字幕列表 影片播放 列印英文字幕 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.