You probably misunderstand the double slit experiment

[00:00:00]The double slit experiment is the probably most famous experiment ever and also the most misunderstood one. In this video, I'll tell you why it confuses even physicists, what the double slit experiment has to do with Einstein's spooky action, what Elon Musk got wrong about it, and why Sabine is permanently grumpy. The typical illustration of the double slit experiment looks something like this. If particles did not have quantum properties, they'd behave like little balls and just form two stripes on the screen. But if they do have quantum properties, they form an interference pattern, which builds up from single particles. Okay, but so what? All this shows is that particles have wave-like properties. Particles aren't little points. They spread out like waves, so they can go through both slits at the same time. And that's why the parts of the wave coming from each single slit can overlap even for a single particle. That's what creates the pattern. The only thing that's weird is that some people think elementary particles should behave like little balls. Well, the explanation goes, the weird part is that if you observe which of the slits the particle went through, then the particle stop behaving this way. They form just two stripes on the screen. That is partly wrong and partly right. The wrong part is that the particles allegedly stop behaving like waves or form two stripes on the screen.

[00:01:35]This just never happens. If you observe which slit the particle goes through, say the right one, then the particle still behave like a wave. It's just that it's now wave that goes through only one slit. It'll then form a diffraction pattern on the screen, which is also an interference effect. And likewise for the right slit. If you observe the particles, they will not give you two stripes on the screen. Instead, you'll get an overlap of the two diffraction patterns. What's correct is that if you know which slit the particle went through, you do not get an interference from both slits. You get a sum of left slit and right slit. So, what happens in the double slit experiment is that if you take the result of only the right slit and add it to the result of only the left slit, you do not get the same as having both slits open at once. The math The mathematical reason for this is rather simple. If you have two slits, you add the wave functions from each slit, but the probability is the absolute square, and the square of the sum is not the sum of the squares. So, for the double slit, you get an extra term. That's the interference term. But then, the double slit experiment just seems to say that if you know the particle only goes through one slit at a time, then you get the pattern from only one slit at a time. And again, I'd like to ask, well, what's surprising about this? Here is the part that's actually surprising about the double slit experiment. It's that you can decide whether or not to measure which slit the particle goes through after you sent the particle on the way. Suppose you sent the particle from here. There are always some particles that hit outside the slits, but these are not relevant, so we can just ignore them. And you put a detector in one of the slits, let's say the right one. You can either leave the detector off, or you turn it on after the particle left the source and before it appeared on the screen. Now, we've just seen that to explain what happens on the screen if we don't know which slit the particle went through, it needs to go through both slits. And if we do know which one it goes through, it goes through only one. The issue is then that if you turn the detector on, the wave function of the particle must suddenly update on both slits. It must jump from a wave function that goes through both slits at once to one that either goes left or goes right, but not both. And that update is instantaneous, faster than light. This is what Einstein called a spooky action at a distance. You make a measurement at one slit and the wave function on the other slit changes immediately.

[00:04:24]And note that this is the case even if the particle does not trigger the detector. If it doesn't go into the detector, you know it must have gone through the other slit. But if the particle went through the other slit, you never interacted with it. You didn't touch the particle. It just knew you were trying to.

[00:04:48]The only way that you can avoid this faster-than-light update is that the particle knows whether you'll measure it before you've made the decision whether to measure it or not. This is what's called superdeterminism.

[00:05:04]And this is what's so weird about the double slit. You either have to accept the faster-than-light update of the wave function, the spooky action, or you have to accept that the future's fully determined already and know this actually has nothing to do with what happens on the screen. The screen is just the evidence for what happened at the slits.

[00:05:27]I know you've been making fun of me for biting into my armrest each time someone gets quantum mechanics wrong, which is why I'm happy to say that I have a new chair from today's sponsor, FlexiSpot.

[00:05:43]This chair is called the C7 Max and it's absolutely great. For one thing, it's super comfortable to sit in even during a heat wave. You don't stick to the chair and you can adjust everything about it. The arm rest, the headrest, the seat on the back and of course the height. I actually had a pillow on the old chair most of the time because the cushion was so thin it was basically sitting on steel. Now, what do I do with the pillow? But the coolest part is this. It has a footrest.

[00:06:13]Only downside is that now my kids want one, too. I'm happy to recommend the chair from FlexiSpot. It's simply a great product and it comes with a 5-year warranty and a 30-day return policy. And yes, of course, I have a special offer.

[00:06:28]Head to flexispot.com and use my code for an amazing discount. And now back to the physics. So, this, I think, is the major misconception about the double slit experiment. It's not that if you measure which slit the particle goes through, it loses its wave properties.

[00:06:46]It's that the particle must have known whether you'd measure it or not before you yourself knew. That or faster-than-light spooky action.

[00:06:57]Speaking of misconceptions, here is what Elon Musk recently had to say about the double slit experiment. It's consistent with the simulation hypothesis. Like a video game, objects generated with positional certainty only when observed.

[00:07:13]The simulation hypothesis is the idea that the entire universe, including us, is a computer simulation. The problem with the idea is that no one knows what this means. This is why everything is compatible with it. It's not even wrong, as Pauli said so aptly. The second problem is that objects in a video game are usually not truly randomly generated. While this is possible in principle, In practice, it's rarely done because the only truly random processes are quantum mechanics. Usually, video game algorithms are pseudo random, which means they're created algorithmically just by an algorithm that's very hard to predict. If it were the case that particles in a double slit experiment were indeed generated like in video games, that is following a deterministic algorithm, that would contradict the current standard interpretation of quantum mechanics. And I'd really like to know the algorithm. And third, it's not true that measuring an object gives it position a certainty. That depends on what you measure. If you measure momentum, then it's the momentum that obtained certainty, not the position.

[00:08:34]While I'm at it, though, there are two other common misunderstandings about the double slit experiment. One is that it's something to do with consciousness. That just isn't so. The measurement doesn't have to be done by a conscious observer.

[00:08:48]Some apparatus will do. The other thing that even physicists are often confused about is momentum conservation. Because suppose the initial particles you sent went straight ahead and at the plate, but then you measure a particle over here. Clearly, that's changed its direction. But that should be impossible. It violates momentum conservation. Now, if you look at the average of all the particles, then the total momentum again goes straight ahead, and that's fine. But it still seems that in single runs, it isn't conserved. That is wrong. What actually happens is that the particle becomes entangled with the double slit itself.

[00:09:30]If the particle gets a small momentum kick into this direction, then the screen gets a little kick into the other direction and the other way around.

[00:09:40]Momentum is conserved in every single run. It's just that because the double slit is so much heavier than a single particle, you can't measure this tiny kick. And the corollary is if the slit was so small that you could measure its momentum, then that would at least partially destroy the interference again. You can do this if you use individual atoms as the slits. Indeed, this was done in a recent paper which I talked about in an earlier video. So, the reason why Sabine is constantly grumpy is that most of the talk about why quantum mechanics is supposedly weird is just nonsense and each time I look, there's more nonsense. And all this nonsense is why we aren't making any headway on the one real problem, namely how is the instantaneous update of the wave function compatible with Einstein's speed of light limit. This is a perfectly obvious research problem that basically no one's working on despite the fact that it's the one topic that could actually return the foundations of physics to relevance. But at least I have a new chair now. Life is good. So, head to flexispot.com and grab yours.

[00:10:54]By the way, this t-shirt is available in my store. It's an easy way to support this channel. Thanks for watching. See you tomorrow.