The biggest lie about the double slit experiment

0:00 There is a lie about the double slit experiment that's told by everyone

0:04 from famous physicists to YouTubers and even me when I used to believe it.

0:09 The lie is when you shoot electrons at two slits,

0:13 if you don't watch which way they go through, they'll act like waves.

0:17 But if you do watch which way they went, then they act like particles.

0:22 They go straight through and they end up in two clumps.

0:26 This actually contradicts the results of the experiment and it's

0:29 completely the wrong lesson to take from the double slit experiment.

0:33 But this lie has been told so often that it's kind of got a life of its own.

0:37 The real lesson of this experiment is way more interesting.

0:40 And I want to tell you about that.

0:42 But first, let's find out what the electrons actually do when you observe them.

0:46 Because it's not this.

0:49 Imagine that we do have a bunch of particles like

0:52 this that are going to go through this double slit experiment.

0:55 You expect to have like two bunches of particles

0:59 and actually these slits are very narrow and very close together.

1:03 So you'd kind of expect a bit of overlap between those two heaps.

1:07 But here's what would happen for a wave.

1:09 Okay, so I love this website because it's

1:10 a wave simulator and I'll link it in the description.

1:13 But basically what we're doing here is we have this wall with two

1:18 narrow slits and we're going to see what happens when these waves go through.

1:25 Okay.

1:26 So what you're seeing happen here is that there

1:29 are these two waves coming from each of the slits.

1:32 But when they touch each other something weird happens.

1:36 They interact with each other and kind of overlap.

1:40 And if we let this go further,

1:45 these interference patterns always kind of look like this, where

1:48 there's a very strong bit of wave in the middle,

1:51 then there's not much, and then there's more wave,

1:54 and then again there's a blank spot, and then another strong spot, etc.

1:59 And as we saw in that simulation, what you'd expect for a wave is that it

2:04 hits this back wall in this very funny pattern.

2:07 There are some spots like this one where it's very intense and then

2:11 there are some spots like this where it's not intense at all.

2:14 And there are these little bands of you know intensity and no intensity.

2:19 And when you do this experiment with light that's exactly what

2:22 you see which suggests very strongly that light is a wave.

2:27 In fact the double slit experiment was the thing

2:30 that really convinced people that light is a wave after all.

2:33 And I've done the double slit experiment

2:35 in a couple of videos before and I always get a lot of comments um being like

2:40 this is just classical optics and that is true.

2:44 But then they say therefore it's irrelevant to quantum mechanics

2:47 and that I disagree with and I'll explain why a little bit later.

2:51 But first let's think about the same experiment with electrons.

2:55 Is it going to turn out more like the particle case or more like the wave one?

2:59 Well, this is footage from an actual double slit experiment using electrons.

3:04 So, let's see what the answer is.

3:06 Well, it's looking very promising for particles because unlike with a wave,

3:10 the result isn't just sort of smeared out.

3:12 It's coming in these individual dots.

3:15 But if we let the pattern develop,

3:18 you start to see that it actually isn't in two lumps.

3:23 It looks much more like the double slit interference pattern for waves.

3:29 So something really odd seems to be happening with these electrons.

3:33 There seems to be some sort of wave

3:35 associated with the electrons and the wave spreads out.

3:40 And just like a water wave or a light wave, it goes through both slits.

3:48 The only real difference between an electron and a wave that goes

3:52 through both slits is that the electron still ends up as dots.

3:57 It isn't smeared out the way that waves usually are.

4:00 But this feels wrong because for something like light, I mean,

4:04 it's perfectly fine to accept that it's some sort of wave.

4:08 But for electrons, you know,

4:10 we're used to thinking of them as little bullets like this, little particles.

4:15 And a particle couldn't be going through both of these slits at the same time.

4:20 That's why it's really natural to think

4:22 that if electrons really are going through both slits,

4:25 then maybe let's try and catch them in the act.

4:27 Let's put two measuring devices here and here that will watch both

4:32 of these slits and we'll tell you

4:34 whether the electron actually goes through both slits.

4:37 The idea is that we want to be able

4:39 to catch the electrons doing both things at the same time.

4:43 But measurement in quantum mechanics doesn't work like that.

4:48 So in quantum mechanics when you try

4:50 and measure an object that's in a superp position

4:53 of two different things like this electron going through

4:56 both slits then you will see a particular result.

5:00 For example, this detector might see an electron go by.

5:05 But now in quantum mechanics what

5:07 happens is that measurement changed the electron.

5:11 Now it's only going through one of these slits, the one you observed.

5:15 So far this is all correct.

5:18 But here comes the myth.

5:19 Many people will tell you that now

5:21 that the electron has been measured it becomes a particle.

5:25 And so this particle will act like any normal particle will sort of end up here.

5:31 And so you can imagine that if we keep measuring which way every electron goes,

5:37 then you end up with two piles of electrons.

5:40 People will draw the results just like this as two

5:45 clumps or at most as two overlapping clumps.

5:48 And so before I ever did these experiments myself,

5:52 I assume that this is what happens.

5:54 But that's not at all the case.

5:57 And I'll get a curve that will probably look more or less like this.

6:00 I wish I had experimental footage of someone actually doing this experiment

6:05 with electrons to show you that this is not what happens, but I don't.

6:10 And I also can't do this experiment myself

6:13 because uh working with the electrons is really hard.

6:15 You need like a vacuum pump and everything.

6:17 But the results that you get when you

6:19 do the experiment with light are very very similar.

6:22 And so I did that experiment in my previous video.

6:26 And this is the result that I got.

6:29 What on earth is this crazy pattern?

6:32 It doesn't look anything like two lumps.

6:35 This is actually called the single slit interference pattern.

6:38 And it's still very much wave behavior.

6:41 Let me show you.

6:43 So this is what the interference pattern look

6:45 like with the two slits here and here.

6:47 But what happens if we take that away and instead just have one slit.

6:53 So here I can change this from double slit to single slit.

6:58 And let's see what happens.

7:04 Okay.

7:04 So, it's much less distinct now,

7:06 but you can still see some funny pattern going on here.

7:09 So, look what happens at this bit where the slit is.

7:13 You can see that there's like a funny sort of edge effect on both sides.

7:17 And that edge, these two edges are what are going to interfere with each other.

7:21 So, the edges of that single slit interfere so

7:25 that you get this kind of weird interference pattern.

7:28 It looks quite similar to the double slit one, but just with like less fringes.

7:32 So there's this bright fringe and then a dark fringe

7:36 and then a much less bright fringe and a dark fringe etc.

7:41 The single slit interference pattern looks something like this.

7:44 But again when we do this experiment with electrons,

7:48 not actual waves, what we get is a slightly different pattern.

7:51 We still get the electrons arriving in individual dots like this.

7:56 But it's just that the dots are going to be according to this pattern.

8:01 If you're wondering how this is different from the double slit experiment,

8:04 remember for that there were all these little dips

8:07 inside of the bigger single slit pattern like this.

8:14 So when you're not observing and you let the electrons go through both slits,

8:18 then you get even more interference.

8:21 But even when you just allow the electron

8:23 to go through one slit by observing it,

8:26 you still get a wave interference pattern.

8:29 So what's going on?

8:32 Well, let me tell you the correct explanation

8:34 of the double slit experiment according to quantum mechanics.

8:38 So for both light and for electrons,

8:41 there is this thing called the wave function which I've drawn here.

8:44 It's the wave associated with that object.

8:47 And when the two slits are open and not observed,

8:52 the wave will go through both of those slits and interfere with itself.

8:57 But in quantum mechanics, if you ever try and observe the position of an object,

9:02 it can no longer be spread out like this.

9:04 So when you do this observation,

9:07 it forces the electron to collapse to just one of these.

9:11 And so the electron acts like a wave even after it's been observed.

9:16 And we know that because you end up with this single slit interference,

9:20 which is just impossible to explain with purely particle behavior.

9:25 If it's a wave, then shouldn't we expect a big

9:28 smear across this wall rather than these individual particle-like things?

9:33 Well, again, our measurement rule comes into play.

9:36 In quantum mechanics, you can have an object that is spread out like a wave,

9:42 but when you measure the position of that object,

9:46 it's forced to collapse to a smaller position.

9:50 So if we have an electron that goes through here,

9:53 when we measure it at the wall, it has to choose where to turn up and it's going

9:58 to be more likely to turn up in one of these bigger regions.

10:03 So are electrons waves or particles?

10:07 I think that even in this case where you're measuring the electron,

10:11 it clearly acts in a way that's governed by this wave function.

10:15 The wave is what determines what it does.

10:18 But there is a sense in which it's particle-like.

10:21 And that is because when it turns up on the wall,

10:25 you find that there's always a full electron that turns up in any one spot.

10:30 Like this electron has the same mass as that electron.

10:35 So the fact that they arrive as these discrete and countable

10:39 and always sort of the same size packages is what makes them particle-like.

10:44 It's the fact that they can kind of be counted.

10:47 And here's where it gets a bit weird.

10:50 Light is exactly the same.

10:52 In my previous videos about the double slit experiment with light,

10:55 I got a lot of comments saying that the experiment that I was doing was

10:59 just an experiment from classical optics and all

11:03 it showed is that light is a wave, which we've known for a long time.

11:08 And that is all true.

11:09 But I think that the quantum version

11:12 of that experiment is not what people think.

11:15 I got a lot of comments saying, "Why don't you do the single photon at a time

11:19 version of this experiment?" And the thing is,

11:21 I would love to do that experiment,

11:23 but I actually think it's going to be a little

11:25 bit underwhelming cuz basically this is how you would do it.

11:29 Say that this is my source for the light.

11:32 Then to do the single photon at a time version of this experiment,

11:36 all I'd need to do is to turn down this light.

11:39 So, I've just maybe put some filters

11:42 in here that block the majority of the light.

11:45 So, there's only a little bit going through.

11:47 Now, our intuition tells us that if light is a particle,

11:52 you know, this thing called a photon, then surely it needs to be a particle uh

11:56 like those little bullets that we were talking about,

11:58 in that case, it would make total sense to talk

12:01 about a single photon going through this experiment at a time.

12:06 But that's not how light works.

12:09 If I just put a filter in front of this laser,

12:13 all I'm going to do is I'm going to make this wave weaker.

12:17 From the side on view, the wave would look something like this, right?

12:21 And as you put more filters in front,

12:24 all that will happen is that the wave will get smaller and smaller.

12:30 But no matter how many filters you put here, it's still a wave.

12:35 At no point does it become a particle like this.

12:38 And so you can see that no matter how weak these waves become,

12:42 that doesn't stop them from spreading out like this.

12:45 And the spreading is what really causes the double slit interference pattern.

12:50 And so you still get the interference pattern.

12:53 However, there is one really different thing about the result.

12:57 Instead of getting a smear like you usually do with the double slit experiment,

13:02 you're going to find that actually the light

13:06 arrives in little dots just like the electrons.

13:11 But how is that possible?

13:12 I said that nothing really changed when

13:15 you turn down the intensity of this light.

13:17 Well, the thing is when we have this light at full brightness,

13:21 there are individual dots turning up here,

13:23 but we can't see them because there's so many coming

13:26 per second that it does just look like a smear.

13:30 And it's only by turning down the light

13:32 that we end up seeing these individual photons.

13:36 So, photons are particles much like electrons are particles.

13:41 They're not really.

13:43 They're more waves.

13:44 But a photon or an electron is more

13:47 a property of what happens when they get measured.

13:51 Whenever you have a quantum wave function like

13:54 this and you try and measure the position of that object,

13:57 it's forced to collapse to one particular spot.

14:02 After I finished recording,

14:04 I found out that there's this amazing paper where they

14:06 explain how you can see the individual photons of light arriving.

14:11 Um, it is from 2016 and I'll link this in the description, but here's the video.

14:17 So, as you can see here,

14:19 there are these individual photons and it kind of looks random at first,

14:24 but you can already start to see how

14:26 it seems to be gathering around these fringes,

14:31 which is exactly what you would expect for a double slit experiment.

14:35 I'd seen this experiment with electrons before,

14:37 but it was very cool to see it with photons.

14:39 And as you can see, the two experiments look nearly identical.

14:44 Another way that photons are very like electrons is that each

14:48 individual photon has the same amount of energy as the other.

14:53 Just like with electrons,

14:55 they come in these discrete packages that are always of the same size.

15:00 So for both electrons and photons,

15:03 they don't really exist as particles throughout this part of the experiment.

15:08 They only really become particles here when they're measured.

15:12 But then you might wonder,

15:13 how can we even talk about doing this experiment one photon at a time?

15:17 Well, the way that you do it is you kind of turn down

15:20 this light a whole lot so that there's only a few of these photons coming,

15:26 let's say, every second.

15:28 And it takes much less than a second for any light

15:31 to have gone from the source all the way to the wall.

15:35 And so you kind of say, well,

15:37 that means that that there couldn't have been more than one

15:40 photon sort of going through this whole thing at a time.

15:43 So that's a one photon at a time experiment.

15:47 And I think that that just is bad terminology.

15:51 Like there are no photons really until you get to here.

15:55 For both electrons and photons,

15:57 we imagine these as sort of individual balls coming out of the source.

16:02 And that's what's the source of the confusion because we think

16:05 that a single particle can't go through two slits at the same time.

16:10 But there is no contradiction when you see

16:13 that both of them are actually just waves.

16:17 And waves absolutely can go through both slits at the same time.

16:22 They really only become particles when they're measured at the wall.

16:26 That's why I think that experiments with light

16:28 are still very interesting quantum mechanically because

16:32 we know from the maths of quantum

16:34 mechanics that light and electrons behave almost identically.

16:39 Like in my last video where I did the quantum eraser experiment

16:43 with light and it was very easy and it just involved polarizers.

16:47 A lot of people pointed out that you can totally explain

16:50 this just using the regular polarization of light in classical optics.

16:54 But did you know that you could do that exact

16:57 same experiment again using the spin of the electrons?

17:02 The maths is the same.

17:04 So the real lesson from the double slit experiment is that both electrons which

17:10 we think of as particles and light which we think of as waves act identically.

17:17 And that's a profound result because it means that both

17:21 of them are kind of neither particles or waves.

17:25 Exactly.