Kind: captions
Language: en
In 1935, Einstein [music]
came up with a thought experiment that
showed quantum mechanics breaks one of
the most sacred principles [music] in
physics, that nothing can go faster than
the speed of light. Physicists assumed
he was wrong. [music] They thought that
at 56, Einstein was an old man past his
prime and just unable to accept the new
theory of physics because it [music] was
too radical. But 30 years later, one man
stumbled across Einstein's forgotten
paper when he realized something. The
prediction could actually be tested.
When scientists ran the experiment, they
were shocked. Quantum physics really
does break the universal [music] speed
limit. We're obliged to invoke something
like actions going faster than light
from one place to another.
>> This is a video about one of the
spookiest [music] and most misunderstood
experiments in all of physics. And it
[music] might even be the strongest
evidence we have that we live in many
worlds.
If the sun were to disappear all of a
sudden, how long would it take until we
noticed and were released out into
space? Newton's theory says that gravity
acts instantly across any distance. So,
if there's a change in gravity, we
should feel it immediately. But Newton
himself was disturbed by this. That one
body may act upon another at a distance
is to me so great an absurdity that I
believe no man who has a competent
faculty of thinking can ever fall into
it.
But in 1905, Einstein realized [music]
action at a distance isn't just absurd.
It leads to outright paradoxes. Einstein
had discovered that observers moving at
different speeds can disagree about when
events happened. Let's say you see two
things happen at the same time. An
observer speeding past would see it
differently. To them, one of these
happened first, and both points of view
are equally valid. But in the case of
gravity, this leads to disaster. Say you
see the sun disappearing and earth
flying off at the same time as Newton
predicted. Then the other observer sees
something impossible. They see the earth
flying off first even while the sun is
still there. And it should of course be
pulling the earth in. So to them it
looks like cause and effect are
reversed. The only way out of this
paradox is to reject the assumption we
started with. So gravity can't be
instant. It took Einstein 10 years to
fix this issue and in the process he
completely overhauled our understanding
of gravity. Gravity is caused by the
bending of spacetime. [music]
When there's a change in gravity that
only affects the local spacetime and
then that ripple spreads out to nearby
regions which spread farther out until
eventually [music] they reach us. This
theory of gravity is local because
effects spread from place to place at
the speed of light instead of being
instant. From our frame of reference, if
the sun disappeared, that ripple would
take about 8 minutes to reach us.
Another observer might disagree about
the length of the delay, but now we all
agree that the sun disappeared first.
This is why nothing can go faster than
light. The delay between cause and
effect [music] ensures all observers
agree on the order. After Einstein fixed
gravity, all of classical physics obeyed
this important rule. But then Einstein
studied the new theory of quantum
mechanics and made a terrible discovery.
This is one of the most famous
photographs in physics. It was taken at
the 1927 SVE conference where the
architects of the brand new quantum
theory gathered to discuss it. Around
60% of the attendees would win Nobel
prizes. But Einstein thought they'd
gotten something fundamentally wrong,
and this was his chance to prove it. So,
he took to the stage with a thought
experiment. Imagine you fire a single
electron through a narrow slit toward a
circular detection screen. Well, quantum
mechanics says that this electron has
some sort of wave associated with it
called a wave function which spreads out
through space as it travels. When the
electron hits the screen, you detect it
at a single point. Where it turns up
depends on the amplitude of the wave. If
the wave is very big in a particular
area, the electron is more likely to
turn up there. Let's say it appears
here. So far, everyone was following.
This is what quantum mechanics predicts.
But Einstein's next question surprised
them. Why doesn't the electron turn up
at this other spot a moment later?
There's only one electron, so we can't
detect it twice. But the way quantum
mechanics ensures this is that when the
electron was detected at the first spot,
its wave function collapsed to zero
everywhere else instantly. That's why
the probability of finding it at the
second spot is now zero. There's no
longer any wave there. But Einstein
asked the audience to think about what
this means. The measurement here must
instantly affect the wave function over
here. No matter how far apart these
locations are. In other words, quantum
mechanics requires instant influences
across distance. It violates locality.
Einstein concluded his talk by saying,
"This is an entirely peculiar mechanism
of action at a distance." And that this
implies to my mind a contradiction with
the postulate of relativity. Einstein's
argument was so simple and his talk so
short that people didn't know what to
make of it. One audience member said, "I
feel myself in a very difficult position
because I don't understand what
precisely is the point which Einstein
wants to make. No doubt it is my fault.
Batman was Neils Boore, the most
influential figure in quantum physics at
the time."
Boore's institute [music] in Copenhagen
had become the hub for the new field.
Dozens of young scientists like Verer
Heisenberg came to learn from him. As
one of his disciples remembers, Boore
had invited a number of us to his home
where we sat close to him, some
literally at his feet on the floor so as
not to miss a word. Boore wasn't the one
who wrote the mathematical rules of
quantum [music] mechanics. Instead, he
told everyone what they meant. While
others were confused by the theory,
Boore offered answers. His philosophy
became known as the Copenhagen
interpretation of quantum mechanics. My
general understanding of the Copenhagen
interpretation is you have the wave
function. It describes everything that
you can know about a particle or a
system and it evolves according to the
Schroinger equation and at some point
you're going to make a measurement and
at that point the wave function
collapses
>> and I think that one bit of that that
you said was like um the wave function
is all you can know about the particle
and I think that was like a pretty
important point to Bore
>> as Bore would put it. It's wrong to
think that the task of physics is to
find out how nature is. The job of
physics is just to predict measurements
in the lab, which quantum mechanics does
incredibly well. As for what the
electron is doing when you're not
looking, well, to bore, that question
didn't even [music] make sense to ask.
The wave function tells you everything
physics can or should tell you. Einstein
couldn't stand the Copenhagen
interpretation. In a letter to his ally
Schroinger, he called it a tranquilizing
philosophy or religion. Einstein felt
his thought experiment exposed a
critical weakness in the Copenhagen
interpretation. He'd shown that the way
the wave function collapses is
non-local. And so he reasoned maybe the
wave function is the problem. Maybe it's
not the best way to describe [music] the
electron. After all, he may not have
convinced Boore of this during his talk,
but he was determined to do it during
the rest of the conference. Physicists
tell a version of this story, you know,
that you will find in physics textbooks
and in pop science books and that, you
know, physicists tell amongst ourselves
that what happened was Einstein and Bore
had a great debate and Einstein was
unhappy with quantum mechanics because
it was fundamentally probabilistic. He
tried to show that there were
perceivable experiments that you could
use to get around those uncertainty
relations. And Boore showed over and
over and over again that you couldn't do
that. And eventually everybody agreed
with Boore.
>> That's Adam [music] Becker, author of
What is Real? A great book about the
history of quantum mechanics. As he
explained to us, Boore may have just
misunderstood the purpose of Einstein's
thought experiments. We have documented
evidence of this in at least one case.
Einstein described a thought experiment
that involved a box of photons and a
mirror. [music]
Its purpose was to show the non-locality
of the Copenhagen interpretation in
action.
Bour [music] just misunderstood it. And
when he recounted it to others later on,
he drew a little diagram of what [music]
Einstein's thought experiment setup was.
It just didn't have the mirror in it at
all. And yet, this is taken as like the
great victory for bore over Einstein,
which [music] is crazy. History is
written by the victors, right?
>> To understand what Einstein was arguing
for, think of the relationship between
Newton's gravity and general relativity.
Newton's theory works well in most
situations. But in that theory, gravity
is a non-local force, leading to
paradoxes. This was the motivation for
coming up with Einstein's general
relativity, [music] which is local.
Einstein believed the same logic applied
to quantum mechanics. His thought
experiment revealed that quantum theory
is non-local. So just like with Newton's
gravity, quantum mechanics must not be
the final theory. There must be a local
one that will replace it. And as a
bonus, he thought this new theory might
even unify gravity with the quantum
world.
>> It would be hard to imagine coming to
the [music] final theory right away. And
and yeah, and the fact that you can see
paradoxes like this would make you think
there's got to be more to it that we
just don't have yet.
>> Absolutely.
>> But Einstein hadn't even persuaded Bore
that quantum mechanics really is
non-local. So in 1935 he made one last
attempt to convince the community that
there was a contradiction between
quantum mechanics and relativity. With
the help of two younger colleagues Boris
Podolski and Nathan Rosen he formulated
another even more striking thought
experiment that shows the non-locality
of quantum mechanics. This paper is now
known as the EPR paper after its
authors.
Here is a simplified version of their
thought experiment. [music] Imagine a
single high energy photon suddenly
becomes two particles. One of them is an
electron and to conserve total charge
the other is a posetron. Since one is
negative and the other is positive they
cancel out. But both electrons and
posetrons have a property called spin.
And like electric charge this also needs
to be conserved. If the light started
out with zero spin well then the two
particles together must have zero total
spin [music] as well. For example, if
the direction of the electron spin is
this, the posetron has to have [music]
spin in the opposite direction so that
they perfectly cancel out. But the
electron spin could have been this
instead or this. All of these
possibilities are valid. So the rules of
quantum mechanics say that the electron
does all of these possible things at
once until it's measured. It's not just
that we don't know what the spin [music]
is. The electron really is doing
everything. The only restriction is
whatever the electron is doing, the
posetron must do the exact [music]
opposite. This also means that when the
electron is measured and its state is
determined, so is the posetrons. This is
what we mean by entanglement. The two
particles states depend on each other.
But how do we measure the particles and
force them to do one thing? Well, for
that we use the Stern Gerlock machine.
It's essentially a strangely shaped
magnet, and it's how we measure spin.
The orientation of the magnets
determines what axis you're measuring
the spin in. For example, if the machine
is like this, and we shoot in a particle
with spin in the positive Z direction,
it will certainly go to this spot, we'll
call plus. If instead a particle has
negative Z spin, it will certainly go
down to minus. [music] So this Stern
Gerlock machine measures spin in the
Z-axis. So what happens when we put in
one of our entangled particles? When the
electron goes into this machine, it
either goes to plus or to minus with
50/50 probability. Let's say our
electron goes to plus. Well, this means
it went from being in an indeterminant
state to positive Z spin. But what about
the posetron? Well, the only way to
conserve spin is if it's now in the
negative Z spin [music] state. When it's
measured, there is a 100% chance it's
minus. It has to be that way to conserve
spin. But the authors of the paper
realized there's something very odd
about this result.
>> To see what's wrong with this, let's
imagine that the electron and the
posetron [music] carry these envelopes
with them. These envelopes represent the
state of the two particles. until
they're measured. Both of the particles
are in a superp position of [music]
being plus and minus at the same time.
So both options are in the envelope.
But now let's move the posetron to
someone [music] who's far far away.
In this analogy, opening the envelope is
like measuring the spin of the [music]
electron. But that causes the wave
function of the electron to collapse to
just [music] one possibility. In this
case, it's plus. But what happens to the
other envelope far away? Well, it needs
to instantly collapse to [music] minus
because otherwise when the experimental
opens their envelope, they have [music]
a chance of seeing plus, which would
violate the conservation of spin. But if
it needs to collapse instantly when
[music] the electron is measured, then
how does it know what to collapse to? It
must receive intel from the far away
[music] electron. But that message has
to travel much faster than the speed of
light to get to the posetron in time.
And so with this argument, Einstein,
Podolski, and Rosen had [music] shown
that the Copenhagen interpretation of
quantum mechanics really is non-local.
Einstein had already shown this in his
conference talk, but this argument was
even more decisive.
>> It does seem like it's the same thing,
>> but now it's ramped up and you've got
these two separate particles to do those
two separate measurements and one
measurement influencing the other
measurement definitely feels wrong.
>> Yeah, exactly. I think he really
realized that it's measurements that are
the problem in quantum mechanics. The
wave function of a single particle or of
this pair of particles can end up spread
over vast distances. That isn't itself
an issue. But when the wave function
collapses, the information about that
collapse needs to spread everywhere the
wave function is. That's what makes
quantum mechanics non-local. The EPR
paper didn't just point out this
non-locality issue. They proved that
there is only one local alternative
theory for explaining this experiment.
In this local story, instead of the
electron choosing whether to be plus or
minus when it's measured, it actually
makes that choice when it's still in
contact with the posetron. There's some
random way that this plus and minus gets
put into these two envelopes,
which is why the plus and minus are
called hidden variables. And because
this alternative theory assigns these
hidden variables in a local way while
they're still in contact with each other
rather than over a big distance, we call
this a local hidden variable theory.
Now, this local hidden variable theory
is going to be able to explain this
experiment [music]
really simply. Let's pass away the
posetron.
And now when the electron is measured as
a plus, it doesn't have to rush to tell
the posetron. The posetron already knows
there is no action at a distance. This
local hidden variable story is so much
more sensible than the quantum one.
>> So, we're forced to accept one of two
explanations [music] for this
experiment. Either non-locality like the
Copenhagen interpretation of quantum
mechanics or a local hidden variable
theory. Given that non-local action at a
distance contradicts relativity,
Einstein thought this was definitive
proof that the Copenhagen interpretation
of quantum mechanics is wrong and
therefore there must be some local
hidden variable theory that will replace
it. Einstein showed us that quantum
mechanics allows influences [music] that
seem to travel faster than light. But
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this part of the video. And now back to
Bell's Theorem. The EPR paper certainly
got a lot of attention. Without asking
Einstein, Bodilski leaked the paper to
the press and the story ended up in the
New York Times.
>> Predigious harvest of the day's
intelligence is reached. Ask to read all
about it. Einstein was the most famous
scientist in the world and he was going
after the strange but successful theory
of quantum mechanics. So of course the
press loved it. But what did scientists
think of the argument itself?
So the reaction of the physics community
was at first mixed. You know there were
some people sort of old allies of
Einstein's who were very happy with it.
Schroinger being sort of at the top of
that list. And in fact, in an attempt to
clear up some of the misunderstandings
that people were having about the EPR
paper, Schroinger publishes the thought
experiment known as Schroinger's cat,
sort of back up Einstein and show the
kind of problem that he and Einstein had
with quantum physics. Or meanwhile, was
like, "Oh my god, what what the hell is
this? He must be wrong. How do we show
that he's wrong?" Then Bour ultimately,
you know, in his sort of painful and
complicated style ends up coming up with
a response [music] to the EPR paper.
This response is sort of famously
obscure and difficult to understand. And
and I have I've read it in detail and
[music] I will tell you Bor's reply is
either nonsensical or makes some actual
mistakes. There is a very well turned
sentence which I believe Bore took a
great deal of trouble in formulating and
whose meaning is just absolutely obscure
to me.
>> Bore said in his reply to EPR in his
multiple replies to EPR that there is no
question of anything non-local going on.
So the ultimate reaction of the physics
community at least in the immediate
years and decades following the
publication of EPR and Boore's reply in
1935 was to think that Boore had settled
it with his reply even though people
didn't really understand what Boore had
said.
>> Two decades later Einstein died still
questioning quantum mechanics but the
majority of the physics community had
moved on without him. Boore, however,
never forgot about the EPR paper. In
1962, 7 years after Einstein's death,
Boore gave an interview about Einstein,
and he lamented that Einstein wasted
decades on fruitless thought experiments
because he simply could not accept
quantum mechanics. It was terrible that
Einstein fell in that trap to work with
Bedski. Bor said Rosen is worse from my
point of view. Rosen even today believes
the EPR thought experiment. Bedski has
given it up as far as I know. The whole
idea is absolutely nothing when one
really gets into it. You may think that
I say it too strongly, but it is true.
There's absolutely no problem in it. The
next day, Boore took a nap after lunch
and never woke up.
And so, after many decades, the Einstein
boore debate was over. Boore's authority
was part of the reason the EPR paper
didn't get [music] the attention it
deserved. But there was another reason
physicists ignored it. In the EPR
experiment, both theories, Copenhagen
quantum mechanics and Einstein's local
hidden variable alternative make exactly
the same prediction. You get the same
results either way. Debating two
different interpretations of the same
experimental result seemed like armchair
philosophy, not real physics. The
Copenhagen interpretation makes good
predictions, so why not just teach that
and move on?
>> It just seems like, you know, shut up
and calculate, I think, is the message
that kind of gets pushed. The general
attitude was this is done. Who cares?
None of this matters. It's all settled.
Einstein and Boore had a big debate
about it and Boore won. Do you think
you're smarter than Neil Boore? Do you
think you're smarter than Albert
Einstein?
>> It seemed like it would be impossible to
resolve this debate until another
[music] physicist turned his attention
to it.
>> John Bell was an undergraduate student
shortly after World War II in this new
era of physics. And so of [music] course
he was taught the Copenhagen
interpretation.
>> John Bell's doubts about quantum
mechanics by his own recollection showed
up basically the minute [music] he
learned it. In his first quantum
mechanics class, he was, you know,
getting pretty upset with the
instructors and [music] saying, "You're
being too vague. What the heck do you
mean about measurement?"
>> Bel was never fully satisfied by the
answers he got about the foundations of
quantum mechanics. But when he was doing
his PhD, he was encouraged to study
something a little bit more respectable.
And so he studied nuclear physics and
went on to have a very accomplished
career at CERN. But after 8 years of
working in particle physics, in 1963, he
took an academic sbatical. And finally,
he had time to focus on his doubts about
quantum mechanics. He said, "I always
knew it was waiting for me."
He began by re-examining the old debates
in the papers that most physicists had
long since dismissed as philosophical
distractions, including the EPR paper.
After this research, he said, "I felt
that Einstein's intellectual superiority
over bore [music] in this instance was
enormous, a vast gulf between the man
who saw clearly what was needed and the
obscurantist. He realized Einstein's
logic was sound. One of [music] the two
conclusions is true." The question was,
could you prove which one using [music]
an experiment? An experiment of this
sort seemed much more feasible by Bell's
time. [music] The EPR paper was the
first to consider the idea of
entanglement. These days, entanglement
is a core feature of [music] quantum
mechanics. But Einstein had been first
to even point out entanglement existed.
This is why the EPR paper was set up as
a thought experiment because no one had
made such an exotic state of matter in
1935. But in the intervening decades
between Einstein's work and Bell's
between 1935 and 1964, entanglement had
become a serious topic of study. By
Bell's time, there were reliable ways to
make it in the lab. In fact, [music]
Madame Wu had famously reproduced the
EPR thought experiment as a real
experiment. But simply doing the EPR
experiment in real life isn't enough to
tell you which explanation is correct,
since both predict the same thing. But
Belle wondered if there was another
version of the experiment where the
non-local and local theories would have
to give a different result.
>> If you're asking that question and
you're playing with the EPR setup, then
it's like, oh, it's literally not just
figuratively a twist, right? Here's a
simplified version of Belle's
experiment. Make your entangled electron
and positron again, but now instead of
just measuring them with a stern gerlock
machine like this, the experimenters get
a choice about how to orient their
machine. The three different choices are
0°, 120°, and 240°.
This is the twist on the EPR experiment.
The experimenters get to choose
independently. So, the electron might be
measured at 0° while the positron is
measured at 240°.
If both experimenters happen to choose
the same axis, then we know what
happens. They have to get the opposite
result as each other to conserve spin.
The interesting case is when the
experimenters happen to choose different
axes.
The number we want to predict here is
the disagreement rate. The probability
that the electrons result is different
from the posetrons.
First, let's see what quantum mechanics
predicts for this number.
Let's say the electron is measured with
the 0° axis and comes out as plus. Now
that its spin has collapsed to positive
Z, the posetron spin needs to instantly
collapse to be negative Z. This is the
non-local part of quantum mechanics. But
what happens when this posetron is
measured by a machine tilted at 120°?
You can see that the posetron spin is
already almost facing the plus end of
the machine. So, it's much more likely
to go to plus. In fact, there's a 75%
chance it goes to plus and only a 25%
chance it goes to minus. So the
disagreement rate is 25%. And we can
show that for any two different axes the
experimenters select, the geometry is
analogous. So they all have this same
disagreement rate of 25%.
Anytime the experimenters choose
different axes, they will get the same
outcome 75% of the time and different
outcomes 25% of the time.
Now, let's consider the local hidden
variable alternative theory. The
particles here are on a mission. Their
aim to make you believe that they're
acting according to quantum mechanics
when really they're acting locally. Now,
we are anthropomorphizing, but I think
it is really useful to just imagine them
this way. We're trying to figure out if
it's always possible for them to use a
hidden variable theory to [music] get
the same experimental outcomes as
Copenhagen quantum mechanics [music] or
if in this new situation our scheming
particles won't be able to fool us. You
can think of it like this. Each of the
particles is going to be [music] asked
one of three possible questions and they
need to decide on their answer while
they're still together so that they can
coordinate on their strategy. When
[music] they're done figuring out a plan
for how they would answer any of the
three questions, they pack away those
hidden variables [music]
into three sealed envelopes for each
particle. The question is, what
strategies should our sneaky particles
[music] take to make people believe that
they're following quantum mechanics?
Remember, quantum mechanics predicted
[music] a disagreement rate of 25%.
And so our particles want to match
[music] that. Whenever they happen to be
asked different questions, their answer
needs to disagree about 25% [music] of
the time. So what's the best strategy?
Well, there's actually only two things
that they really can do. The first
strategy is this. The electron answers
the same way for each of its axes, and
the posetron answers in the opposite
way. Let's say the electron answers
[music] with minus
and the posetron with plus.
But this is a terrible idea because
[music] whatever two different axes the
experimenters happen to choose, the
disagreement [music] rate is 100%.
Which is very different from 25%. And so
that strategy [music] doesn't work. But
there's only one other strategy that the
particles could use. Instead of the
electron doing exactly the same thing
for all three axes, it does the same
thing [music] for any two of its axes
and then something different for the
last one. Let's [music] just say for
example that it does this and then the
posetron does the opposite. [music] This
is just one example, but it turns out
for all possible strategies like this,
the disagreement rate is going to
[music] be the same.
Let's imagine first that the
experimentter who's measuring the
electron [music] happens to measure it
in the 120° axis and it gets the answer
minus. They make this choice a third of
the time. [music] And now to calculate
the disagreement rate, we need to see
what happens when the experimentter
who's measuring the posetron [music]
happens to measure a different axis from
this one. So one of these two. But in
either one of these cases, the posetron
is also a minus. And so the two answers
agree [music] with each other. And so
the answers have no disagreement. And so
we can multiply this by zero.
But 2/3 [music] of the time the
experimental who's measuring the
electron will happen to measure it in
one of the other two axes. [music] Let's
say this one.
The experimental measuring the polyron
will measure in one of these two axes.
[music]
But you can see that they only pick an
axis that disagrees a half of the time.
That's 1/3, which is roughly [music]
equal to 33%. Which is a different
number from the quantum one. When our
scheming local particles are
interrogated, their answers for
different questions [music] match just a
little too often. They simply can't fake
the results of Copenhagen [music]
quantum mechanics.
So Bell's proof showed that non-local
and local theories make different
predictions about how often the two
results will disagree when the
experimenters measure different axes.
Non-local quantum mechanics predicts
disagreement only 25% of the time. Local
hidden variables predicts disagreement
at least 33% of the time. So to find out
if there really is a local hidden
variable theory, you just need to do the
experiment.
Okay. So, welcome at the Institute
Optic. You are [music] here in the place
where Alan Aspe performed 40 years ago
his experiments on the measurement of
Bell's inequalities. And here [music]
are some of the original pictures of
this experiment which was much more
challenging than it is today. And it was
a real experimental tool the force.
>> So, is this one of the original
equipment from that? This is one of the
yeah of the polarizer the [music]
it looks like that now [music] uh so
only on this uh small breadboard right
it's our [music] main source and this
beam is directed towards this element
which is the key element of the setup so
it's a pair of crystals that produces
[music] pairs of entangled photons we
will produce
a pair of entangle photons and both
[music] are propagating along each of
these two arms. They are separated. So
here we have the two detection [music]
arms and we can rotate the half wave
plate to [music] change the orientation
of the measurement basis.
>> The experiment that we did with light
was a little bit different from the one
we described earlier with electrons and
protons. So I'm going to explain how
they correspond. Here's a little diagram
of the photon experiment. So, first we
have this element that makes the
entangled pair of particles.
So, that's these two. [music] Then, the
entangled particles go off on separate
arms of the experiment. In our previous
experiment, we could decide the
direction that we're [music] going to
measure the particles in by rotating the
stern gerlac machines. And in this
experiment, it's actually really
similar. So we have these two polarizers
that [music] we're able to rotate
independently and that is going to
decide which direction these particles
are measured in. And so this experiment
with light is [music] completely
equivalent to Belle's experiment.
>> Belle expected that the that the
experiments would uh show that the
predictions of quantum mechanics were
correct and that you know there was some
kind of non-locality in nature. Before
the first Bell test was done, John Bell
said, "In view of the general success of
quantum mechanics, it's very hard for me
to doubt the outcome of such
experiments."
>> You didn't expect quantum physics to be
wrong because who would bet against
quantum physics? You'd have to be crazy.
>> Remember, the two different outcomes for
Bell's theorem depend on how often two
different measurement axes are going to
have results that disagree with each
other. Here's how we measure that
disagreement rate. First, we're going to
start with both of the measurements
being in the same direction. And now we
expect that these two are always going
to disagree with each other because they
have opposite spins. So, we're going to
create a bunch of entangled particles
and find out how many of them disagree
with each other per second. This is
going to give us a measure of the total
number of particles coming per [music]
second. That's because this device is
making loads and loads of entangled
particles. And so we just need to know
how many of them are coming at a time.
Then
we rotate
one of the axes.
And now we measure the number of
disagreeing pairs per second. And then
dividing these two will give us the
disagreement rate. And remember quantum
mechanics predicts that the disagreement
rate will only be [music] a quarter.
Whereas local hidden variables expects
this number to be a third.
>> So I started
uh at 2,000, right? And now I have five
500. So
>> basically [snorts] perfect.
>> That's that really works pretty well.
>> We did do this experiment again and the
number we got very much agreed with
quantum mechanics.
But this is one of the most
misunderstood experiments in all of
physics.
>> You will find in all sorts of physics
textbooks and papers and whatnot that
what Bell's theorem proves is that it
rules out local hidden variables or
local realism.
>> John Bell said that was an error. Um you
know he he said like it's really quite
remarkable how many people make that
error.
>> I always get confused at the conclusion
of Bella's serum. Yeah. because there's
a lot of people who say like, okay, it
rules out hidden variables or things
have to be non-local or whatever. But
what do you think?
>> Yeah, I think it is super confusing. And
when I first learned Bell's theorem, um
I was told that it rules out local
hidden variables.
>> I've heard this other argument that it
sort of disproves either locality or
realism. [music]
>> If you say, okay, it means that you give
up local realism.
Um, and so that means you somehow have a
choice between giving up locality and
giving up realism. If you're giving up
realism, realism about what?
Like, like you got to you got to tell me
because like for most definitions of
that word,
you'd also be giving up locality. So,
what the hell are you saving? Um, like I
just don't Yeah, it's it's a really deep
misunderstanding that shows up in almost
every single textbook on the subject.
So, what does Bell's theorem really
prove? Well, here's the logic. Start by
assuming locality for the entangled
particles using the EPR argument. The
only way for them to coordinate their
outcomes is using local hidden
variables. Then, Belle's proof showed
that local hidden variables predict an
incorrect experimental result.
Therefore, the assumption of locality
must have been wrong.
>> We're obliged to invoke something like
actions going faster than light from one
place to another. The EPR paper by
itself had shown that the Copenhagen
interpretation is non-local, which is
why Einstein thought there must be an
alternative way to describe the
experiment that is local. But Bell's
theorem says that's not true. Any theory
that correctly describes this experiment
must be non-local.
>> But I I still I would hesitate to say
that that means that Einstein was wrong,
right? Because what I would I would say
is this shows that Einstein was right to
be concerned about all of [music] this.
People often claim that Einstein's
problem was that he simply couldn't
accept quantum mechanics. But it was
only because he refused to shut up and
calculate that he discovered two of the
most important aspects of quantum
mechanics, entanglement and
non-locality. The heart of the debate
between Einstein and Boore was about
whether there was a problem, whether
there was something to be concerned
about. And the major concern that
Einstein brought to the table from the
beginning was about locality. But you
know [music] what Belle showed was, oh
yeah, all that stuff that Einstein was
concerned about about locality, he was
completely right to be worried about it.
We have a problem.
>> If these particles really are acting
non-locally, this should cause
paradoxes, shouldn't it? Well, it does,
but the paradox seems to be surprisingly
tame. Imagine you and your friend are
measuring a pair of entangled particles.
[music]
Suppose an observer sees you measure
yours first and then your friend
measures hers. That observer thinks that
you collapse the overall state of both
particles and your friend just finds out
the result when she measures. But
another observer will see the situation
in reverse. They see her measure first
and then you. To them, it was her
measurement that caused the collapse,
not yours. But who's right? Which
measurement was the cause of the
collapse and which was the effect? It
seems to depend on your frame of
reference.
This paradox is worrying, but it isn't
as bad as the usual faster than light
paradoxes. [music] In relativity, if you
can communicate faster than light, then
you can exploit how different observers
disagree about timing. If your friend
who's on a rocket sends you an instant
message and you send an instant message
back, in some frames of reference, your
message can arrive before she even sent
the first one. If your message says,
"Don't send your original message," and
so she doesn't, then you've got yourself
a paradox. What prompted you to send
this message if she never sent you
anything in the first place? Quantum
mechanics sidesteps [music] these
paradoxes through a fundamental
constraint. The outcomes are random. So,
you can't send messages faster than
light. When you measure your particle,
you get a plus or a minus completely at
random. Your friend measuring their
particle also gets a random result. Now
the results will be correlated but
they're still completely random. [music]
So there's no way to send any faster
than light message in this way. That's
what prevents us from sending messages
back in time using quantum mechanics. So
quantum mechanics is non-local, but it
doesn't lead [music] to the sort of
catastrophic paradoxes you might expect
from relativity. But it's an uneasy
truce. Quantum mechanics may not break
the letter of relativity's laws, but it
certainly violates the spirit. And
non-locality isn't the only troubling
thing about quantum mechanics. The
Copenhagen interpretation still doesn't
explain what an electron is really doing
and why it acts so differently when
measured. Despite this, many [music]
physicists took Bell's theorem to mean
that the Copenhagen interpretation was
right all along. Bell himself rejected
this. He spent the rest of his life
championing alternative interpretations
of quantum mechanics, including the
hidden variable interpretation called
pilot wave theory or bombium mechanics.
Bell's theorem doesn't rule this
interpretation out because the pilot
wave theory is non-local just like the
Copenhagen interpretation.
It was Bell's theorem and Bell's
subsequent tireless work that made
studying the meaning of quantum
mechanics respectable again.
>> He showed that mere armchair philosophy
and thought experiments can have real
consequences in physics.
>> We need to be teaching quantum physics
in a different way. We need to be
teaching Bell's theorem in a different
way. We do often teach Bell's theorem to
physics students and it's taught us
something that rules out local hidden
variables. That's just not true.
Bell's theorem, you know, says that
quantum physics is in very serious
tension with relativity on the issue of
locality.
>> John Bell passed away suddenly at the
age of 62. He didn't know it, but he had
been nominated for the Nobel Prize just
a year earlier.
>> In a talk he gave in Geneva in January
1990.
He said, "I think you're stuck with the
non-locality.
I don't know any conception of locality
which works with quantum mechanics."
That was 8 months before he died. Um, so
pretty much his last word on the
subject. [music]
>> And so that's it. There really are
faster than light influences in the
universe. Bell's theorem proves it. But
maybe there is a way out. There is
another way to interpret quantum
mechanics that's even more bizarre than
the Copenhagen interpretation. Imagine
the EPR thought experiment again. We can
think of the entangled state as being in
a superp position of the electron being
up and the posetron down and the
electron being down and the posetron
being up. In the Copenhagen
interpretation, when you measure a
particle and you get only one result,
say plus, the other part of the superp
position collapses. But in our examples,
we've seen that measurement collapse
seems to be the source of non-locality.
So why don't we just get rid of collapse
altogether? This is what the many worlds
interpretation of quantum mechanics
proposes. When you measure a particle,
instead of you collapsing the particle
to one outcome, both outcomes happen and
there's two parallel versions of you who
sees each outcome. You have become
entangled with your particle because
your state depends on what the particle
is doing. It sounds strange, but there's
one [music] huge benefit of this
interpretation. When your friend is
about to measure her electron, your
posetron doesn't need to rush to tell
the electron what the answer will be.
There are already two versions of the
electron and they contain the right
answer for each version of her. There
was no need for faster than light
communication to explain the EPR
experiment. But how is that possible?
Doesn't Bell's theorem prove that the
two particles must communicate faster
than light? Well, in Bell's proof, we
assumed that all measurements have just
one outcome, but that assumption just
isn't true in many worlds. [music]
This means that technically that proof
doesn't even apply in the many worlds
case. So, is many worlds local? In one
sense, no, because just like in
Copenhagen quantum mechanics, entangled
pairs can be separated by a huge
distance and still share their state.
However, it is local unlike Copenhagen
in the sense that these farway entangled
particles do not influence each other
faster than light. Many worlds obeys
Einstein's universal speed limit. But is
it really worth accepting that there are
many versions of you in parallel
universes just to recover locality?
Well, locality isn't the only reason
Many Worlds has become more and more
popular. I also really like Many Worlds.
I cuz [music] Copenhagen never sits
right. And when you start telling the
story right of like what happens at
measurement, it's like well what is a
measurement? It's when you have like
this quantum system and there's some
other system which is like much larger
and so you know but it it always feels a
little bit arbitrary. Whereas this this
argument that every time two quantum
particles are interacting their wave
functions are essentially you know
combining and becoming entangled.
>> Um that [music] to me feels more
consistent.
>> Yeah. I I think that's right. What do
you think are like the problems with
many worlds?
>> The [music] biggest problem is I think
people's struggle to deal with sort of
the infinity that that brings forth.
>> For sure.
>> But I I don't know that that's
necessarily an argument against it. Just
cuz [music] like just cuz it's hard to
imagine doesn't mean
>> it's not what's happening.
>> If Many Worlds is right, everything
changes. The conflict [music] between
quantum mechanics and relativity
vanishes. Physicists have been
struggling for decades [music] to unite
quantum mechanics with general
relativity to build a theory of quantum
gravity. And maybe we've [music] been
failing because we've been trying to
marry relativity to a non-local theory.
But if quantum mechanics [music]
ultimately turns out to be local, well
then Einstein's dream of a local
description of reality might not be dead
after all.