Bell's Theorem and Quantum Nonlocality: Einstein, Bohr, and Many Worlds

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

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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.