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

0:00 So for years, I believe that light is made out

0:03 of particles because of this book by Richard Feynman called QED,

0:07 because in it he's super insistent that light behaves as particles.

0:14 I read this just before going to university and it had a huge influence on me,

0:19 even though in all my classes they were

0:22 explaining the ways that light is very wavelike.

0:24 I thought that fundamentally light has to be a particle.

0:27 I guess because Fey said so,

0:30 and it wasn't until a few years ago where I started doing experiments with light

0:35 by myself that I started to see why the wave picture is actually really natural.

0:41 Seeing this was the first time that I actually

0:44 felt in my gut that light is somehow wavelike,

0:47 and so I converted to thinking about light more in terms of waves.

0:52 Until I saw this video by ver potassium,

0:55 they do this really cool experiment in it.

0:58 It seems like you can't explain this experiment

1:00 except by thinking of light as particles.

1:03 Yeah, this is crazy.

1:04 'cause it just seems to prove fine and right, which really shocked me.

1:08 And so I decided that I needed to do

1:11 this experiment for myself to understand what's going on.

1:14 Okay, I see it now.

1:17 So here's the setup.

1:19 There's a laser here that reflects off this mirror.

1:23 So the path of the laser looks like this.

1:26 At least that's what common sense would say,

1:29 that the laser only goes along this path,

1:31 hits this bit of the mirror, and then reflects.

1:35 But Feynman would say that that's not what really happens.

1:39 Actually, each particle of light coming out of this laser

1:44 decides to go on all possible paths at the same time.

1:49 So for example, this is another path that the light

1:52 could have gone on, so it would've hit the mirror here.

1:56 So, according to Feynman's theory,

1:58 the light coming out of here is coming out in discreet particles,

2:03 but each particle goes on.

2:05 Every single possible path to the destination,

2:08 meaning each particle of light hits the mirror at all of these different points.

2:13 You might wonder then why don't we see light

2:17 bouncing off all of the parts of the mirror?

2:19 Why do we only see it bouncing off here?

2:21 Well, according to Feynman,

2:23 the reason is because these weird paths will cancel out.

2:26 For example, this path here will cancel out with other paths that are.

2:32 Almost the same, but slightly different like these.

2:37 The exact mechanism of that cancellation doesn't matter for us right now,

2:43 so we won't worry about it.

2:45 But the main thing we wanna get from this is that according to Feynman's theory,

2:49 there actually is some light that hits this part of the mirror.

2:53 We just don't usually see it.

2:55 And you might think that's crazy, but supposedly there is a proof.

3:00 First, we'll get some black paper to just cover up the majority of the mirror.

3:05 This is where the black piece of paper goes in the other setup,

3:09 and you can see that it's blocking a fair

3:11 bit of the mirror as well as the reflection point.

3:14 This isn't actually necessary,

3:15 but the reason we're gonna put it there is so we don't get

3:17 distracted by the usual part of the mirror where it would have a reflection.

3:21 So I'm gonna get rid of that reflection coming from the mirror.

3:26 And now the actually important bit is this stuff,

3:30 it's called a diffraction grading.

3:32 And I won't go into exactly how it works,

3:36 but it does something very cool in this experiment.

3:40 So if I put this diffraction grading here in front

3:42 of the bit of the mirror that's not being covered up,

3:45 something kind of weird happens.

3:47 So I told you that the paths close by cancel out this strange path.

3:53 But what this does is it selectively cancels certain paths already,

4:00 which means that now this weird path is

4:03 actually not going to be canceled out anymore.

4:07 And so we should see it.

4:09 We should see some light bouncing off this part of the mirror,

4:13 which is very far away from the usual reflection point.

4:16 So let's see if we do.

4:18 Yeah, this is crazy 'cause it just seems to prove Feynman right.

4:22 I mean, this main laser beam is blocked over there and yeah,

4:26 you can see the reflection of it here.

4:29 That's not surprising.

4:30 But what is this extra light?

4:32 The only place it could have come from is from this mirror,

4:37 which we can confirm because once we're

4:39 not on the diffraction grading, that one's gone.

4:42 So essentially it looks like.

4:44 The light has hit this bit of the mirror, like right here.

4:49 You can tell 'cause like when I block that little bit of the mirror,

4:55 the light's blocked.

4:56 Um, and then it's hitting our camera,

4:58 which means that there must have been some light coming from here

5:02 and hitting that bit of the mirror to start off with.

5:07 So doesn't this prove Feynman's theory?

5:10 Find's theory that light is a particle that goes all possible directions.

5:15 Has no problem explaining this experiment because it says that there is

5:19 some light that's going to hit this weird bit of the mirror,

5:23 and with the diffraction grading there, we'll be able to see it.

5:27 But there is another way to explain light in terms of waves,

5:31 and here's what Feynman has to say about the wave theory of light.

5:35 It is very important to know that light behaves as particles,

5:39 especially for those of you who have gone to school

5:43 where you are probably told something about light behaving as waves.

5:47 I'm telling you the way it does behave like particles.

5:51 So in this book, heat insists that all the experiments in here, um,

5:58 including this very experiment,

6:00 can only be explained by his particle theory of light.

6:05 But it's, it's honestly hilarious because I know

6:08 that these experiments can be explained by waves just as well,

6:12 and he does too, because they're very clearly equivalent.

6:16 Let me show you why.

6:17 Let's say that we have a torch here instead of a laser,

6:21 and we know that the light from this torch is going to spread outwards,

6:25 and so the wave theory of light would say

6:29 that there's light emanating from this torch in waves like this.

6:36 Basically light is like a water wave that's rippling out from this torch,

6:40 and it spreads as it goes.

6:43 Now let's look at this exact same situation.

6:45 From the particle point of view,

6:47 it says that each particle of light goes on every possible path off.

6:58 But even visually, you can see why this is the same thing.

7:02 Outside a certain region, the light wave is going to be basically zero.

7:07 And so when Feynman says that a particle of light goes on all possible paths,

7:12 really what he means is all possible paths where the wave is not zero.

7:18 And so these two theories are completely consistent.

7:21 The Wave Theory says that light spreads out as it goes,

7:25 and the particle theory says that each particle goes on every possible path,

7:29 and these paths spread out as long as the boundary

7:33 of the waves and the boundary of these paths are the same.

7:38 Then their equivalent theories that are basically saying the exact same thing,

7:43 which is just light spreads out.

7:46 But this is the bit that really confused me

7:50 about the laser experiment because lasers don't really spread.

7:55 If I put a laser here instead of a torch,

7:59 things really change in the wave picture.

8:02 So, as you know, with a laser, most of the light will just be on this beam,

8:08 which described in the wave theory says that there is a very strong bit of wave.

8:14 On the beam, but off the beam, it falls off very quickly.

8:22 So according to the wave theory, it seems like there's almost no wave

8:27 touching this bit of the diffraction grading.

8:30 There's no light here, and so it's not possible for the diffraction

8:34 grading to make it reflect from this point anymore.

8:37 In the torch scenario,

8:38 as long as there was some light touching this bit of the mirror,

8:42 in other words, some of the wave had gotten to this part,

8:45 then the wave theory had no problem predicting that there

8:48 would be a reflection from this part of the diffraction grading.

8:52 But now things have really changed because

8:55 there is apparently no wave touching this bit.

9:00 All of the wave is concentrated on just this narrow beam,

9:03 and it falls off to zero pretty quickly outside of the beam.

9:08 And so it seems like the wave theory couldn't

9:11 possibly predict that there was some reflection from here.

9:15 On the other hand, this seems to really validate fireman's theory,

9:19 that the light takes all possible paths because there

9:23 is light coming from this part of the mirror.

9:27 And surely then it must have come from light going

9:30 on this path even though there is no wave in this area.

9:35 So this really shocked me because this seems to be a circumstance

9:40 where the wave theory and the light as a particle theory.

9:44 Don't seem to make the same prediction.

9:47 They're not equivalent, and it seems like the particle theory comes out on top.

9:52 Thankfully though, doing the experiment made

9:54 me realize what I was getting wrong.

9:57 The laser is hitting this part of the mirror.

10:00 You can actually see that like, look, my hand is going slightly green.

10:04 The original laser light is definitely hitting the mirror.

10:07 I mean, look at this, you can see how much light there is further away.

10:15 Look, there's plenty of light hitting this bit of the mirror

10:18 where that reflection is supposed to be coming from.

10:21 So that's surprising to us because we usually

10:23 only see the main beam of the light.

10:26 But that doesn't mean that it's the only light that this laser is outputting.

10:30 It's also outputting light off that main beam as well.

10:34 You see, I was incorrectly assuming that the wave

10:38 is basically only on the main beam, but that's not true.

10:42 Even for a laser.

10:43 The light spreads out quite a lot.

10:46 I know that's hard to visualize,

10:49 but, and you can see that the light actually spreads out quite far.

10:54 So really the light here is spreading just like this.

11:00 And so there's plenty of wave that touches

11:03 all of these weird bits of the mirror, and that's why the diffraction grading is

11:09 able to have a reflection from that point.

11:12 If you want a good explanation actually of why the wave

11:15 theory predicts that the diffraction grading

11:17 will give you this weird reflection,

11:20 then you should check out the Feynman lectures.

11:23 Ironically, most of the experiments described in here in terms of particles

11:29 are also described by Feynman in the Feynman lectures in terms of waves,

11:35 and they're some of the best explanations

11:37 of the wave theory of light that I've seen.

11:39 So I highly recommend that source if you

11:42 wanna understand the equivalence between these two theories.

11:46 So is light a particle or a wave?

11:49 I think the answer is that it's neither,

11:51 but both of these pictures can be different

11:54 and helpful ways to see the same thing.

11:56 And it all comes down to the mental

11:59 model that you use to understand something like light.

12:03 So for example, I find the mental model

12:05 of waves quite helpful when I'm trying to think,

12:08 you know, what would light do if it encountered a sharp edge?

12:11 All I have to do is think what would a water wave do in that situation?

12:16 And it turns out that it would spread.

12:18 And so I can predict that light would spread in that situation, and it does.

12:24 Similarly for a double slit experiment, what does water do?

12:27 And it turns out light does the same thing.

12:30 So I find that mental model easier for me to work with.

12:34 On the other hand, I find the model in here

12:36 a lot more difficult for me to think about.

12:39 That's because for each possible path that the light

12:41 could go on, you need to calculate an arrow,

12:44 which tells you the phase of that path,

12:47 and here's how you calculate the direction of the arrow.

12:51 When a photon leaves the source, we start the stopwatch.

12:54 As long as the photon moves, the stopwatch hand turns.

12:57 When the photon ends up at the photo multiplier, we stop the watch.

13:01 The hand ends up pointing in a certain direction.

13:03 That is the direction.

13:05 We'll draw the arrow.

13:07 I remember reading that when I was straight outta school and really,

13:12 really enthusiastic about physics.

13:13 And I hated it.

13:15 I mean, I believed it, of course,

13:17 but I found it extremely disheartening because I came to physics 'cause I

13:21 wanted to understand how things worked and this just seemed like a horrible,

13:26 prescriptive formula for getting the right answer,

13:28 but not understanding anything at all.

13:30 Whereas for me, the wave picture has really helped

13:34 me have some sort of like feeling of understanding light.

13:37 But my view on the particle theory of light

13:40 has softened a bit recently because I think

13:42 that that theory of light is quite helpful when

13:44 it comes to thinking about light being absorbed by matter,

13:47 which I don't feel like I understand anywhere near enough.

13:50 And so I want to go and explore that topic a whole lot more.

13:55 And probably this will be quite a useful, you know, tool to have in the toolkit.

14:00 Light is just more nuanced than being a particle or a wave,

14:04 and I think having both of those mental models

14:11 will help me just get closer to the truth.