Thank you to Brilliant for supporting PBS. If we discover how to connect quantum mechanics
with general relativity we’ll pretty much win physics. There are multiple theories that claim
to do this, but it’s notoriously difficult to test them. They seem to require absurd
experiments like a particle collider the size of a galaxy. Or we could try to physics smarter,
instead of physicsing harder. Let’s talk about some ideas for quantum gravity experiments that
can be done on a non-galaxy-sized lab bench, and in some cases already have been done.
It’s been almost right on 100 years since quantum mechanics was discovered, and 110 since general
relativity. Together these theories explain almost everything—quantum gives us the behaviour of the
subatomic world and so the building blocks of all matter and their interactions via the quantum
forces, and GR gives us the the background in which that matter lives—space and time along with
the force of gravity. Despite their incredible successes in their own domains, the two seem
to contradict each other in deep ways. There must be some master theory that encapsulates
both and resolves these contradictions, but in the century following the discoveries of
general relativity and quantum mechanics, our efforts to find that theory have come up short.
The common approach to reconciling general relativity with quantum mechanics has been
to quantize gravity—to make the gravitational field and so the geometry of spacetime obey the
strange rules of quantum mechanics. And why not? It’s worked for everything else; for example we
made the electromagnetic field “quantum”, showing it was mediated by force-carrying
photon—and that was a stunning success. But quantizing gravity proved extraordinarily
difficult. The hypothetical force-carrier for gravity—the graviton—has an energy so tiny that
it’s hard to even conceive of an experiment that could detect it directly. And then there’s the
fact that the field of gravity is the fabric of spacetime itself. We quantized the other
forces of nature with respect to a continuous background, but so far it’s been exceptionally
challenging to quantize that background itself. The most prominent approaches, like string theory
and loop quantum gravity, may have made progress, but they struggle to make testable predictions.
So what if we’ve been looking at it the wrong way around. What if, instead of quantizing gravity,
we should be trying to “gravitize the quantum”. That is, instead of trying to give quantum
properties to gravity, we should accept that gravity is fundamentally classical and try
to understand how truly quantum matter can result in truly classical gravity.
Now we talked about approaches in which gravity stays classical in the last episode,
when we explored the new Postquantum gravity hypothesis by Jonathan Oppenheim, in which random
fluctuations in the gravitational field act on the quantum wavefunction, causing it to collapse.
We also talked about the Diose-Penrose model a while ago, in which a superposition of
different spatial distributions mass and energy is ultimately forced to choose one
such distribution consistent with a singular gravitational field.
There are compelling reasons on both sides of the discussion,
but which is it? Is gravity quantized or is the quantum gravitized? Unlike many questions about
quantum gravity, this one may be possible to answer with actual doable experiments.
Let’s consider the two scenarios and see what
predictions they make that we might be able to test.
One: Gravity is quantum - this means gravity and by extension the fabric of spacetime exhibit the
various weird properties of quantum world, like superposition and entanglement. If we
can experimentally demonstrate one of these things then we know gravity is quantum.
Two: The quantum is gravitized - this can be tested in the context of specific scenarios
like the Diosi-Penrose theory or Oppenheim’s Postquantum gravity. I’m going to talk about
this first because it’s shorter, then I’ll get back to tests of actual quantum gravity.
In both the Diosi-Penrose and Oppenheim approaches, the gravitational field is responsible
for collapsing the quantum wavefunction. In these ideas, there’s only one gravitational field,
only one spacetime, and quantum systems do quantum stuff within that classical background.
One of the things quantum systems can do is be in superpositions of states—they can exist
simultaneously in multiple states at once—like Schrodinger’s alive-and-dead cat. The job of the
quantum wavefunction is to describe this range of possible states. Measurement or observation
“collapses the wavefunction” - it forces the system to choose one state from the possibilities.
If gravity is classical, then the superposition of its contents shouldn’t cause a superposition
of the gravitational field. A superposition might involve different distributions of
mass—which normally would mean different spacetime curvatures—perhaps a superposition of spacetimes.
But if gravity is classical then there’s only one spacetime regardless of its contents. This could
lead to a tension between the gravitational field and the matter producing that field.
The Diosi-Penrose approach says that when this tension gets too large,
the wavefunction collapses. Another way to think about it is that the fuzziness
in the matter distribution is within a certain range of matching a single spacetime curvature
And in that case it can stay in superposition. But if the distribution of matter strays outside that
range then the superposition collapses into a well-defined matter distribution.
Oppenheim’s postquantum gravity is slightly different:
it states that the gravitational field contains random fluctuations—called
gravitational diffusion—that mess up quantum superpositions, causing them to collapse.
Both the Oppenheim and Diosi and Penrose make a similar prediction:
that there should be a stochastic quality to the gravitational force or its effect.
This gravitational diffusion might be the most straightforward thing to test if it exists. It
would imply that there’s a fundamental limit to the precision with which any mass can be
measured. So if we can build a set of scales with a certain very high precision, and if the
weights it measures appear to fluctuate outside that precision, it may suggest an intrinsic
uncertainty in the gravitational field. The most accurate experiment measuring a mass conducted
so far has led researchers to rule out versions of the theory with rapid spacetime diffusion,
implying that any gravity-induced collapse from this mechanism is very weak. Oppenheim and team
propose other tabletop experiments that could constrain this better in the future.
Rather than look for the funky behavior of the gravitational field
we can also approach this from the other side—by trying to see when the collapse of
the wavefunction happens. If we can see where quantum superposition gives way to classical
matter we may be able to figure out if it's gravity causing that collapse.
Both the Oppenheim and Diosi-Penrose models, as well as any of the family of models known
as objective collapse theories predict that wavefunction collapse becomes increasingly
likely as an object grows in size or mass. If any of these ideas are correct, there should
be a limit to how large an object we can put into superposition for any appreciable amount of time.
So far we haven’t found such a hard limit, but we’ve been able to put some pretty big
things in superposition—for example, getting interference patterns from large molecules in
double-slit experiment. Based on the largest systems that have been put in superposition,
we can rule theories where the gravity-induced collapse is very strong.
So we have some constraints on these ideas where a classical gravity is influencing
the quantum wavefunction of matter, and more advanced tests are being planned
that may turn up something. Of course, it may be that gravity really isn’t classical
in the way these theories propose. It may be that gravity really is quite quantum,
just like matter. If that’s the case then, we’ll need different experiments.
Two of the strangest properties of quantum objects are superposition—which we just
talked a lot about—and entanglement. Quantum entanglement is when you have
two quantum systems that each may be in a superposition of states,
but the systems are also correlated so that the state of one depends on the state of the other.
For example, if a “quantum coin” was in a superposition of both heads and tails,
while a pair of quantum coins could be both in superposition, but also be entangled so be in
the opposite state to each other. Even though each of the coins is fundamentally undefined in
its state. If the first coin is measured so that it’s wavefunction collapses to either heads or
tails, its entangled partner’s wavefunction will immediately collapse to the opposite.
Quantum entanglement sounds like an exotic phenomenon, but it’s not; quantum particles are
constantly getting entangled with each other. In fact, every interaction between particles
generates entanglement, even if it doesn’t last very long. We’ve observed these entanglements
when the interaction happens via the quantum forces—for example, when particles interact by
exchanging the photons of the electromagnetic force. So what if we could cause particles to
become entangled by a gravitational interaction? As I’ll try to convince you, this would mean
that the gravitational field itself has to be part of the entanglement of those objects and
spacetime has to be in a superposition of states. Which would mean spacetime has to be quantum.
One particular proposal for this type of experiment has recently been put forward,
known as a Quantum Gravity-induced Entanglement of Masses, QGEM, experiment. I’ll describe the
original version proposed by Sougato Bose and collaborators in 2017.
It uses a Stern-Gerlach interferometer. This is the device that first demonstrated
the existence of quantum spin back in the 20s. The original version used silver atoms,
which have a single unpaired electron in their outer shell, so the atom takes on
the quantum spin of that lone electron. That gives the atom a tiny dipole magnetic field.
This causes the atom to be deflected when it passes through the external magnetic field of
the Stern-Gerlach device. It’s deflected upwards if its spin axis is point up and downwards
if its spin is down. Because quantum spin is quantum, all atoms will be measured to have
either spin fully up or spin fully down (reflected all the way up or reflected all the way down)—
there are no in-betweens for quantum properties like this.
If the atom starts in a superposition state of being both spin-up and spin-down then it’ll end
up in a superposition state of being physically up or down just before it hits the detector
screen. So we've turned a spin superposition into a spatial superposition. At the screen,
the superposition collapses into one or the other location.
But we can also remove the screen and use more magnets to bring the spatial
superpositions back together, in which case we turn the spatial superposition
back into a spin superposition. Now if we measure that spin we should find
it collapses completely randomly into 50-50 spin up or spin down.
If we were to run a second Stern-Gerlach interferometer next to the first, the spins
we’d measure at the end of the second device would have no relationship to the spins measured
at the end of the first. Atoms in the different devices are uncorrelated—they are not entangled.
But now let’s move the devices really, really close together. The spin-up path
of interferometer one is right alongside the spin-down path of interferometer two.
Let’s pretend for a moment that the silver atoms can interact gravitationally. The
spin-up-one and spin-down-two atoms tug on each other ever so slightly,
while the other paths are too far for the atoms to interact. Remember that the spin measurements
at the output of interferometer one versus two used to be random—uncorrelated. Up-up, down-down,
up-down, down-up should all be equally likely. Any quantum interaction should generate entanglement,
so now, if gravity is quantum, the outputs of the two interferometers are entangled and
correlated. For example, depending on how this is set up you may find that that the interferometers
produce opposite spin results to each other very very slightly more often than same spin results.
Now single atoms are way too light to produce a measurable entanglement. But what if we
could do this with many atoms? Specifically, with a nanodiamond. These are tiny diamonds
with widths measured in nanometers. A little defect inside the diamond crystal structure
can be created by removing one carbon atom and replacing it with a neighbouring nitrogen atom.
This leaves an unpaired electron which gives the nanodiamond a net spin. The
same technology is actually a candidate for qubit storage in quantum computers.
So we create nanodiamonds and put their unpaired electrons in a spin superposition,
then shoot them through the pair of Stern-Gerlach devices. In principle we should get a spatial
superposition of these relatively massive crystals, and if gravity is quantum, we should
get entanglement between the devices that will be reflected in correlations in their final spins.
What would all of this mean if it proved to work?
Well from the point of view of particle physics, the masses get entangled when
gravitons are exchanged between the two masses. This is just like in electromagnetism, where
photon exchange leads to the electromagnetic force, and sometimes entanglement. We’ve
made a video before on how hard it is to detect single gravitons even in principle,
so this entanglement between the two masses in the QGEM experiment might be considered indirect
evidence for the existence of gravitons—the hypothetical quantum carrier of gravity.
From the point of view of general relativity, the superposition of
gravitational forces could be interpreted as a superposition of spacetime geometries. So,
spacetime itself would be exhibiting one of the most quantum of all properties. In general,
a positive QGEM result would be strong evidence that gravity is indeed quantum.
These are just some examples of experiments that have been proposed—or actually conducted—to test
some of these ideas about the link between quantum mechanics and gravity. Whether to find
quantum effects in the gravitational field, or to show that gravity doesn’t even need
to be truly quantum. There are lots of other ideas and efforts in progress beyond these,
and there’s a good chance that some will bear fruit—perhaps finally
unlocking that century-old mystery of the quantum nature of spacetime.
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