. See, quantum mechanics is a type of wave  

01:36:35.555 --> 01:36:41.315
mechanics. a very weird type. However, it turns 
out that something like the uncertainty principle  

01:36:41.315 --> 01:36:48.515
arises in any wave mechanics. So, let's choose 
a type of wave that's a little more intuitive.  

01:36:48.515 --> 01:36:55.395
Sound waves. You can describe a sound wave just 
as the intensity of the wave as it passes by. So,  

01:36:55.395 --> 01:37:01.475
intensity changing over time. It can take really 
any shape. That shape determines what the wave  

01:37:01.475 --> 01:37:08.595
sounds like to our ears. The soundwave for a 
simple pure tone like a middle C is a sinosoidal  

01:37:08.595 --> 01:37:14.675
wave with the frequency determining the pitch of 
the tone. The soundwave from say an orchestra is  

01:37:14.675 --> 01:37:22.915
extremely complex but amazingly it can always be 
broken down into a combination of many simple sine  

01:37:22.915 --> 01:37:30.035
waves of different frequencies. This is Furier's 
theorem after French mathematician Jean Baptiste  

01:37:30.035 --> 01:37:36.915
de Joseph Furier. It states that any complex 
soundwave can be decomposed into a number of  

01:37:36.915 --> 01:37:42.835
sine waves of different frequencies, each with a 
different strength stacked on top of each other  

01:37:42.835 --> 01:37:49.235
or superposed. In fact, instead of representing 
a sound wave in terms of intensity changing with  

01:37:49.235 --> 01:37:55.235
time, you can also represent it in terms of its 
frequency components, each with its own waiting  

01:37:55.235 --> 01:38:02.355
or strength. When you switch between a time and 
a frequency representation, you're doing a forier  

01:38:02.355 --> 01:38:09.155
transform. In fact, digital audio equipment 
stores, manipulates, and transmits sound in its  

01:38:09.155 --> 01:38:15.875
frequency representation. In the physics of sound, 
time and frequency have a special relationship  

01:38:15.875 --> 01:38:22.035
because any sound wave can be represented in 
terms of one or the other. We call them fier  

01:38:22.035 --> 01:38:29.875
pairs also sometimes conjugate variables. Okay. So 
we can make any shaped soundwave with a series of  

01:38:29.875 --> 01:38:36.435
sine waves of different frequencies. For example, 
you can build a wave packet by adding frequency  

01:38:36.435 --> 01:38:43.475
components with the right phases to destructively 
interfere everywhere except within a small region.  

01:38:43.475 --> 01:38:48.595
The tighter you want to make that time window for 
the wave packet, the more frequency components you  

01:38:48.595 --> 01:38:54.355
need to use. In fact, to get those steep edges of 
the wave packet, you need to add higher and higher  

01:38:54.355 --> 01:38:59.235
frequencies because the high frequency components 
are the ones that give you rapid changes in  

01:38:59.235 --> 01:39:05.955
intensity. So, what if you try to compress the 
wave packet to a a single spike, a blip of sound  

01:39:05.955 --> 01:39:12.195
that exists for only one instant in time? Is it 
even possible to make an instantaneous spike at  

01:39:12.195 --> 01:39:19.075
one point in time out of a bunch of sine waves 
that themselves extend infinitely through time? In  

01:39:19.075 --> 01:39:25.635
fact, it is. However, to get a spike at one point 
in time, you need to use infinitely many different  

01:39:25.635 --> 01:39:32.435
frequency sine waves, each of which exist at all 
points in time. So then if we make a sound that  

01:39:32.435 --> 01:39:38.595
is perfectly located in time, it doesn't have 
a frequency or it has all frequencies. At the  

01:39:38.595 --> 01:39:44.675
same time, a sound wave with a perfectly known 
frequency is a simple traveling sine wave that  

01:39:44.675 --> 01:39:51.235
extends infinitely in time. So the time of its 
existence is undefined. That sounds an awful  

01:39:51.235 --> 01:39:56.675
lot like a frequency time uncertainty principle 
for sound waves. Now it's not really a statement  

01:39:56.675 --> 01:40:02.675
about the fundamental knowability of a soundwave 
as is Heisenberg's uncertainty principle is more a  

01:40:02.675 --> 01:40:08.675
statement about the sampling of frequencies needed 
to produce a given wave packet. But the underlying  

01:40:08.675 --> 01:40:14.755
idea is the same. So how does this relate to the 
quantum world? Well, before we get back to quantum  

01:40:14.755 --> 01:40:21.075
fields, let's think about the wave function, the 
solution to the Schrodinger equation that contains  

01:40:21.075 --> 01:40:27.155
all of the information about a quantum system. 
Like the soundwave, it oscillates through space at  

01:40:27.155 --> 01:40:31.395
a particular frequency. To keep things simple, 
we're just going to consider a wave function  

01:40:31.395 --> 01:40:37.395
that doesn't vary in time. It only changes with 
position in space. This is more like a standing  

01:40:37.395 --> 01:40:43.795
soundwave inside an organ pipe rather than the 
traveling soundwave from earlier. So position  

01:40:43.795 --> 01:40:50.595
rather than time becomes the first of our fier 
pair. The second of this pair is momentum,  

01:40:50.595 --> 01:40:56.275
not frequency. See, momentum is sort of the 
generalization of frequency for what we call  

01:40:56.275 --> 01:41:01.795
a matter wave. In the early days of quantum 
mechanics, it was realized that photons are  

01:41:01.795 --> 01:41:08.515
electromagnetic wave packets whose momentum is 
given by their frequency. Louis de Bruy extended  

01:41:08.515 --> 01:41:14.355
this idea to particles and his de Bruy relation 
generalizes the relationship between frequency  

01:41:14.355 --> 01:41:21.395
and momentum of a matter wave. we now call matter 
waves wave functions and we can describe them in  

01:41:21.395 --> 01:41:28.275
terms of position or momentum just as a traveling 
sound wave can be expressed in terms of time or  

01:41:28.275 --> 01:41:34.595
frequency. So any particle any wave function can 
be represented as a combination of many locations  

01:41:34.595 --> 01:41:40.355
in space with accompanying intensities. Think of 
it as the particle being smeared over possible  

01:41:40.355 --> 01:41:46.115
positions or as a combination of many momenta 
with accompanying intensities in which case the  

01:41:46.115 --> 01:41:52.515
particle would be smeared in momentum space. And 
of course this means that position and momentum  

01:41:52.515 --> 01:41:59.155
have the same kind of uncertainty relation that 
time and frequency had in the soundwave. But what  

01:41:59.155 --> 01:42:05.475
does it even mean for a particle to be comprised 
of waves of many different positions or momenta?  

01:42:05.475 --> 01:42:11.955
To answer this, we need one more bit of physics. 
The interpretation of the wave function itself,  

01:42:11.955 --> 01:42:18.595
known as the Bourne rule. The magnitude of 
the wave function squared is the probability  

01:42:18.595 --> 01:42:23.715
distribution for the particle. If we're expressing 
the wave function in terms of position, then  

01:42:23.715 --> 01:42:29.795
applying the B rule tells us how likely we are to 
find the particle at any given point when we make  

01:42:29.795 --> 01:42:35.715
a measurement. Or put another way, the range of 
positions in which the particle is likely to be  

01:42:35.715 --> 01:42:42.595
located were we to look. If we apply the Bourne 
rule to the momentum wave function, then we learn  

01:42:42.595 --> 01:42:48.755
the range of momenta the particle is likely to 
have. So if we measure a particle's position,  

01:42:48.755 --> 01:42:54.835
then from our point of view, its wave function 
is highly localized in space. We know where the  

01:42:54.835 --> 01:43:01.075
particle is. The resulting particle wave packet 
now constrained in position can only be described  

01:43:01.075 --> 01:43:08.435
as a superposition of waves with a very large 
range of different momenta via a foria transform.  

01:43:08.435 --> 01:43:14.595
The result is a very fat momentum wave function 
that gives a wide range of possible momenta. The  

01:43:14.595 --> 01:43:19.475
more precisely we try to measure position, the 
narrower we make its position wave function and  

01:43:19.475 --> 01:43:25.475
so the less certain we become about its momentum 
as that momentum wave function gets wider.  

01:43:25.475 --> 01:43:31.715
This is all super abstract, but a concrete 
example is single slit defraction. If we  

01:43:31.715 --> 01:43:37.315
increase our certainty of the position of a 
particle by narrowing the slit, we also increase  

01:43:37.315 --> 01:43:42.835
the uncertainty of its momentum as it passes 
the slit. This results in an increasing spread  

01:43:42.835 --> 01:43:49.315
in final locations. Check out Veritassium's 
excellent video to see this in action. So,  

01:43:49.315 --> 01:43:54.115
that's exactly the uncertainty principle. 
It's a statement about how much of a quantum  

01:43:54.115 --> 01:44:00.675
systems information is accessible at a fundamental 
level. It's an unavoidable outcome of describing  

01:44:00.675 --> 01:44:05.795
particles as the superp position of waves. 
Waves that can be represented in terms of either  

01:44:05.795 --> 01:44:12.115
position or momentum. The fact that both can't 
be known simultaneously with perfect precision  

01:44:12.115 --> 01:44:17.555
is a property of the nature of the wave function 
itself. Precision in one is actually constructed  

01:44:17.555 --> 01:44:23.315
by the uncertainty in the other. Okay. So what 
does this old school quantum mechanics have to do  

01:44:23.315 --> 01:44:28.995
with quantum field theory and Hawking radiation? 
Well, the key to understanding these things is to  

01:44:28.995 --> 01:44:34.675
be able to switch between thinking about quantum 
fields in terms of position versus momentum. See,  

01:44:34.675 --> 01:44:40.755
a single particle, a quantum field vibration, 
perfectly localized at one spot in space,  

01:44:40.755 --> 01:44:47.395
can also be described as infinite oscillations 
in momentum space spanning all possible momenta.  

01:44:47.395 --> 01:44:53.075
But each of these oscillations in momentum space 
are equivalent to particles with highly specific  

01:44:53.075 --> 01:44:58.115
momenta. The uncertainty principle therefore tells 
us that they must be completely unconstrained  

01:44:58.115 --> 01:45:05.555
in position. So a perfectly specially localized 
particle is equally an infinite number of momentum  

01:45:05.555 --> 01:45:11.795
particles that themselves occupy all locations in 
the universe. It's only by manipulating quantum  

01:45:11.795 --> 01:45:18.275
fields in this strange momentum space by adding 
and removing these spatially infinite particles  

01:45:18.275 --> 01:45:23.795
that we can describe how the quantum vacuum 
changes to give us phenomena like UNRA and  

01:45:23.795 --> 01:45:34.595
Hawking radiation which you'll soon understand 
as some of the weirdest behaviors of spaceime.

01:45:34.595 --> 01:45:39.715
Does quantum mechanics allow the future to 
retroactively influence the past, as in the  

01:45:39.715 --> 01:45:45.875
infamous delayed choice quantum eraser experiment? 
Well, how about we get an actual quantum physicist  

01:45:45.875 --> 01:45:52.035
who many of you already know to show us how 
to do this experiment at home and hopefully  

01:45:52.035 --> 01:45:57.875
put the matter to rest once and for all. Richard 
Feineman said of the double slit experiment that  

01:45:57.875 --> 01:46:05.795
it has in it the heart of quantum mechanics. In 
reality, it contains the only mystery. Well, the  

01:46:05.795 --> 01:46:11.955
mystery he was talking about is superp position. 
In the double slit experiment, that means a superp  

01:46:11.955 --> 01:46:19.795
position of a single particle traveling through 
both of the double slits at once. The experiment  

01:46:19.795 --> 01:46:25.875
hints that prior to detection at a screen, a 
quantum particle plays out all possible realities  

01:46:25.875 --> 01:46:31.875
that may lead to that one measurement result. 
And the shadow of these overlapping realities,  

01:46:31.875 --> 01:46:38.435
these interacting trajectories is cast on the 
screen in the form of an interference pattern.  

01:46:38.435 --> 01:46:46.355
But if you prune the reality tree early, say by 
measurement at one of the slits, the final pattern  

01:46:46.355 --> 01:46:52.835
of particle locations reflects that. As if the 
standard double slit experiment wasn't strange  

01:46:52.835 --> 01:46:59.315
enough, then we have the delayed choice quantum 
eraser experiments in which the measurement of  

01:46:59.315 --> 01:47:07.875
the particles path to the screen is made after the 
pattern is recorded on the screen. The most famous  

01:47:07.875 --> 01:47:17.555
example is the one by Kim Yu Kulik Shei and Scully 
in 1999. Such experiments have been interpreted as  

01:47:17.555 --> 01:47:25.715
saying that a future measurement can influence 
a past measurement retrocausal influence. We  

01:47:25.715 --> 01:47:32.835
did an episode on the delayed choice quantum 
arrays a long time ago. Now, we didn't quite  

01:47:32.835 --> 01:47:39.155
promote the retrocausal interpretation, but 
we certainly failed to give any alternative.  

01:47:39.155 --> 01:47:46.275
Well, there are now some really solid counter 
explanations to the idea of retrocausality. A few  

01:47:46.275 --> 01:47:51.875
papers have been published which get at aspects of 
it and various YouTube videos also explain some of  

01:47:51.875 --> 01:47:58.515
the errors in the retrocausal interpretation. 
So, it's time for us to weigh in. Today,  

01:47:58.515 --> 01:48:06.115
we're actually going to do a real experiment. And 
by we, I mean Mithina from Looking Glass Universe,  

01:48:06.115 --> 01:48:12.995
who's figured out a way to do the quantum eraser 
in a homemade setup. She's going to show you how  

01:48:12.995 --> 01:48:19.075
this experiment gives the illusion of information 
traveling backwards in time. I'm really excited  

01:48:19.075 --> 01:48:23.875
to show you this experiment because 12 years 
ago, I did a video on the delayed choice quantum  

01:48:23.875 --> 01:48:31.235
eraser where I got it completely wrong. And it was 
only doing this experiment that made it finally  

01:48:31.235 --> 01:48:37.075
click for me. The delay choice quantum eraser 
is just a variation on the usual double slit  

01:48:37.075 --> 01:48:43.475
experiment. So for that experiment, all you'll 
need is some light and two very narrow beams that  

01:48:43.475 --> 01:48:50.035
you can shine that laser at. What do we expect to 
see? Well, it depends on how we think about light.  

01:48:50.035 --> 01:48:54.595
On the one hand, we know that light is a wave in 
the electromagnetic field. And like any wave, it  

01:48:54.595 --> 01:49:00.835
should produce interference patterns as components 
of the wave passing through the different slits  

01:49:00.835 --> 01:49:06.915
overlap, stacking up or canceling out. At the 
wall, we get the classic double slit interference  

01:49:06.915 --> 01:49:12.995
pattern. On the other hand, we also think of light 
as particles, which is fair because the classical  

01:49:12.995 --> 01:49:18.595
electromagnetic wave is made up of indivisible 
quantum particles that we call photons.  

01:49:18.595 --> 01:49:24.915
An intuitive but naive picture for particles 
is as a bunch of tiny balls. Spray balls at  

01:49:24.915 --> 01:49:29.635
our pair of slits and they should end up forming 
two piles corresponding to passage through each  

01:49:29.635 --> 01:49:36.835
slit. Photons are really some weird in between 
thing between waves and particles. If you fire  

01:49:36.835 --> 01:49:42.355
individual photons, they do look like they make 
single hits on the wall as though they were tiny  

01:49:42.355 --> 01:49:47.795
balls. But after many firings, the double slit 
pattern becomes visible. The interpretation  

01:49:47.795 --> 01:49:53.155
is that each photon really passes through both 
slits as a wave and then the final destination  

01:49:53.155 --> 01:49:58.835
of the particle-like spot is determined by the 
interference pattern made by that wave. In quantum  

01:49:58.835 --> 01:50:05.715
mechanics, we say that the position wave function 
of the photon passes through both slits or that it  

01:50:05.715 --> 01:50:11.395
exists in a superp position of both trajectories 
and in fact of all possible trajectories that  

01:50:11.395 --> 01:50:18.755
reach the wall. But when the wall measures the 
photon location, that superp position collapses  

01:50:18.755 --> 01:50:24.595
and a single position is chosen. Even then, 
the path the photon took to get there remains  

01:50:24.595 --> 01:50:30.035
undefined. If you want to know the path taken, you 
have to measure it before the wall. For example,  

01:50:30.035 --> 01:50:35.395
you could place a little detector at the slits 
that clocks the passing photon without actually  

01:50:35.395 --> 01:50:40.195
stopping it. Essentially, we're collapsing 
the position wave function of the slits,  

01:50:40.195 --> 01:50:44.595
not at the wall. Now that we know which slit the 
photon passed through, everything we see after  

01:50:44.595 --> 01:50:49.875
that has to be consistent with it only passing 
through that slit, which means no double slit  

01:50:49.875 --> 01:50:56.435
interference pattern. One common misconception 
is that if a photon passes through just one slit,  

01:50:56.435 --> 01:51:02.835
it lands ultimately just like a classical particle 
in a single pile of points. But actually single  

01:51:02.835 --> 01:51:09.715
slit interference looks like this. It looks a lot 
like double slit interference. But if you compare  

01:51:09.715 --> 01:51:15.155
them side by side, you can see the difference. 
Single slit interference still has dark fringes,  

01:51:15.155 --> 01:51:21.315
but just not as many as double slit. This is a 
form of frownhoffer defraction with the edges  

01:51:21.315 --> 01:51:27.875
of the slits causing the interference in this 
case. So photons keep behaving like waves even if  

01:51:27.875 --> 01:51:33.635
we measure which slit they traversed. If you want 
to understand single slit defraction better, Mina  

01:51:33.635 --> 01:51:39.395
did a video explaining why this is and what it 
means. The transition from double to single slit  

01:51:39.395 --> 01:51:44.355
interference occurs when we measure which slit 
the photon passed through. We sometimes call this  

01:51:44.355 --> 01:51:50.435
a whichway measurement. That's weird enough, but 
at least in this case, we have a sensible causal  

01:51:50.435 --> 01:51:56.195
ordering. You make the measurement, it collapses 
the wave function, stopping it from forming a  

01:51:56.195 --> 01:52:01.235
double slit pattern further down the line. Things 
get a lot weirder in the case of the delayed  

01:52:01.235 --> 01:52:07.395
choice quantum eraser. In the classic experiment 
by Kim and collaborators, the photons passing  

01:52:07.395 --> 01:52:14.755
through the slits are actually cloned using a beta 
barium borate or BBO crystal which converts each  

01:52:14.755 --> 01:52:21.635
incoming photon into a pair of near identical 
outgoing photons that are quantum entangled.  

01:52:21.635 --> 01:52:28.995
The key is that both photons in this pair carry 
the which way information. They both know which  

01:52:28.995 --> 01:52:36.755
slit their progenitor photon passed through before 
the BBBO crystal duplicated it. So now we send one  

01:52:36.755 --> 01:52:43.715
of the new pair, call it photon A, to our usual 
double slit detector to be observed by Alice. The  

01:52:43.715 --> 01:52:49.715
other photon B goes to Bob who can extract the 
whichway information or alternatively can choose  

01:52:49.715 --> 01:52:56.115
to erase that info. The device is simple enough. 
The path of photon B depends on which slit it  

01:52:56.115 --> 01:53:02.595
came through. So we can place a detector on both 
paths and see which one is triggered. We can also  

01:53:02.595 --> 01:53:07.795
choose to scramble the whichway information by 
recombining those two paths. So there's no way  

01:53:07.795 --> 01:53:14.995
to tell them apart anymore. The bizarre result of 
the Kim paper and similar experiments is that if  

01:53:14.995 --> 01:53:22.995
the whichway information is measured from photon 
B, then photon A acts as though we also measured  

01:53:22.995 --> 01:53:31.795
which slit it came from. Alice sees that those 
photon A's have their whichway info measured and  

01:53:31.795 --> 01:53:39.475
fall according to a single slit pattern. But if 
the whichway information in photon B is scrambled,  

01:53:39.475 --> 01:53:45.875
Alice sees the corresponding photon A's landing 
according to the classic double slit pattern. It  

01:53:45.875 --> 01:53:52.115
acts as though it did pass through both slits. 
It's like we change photon A's behavior without  

01:53:52.115 --> 01:53:56.915
actually touching it, only by measuring 
its twin. And if that wasn't weird enough,  

01:53:56.915 --> 01:54:01.395
in the Kim experiment, the choice of whether 
to keep or discard the whichway information  

01:54:01.395 --> 01:54:09.315
in photon B is made after its entangled partner 
photon A reached its detector. That makes it seem  

01:54:09.315 --> 01:54:14.435
like photon A landed according to a choice that 
hasn't been made yet. And this has been referred  

01:54:14.435 --> 01:54:21.795
to as retrocausal influence. And it's what makes 
the delayed choice quantum eraser so befuddling.  

01:54:21.795 --> 01:54:26.515
To explain how we can get around this paradox, 
I'm going to do an analogous experiment that you  

01:54:26.515 --> 01:54:33.155
can do very easily at home. And when I say it's 
analogous, what I mean is that it's mathematically  

01:54:33.155 --> 01:54:39.155
equivalent. Like if you looked at the maths of 
the original experiment versus this experiment,  

01:54:39.155 --> 01:54:45.715
it's exactly the same except for how you label 
a few things. That's not to say though that this  

01:54:45.715 --> 01:54:51.315
experiment is as cool as the other one because 
the other one involves actual entanglement  

01:54:51.315 --> 01:54:58.995
whereas this uses a little trick to kind of mimic 
entanglement without actually making an entangled  

01:54:58.995 --> 01:55:06.275
photon. Show is able to do but BBO crystals are 
very expensive. So we've got our laser here and  

01:55:06.275 --> 01:55:12.835
we've also got a double slit. In the original 
experiment, for every photon A that goes through  

01:55:12.835 --> 01:55:20.515
the slits, they make a photon B which knows the 
slit that photon A took. So photon B is what  

01:55:20.515 --> 01:55:27.475
stores the information about which way photon A 
went. So we need in our experiment some other way  

01:55:27.475 --> 01:55:33.235
to store that information. Well, we're going to 
do it in a really simple way. Okay, so here's our  

01:55:33.235 --> 01:55:40.195
double slit and we've put some filters in front of 
it. two different filters in front of each of the  

01:55:40.195 --> 01:55:46.915
slits. In front of slit one, we have a horizontal 
polarization filter and in front of slit two,  

01:55:46.915 --> 01:55:52.835
we have a vertical polarization filter. Well, now 
we know that any light that makes it through this  

01:55:52.835 --> 01:55:58.115
double slit is marked. If it's horizontally 
polarized, it must have come through slit  

01:55:58.115 --> 01:56:04.035
one. Whereas, if it's vertically polarized, it 
must have come through slit two. In other words,  

01:56:04.035 --> 01:56:09.875
the polarization is what's now marking whether 
the light went through slit one or slit two,  

01:56:09.875 --> 01:56:16.995
which is convenient for us because we can use this 
crystal to split the light accordingly. So, this  

01:56:16.995 --> 01:56:23.955
is called calsite. It's not as cool as BBO, which 
is what they used in the original experiment. But  

01:56:23.955 --> 01:56:31.555
it does have one very nice property. If I put it 
here at this angle, it splits the light into two.  

01:56:31.555 --> 01:56:37.475
These two bits of light represent the two slits 
of the double slit experiment. If the light is  

01:56:37.475 --> 01:56:44.915
vertical, it came from slit one. Whereas, if 
it's horizontal, it came from slit two. The  

01:56:44.915 --> 01:56:50.115
light in the top pattern only went through slit 
one. And the light from the bottom pattern only  

01:56:50.115 --> 01:56:56.275
went through slit two. So, we'd expect that each 
of these patterns are going to be single slit  

01:56:56.275 --> 01:57:02.115
patterns. And if you look closely at them, you 
can see that they are a double slit pattern would  

01:57:02.115 --> 01:57:08.915
have a lot more dark fringes in it. So the calsite 
oriented this way is acting just like Bob's which  

01:57:08.915 --> 01:57:16.995
way measurement and collapsing the light into a 
single slit. But if we rotate the calite by 45°,  

01:57:16.995 --> 01:57:22.515
we'll see why this new measurement is now going to 
look like Bob's arasia measurement and we're going  

01:57:22.515 --> 01:57:28.035
to get the double slit pattern back. Okay, so that 
took me a while, but I managed to rotate this by  

01:57:28.035 --> 01:57:34.195
45°. And I haven't yet put the double slit in 
there, but I just want to show you what rotating  

01:57:34.195 --> 01:57:42.355
the calite actually does. So, if you look over 
there, you can see that there are two dots. Well,  

01:57:42.355 --> 01:57:47.795
there are roughly two dots. Again, it's a little 
bit noisy because my cows site here is nowhere  

01:57:47.795 --> 01:57:53.475
near perfect, but roughly it's two dots. And we're 
going to give the two dots names. So, we're going  

01:57:53.475 --> 01:57:59.155
to call the top one plus, and we're going to 
call the bottom one minus. And it's annoying,  

01:57:59.155 --> 01:58:04.515
but it's an inevitability of this geometry that 
they're not aligned with each other anymore. So,  

01:58:04.515 --> 01:58:09.955
they're not one on top of each other. So, with the 
magic of post-prouction, we'll stack these so that  

01:58:09.955 --> 01:58:14.515
they are one on top of each other. That's going to 
make everything a little bit easier in a second.  

01:58:14.515 --> 01:58:20.915
To figure out why calsite oriented like this 
is going to be an arasia measurement. Let's see  

01:58:20.915 --> 01:58:28.275
what happens when we put in vertically oriented 
light into this calite. You can see that the total  

01:58:28.275 --> 01:58:36.035
amount of light decreases but both dots are still 
there. It's not like last time where horizontal  

01:58:36.035 --> 01:58:42.115
light all goes to the same dot and vertical light 
all goes to the other dot. With this plus and  

01:58:42.115 --> 01:58:48.115
minus measurement, half of the vertical light 
will go to plus and half will go to minus. And  

01:58:48.115 --> 01:58:53.475
same for horizontal. But then what does that tell 
us about light that's going to go through this  

01:58:53.475 --> 01:58:59.155
marked double slit? So light that goes through the 
first slit is horizontally polarized whereas light  

01:58:59.155 --> 01:59:05.075
that goes through the second slit is vertically 
polarized. And previously our calite was letting  

01:59:05.075 --> 01:59:12.195
us measure the polarization and therefore measure 
whether it went through slit one or slit two. But  

01:59:12.195 --> 01:59:19.315
now think about what this plus minus capsite is 
doing. It's going to take light from the first  

01:59:19.315 --> 01:59:25.715
slit and it's going to have an equal probability 
of ending up at plus or at minus. And same for the  

01:59:25.715 --> 01:59:31.475
second slit. So now just looking at the light 
after it goes through the calite isn't enough  

01:59:31.475 --> 01:59:38.035
to tell us anything at all about which way the 
light went. In fact, it scramles up the light  

01:59:38.035 --> 01:59:44.355
from both paths. So it basically erases all of 
that information that we had gotten by marking  

01:59:44.355 --> 01:59:53.795
the paths. So what do we expect to happen then? 
Well, let's find out. Oh man, that looks so good.  

01:59:53.795 --> 02:00:05.075
Okay. Um, that kind of worked out better than I 
expected. Um, okay. So, have a look and hopefully  

02:00:05.075 --> 02:00:12.595
you can see two double slit patterns. So, if 
we use the calite in its previous orientation,  

02:00:12.595 --> 02:00:18.835
then we're using it to measure which way the light 
went and that collapses the light into going into  

02:00:18.835 --> 02:00:25.395
single slit interference. But if we use the calite 
in this orientation, the plus minus orientation,  

02:00:25.395 --> 02:00:31.315
we're using it as an eraser, it erases the 
information about which slit the light went  

02:00:31.315 --> 02:00:37.155
through. And so we're back to the double slit. If 
you look really closely at these two double slits,  

02:00:37.155 --> 02:00:44.115
you'll see that they don't exactly line up. And 
in fact, they seem to be offset from each other  

02:00:44.115 --> 02:00:51.395
the exact right amount that if you superimpose 
them, it looks like the single slit interference  

02:00:51.395 --> 02:00:58.115
pattern. So what's going on? Well, it turns 
out that this is the solution to the entire  

02:00:58.115 --> 02:01:03.795
paradox. Let's go back to the original 
setup to see why. In the Kim experiment,  

02:01:03.795 --> 02:01:09.955
photons are cloned by the BBO crystal and the 
whichway analysis is done with only one of those.  

02:01:09.955 --> 02:01:17.075
We called it photon B. Just as our experiment 
sorts the photons into two interference patterns,  

02:01:17.075 --> 02:01:24.675
the original experiment sorts the B photons into 
two bins. For the measure case, those bins tell  

02:01:24.675 --> 02:01:30.515
you which slit the corresponding photon A also 
went through. For the eraser case, there are still  

02:01:30.515 --> 02:01:37.075
two bins, but there is no whichway information 
in how photons are sorted into them. Alice never  

02:01:37.075 --> 02:01:42.595
sees a double slit interference pattern until 
something very specific occurs. At first, she  

02:01:42.595 --> 02:01:50.035
sees just a blob of positions for all photon A's. 
Then Bob comes and tells her which of those had a  

02:01:50.035 --> 02:01:57.475
twin photon B for which the way info was measured 
and for which it was erased. The measured cases  

02:01:57.475 --> 02:02:04.275
are still a blob. Basically a blurry single slit 
interference. Now, the sum of all of the cases in  

02:02:04.275 --> 02:02:13.075
which the whichway info was erased are actually a 
very similar mess, a blob. So, Bob also needs to  

02:02:13.075 --> 02:02:20.915
tell Alice which of those erased photon B's ended 
up in each of the separate eraser detectors. Only  

02:02:20.915 --> 02:02:28.595
then can Alice disentangle the featureless blob of 
photon A positions and see interference bands. and  

02:02:28.595 --> 02:02:35.395
her patterns look a lot like mnas, clear peaks 
and valleys that are slightly offset. And when  

02:02:35.395 --> 02:02:41.635
you merge them, you get a featureless pile. This 
is a critical point that's often glossed over in  

02:02:41.635 --> 02:02:48.355
descriptions of the quantum eraser. So to recap, 
the solution is Alice always sees the single slip  

02:02:48.355 --> 02:02:55.155
pattern, and that's regardless of what Bob chooses 
to do on his side. But a single slip pattern is  

02:02:55.155 --> 02:03:01.155
really just two double slip patterns on top of 
each other. The plus version and the minus version  

02:03:01.155 --> 02:03:08.035
that's slightly offset. Now, if Bob does happen 
to do the eraser measurement, then he and Alice  

02:03:08.035 --> 02:03:15.475
can reconvene afterwards and sort Alice's photons 
into the plus and minus groups and they find those  

02:03:15.475 --> 02:03:21.635
two double slit patterns. In our experiment, 
we did this sorting via calsite, but in the  

02:03:21.635 --> 02:03:27.315
real version of the experiment, it just looks like 
post-processing the data to make the single slit  

02:03:27.315 --> 02:03:33.795
interference pattern into two double slit ones. 
So, instead of Bob's arasia measurement instantly  

02:03:33.795 --> 02:03:39.235
uncolapsing the wave function and turning the 
single slit interference into the double slit,  

02:03:39.235 --> 02:03:46.835
you only see this effect of his measurement much 
later once he and Alice sift through the data.  

02:03:46.835 --> 02:03:53.235
The story told by the retrocausal interpretation 
of the delayed choice quantum araser is that Bob's  

02:03:53.235 --> 02:03:59.795
choice of measurement retroactively influences 
where Alice's photon lands. But the same logic  

02:03:59.795 --> 02:04:06.675
works the other way around. The position of 
photon A measured by Alice absolutely influences  

02:04:06.675 --> 02:04:13.315
where photon B can go, where it's measured 
by Bob. Just for reference, I've drawn the  

02:04:13.315 --> 02:04:19.555
plus double slit pattern here and the minus one 
here just so that we can kind of see where they  

02:04:19.555 --> 02:04:30.515
are. And then let's say that Alice measures 
her photon and it happens to land right here.  

02:04:30.515 --> 02:04:38.515
This lines up perfectly with the minus double 
slit pattern, but not at all with the plus one.  

02:04:39.315 --> 02:04:45.235
Now after that Bob measures his photon B and he 
chooses to do an arasia measurement. So that means  

02:04:45.235 --> 02:04:52.035
that there are two possible outcomes. He could get 
a plus or he could get a minus. But could he get a  

02:04:52.035 --> 02:04:58.915
plus? I mean that wouldn't be consistent with the 
result that Alice has already got. And so actually  

02:04:58.915 --> 02:05:05.955
Alice's result forces Bob's result to also be 
minus. I should add that Alice's measurement of  

02:05:05.955 --> 02:05:12.595
the position of photon A only affects one thing 
for Bob, and that's which of the eraser detectors  

02:05:12.595 --> 02:05:20.275
photon B will land in, assuming photon B makes it 
all the way to the eraser section. But then one of  

02:05:20.275 --> 02:05:28.115
the eraser detectors will strongly favor photons 
whose twin landed in one set of bands on Alice's  

02:05:28.115 --> 02:05:34.995
screen, while the other favors the complimentary 
set of bands. So, we've had this causality thing  

02:05:34.995 --> 02:05:41.395
the wrong way around this whole time. We thought 
that it was Bob's measurement that forced Alice's  

02:05:41.395 --> 02:05:47.555
photon to act in a particular way, but now we 
can see it's the other way around. If Alice's  

02:05:47.555 --> 02:05:53.955
measurement happens first, then her outcome is 
what determines what happens when Bob does his  

02:05:53.955 --> 02:06:00.915
arasia measurement. In other words, Alice's 
outcome is what forces Bob's to happen in the  

02:06:00.915 --> 02:06:08.435
way it does. But that's only the case if Alice's 
measurement happens first and then Bob's. But  

02:06:08.435 --> 02:06:16.435
here's the twist. Does Alice's measurement always 
happen first? Or does that depend on your frame of  

02:06:16.435 --> 02:06:22.515
reference? It's possible to set up this experiment 
so that Bob's detections actually happen before  

02:06:22.515 --> 02:06:30.675
Alice's just by changing the path lengths for 
each section. If we do that, we expect exactly  

02:06:30.675 --> 02:06:36.355
the same result. The same interference fringes are 
revealed when photon A is filtered according to  

02:06:36.355 --> 02:06:44.035
where photon B lands in the eraser. In that case, 
it seems that B caused A, but with the same result  

02:06:44.035 --> 02:06:51.395
as when A caused B. This makes it harder to assign 
a causal power to either case. It's also hard to  

02:06:51.395 --> 02:06:57.635
even imagine a causal path connecting Alice and 
Bob's observations beyond the sort of spooky  

02:06:57.635 --> 02:07:05.555
action at a distance that Einstein so hated. That 
said, there is a way to explain the causation that  

02:07:05.555 --> 02:07:11.875
connects Alys's and Bob's observations that 
doesn't need to assign a causal direction. It  

02:07:11.875 --> 02:07:17.555
all depends on what mysterious property really 
determines how our photons get sorted into the  

02:07:17.555 --> 02:07:23.475
two detectors in the quantum array or how they 
are sorted into the two interference patterns  

02:07:23.475 --> 02:07:28.995
by the calsite crystal in Mina's experiment. 
What property is that? Well, you're going to  

02:07:28.995 --> 02:07:36.115
have to wait for an upcoming episode for that one. 
But in the meantime, go check out Mith's channel  

02:07:36.115 --> 02:07:43.155
for quantum deep dives and more DIY quantum 
experiments. It's looking glass universe and it's  

02:07:43.155 --> 02:07:50.035
amazing. I also had a long and really fun chat 
with Mithna about quantum mysteries, her work, and  

02:07:50.035 --> 02:07:55.395
a lot more. It's in our new series of space-time 
conversations that you can find in our community  

02:07:55.395 --> 02:08:02.995
tab. The delayed choice quantum eraser. It's 
weird, but no weirder than any doubles experiment.  

02:08:02.995 --> 02:08:26.435
And in case you don't believe me yet, we'll 
finalize the debunking in an upcoming spaceime.

02:08:02.995 --> 02:08:10.595
tab. The delayed choice quantum eraser. It's 
weird, but no weirder than any doubles experiment.  

02:08:02.995 --> 02:08:26.435
And in case you don't believe me yet, we'll 
finalize the debunking in an upcoming spaceime.