The biggest lie about the double slit experiment

There is lie about the double slit experiment that's told by everyone from famous physicists to YouTubers and even me when used to believe it. The lie is when you shoot electrons at two slits, if you don't watch which way they go through, they'll act like waves. But if you do watch which way they went, then they act like particles. They go straight through and they end up in two clumps. This actually contradicts the results of the experiment and it's completely the wrong lesson to take from the double slit experiment. But this lie has been told so often that it's kind of got life of its own. The real lesson of this experiment is way more interesting. And want to tell you about that. But first, let's find out what the electrons actually do when you observe them. Because it's not this. Imagine that we do have bunch of particles like this that are going to go through this double slit experiment. You expect to have like two bunches of particles and actually these slits are very narrow and very close together. So you'd kind of expect bit of overlap between those two heaps. But here's what would happen for wave. Okay, so love this website because it's wave simulator and I'll link it in the description. But basically what we're doing here is we have this wall with two narrow slits and we're going to see what happens when these waves go through. Okay. So what you're seeing happen here is that there are these two waves coming from each of the slits. But when they touch each other something weird happens. They interact with each other and kind of overlap. And if we let this go further, these interference patterns always kind of look like this, where there's very strong bit of wave in the middle, then there's not much, and then there's more wave, and then again there's blank spot, and then another strong spot, etc. And as we saw in that simulation, what you'd expect for wave is that it hits this back wall in this very funny pattern. There are some spots like this one where it's very intense and then there are some spots like this where it's not intense at all. And there are these little bands of you know intensity and no intensity. And when you do this experiment with light that's exactly what you see which suggests very strongly that light is wave. In fact the double slit experiment was the thing that really convinced people that light is wave after all. And I've done the double slit experiment in couple of videos before and always get lot of comments being like this is just classical optics and that is true. But then they say therefore it's irrelevant to quantum mechanics and that disagree with and I'll explain why little bit later. But first let's think about the same experiment with electrons. Is it going to turn out more like the particle case or more like the wave one? Well, this is footage from an actual double slit experiment using electrons. So, let's see what the answer is. Well, it's looking very promising for particles because unlike with wave, the result isn't just sort of smeared out. It's coming in these individual dots. But if we let the pattern develop, you start to see that it actually isn't in two lumps. It looks much more like the double slit interference pattern for waves. So something really odd seems to be happening with these electrons. There seems to be some sort of wave associated with the electrons and the wave spreads out. And just like water wave or light wave, it goes through both slits. The only real difference between an electron and wave that goes through both slits is that the electron still ends up as dots. It isn't smeared out the way that waves usually are. But this feels wrong because for something like light, mean, it's perfectly fine to accept that it's some sort of wave. But for electrons, you know, we're used to thinking of them as little bullets like this, little particles. And particle couldn't be going through both of these slits at the same time. That's why it's really natural to think that if electrons really are going through both slits, then maybe let's try and catch them in the act. Let's put two measuring devices here and here that will watch both of these slits and we'll tell you whether the electron actually goes through both slits. The idea is that we want to be able to catch the electrons doing both things at the same time. But measurement in quantum mechanics doesn't work like that. So in quantum mechanics when you try and measure an object that's in superp position of two different things like this electron going through both slits then you will see particular result. For example, this detector might see an electron go by. But now in quantum mechanics what happens is that measurement changed the electron. Now it's only going through one of these slits, the one you observed. So far this is all correct. But here comes the myth. Many people will tell you that now that the electron has been measured it becomes particle. And so this particle will act like any normal particle will sort of end up here. And so you can imagine that if we keep measuring which way every electron goes, then you end up with two piles of electrons. People will draw the results just like this as two clumps or at most as two overlapping clumps. And so before ever did these experiments myself, assume that this is what happens. But that's not at all the case. And I'll get curve that will probably look more or less like this. wish had experimental footage of someone actually doing this experiment with electrons to show you that this is not what happens, but don't. And also can't do this experiment myself because working with the electrons is really hard. You need like vacuum pump and everything. But the results that you get when you do the experiment with light are very very similar. And so did that experiment in my previous video. And this is the result that got. What on earth is this crazy pattern? It doesn't look anything like two lumps. This is actually called the single slit interference pattern. And it's still very much wave behavior. Let me show you. So this is what the interference pattern look like with the two slits here and here. But what happens if we take that away and instead just have one slit. So here can change this from double slit to single slit. And let's see what happens. Okay. So, it's much less distinct now, but you can still see some funny pattern going on here. So, look what happens at this bit where the slit is. You can see that there's like funny sort of edge effect on both sides. And that edge, these two edges are what are going to interfere with each other. So, the edges of that single slit interfere so that you get this kind of weird interference pattern. It looks quite similar to the double slit one, but just with like less fringes. So there's this bright fringe and then dark fringe and then much less bright fringe and dark fringe etc. The single slit interference pattern looks something like this. But again when we do this experiment with electrons, not actual waves, what we get is slightly different pattern. We still get the electrons arriving in individual dots like this. But it's just that the dots are going to be according to this pattern. If you're wondering how this is different from the double slit experiment, remember for that there were all these little dips inside of the bigger single slit pattern like this. So when you're not observing and you let the electrons go through both slits, then you get even more interference. But even when you just allow the electron to go through one slit by observing it, you still get wave interference pattern. So what's going on? Well, let me tell you the correct explanation of the double slit experiment according to quantum mechanics. So for both light and for electrons, there is this thing called the wave function which I've drawn here. It's the wave associated with that object. And when the two slits are open and not observed, the wave will go through both of those slits and interfere with itself. But in quantum mechanics, if you ever try and observe the position of an object, it can no longer be spread out like this. So when you do this observation, it forces the electron to collapse to just one of these. And so the electron acts like wave even after it's been observed. And we know that because you end up with this single slit interference, which is just impossible to explain with purely particle behavior. If it's wave, then shouldn't we expect big smear across this wall rather than these individual particle-like things? Well, again, our measurement rule comes into play. In quantum mechanics, you can have an object that is spread out like wave, but when you measure the position of that object, it's forced to collapse to smaller position. So if we have an electron that goes through here, when we measure it at the wall, it has to choose where to turn up and it's going to be more likely to turn up in one of these bigger regions. So are electrons waves or particles? think that even in this case where you're measuring the electron, it clearly acts in way that's governed by this wave function. The wave is what determines what it does. But there is sense in which it's particle-like. And that is because when it turns up on the wall, you find that there's always full electron that turns up in any one spot. Like this electron has the same mass as that electron. So the fact that they arrive as these discrete and countable and always sort of the same size packages is what makes them particle-like. It's the fact that they can kind of be counted. And here's where it gets bit weird. Light is exactly the same. In my previous videos about the double slit experiment with light, got lot of comments saying that the experiment that was doing was just an experiment from classical optics and all it showed is that light is wave, which we've known for long time. And that is all true. But think that the quantum version of that experiment is not what people think. got lot of comments saying, "Why don't you do the single photon at time version of this experiment?" And the thing is, would love to do that experiment, but actually think it's going to be little bit underwhelming cuz basically this is how you would do it. Say that this is my source for the light. Then to do the single photon at time version of this experiment, all I'd need to do is to turn down this light. So, I've just maybe put some filters in here that block the majority of the light. So, there's only little bit going through. Now, our intuition tells us that if light is particle, you know, this thing called photon, then surely it needs to be particle like those little bullets that we were talking about, in that case, it would make total sense to talk about single photon going through this experiment at time. But that's not how light works. If just put filter in front of this laser, all I'm going to do is I'm going to make this wave weaker. From the side on view, the wave would look something like this, right? And as you put more filters in front, all that will happen is that the wave will get smaller and smaller. But no matter how many filters you put here, it's still wave. At no point does it become particle like this. And so you can see that no matter how weak these waves become, that doesn't stop them from spreading out like this. And the spreading is what really causes the double slit interference pattern. And so you still get the interference pattern. However, there is one really different thing about the result. Instead of getting smear like you usually do with the double slit experiment, you're going to find that actually the light arrives in little dots just like the electrons. But how is that possible? said that nothing really changed when you turn down the intensity of this light. Well, the thing is when we have this light at full brightness, there are individual dots turning up here, but we can't see them because there's so many coming per second that it does just look like smear. And it's only by turning down the light that we end up seeing these individual photons. So, photons are particles much like electrons are particles. They're not really. They're more waves. But photon or an electron is more property of what happens when they get measured. Whenever you have quantum wave function like this and you try and measure the position of that object, it's forced to collapse to one particular spot. After finished recording, found out that there's this amazing paper where they explain how you can see the individual photons of light arriving. it is from 2016 and I'll link this in the description, but here's the video. So, as you can see here, there are these individual photons and it kind of looks random at first, but you can already start to see how it seems to be gathering around these fringes, which is exactly what you would expect for double slit experiment. I'd seen this experiment with electrons before, but it was very cool to see it with photons. And as you can see, the two experiments look nearly identical. Another way that photons are very like electrons is that each individual photon has the same amount of energy as the other. Just like with electrons, they come in these discrete packages that are always of the same size. So for both electrons and photons, they don't really exist as particles throughout this part of the experiment. They only really become particles here when they're measured. But then you might wonder, how can we even talk about doing this experiment one photon at time? Well, the way that you do it is you kind of turn down this light whole lot so that there's only few of these photons coming, let's say, every second. And it takes much less than second for any light to have gone from the source all the way to the wall. And so you kind of say, well, that means that that there couldn't have been more than one photon sort of going through this whole thing at time. So that's one photon at time experiment. And think that that just is bad terminology. Like there are no photons really until you get to here. For both electrons and photons, we imagine these as sort of individual balls coming out of the source. And that's what's the source of the confusion because we think that single particle can't go through two slits at the same time. But there is no contradiction when you see that both of them are actually just waves. And waves absolutely can go through both slits at the same time. They really only become particles when they're measured at the wall. That's why think that experiments with light are still very interesting quantum mechanically because we know from the maths of quantum mechanics that light and electrons behave almost identically. Like in my last video where did the quantum eraser experiment with light and it was very easy and it just involved polarizers. lot of people pointed out that you can totally explain this just using the regular polarization of light in classical optics. But did you know that you could do that exact same experiment again using the spin of the electrons? The maths is the same. So the real lesson from the double slit experiment is that both electrons which we think of as particles and light which we think of as waves act identically. And that's profound result because it means that both of them are kind of neither particles or waves. Exactly.