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for a long time I had a big
misunderstanding about stringer's cat
which Shing just cat you ask well it's a
thought experiment that Irvin schinger
along with Einstein came up with to
express how unhappy they were with
quantum mechanics but what exactly is
the thought experiment well you trap a
cat in a box with a radioactive atom
which if it decays it triggers a poison
killing the cat don't worry it's just a
thought experiment now if we close the
lid so that nobody can see what's inside
the Box the question is what is the
state of the cat right now now here's
what our classical thinking says okay
radioactivity is completely random which
means there's a 50% chance that atom is
decayed or not it's either one of them
right and as a result right now there's
a 50% chance that the cat is either dead
or not it's one of them right but that's
not what quantum mechanics says here's
what I thought was going on I thought
that according to quantum mechanics as
long as you don't open the box as long
as nobody sees what's inside the box
that atom is neither decayed nor not
decayed it's kind of both at the same
time and therefore that cat is neither
dead nor alive it's kind of both at the
same time I thought this was the meaning
of the word Quantum superposition kind
of like Nature has not yet made a
decision but it's only when you open the
box one of them becomes reality
now this sounds pretty mindboggling
right but I had so many questions about
this like first first of all what does
it mean to say that that cat is both
dead and alive at the same time and
secondly why does reality change based
on whether you're looking at it or not
how does nature know and thirdly how do
we know what's happening inside the box
when no one's looking at inside of it
like how do we know how can we even
comment about it we are not looking
inside of it I didn't get any
satisfactory answer for a long time
until I read fin lectures just by going
through the first chapter of his volume
3 I realized that it's wrong to say that
that cat is both dead and alive at the
same time that's not what Quantum
superposition is it's much more profound
and so the goal of this video is not
only to get a deeper understanding of
what Quantum superposition is but also
to ReDiscover all of these ideas step by
step ourselves so that we don't have to
take anybody's word for it so if you're
ready for this let's begin so fan where
do we start fan says if you want to
ReDiscover things yourself mahes then
the best place is to look at experiment
and try to construct scientific models
all by yourself there are ton of
experiments we can look at but the one
that Fon loves the most is the electron
double slit experiment now the moment he
says this I'm like fine I'm going to
stop you right there I already know the
electron double slit experiment it's
pretty famous in our times now so I tell
him that hey look you take an electron
gun and you shoot one electron at a time
through the two slits what we would
expect is a pattern somewhat like this
right where you get two smears right
behind the two Blu slets but that's not
what we get instead we get an
interference pattern and so we now
wonder what's going on this helps us
understand that electrons don't just
behave like particles they kind of
behave like probability waves which
passes through both the slits and then
the wave interferes with itself and then
when it finally reaches the screen the
kind wave collapses and that's where you
get the particle that's where you get
the electron and that's basically what's
going on over here right finally
right and fan says Mahesh the only thing
that's interfering over here is your
superficial half knowledge it's fooling
you into thinking that you understand
quantum mechanics please please for the
love of God keep all your preconceived
notions aside at least for the next few
minutes and think about what I have to
say with a fresh mind because I'm going
to show you a perspective that you
probably never thought of before and I'm
like all right fine I'll play along and
I did get to see a perspective on the
double experiment that I had never
thought of before so again we're going
to take our electron gun shoot one at a
time through the double slit and wait
for the pattern to generate and
eventually the pattern that we get looks
like a interference pattern that we get
when you send waves through it so now
fan asks Mahesh the question over here
is which slit did the electrons go
through now I'm like in the back of my
head I already know the answer to this a
single electron actually goes through
both the slits and then sort of
interferes with itself that's the only
way to to get that pattern but fan has
asked me to keep my precons Notions
aside so I'm not going to talk about
that so I'm like okay fan how do we
answer this question fan says well let's
go through some options maybe the first
option is all the electrons went through
slit one now I know that that's wrong
right but fan says okay but how do you
test it how do you test whether this
option is right or wrong here's how you
do it he says you close slit 2 and redo
the experiment and think about what
pattern we would get now if the pattern
we got is the same as the pattern we
originally got with the double slit then
that option might be right if we don't
get the same pattern then that option
would be wrong so fan asks mahes if I
close one of those slits what pattern
would you expect well if I close one of
those slits the pattern we get would
look somewhat like this we'll get a lot
of electrons right behind the slit one
only and clearly that's not the pattern
that we got in the double slit so from
this we can say that hey all the
electrons did not go through slit one so
this is how we're going to answer all
our all our questions experimentally
okay all right so the next option is
maybe all the electrons went through
slate two now how do we check that well
the same way now you close slit one and
see what pattern we would have gotten
the pattern that we would have gotten
would look like this we would get a
patch right behind the slit to if this
was the case but again our pattern is
not that we get an interference pattern
so clearly they're not the same which
means the electrons did not go through
slit 2 all the electrons did not go
through slit 2 okay what other options
are there well maybe we would say some
electrons went through slit one and
others went through slit two again how
do we experimentally check for this well
if this was the case we would have
gotten a combination of the previous two
because we know what pattern we get when
electrons go to Slit one we know what
pattern we get when the electrons go to
Slit two so if some go to Slit one some
go to Slit two then we should get a
combination that would look like this
but again that's not what we got we got
an interference pattern so from that we
can say that that's not what happened
happened and so now f is like mahes what
else is there well I'm like all right
fan what if each electron is somehow
going through both the slit huh isn't
that the answer and fan is like well
let's test it if a single electron is
going through both the slit then if I
were to bring the screen all the way
close to both the slits then every
single electron should give me two dots
right right and I'm like no no no I I
don't mean the individ actual electron
the probability wave is what is going
through and F is like Mah what did I
tell you do not use your preconceived
notion let's think about this rigorously
you said that each electron is going
through both the slit what is the
meaning of that if each electron is
going through both the slit then I
should get two dots right behind the
slits right and I'm like yeah right but
does that happen it doesn't you will
always always find if you keep the
screen right next up to this SL you'll
always always find only one dot which
means this option is also wrong each
electron does not go through both the
slits
okay okay okay what if the electrons are
going through neither of the slits huh
that's possible right and five minut is
like mahes now we thinking rigorously
but again how do we test it well if
electrons are going to neither of the
slits let's redo the experiment this
time we'll close both the slits and see
what we'll get this is what it means for
electrons to go through neither of the
slits right but if you close both the
holes then you would get nothing on the
screen so clearly that's not what we got
we did get something on the screens that
means even this option is wrong what
else and I'm like oh my God I'm running
out of options over here but wait wait
we're not going to give up maybe
electrons are doing something way more
complicated what if an electron is say
going through slit one and then coming
back to Slit two and then again going
like this and Fon says again you can
test for it if an electron is going to
Slit one and then coming back to Slit
two then if I keep a barrier over here I
should be able to detect it but that
never happens if I keep a barrier I will
never ever detect electron being
detected over here which means that's
also wrong any complicated path that you
can think of that's also wrong and I'm
like we've exhausted all the logical
options that we can think of I mean
there is no other possibility right and
yet electrons are somehow going through
both the slits in a way that's not any
of this but it's
inconceivable it's almost as
out the link is also in the description now back to the video so Fon what is the
answer to this question which slit did
the electrons really go through I mean
we've looked at all the possible options
that we can logically think of and all
of them are wrong so what do we do and
fan says well
apparently this means electrons are
going through both these slits in a way
that we can't even logically
conceptualize it means that we can't use
our words or vocabulary or pictures to
communicate what they're doing what
they're doing is a completely new
behavior and that's why we come up with
the new name for it we call it the
quantum superposition we say that the
electrons are in a Quantum superposition
of going through slit one and going
through slit 2 but I'm like what does
that
mean okay and fman says I cannot tell
you what that means using any words that
we use to communicate or any pictures if
I try to do that I will only cause
miscommunications so the best I can do
mahes is I can tell you what Quantum
superos is not it's of the above it's
its own new thing that cannot be
explained using any prior knowledge that
we have at first this is deeply
frustrating right but if you think a
little bit more about it it actually is
quite satisfying because fan is actually
showing us why it is impossible to use
our language our words or pictures to
explain what Quantum Superion is if you
try to do that you're going to cause
misconceptions and that's why it's even
wrong to say Quantum superation is kind
of like the electrons are going through
both the slits no wrong don't use any
words you can only explain what Quantum
superposition is not you cannot use
words or pictures or animations to
describe B it is it is a brand new thing
it's its own thing and you cannot use
any prior knowledge to explain what it
is and that is truly profound that is
that is
beautiful but what's even more beautiful
is that you can use math to describe
exactly what Quantum superposition is
and you can communicate it
mathematically and that's why math is
the language of quantum mechanics so fin
one are you saying that it's wrong to
think of electrons as a probability wave
that passes through both the slits and
then interferes with each other and then
they collapse right well finally say
it's not wrong to say that but you need
to understand that is an interpretation
of what the underlying math is telling
us this is called The Copenhagen
interpretation is the most famous one
there are other interpretations first of
all but more importantly science never
tells us which interpretation is right
okay and that's important this is just
the way we interpret the the results the
the math over here that's important okay
it's not what the experiments are
telling us that that's a fact but more
importantly when we say for example
probability wave what do we really mean
I mean when you think about a wave what
comes to your mind is that something is
going up and down and then energy is
being transferred but over here none of
that is happening so it's not a wave in
any traditional sense it's just called
as a wave because the mathematical
equation that describes it is very
similar to the wave equation and so
these are there are all these caveats
and disclaimers that come when we using
those words and we need to be very
careful about it and that's what fman is
saying when you're trying to use this
words you need to be very careful
otherwise we might just fool ourselves
okay so the final question we could be
having then is why doesn't Quantum
superposition happen at a macroscopic
level if it did then we would all have
intuition for it right why does it only
happen at the microscopic level the FI
the fan says that's a great question for
that we can go back to our experiment
see over here people were really annoyed
with the fact that none of the above
none of these like you know electrons
are not doing any of these things right
they're doing something completely
different so they really wanted to find
out if they could somehow detect what
the electrons are really doing and so
for that they decided to use a detector
see so far they were using they were
actually doing this experiment in
complete darkness so that nothing was
interacting with the electron not even
light but now they thought what if they
use a tiny light source they keep it
between the two slits so that when
electron passes close through a slit the
light can be scattered off and then if
it reaches our eyes we can see a Flash
and by looking at that flash we may be
able to identify what where the
electrons are really going so for
example if we conduct this experiment
with that light source and we see a
flash going going in front of this let's
say slit one then I know electrons went
to Slit one if it goes in slit two then
I know electrons are going to Slit two
so this way I can really detect what's
going on so people red this experiment
with that with that light source and
guess what they got this time they
actually found flashes in front of slit
one and slit two which means that the
electrons were indeed some of them were
going to Slit one and some of them were
going to Slit two so that was indeed
what's happening
but when we look at the pattern now we
no longer get an interference pattern we
get the pattern that we would expect
from particles we get two patches behind
the slits so what does it all mean and
I'm like I know fman what this means
this means when there is no Observer you
do this in complete darkness we get an
interference pattern but when we use
when we have an observer that
interference pattern is gone that means
the act of observing changes reality
that means Consciousness affects reality
right and fan says again mahes no all
that is Hocus pokus no none of that
happens what's really happening again if
you were to be rigorous and be
scientific what's really happening from
this experiment what we can conclude is
that when there was no light source the
electrons were
behaving they were in a Quantum superp
position but because of the light source
the electron started interacting with
the light when they interact with the
light now they are going through one of
the slits that means they're no longer
in Quantum superposition so that means
when you interact with these particles
with light or with anything else the
quantum superposition is lost so this is
not just true for electrons going
through the slits it's true for any
Quantum properties for example consider
the spin when you measure electron spin
you might get it to be spin up or spin
down but if there are no measurements
being done if there are no interactions
then the experiment show us that the
electron doesn't have enough up spin nor
a down spin nor both nor neither they
would be in a Quantum super position of
being up and down and again that does
not mean they're up and down at the same
time and I cannot show any visuals or
any animations to explain what Quantum
suppos is but when you measure it you'll
always find it to be spin up or spin
down with a 50% chance and this is what
Irvin shinger and Einstein didn't like
because this meant that nature is no
longer deterministic I cannot with 100%
certainty predict the outcome of what
the measurement will give me quantum
mechanics was saying that nature is
inherently probabilistic and they didn't
like that and to express that
dissatisfaction they came up with the
thought experiment and now the thought
experiment makes sense when you close
the Box you're basically saying assume
there are no interactions happening
inside then what is the state of the
atom that atom is not decayed it's also
not not decayed it's in a Quantum
superposition of being decayed and not
decayed that does not mean it's both
decayed and not decayed at the same time
hope you understand that now right and
therefore what is the state of that cat
it's not dead it's not alive it's also
not both it's also not neither it's a
the state is a Quantum superation of
being dead and alive and it's only when
you open the box the state changes the
quantum superation is gone the state
changes to being either dead or alive
and so their argument was does it make
sense for that cat to be in Quantum
superposition no right and therefore
Quantum superposition is not real but
today we know it is real with all the
experiment the double set experiments
the experiment with the spin we have a
lot of experiments that show that the
quantum superposition is indeed real but
of course it doesn't apply to
macroscopic objects like cats because
there are a lot of interactions
happening remember it has nothing to do
with conscious observers looking at it
it has something to do with interactions
macroscopic objects have a lot of
interactions so it doesn't apply to
microscopic objects but Quantum soltion
is real as long as you don't measure
something as long as something is not
interacting their state is in a Quantum
superp position so the final question we
could be having for fan is since Quantum
superposition only Act at the
microscopic level and that too when
there are no interactions happening and
we live in a microscopic world with so
many interactions around does that
concept really have any significance and
fan says think about this consider an
electron that is going around a nucleus
now imagine another nucleus comes close
by then that electron has a choice of
orbiting the nucleus one or it could be
orbiting nucleus 2 or it has a choice to
orbit both the nuclei or it has a choice
to not orbit any of the nuclei what will
that electron do none of the above it
will now be in a Quantum superposition
of orbiting the nucleus one and the
nucleus 2 and by the way when we say the
word orbit that's wrong electrons are
not going around the nucleus even here
we can think that electron is in a
Quantum superp position of different
places that it can all be this is what
we call an orbital but again that's a
side note anyways over here with the two
nuclei what you can see is that this
Quantum
superposition is what is keeping the two
nuclei together is creating a chemical
bond which we call the covalent bond a
covalent bond is a Quantum super
position it's not really that electrons
are being shared between the two nuclear
that's a word that we use to make things
easier to communicate but that's wrong
it's in a Quantum super position without
it we wouldn't have covalent bonds which
means we wouldn't have water molecules
we wouldn't have molecules in your DNA
proteins life itself wouldn't exist
without Quantum
superposition so if you think about it
Shing an original came up with the
thought experiment along with einan
actually to tell us why they were
unhappy with quantum mechanics but it
turns out that the thought experiment is
actually useful in explaining why you
and I and this world exists in the first
place if you like this video you'll also
probably like my previous video on how a
simple accident a lab accident unlocked
the whole Quantum Revolution do check
that out over here see you