You Probably Misunderstand the Double Slit Experiment

[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  

[00:12] 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  

[00:24] 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,  

[00:40] but so what? All this shows is that particles  have wavelike 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  

[00:55] 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  

[01:08] 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 particles stop behaving this  

[01:21] 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. This  

[01:35] just never happens. If you observe which slit  the particle goes through, say the right one,   then the particles still behave like a wave. It's  just that it's now a wave that goes through only  

[01:47] one slit. It'll then form a defraction pattern on  the screen which is also an interference effect.   And likewise for the right slid. If you observe  the particles, they will not give you two stripes   on the screen. Instead, you'll get an overlap  of the two defraction patterns. What's correct  

[02:03] 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,  

[02:20] you do not get the same as having both slits  open at once. The mathematical reason for this   is rather simple. If you have two slits, you  add the wave functions from each slit. But the  

[02:32] 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  

[02:45] 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  

[03:01] experiment. It's that you can decide whether  or not to measure which slit the particle goes   through after you send the particle on the way.  Suppose you sent the particle from here. There  

[03:14] 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  

[03:31] 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  

[03:43] 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  

[03:59] 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 night. This is what Einstein called a spooky  

[04:13] action at a distance. You make a measurement at  one slit and the wave function on the other slit   changes immediately. And note that this is the  case even if the particle does not trigger the  

[04:30] 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  

[04:43] the particle. It just knew you were trying to. The  only way that you can avoid this faster the light   update is that the particle knows whether you'll  measure it before you've made the decision whether  

[05:00] to measure it or not. This is what's called super  determinism. And this is what's so weird about the   double slit. You either have to accept a faster  than light update of the wave function, the spooky  

[05:13] action, or you have to accept that the future's  fully determined already. And no, this actually   has nothing to do with what happens on the screen.  The screen is just the evidence for what happened  

[05:25] at the slits. 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,  

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[06:25] policy. And yes, of course, I have a special  offer. 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  

[06:39] the double sit experiment. It's not that if you  measure which slit the particle goes through, it   loses its wave properties. It's that the particle  must have known whether you'd measure it or not  

[06:51] before you yourself knew that or faster than  light spooky action. Speaking of misconceptions,   here is what Elon Musk recently had to say about  the double set experiment. It's consistent with  

[07:04] the simulation hypothesis. Like a video game,  objects are randomly generated with positional   certainty only when observed. The simulation  hypothesis is the idea that the entire universe,  

[07:18] 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 Pi said so aptly.  

[07:33] 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  

[07:46] 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  

[08:00] it were the case that particles in a double set  experiment were indeed generated like in video   games that is following a deterministic  algorithm that it contradict the current  

[08:14] 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 positional certainty that depends on what  

[08:27] you measure. If you measure momentum, then  it's the momentum that obtained certainty,   not the position. While I'm at it, though, there  are two other common misunderstandings about the  

[08:39] double sit 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. Some operators will   do. The other thing that even physicists are  often confused about is momentum conservation.  

[08:56] 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  

[09:09] 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  

[09:21] that in single runs it isn't conserved. That  is wrong. What actually happens is that the   particle becomes entangled with the double sit  itself. If the particle gets a small momentum  

[09:33] kick into this direction, then the screen gets  a little kick into the other direction and the   other way around. Momentum is conserved in every  single run. It's just that because the double slit  

[09:46] is so much heavier than a single particle, you  can't measure this tiny kick. And the coral is   if the slit was so small that you could measure  its momentum then that would at least partially  

[09:59] 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  

[10:12] constantly grumpy is that most of the talk about  why quantum mechanics is supposedly weird is just   nonsense. And each time I look, there are more  nonsense. And all this nonsense is why we aren't  

[10:25] making any headway on the one real problem.  Namely, how is the instantaneous update of the   wave function compatible with Einstein speed  of light limit? This is a perfectly obvious  

[10:37] 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  

[10:49] now. Life is good. So head to flexispot.com and  grab yours. 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.