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

- In 1935. Einstein came up with a thought experiment that showed quantum mechanics breaks one

of the most sacred principles in physics, that nothing can go faster than the speed of light. Physicists assumed he was wrong.

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 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 speed limit. - We are obliged to invoke something like actions going faster than light from one place to another.

- This is a video about one of the spookiest and most misunderstood experiments in all of physics, and it 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 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. When there's a change in gravity

that only affects the local space time, and then that ripple spreads out to nearby regions which spread farther out

until eventually 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 eight 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 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 Solvay 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 a 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.'

That man was Niels Bohr, the most influential figure in quantum physics at the time. Bohr's Institute in Copenhagen had become the hub

for the new field. Dozens of young scientists like Werner Heisenberg came to learn from him.

As one of his disciples remembers, 'Bohr 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.' Bohr wasn't the one who wrote the mathematical rules of quantum mechanics.

Instead, he told everyone what they meant. While others were confused by the theory Bohr 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 Schrödinger equation. And at some point you're gonna make a measurement and at that point the wave function collapses.

- I think that one bit of that that you said was like the wave function is all you can know about the particle, and I think that was like a pretty important point to Bohr.

- As Bohr 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 Bohr, that question didn't even 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 Schrodinger, 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 the electron.

After all, he may not have convinced Bohr 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 Bohr had a great debate and Einstein was unhappy with quantum mechanics

because it was fundamentally probabilistic. He tried to show that there were conceivable experiments that you could use to get around those uncertainty relations

and Bohr showed over and over and over again that you couldn't do that. And eventually everybody agreed with Bohr.

- That's Adam Becker, author of What is Real, a great book about the history of quantum mechanics. As he explained to us,

Bohr 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. Its purpose was to show the non-locality

of the Copenhagen interpretation in action. - Bohr just misunderstood it, and when he recounted it to others later on,

he drew a little diagram of what Einstein's thought experiment setup was, and it just didn't have the mirror in it at all.

And yet this is taken as like the great victory for Bohr over Einstein, which is crazy, but 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, 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 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 final theory right away. And yeah, and the fact that you can see paradoxes like this,

would make you think there's gotta be more to it that we just don't have yet. - Absolutely. But Einstein hadn't even persuaded Bohr

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 Podolsky 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. 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 positron since one is negative and the other is positive, they cancel out.

But both electrons and positrons 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 as well. For example, if the direction of the electron spin is this,

the positron has to have 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 is, the electron really is doing everything.

The only restriction is whatever the electron is doing. The positron must do the exact opposite. This also means that when the electron is measured

and its state is determined, so is the positrons. 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-Gerlach 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. So this Stern-Gerlach 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 indeterminate state to positive Z spin.

But what about the positron? Well, the only way to conserve spin is if it's now in the negative Z spin 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 positron 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 superposition

of being plus and minus at the same time. So both options are in the envelope, but now let's move the positron

to someone who's far far away. In this analogy, opening the envelope is like measuring the spin of the electron,

but that causes the wave function of the electron to collapse to just one possibility. In this case, it's plus,

but what happens to the other envelope far away? Well, it needs to instantly collapse to minus because otherwise when the experimenter opens their envelope,

they have a chance of seeing plus, which would violate the conservation of spin. But if it needs to collapse instantly when the electron is

measured, then how does it know what to collapse to? It must receive intel from the far away electron, but that message has to travel much faster than the speed

of light to get to the positron in time. And so with this argument, Einstein Podolski and Rosen had 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 positron.

There's some random way that this plus or 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 really simply.

Let's pass away the positron, and now when the electron is measured as a plus, it doesn't have to rush to tell the positron.

The positron 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 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 that seem to travel faster than light,

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I want to thank Nord VPN for sponsoring this part of the video, and now back to Bell's theorem. The EPR paper certainly got a lot of attention.

Without asking Einstein, Podolsky leaked the paper to the press and the story ended up in the New York Times

- Prodigious harvest of the day's. intelligence is reached. - Extra, extra - 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.

Schrodinger 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, Schrodinger publishes the thought experiment known as Schrodinger's cat, sort of back up Einstein

and show the kind of problem that he and Einstein had with quantum physics. Or meanwhile, it's like, oh my God, what?

What the hell is this? He must be wrong. How do we show that he's wrong? Then Bohr ultimately, you know, in his sort of painful

and complicated style ends up coming up with a response to the EPR paper. This response is sort of famously obscure

and difficult to understand and and I have, I have read it in detail and I will tell you Bohr's reply is either

nonsensical or makes some actual mistakes. - There is a very well turned sentence, which I believe Bohr took a great deal

of trouble in formulating, and his meaning is just absolutely obscure to me. - Bohr 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 Bohr's reply in 1935, was to think

that Bohr had settled it with his reply, even though people didn't really understand what Bohr had said.

- Two decades later, Einstein died still questioning quantum mechanics, but the majority of the physics community had moved on without him.

Bohr however, never forgot about the EPR paper. In 1962, 7 years after Einstein's death,

Bohr 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 Podolsky' Bohr said, Rosen is worse from my point

of view, Rosen, even today believes the EPR thought experiment. Podolsky 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, Bohr took a nap after lunch and never woke up, and so after many decades,

the Einstein-Bohr debate was over. Bohr's authority was part of the reason the EPR paper didn't get 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.

- General attitude was, this is done, who cares? None of this matters. It's all settled. Einstein and Bohr had a big debate about it and Bohr won.

Do you think you're smarter than Niels Bohr? Do you think you're smarter than Albert Einstein? - It seemed like it would be impossible

to resolve this debate until another 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 course, he was taught the Copenhagen Interpretation.

- John Bell's doubts about quantum mechanics by his own recollection, showed up basically the minute he learned it.

In his first quantum mechanics class, he was, you know, getting pretty upset with the instructors and saying, you're being too vague.

What the heck do you mean about measurement? - Bell 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 eight years

of working in particle physics, in 1963, he took an academic sabbatical and finally he had time

to focus on his doubts about quantum mechanics. He said, I always knew that it was waiting for me. He began by re-examining the old debates

and 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 Bohr in this, instance was enormous; I've vast gulf between the man who saw clearly

what was needed and the obscurantist.' He realized Einstein's logic was sound. One of the two conclusions is true.

The question was, could you prove which one, using an experiment? An experiment of this sort seemed much more feasible by Bell's time.

The EPR paper was the first to consider the idea of entanglement. These days, entanglement is a core feature

of 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, Madam 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 Bell 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 bell's experiment, make your entangled electron and positron again.

But now, instead of just measuring them with a Stern–Gerlach machine like this, the experimenters get a choice about how

to orient their machine. The three different choices are zero degrees, 120 degrees and 240 degrees.

This is the twist on the EPR experiment. The experimenters get to choose independently, so the electron might be measured at zero degrees while the

positron is measured at 240 degrees. 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 electron's result is different from the positron's.

At first, let's see what quantum mechanics predicts for this number. Let's say the electron is measured with the zero degree axis

and comes out as plus. Now that its spin has collapsed to positive Z, the positron spin needs

to instantly collapse to be negative Z. This is the non-local part of quantum mechanics, but what happens when this positron is measured

by a machine tilted at that the positron 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 outcomes 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 get the same experimental outcomes as Copenhagen quantum mechanics,

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 gonna be 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 they're done figuring out a plan for how they would answer any of the three questions,

they pack away those hidden variables into three sealed envelopes for each particle. The question is, what strategies should our sneaky particles

take to make people believe that they're following quantum mechanics? Remember, quantum mechanics predicted a disagreement rate

of 25%, and so our particles want to match that. Whenever they happen to be asked different questions, their answer needs to disagree about 25% 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 positron answers in the opposite way.

Let's say the electron answers with minus and the positron with plus. But this is a terrible idea

because whatever two different axes the experimenters happen to choose, the disagreement rate is a hundred percent, which is very different from 25%.

And so that strategy 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 for any two of its axes and then something different for the last one,

let's just say for example, that it does this and then the positron does the opposite. This is just one example,

but it turns out for all possible strategies like this, the disagreement rate is gonna be the same. Let's imagine first

that the experimenter who's measuring the electron happens to measure it in the 120 degree axis, and it gets the answer minus they make this

choice a third of the time. And now to calculate the disagreement rate, we need to see what happens when the experimenter who's measuring the

positron happens to measure a different axis from this one. So one of these two, but in either one of these cases,

the positron is also a minus, and so the two answers agree with each other, and so the answers have no disagreement.

And so we can multiply this by zero, but two thirds of the time, the experimenter who's measuring the electron will happen

to measure it in one of the other two axes. Let's say this one, the experimenter measuring the positron will measure in one

of these two axes, but you can see that they only pick an axis that disagrees a half of the time.

That's one third, which is roughly equal to 33%, which is a different number from the quantum one. When our scheming local particles are interrogated,

their answers for different questions match just a little too often. They simply can't fake the results

of Copenhagen 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 Institut d'Optique.

You are here in the place where Alain Aspect performed forty years ago, his experiments on the measurement of Bell's inequalities, and here are some of the original pictures

of this experiment. Which was much more challenging than it is today, and it was a real experimental tour de force.

- So is this one of the original equipment from that? - This is one of the, yeah, of the polarizers. It looks like that now.

So only on this small breadboard, right? This is our 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 pairs of entangled photons.

We will produce a pair of entangled photons and both are propagating along each of these two arms. They're separated.

So here we have the two detection arms and we can rotate the halfway plate to 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 gonna 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.

Then the entangled particles go off on separate arms of the experiment. In our previous experiment, we could decide the direction

that we're going to measure the particles in by rotating the Stern–Gerlach machines. And in this experiment, it's actually really similar.

So we have these two polarizers that we're able to rotate independently, and that is gonna decide which direction these

particles are measured in. And so this experiment with light is completely equivalent to Bell's experiment.

- Bell expected that the, that the experiments would 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. - He 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 gonna start with both of the measurements being in the same direction,

and now we expect that these two are always gonna disagree with each other because they have opposite spins. Though we're gonna 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 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 a quarter, whereas local hidden variables expects this number to be a third.

- So I started at 2000, right? And now I have five, 500. So that's basically 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'll find in all sorts of physics textbooks and papers

and whatnot, that what Bell's theorem proves that it rules out local hidden variables or local realism. John Bell said that was an error, 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 Bell's theorem.

- Yeah. - Because there's a lot of people who say like, okay, it rules out hidden variables,

or things have to be none local or whatever. But what, what do you think? - Yeah, I think it is super confusing,

and when I first learned Bell's theorem, I was told that it rules out local hidden variables. - I've heard this other argument that it's sort

of disproves either locality or realism. - If you say, okay, it means that you give up local realism, 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 gotta, you gotta tell me,

because like for most definitions of that word, you'd also be giving up locality. So what the hell are you saving?

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 Bell's proof showed that local hidden variables predict an incorrect

experimental result. Therefore, the assumption of locality must have been wrong. - We are 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 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 Bohr 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 what Bell 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.

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. In relativity, 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 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 there's still completely random. 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 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 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 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 Bohmian 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 as 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 eight months before he died.

So pretty much his last word on the subject. - 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 superposition

of the electron being up and the positron down, and the electron being down, and the positron being up.

In the Copenhagen interpretation, when you measure a particle and you get only one result, say plus, the other part of the superposition 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 world's 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 huge benefit of this interpretation.

When your friend is about to measure her electron, your positron 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 fast

and the like 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. 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 far away 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,

because 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 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, that to me feels more consistent.

- Yes. But I, I think that's right. What do you think are like the problems with many worlds? - The 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 'cause like, just 'cause it's hard to imagine doesn't mean, yeah, it's not what's happening.

If many worlds is right, everything changes the conflict between quantum mechanics and relativity vanishes. Physicists have been struggling for decades

to unite quantum mechanics with general relativity to build a theory of quantum gravity. And maybe we've been failing

because we've been trying to marry relativity to a non-local theory. But if quantum mechanics ultimately turns out to be local,

well then Einstein's dream of a local description of reality might not be dead, after all.