Does this experiment *actually* prove light is a particle?

So for years, I believe that light is made out of particles because of this book by Richard Feynman called QED, because in it he's super insistent that light behaves as particles. I read this just before going to university and it had a huge influence on me, even though in all my classes they were explaining the ways that light is very wavelike. I thought that fundamentally light has to be a particle. I guess because Fey said so, and it wasn't until a few years ago where I started doing experiments with light by myself that I started to see why the wave picture is actually really natural. Seeing this was the first time that I actually felt in my gut that light is somehow wavelike, and so I converted to thinking about light more in terms of waves. Until I saw this video by ver potassium, they do this really cool experiment in it. It seems like you can't explain this experiment except by thinking of light as particles. Yeah, this is crazy. 'cause it just seems to prove fine and right, which really shocked me. And so I decided that I needed to do this experiment for myself to understand what's going on. Okay, I see it now. So here's the setup. There's a laser here that reflects off this mirror. So the path of the laser looks like this. At least that's what common sense would say, that the laser only goes along this path, hits this bit of the mirror, and then reflects. But Feynman would say that that's not what really happens. Actually, each particle of light coming out of this laser decides to go on all possible paths at the same time. So for example, this is another path that the light could have gone on, so it would've hit the mirror here. So, according to Feynman's theory, the light coming out of here is coming out in discreet particles, but each particle goes on. Every single possible path to the destination, meaning each particle of light hits the mirror at all of these different points. You might wonder then why don't we see light bouncing off all of the parts of the mirror? Why do we only see it bouncing off here? Well, according to Feynman, the reason is because these weird paths will cancel out. For example, this path here will cancel out with other paths that are. Almost the same, but slightly different like these. The exact mechanism of that cancellation doesn't matter for us right now, so we won't worry about it. But the main thing we wanna get from this is that according to Feynman's theory, there actually is some light that hits this part of the mirror. We just don't usually see it. And you might think that's crazy, but supposedly there is a proof. First, we'll get some black paper to just cover up the majority of the mirror. This is where the black piece of paper goes in the other setup, and you can see that it's blocking a fair bit of the mirror as well as the reflection point. This isn't actually necessary, but the reason we're gonna put it there is so we don't get distracted by the usual part of the mirror where it would have a reflection. So I'm gonna get rid of that reflection coming from the mirror. And now the actually important bit is this stuff, it's called a diffraction grading. And I won't go into exactly how it works, but it does something very cool in this experiment. So if I put this diffraction grading here in front of the bit of the mirror that's not being covered up, something kind of weird happens. So I told you that the paths close by cancel out this strange path. But what this does is it selectively cancels certain paths already, which means that now this weird path is actually not going to be canceled out anymore. And so we should see it. We should see some light bouncing off this part of the mirror, which is very far away from the usual reflection point. So let's see if we do. Yeah, this is crazy 'cause it just seems to prove Feynman right. I mean, this main laser beam is blocked over there and yeah, you can see the reflection of it here. That's not surprising. But what is this extra light? The only place it could have come from is from this mirror, which we can confirm because once we're not on the diffraction grading, that one's gone. So essentially it looks like. The light has hit this bit of the mirror, like right here. You can tell 'cause like when I block that little bit of the mirror, the light's blocked. Um, and then it's hitting our camera, which means that there must have been some light coming from

here and hitting that bit of the mirror to start off with. So doesn't this prove Feynman's theory? Find's theory that light is a particle that goes all possible directions. Has no problem explaining this experiment because it says that there is some light that's going to hit this weird bit of the mirror, and with the diffraction grading there, we'll be able to see it. But there is another way to explain light in terms of waves, and here's what Feynman has to say about the wave theory of light. It is very important to know that light behaves as particles, especially for those of you who have gone to school where you are probably told something about light behaving as waves. I'm telling you the way it does behave like particles. So in this book, heat insists that all the experiments in here, um, including this very experiment, can only be explained by his particle theory of light. But it's, it's honestly hilarious because I know that these experiments can be explained by waves just as well, and he does too, because they're very clearly equivalent. Let me show you why. Let's say that we have a torch here instead of a laser, and we know that the light from this torch is going to spread outwards, and so the wave theory of light would say that there's light emanating from this torch in waves like this. Basically light is like a water wave that's rippling out from this torch, and it spreads as it goes. Now let's look at this exact same situation. From the particle point of view, it says that each particle of light goes on every possible path off. But even visually, you can see why this is the same thing. Outside a certain region, the light wave is going to be basically zero. And so when Feynman says that a particle of light goes on all possible paths, really what he means is all possible paths where the wave is not zero. And so these two theories are completely consistent. The Wave Theory says that light spreads out as it goes, and the particle theory says that each particle goes on every possible path, and these paths spread out as long as the boundary of the waves and the boundary of these paths are the same. Then their equivalent theories that are basically saying the exact same thing, which is just light spreads out. But this is the bit that really confused me about the laser experiment because lasers don't really spread. If I put a laser here instead of a torch, things really change in the wave picture. So, as you know, with a laser, most of the light will just be on this beam, which described in the wave theory says that there is a very strong bit of wave. On the beam, but off the beam, it falls off very quickly. So according to the wave theory, it seems like there's almost no wave touching this bit of the diffraction grading. There's no light here, and so it's not possible for the diffraction grading to make it reflect from this point anymore. In the torch scenario, as long as there was some light touching this bit of the mirror, in other words, some of the wave had gotten to this part, then the wave theory had no problem predicting that there would be a reflection from this part of the diffraction grading. But now things have really changed because there is apparently no wave touching this bit. All of the wave is concentrated on just this narrow beam, and it falls off to zero pretty quickly outside of the beam. And so it seems like the wave theory couldn't possibly predict that there was some reflection from here. On the other hand, this seems to really validate fireman's theory, that the light takes all possible paths because there is light coming from this part of the mirror. And surely then it must have come from light going on this path even though there is no wave in this area. So this really shocked me because this seems to be a circumstance where the wave theory and the light as a particle theory. Don't seem to make the same prediction. They're not equivalent, and it seems like the particle theory comes out on top. Thankfully though, doing the experiment made me realize what I was getting wrong. The laser is hitting this part of the mirror. You can actually see that like, look, my hand is going slightly green.

The original laser light is definitely hitting the mirror. I mean, look at this, you can see how much light there is further away. Look, there's plenty of light hitting this bit of the mirror where that reflection is supposed to be coming from. So that's surprising to us because we usually only see the main beam of the light. But that doesn't mean that it's the only light that this laser is outputting. It's also outputting light off that main beam as well. You see, I was incorrectly assuming that the wave is basically only on the main beam, but that's not true. Even for a laser. The light spreads out quite a lot. I know that's hard to visualize, but, and you can see that the light actually spreads out quite far. So really the light here is spreading just like this. And so there's plenty of wave that touches all of these weird bits of the mirror, and that's why the diffraction grading is able to have a reflection from that point. If you want a good explanation actually of why the wave theory predicts that the diffraction grading will give you this weird reflection, then you should check out the Feynman lectures. Ironically, most of the experiments described in here in terms of particles are also described by Feynman in the Feynman lectures in terms of waves, and they're some of the best explanations of the wave theory of light that I've seen. So I highly recommend that source if you wanna understand the equivalence between these two theories. So is light a particle or a wave? I think the answer is that it's neither, but both of these pictures can be different and helpful ways to see the same thing. And it all comes down to the mental model that you use to understand something like light. So for example, I find the mental model of waves quite helpful when I'm trying to think, you know, what would light do if it encountered a sharp edge? All I have to do is think what would a water wave do in that situation? And it turns out that it would spread. And so I can predict that light would spread in that situation, and it does. Similarly for a double slit experiment, what does water do? And it turns out light does the same thing. So I find that mental model easier for me to work with. On the other hand, I find the model in here a lot more difficult for me to think about. That's because for each possible path that the light could go on, you need to calculate an arrow, which tells you the phase of that path, and here's how you calculate the direction of the arrow. When a photon leaves the source, we start the stopwatch. As long as the photon moves, the stopwatch hand turns. When the photon ends up at the photo multiplier, we stop the watch. The hand ends up pointing in a certain direction. That is the direction. We'll draw the arrow. I remember reading that when I was straight outta school and really, really enthusiastic about physics. And I hated it. I mean, I believed it, of course, but I found it extremely disheartening because I came to physics 'cause I wanted to understand how things worked and this just seemed like a horrible, prescriptive formula for getting the right answer, but not understanding anything at all. Whereas for me, the wave picture has really helped me have some sort of like feeling of understanding light. But my view on the particle theory of light has softened a bit recently because I think that theory of light is quite helpful when it comes to thinking about light being absorbed by matter, which I don't feel like I understand anywhere near enough. And so I want to go and explore that topic a whole lot more. And probably this will be quite a useful, you know, tool to have in the toolkit. Light is just more nuanced than being a particle or a wave, and I think having both of those mental models will help me just get closer to the truth.