[00:00] The double slit experiment is the probably&nbsp; most famous experiment ever and also the most&nbsp;&nbsp; misunderstood one. In this video, I'll tell&nbsp; you why it confuses even physicists. What the&nbsp;&nbsp;
[00:12] double slit experiment has to do with Einstein's&nbsp; spooky action, what Elon Musk got wrong about it,&nbsp;&nbsp; and why Sabine is permanently grumpy. The typical&nbsp; illustration of the double slit experiment looks&nbsp;&nbsp;
[00:24] something like this. If particles did not&nbsp; have quantum properties, they'd behave like&nbsp;&nbsp; little balls and just form two stripes on the&nbsp; screen. But if they do have quantum properties,&nbsp;&nbsp; they form an interference pattern which&nbsp; builds up from single particles. Okay,&nbsp;&nbsp;
[00:40] but so what? All this shows is that particles&nbsp; have wavelike properties. Particles aren't little&nbsp;&nbsp; points. They spread out like waves. So they can&nbsp; go through both slits at the same time. And that's&nbsp;&nbsp;
[00:55] why the parts of the wave coming from each single&nbsp; slit can overlap even for a single particle.&nbsp;&nbsp; That's what creates the pattern. The only thing&nbsp; that's weird is that some people think elementary&nbsp;&nbsp;
[01:08] particles should behave like little balls. Well,&nbsp; the explanation goes the weird part is that if&nbsp;&nbsp; you observe which of the slits the particle went&nbsp; through, then the particles stop behaving this&nbsp;&nbsp;
[01:21] way. They form just two stripes on the screen.&nbsp; That is partly wrong and partly right. The wrong&nbsp;&nbsp; part is that the particles allegedly stop behaving&nbsp; like waves or form two stripes on the screen. This&nbsp;&nbsp;
[01:35] just never happens. If you observe which slit&nbsp; the particle goes through, say the right one,&nbsp;&nbsp; then the particles still behave like a wave. It's&nbsp; just that it's now a wave that goes through only&nbsp;&nbsp;
[01:47] one slit. It'll then form a defraction pattern on&nbsp; the screen which is also an interference effect.&nbsp;&nbsp; And likewise for the right slid. If you observe&nbsp; the particles, they will not give you two stripes&nbsp;&nbsp; on the screen. Instead, you'll get an overlap&nbsp; of the two defraction patterns. What's correct&nbsp;&nbsp;
[02:03] is that if you know which slit the particle went&nbsp; through, you do not get an interference from both&nbsp;&nbsp; slits. You get a sum of left slit and right slit.&nbsp; So what happens in the double slit experiment is&nbsp;&nbsp; that if you take the result of only the right slit&nbsp; and add it to the result of only the left slit,&nbsp;&nbsp;
[02:20] you do not get the same as having both slits&nbsp; open at once. The mathematical reason for this&nbsp;&nbsp; is rather simple. If you have two slits, you&nbsp; add the wave functions from each slit. But the&nbsp;&nbsp;
[02:32] probability is the absolute square and the square&nbsp; of the sum is not the sum of the squares. So for&nbsp;&nbsp; the double slit, you get an extra term. That's&nbsp; the interference term. But then the double slit&nbsp;&nbsp;
[02:45] experiment just seems to say that if you know the&nbsp; particle only goes through one slit at a time,&nbsp;&nbsp; then you get the pattern from only one slit&nbsp; at a time. And again, I'd like to ask, well,&nbsp;&nbsp; what's surprising about this? Here is the part&nbsp; that's actually surprising about the double slit&nbsp;&nbsp;
[03:01] experiment. It's that you can decide whether&nbsp; or not to measure which slit the particle goes&nbsp;&nbsp; through after you send the particle on the way.&nbsp; Suppose you sent the particle from here. There&nbsp;&nbsp;
[03:14] are always some particles that hit outside the&nbsp; slits, but these are not relevant, so we can&nbsp;&nbsp; just ignore them. And you put a detector in one of&nbsp; the slits, let's say the right one. You can either&nbsp;&nbsp; leave the detector off or you turn it on after the&nbsp; particle left the source and before it appeared on&nbsp;&nbsp;
[03:31] the screen. Now, we've just seen that to explain&nbsp; what happens on the screen. If we don't know which&nbsp;&nbsp; slit the particle went through, it needs to go&nbsp; through both slits. And if we do know which one&nbsp;&nbsp;
[03:43] it goes through, it goes through only one. The&nbsp; issue is then that if you turn the detector on,&nbsp;&nbsp; the wave function of the particle must suddenly&nbsp; update on both slits. It must jump from a wave&nbsp;&nbsp;
[03:59] function that goes through both slits at once to&nbsp; one that either goes left or goes right, but not&nbsp;&nbsp; both. And that update is instantaneous, faster&nbsp; than night. This is what Einstein called a spooky&nbsp;&nbsp;
[04:13] action at a distance. You make a measurement at&nbsp; one slit and the wave function on the other slit&nbsp;&nbsp; changes immediately. And note that this is the&nbsp; case even if the particle does not trigger the&nbsp;&nbsp;
[04:30] detector. If it doesn't go into the detector, you&nbsp; know it must have gone through the other slit.&nbsp;&nbsp; But if the particle went through the other slit,&nbsp; you never interacted with it. You didn't touch&nbsp;&nbsp;
[04:43] the particle. It just knew you were trying to. The&nbsp; only way that you can avoid this faster the light&nbsp;&nbsp; update is that the particle knows whether you'll&nbsp; measure it before you've made the decision whether&nbsp;&nbsp;
[05:00] to measure it or not. This is what's called super&nbsp; determinism. And this is what's so weird about the&nbsp;&nbsp; double slit. You either have to accept a faster&nbsp; than light update of the wave function, the spooky&nbsp;&nbsp;
[05:13] action, or you have to accept that the future's&nbsp; fully determined already. And no, this actually&nbsp;&nbsp; has nothing to do with what happens on the screen.&nbsp; The screen is just the evidence for what happened&nbsp;&nbsp;
[05:25] at the slits. I know you've been making fun of me&nbsp; for biting into my armrest each time someone gets&nbsp;&nbsp; quantum mechanics wrong, which is why I'm happy to&nbsp; say that I have a new chair from today's sponsor,&nbsp;&nbsp;
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[06:25] policy. And yes, of course, I have a special&nbsp; offer. Head to flexispot.com and use my code for&nbsp;&nbsp; an amazing discount. And now back to the physics.&nbsp; So this, I think, is the major misconception about&nbsp;&nbsp;
[06:39] the double sit experiment. It's not that if you&nbsp; measure which slit the particle goes through, it&nbsp;&nbsp; loses its wave properties. It's that the particle&nbsp; must have known whether you'd measure it or not&nbsp;&nbsp;
[06:51] before you yourself knew that or faster than&nbsp; light spooky action. Speaking of misconceptions,&nbsp;&nbsp; here is what Elon Musk recently had to say about&nbsp; the double set experiment. It's consistent with&nbsp;&nbsp;
[07:04] the simulation hypothesis. Like a video game,&nbsp; objects are randomly generated with positional&nbsp;&nbsp; certainty only when observed. The simulation&nbsp; hypothesis is the idea that the entire universe,&nbsp;&nbsp;
[07:18] including us, is a computer simulation. The&nbsp; problem with the idea is that no one knows what&nbsp;&nbsp; this means. This is why everything is compatible&nbsp; with it. It's not even wrong, as Pi said so aptly.&nbsp;&nbsp;
[07:33] The second problem is that objects in a video&nbsp; game are usually not truly randomly generated.&nbsp;&nbsp; While this is possible in principle, in practice&nbsp; it's rarely done because the only truly random&nbsp;&nbsp;
[07:46] processes are quantum mechanics. Usually&nbsp; video game algorithms are pseudo random,&nbsp;&nbsp; which means they're created algorithmically just&nbsp; by an algorithm that's very hard to predict. If&nbsp;&nbsp;
[08:00] it were the case that particles in a double set&nbsp; experiment were indeed generated like in video&nbsp;&nbsp; games that is following a deterministic&nbsp; algorithm that it contradict the current&nbsp;&nbsp;
[08:14] standard interpretation of quantum mechanics and&nbsp; I'd really like to know the algorithm. And third&nbsp;&nbsp; it's not true that measuring an object gives&nbsp; it positional certainty that depends on what&nbsp;&nbsp;
[08:27] you measure. If you measure momentum, then&nbsp; it's the momentum that obtained certainty,&nbsp;&nbsp; not the position. While I'm at it, though, there&nbsp; are two other common misunderstandings about the&nbsp;&nbsp;
[08:39] double sit experiment. One is that it's something&nbsp; to do with consciousness. That just isn't. So,&nbsp;&nbsp; the measurement doesn't have to be done by&nbsp; a conscious observer. Some operators will&nbsp;&nbsp; do. The other thing that even physicists are&nbsp; often confused about is momentum conservation.&nbsp;&nbsp;
[08:56] Because suppose the initial particles you&nbsp; sent went straight ahead and at the plate,&nbsp;&nbsp; but then you measure a particle over here. Clearly&nbsp; that's changed its direction. But that should be&nbsp;&nbsp;
[09:09] impossible. It violates momentum conservation. Now&nbsp; if you look at the average of all the particles,&nbsp;&nbsp; then the total momentum again goes straight&nbsp; ahead and that's fine. But it still seems&nbsp;&nbsp;
[09:21] that in single runs it isn't conserved. That&nbsp; is wrong. What actually happens is that the&nbsp;&nbsp; particle becomes entangled with the double sit&nbsp; itself. If the particle gets a small momentum&nbsp;&nbsp;
[09:33] kick into this direction, then the screen gets&nbsp; a little kick into the other direction and the&nbsp;&nbsp; other way around. Momentum is conserved in every&nbsp; single run. It's just that because the double slit&nbsp;&nbsp;
[09:46] is so much heavier than a single particle, you&nbsp; can't measure this tiny kick. And the coral is&nbsp;&nbsp; if the slit was so small that you could measure&nbsp; its momentum then that would at least partially&nbsp;&nbsp;
[09:59] destroy the interference. Again you can do this if&nbsp; you use individual atoms as the slits. Indeed this&nbsp;&nbsp; was done in a recent paper which I talked about&nbsp; in an earlier video. So the reason why Sabine is&nbsp;&nbsp;
[10:12] constantly grumpy is that most of the talk about&nbsp; why quantum mechanics is supposedly weird is just&nbsp;&nbsp; nonsense. And each time I look, there are more&nbsp; nonsense. And all this nonsense is why we aren't&nbsp;&nbsp;
[10:25] making any headway on the one real problem.&nbsp; Namely, how is the instantaneous update of the&nbsp;&nbsp; wave function compatible with Einstein speed&nbsp; of light limit? This is a perfectly obvious&nbsp;&nbsp;
[10:37] research problem that basically no one's working&nbsp; on, despite the fact that it's the one topic that&nbsp;&nbsp; could actually return the foundations of physics&nbsp; to relevance. But at least I have a new chair&nbsp;&nbsp;
[10:49] now. Life is good. So head to flexispot.com and&nbsp; grab yours. By the way, this t-shirt is available&nbsp;&nbsp; in my store. It's an easy way to support this&nbsp; channel. Thanks for watching. See you tomorrow.