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  • Today I am going to tell you about one of my favourite results, Bell's theorem, that

  • answers the EPR paradox. But instead of proving the thing here, I've decided to push that

  • to the next video, and instead just tell you the result and explain what it actually means.

  • That's because this is the most misunderstood theorem of science that I know of. I have

  • heard it claimed by so many sources, from popular books, to lecturers, to serious research

  • papers that Bell's theorem proves quantum mechanics is right Let's see if that's true

  • We've already seen that quantum mechanics has a few disturbing aspects. I'd argue that

  • a lot that comes done to one statement in quantum mechanics, the superposition principle;

  • that is that if we have a particle that could be in several possible states at a certain

  • time and we can't tell which one its in, it's in all those states at once. That means that

  • while we have our backs turned, things act way differently to how they act when we're

  • looking. But do we have to really believe that? I mean, we can never, by definition

  • actually catch the objects in the act. The EPR paradox tried to prove that superposition

  • is wrong.

  • I won't go through the EPR argument in detail again because you can just watch that video

  • once more if you need a refresher

  • Remember for the EPR video, we had our two entangled objects and we knew one was red

  • and other white, but we didn't know which was which. Quantum Mechanics insists that

  • all possible things happen so each particle is both red and white. But if we open one

  • box and the object is white, then when we open the other we must have the red one. The

  • only way is if, somehow or another, the particles can talk to each other.

  • We'll call any theory where entangled particles are able to communicate while they're separated

  • like this, a theory with "talking particles"

  • But the EPR argument then showed that

  • If a theory has talking particles, then those messages go faster than light.

  • So what does this mean? If Einstein and the majority of physicists

  • at the time where right and nothing can ever faster then light, then EPR tells us that

  • all theories with talking particles are wrong. That includes quantum mechanics.

  • If nothing truly can go faster than light, how can we explain the fact that entangled

  • particles always have opposite colours to each other? They can't talk talk to each other

  • while they're seperated as EPR shows, so they must decide which colours to be while they're

  • still together. That means that instead of acting all crazy and going into superpositions

  • while they're behind our backs, they instead act pretty normal with just one colour. SO

  • you see, if nothing can go faster than the speed of light, not only is Quantum mechanics

  • done for, we can go back to believing the world isn't doing such crazy things while

  • we're not looking. I.e if particle's can't talk faster than light,

  • then superposition is wrong.

  • What about the other possibility? What if entangled particles do talk faster than light

  • to each other? This is where lots of people get it wrong so I urge you to be careful.

  • if entangled particles do talk faster than light that doesn't necessarily imply superposition

  • is right.

  • That would be like saying If an animal is a raven it is black

  • Means that If an animal isn't a raven then its not black.

  • That's clearly not true. If an animal isn't a raven, all you can say is that its not raven,

  • you can't infer anything about its colour. In the same way, if particles talk faster

  • than light, all you can say is, they do talk faster than light.

  • SO finally we get to the point. What is Bell's theorem. Bell came up with an experiment that

  • would give different results in a world where particles can talk faster than light and one

  • where they can't. This got people really excited because there was a chance to disprove quantum

  • mechanics. 15 years later, people finally came up with the technology to do the experiment.

  • There result? Particles do actually do talk faster than light. Quantum mechanics survived.

  • But like I said before, this doesn't prove Quantum mechanics is right, only that it isn't

  • wrong in this way.

  • This brings up a lot questions. Firstly, what about relativity? Remember I told you that

  • the biggest assumption in relativity is that nothing goes faster than light, but apparently

  • that's not true! I don't know enough relativity to teach it to you, but in relativity, if

  • it's possible for Alice to send Bob a message faster than light, all kinds of crazy stuff

  • can happen. For example, to some people it will look like Bob gets Alice's message before

  • she sends it. It's even possible for him to reply before she sends it- but then -what

  • if she decided not to send her original message afterall?

  • That is an aweful logical mess. So what's the solution? Physicists say that they have

  • come up with it. They claim that it's impossible to manipulate our entangled talking particles

  • to send the messages we want. This is called the No Communication theorem. I don't know

  • if we should just trust though, so we'll have a closer look at it later.

  • Another question is that, sure, this experiment doesn't prove that superposition is true,

  • but is it really possible to explain all the weird experiments we've been talking about

  • with out it? Turns out it might be. There is at least one very promising alternative

  • to quantum mechanics that does have talking particles so it's not ruled out, but doesn't

  • have superposition. It's called Bohmian Mechanics, and I might make a video about it cos it's

  • pretty amazing.

  • So that's the end of this video but if you want to see how Bell's theorem proves that

  • particles can talk faster than light, then I will be putting up videos about that. They'll

  • be one to explain something called Spin and then the actual proof. If you don't know too

  • much about spin, you should watch that one first.

Today I am going to tell you about one of my favourite results, Bell's theorem, that

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Is Quantum Mechanics True? Bell's theorem explained | Quantum ep 9

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    林婉蓉 posted on 2015/05/01
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