Classic Episode - Is Light a Particle or a Wave?

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Speaker 1: Is it possible to be two different things at the same time?

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Speaker 2: Can you like dogs and cats?

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Speaker 1: Can you be a horse and a giraffe at the same time?

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Speaker 2: Can something taste salty and sweet? Can a dress be black and blue and white and gold?

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Speaker 1: In today's podcast, we talk about the centuries old scientific debate about light.

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Speaker 2: Is light a particle or a wave?

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Speaker 1: Or is it both? Hello, I'm Jorge and I'm Daniel.

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Speaker 2: Welcome to Daniel and Jorge Explain the Universe.

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Speaker 1: In which we try to explain the whole universe and everything in it, including light.

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Speaker 2: Now, I'm a cartoonist. I draw something called PhD comics.

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Speaker 1: And I'm a particle physicist. During the day, I smash particles together at the large Hadron Collider.

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Speaker 2: Yeah. Well, today on the program, we're going to talk about the nature of light.

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Speaker 1: That's right. People have been arguing for centuries what is light? Is it made out of particles? Is it made out of waves? It's something else? Is it tiny little puppies screaming through space? People have gone back and forth on the issue, and today even the topic is not yet totally settled. So we're going to be taking you through that history and breaking it down.

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Speaker 2: It's one of the most mind blowing questions in human scientific history.

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Speaker 1: That's right, what is light made out of? So, as usual, before we dig into it, we went out and we asked people on the street. What do you think light is made out of? What do people know about light?

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Speaker 2: Is light a particle or is it a wave?

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Speaker 1: Here's what people had to say. Do you think light is made out of particles or waves or both or neither? Photons? Yeah? Photons? Yeah, so you think light it's a particle. I think it's waves, youth waves. Yeah, cool, it's both, I think because it moves like a wave, but it also has properties of a particle and there's nothing saying it can't people. Okay, light, I think they're made of wavelength.

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Speaker 2: Yeah all right. Well it's interesting because I think all of the answers are right.

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Speaker 1: Or none of them are or both.

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Speaker 2: Yeah.

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Speaker 1: Well, it seems like a lot of people reflected the fact that there is a controversy like that. You know, it's not really well described, but either those some people went all in, you know, like it's a photon or it's a wave, or it's a wave.

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Speaker 2: Length right, Yeah, that was my favorite one.

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Speaker 1: I want to be a wavelength.

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Speaker 2: Like I've heard of this word. It sounds really cool and scientific. I'm just going to throw it up.

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Speaker 1: There, that's right. Yeah, maybe I get some points. We award no points to people, no points, that's right. There's no price. Your price is you get to be on our podcast and maybe we even make fun of you. Yeah.

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Speaker 2: Yeah, but yeah, I guess what you mean is nobody sort of fell for the trap, right, Like nobody said, oh of course it's a particle, or nobody said, oh of course it's a wave. Most people sort of knew that there's some sort of duality there, something weird going on.

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Speaker 1: That's right. That science is having some trouble, some difficulty coming up with a way to describe what light is. And that might seem surprising you because light is everywhere, right, and it runs the universe. It's streaming through the Solar system from the Sun, illuminating our lives, empowering everything on Earth. So you think this would be sort of a high priority topic to figure out, like what is this stuff? What is it made out of?

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Speaker 2: Yeah? I mean, like, what are we paying you for, Daniel, if not to figure these kinds of questions out.

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Speaker 1: I was just about to figure out what light was when you called and said it's time to do this podcast.

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Speaker 2: Sorry, science was totally you're a train of thought there, that's right.

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Speaker 1: Reflect on that for a minute.

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Speaker 2: Or but no, Yeah, I'm a California taxpayer. Part of my salary goes to paying your salary, like you know, one million of a percent.

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Speaker 1: That's true. Yeah, so you're saying you did pay taxes last year.

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Speaker 2: There you go again.

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Speaker 1: That's another topic.

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Speaker 2: Seekers on air, Daniel. Anyway, So that's an interesting question, like is light a waver particle? And it's weird that we don't know, But maybe let's break it down a little bit. What is it? Like, what are we actually talking about when we say that light could be a particle or light could be a wave, like you know, most people probably think of light as just like just like brightness, right.

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Speaker 1: Yeah. The thing to understand here is that we try to describe light in terms of things we know, and that's what science is. Right. You see something weird and new and you wonder, oh, is it like this other thing I know? So we've observed different kinds of phenomena in the world, like you've seen waves, right, you go to the beach, you see waves and water, you drop a rock in a small puddle, you see waves. We know what waves are, and we see different phenomena. We try to categorize them in terms of things we know, right, So like when people were studying sound, they discovered, oh, sound is actually a wave. You know, it's a compression wave in the air. And that's cool because he says, oh, I already know how the math for waves works. Right, I've seen waves and water. I've seen waves and other stuff.

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Speaker 2: You can describe it with like equations, right, yeah, wavy equations.

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Speaker 1: That's right, very solid unwavy physics to describe waves. And there's a lot of science that's gone into understanding waves. So if you can cram it into that box and say, oh, this is just another example of something we already know, then you're taking a huge leap forward. Right. So that's something people try to do, is say, like, look, can we describe this in terms of other things we know?

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Speaker 2: So we need like, you know, we know about light, but we want to know how it behaves and what makes it work.

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Speaker 1: Yeah, and just on a more general level, you try to see something new, try to describe in terms of things you know, Like say you taste a new kind of fruit and you'd be like, Oh, it's a little bit like a cherry, and a little bit like an apple, and a little bit like you know, it's got a hint of smokiness to it or whatever. You know.

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Speaker 2: You're like, it's a chapel.

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Speaker 1: It's a chapel. How has nobody ever invented that? The cherry apple chapel? Oh my gosh, somebody, if our lawyer is listening, get on that right away. Copyright that idea.

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Speaker 2: I'll reserve ww dot chapel dot com. Up.

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Speaker 1: That's right. So that's the basic idea is we have these things we've seen. You see something new, you don't want to create a whole new category. You want to fit into one of the existing categories.

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Speaker 2: So we sort of knew about light. It came from the sun. It you know, if you light a fire, it spreads out into a room. And so we're like, what's going on? Like what what best describes how this light you know, comes from a source and bounces off the walls and stuff.

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Speaker 1: Exactly exactly, that's the question. And so we'd seen things like waves, So what do we mean when we say a wave? Like how could a light be a wave?

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Speaker 2: Well, how can anything be a wave?

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Speaker 1: Yeah? How can anything be a wave? A wave is a funny thing because it's not a thing itself. It's a property of some medium.

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Speaker 2: Like it's the words like a ripple on something.

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Speaker 1: Yeah, that's right. Like if you do the wave at a baseball game, you know, there's nothing to the wave itself. It's just a bunch of people moving up and down and waving their hands, right.

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Speaker 2: Or like a sound wave is just like air molecules kind of bumping forward.

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Speaker 1: That's right, yeah, exactly. Or a wave in the ocean is just it's an arrangement of the water, right, It's a way the water gets compressed and then stretched out and compressed and then gets stretched out. So that's an important thing about a wave is that it moves in this way through a medium.

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Speaker 2: M Okay, so that's a wave. It's like a propagation, it's like a ripple through something. But then, so then what what would you call a particle? A particle is different than that.

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Speaker 1: A particle is different than that, and it's a totally different kind of thing, you know, And to be a particle physicist, it's kind of odd, but The concept of a particle is not that really well defined, you know. But when I think of a particle, I think of taking matter and breaking it down to its smallest pieces. If something's made out of particles, it means that at its smallest level, it's made out of these little bits that can't be chopped into smaller bits, and that they're localized. They're like small and contained. Right. If you discover that something is made of particles, you expect it to be like mostly empty space, but with these little dots of matter, right.

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Speaker 2: Like you would take something and then you smash it to bit and just keep smashing, and at some point you're going to get to these little baby balls or like little tiny pellets that you can't break down anymore.

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Speaker 1: That's right. Yeah, It's like seeing a picture on your computer screen and discovering it's made out of pixels, right, and that those pixels are the basic elements and they come together to make the whole picture. So figuring out that something is made of particles means that there's made out these these little bits that are not connected to each other, right, they're separated. So a wave and a particle in nature are totally different. Kinds of things. Right. Now, water of course is made of particles, but can have waves in it.

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Speaker 2: Right, But I think maybe what's important here is that, you know, particles we tend to think of as little tiny bits that can bounce around, right, and like go in a street line and then hit something else and then bounce back, or you know, kind of fly through space right in a discrete little package exactly.

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Speaker 1: That's exactly the right way to say. It's a discrete little package. Right. So things that are made of particles we think of as being discrete little bits, and they they're broken up into these little pieces, and you're right, they move in straight lines. Right, Like you throw a rock, you roll a smooth ball across the surface, you expect it to move in a straight line.

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Speaker 2: So that's kind of what we mean by a wave and a particle, that's right.

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Speaker 1: Yeah.

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Speaker 2: And so the question is like, is light a ripple on a medium? Is that what light is? Or is it like actual little things and move around in space?

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Speaker 1: Right? Does it have its own stuff to it? Right? Or is it just a way something else moves right? That's sort of another way to phrase the question.

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Speaker 2: Right, And those are two pretty different pictures. Is of reality, right, Yeah, light could be little pellets flying around, or it could be some sort of ripple on a medium. To us in our intuitive sense, it couldn't be any more different, right.

00:10:10
Speaker 1: That's right. Yeah. It's like you can't be a Democrat and a Republican, you know, just you have to pick one, you know. Yeah, if you vote, you can be or you could be neither. I suppose you shouldn't be both though, Yeah, that would be a violation of some election law not recommended to violate elections, right, that's right. Yeah. So, speaking of political shouting matches, this one, this historical scientific shouting match, began all the way back with the Greeks, right, Democratus he's the guy sort of the first atomist. He's the first person to look at the world and to say, you know, maybe everything's made out of tiny little bits, not just light, but also matter. And that was sort of the birth of that idea that maybe everything around us that seems macroscopic is made out of tiny little things, smaller than we can see. And you know, as usual, when somebody comes up with a good idea, they overextended. They're like, well, maybe if rocks were made out of stuff. Then water is also made out of particles, and maybe even light is made out of particles. You know, at the time, seemed like a totally a crazy reach, And.

00:11:12
Speaker 2: That makes sense, right, because light seems to go in a straight line. It seems to bounce off of things. So why couldn't light just be like little tiny little pellets that bounce around the room and then eventually hit your eye and then that's how you see something.

00:11:24
Speaker 1: Yeah, it certainly seems to have some of those particle like properties, right, it moves in straight lines, it certainly would be going really really fast. At the time, people thought that light traveled instantly, right. They thought that light instantaneously went from like the sun to the Earth, or or if you started a fire, that the light would immediately illuminate the room. Now, we of course know that it just happens super duper crazy fast, too fast for those folks to ever measure, so it's almost like it's instantaneous. But they thought that these things just moved instantly through space and filled up the room. Okay, and I want to talk a little bit more about that, but first a quick break.

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Speaker 2: So, initially we thought light was or the Greeks thought that light was a.

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Speaker 1: Particle, right, And I think we have to qualify that because it makes the Greeks sound really smart. Just come up with this idea of atoms and all that stuff.

00:12:25
Speaker 2: And you say this before you're really down into Greeks.

00:12:27
Speaker 1: Well, I think people give the Greeks too much credit for that, because, as I've probably said to you before, the Greeks had lots and lots of ideas. You know, they had like thousands of these ideas about how the way the world works, and yeah, one of them was close to true. But like, if we're going to do some accounting, let's also remember the nine hundred and ninety nine ones that were totally off base, you know, and give them credit for those.

00:12:48
Speaker 2: Yeah, find that Greek who thought life was just little puppies, and be like seeing you guys also thought they were puppies. You can't be that.

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Speaker 1: Smart, that's right. But he's a cool idea. So give them credit for having that idea. Know what they were smoking when they came up with it, But I'd like to figure out where to find some. And then it was thousands of years later before people had another idea. It was a Descartes, the guy who's famous for. You know, I think therefore I am he thought about He was one of the earliest scientists, not just philosopher, but a scientist, back in the day when you know, science really was part of philosophy, and he thought that light was waves.

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Speaker 2: What made him think it was waves?

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Speaker 1: You know, I don't think he had much justification for it. This is back in the early days when science wasn't really an empirical study where you didn't like go out and do experiments to test your hypothesis. It just made more sense to him for light to be like these wavelike disturbances.

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Speaker 2: Right, which kind of makes sense, right, Like if you have a speaker in a room emitting sound waves, it's not that different from like a light bulb in the middle of the room emitting light all around it.

00:13:50
Speaker 1: Right. Yeah. And there's some things that light does that don't really that don't really seem consistent with particles, you know, like the way light bends through a lens, right, it's called we call it in science, we call that refraction. You know, with light changes from going through air to glass, it bends in this weird way. That's something that's very common for waves, right.

00:14:10
Speaker 2: And a particle wouldn't bend inside of a lens.

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Speaker 1: No, No, a particle that's definitely a wave like behavior. Yeah, oh, not particle like behavior. And so Descartes saw that and he's like, oh, you know, we have optics, we have these lenses, so maybe light is a wave. But if light is a wave, then it opens this other question. What's doing the waving? Right? I mean with sound, you know it's the air and the water waves. Obviously it's the water. But if light is a wave, then what is waving?

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Speaker 2: Meaning? Like, if light is a ripple, what is it a.

00:14:41
Speaker 1: Ripple of that's right? Yeah, what's doing the rippling? Right? If it's a wave, it has to be a wave in something, because a wave is just a description of some other form of matter rippling, right.

00:14:52
Speaker 2: It couldn't just be like stuff that we can't see.

00:14:56
Speaker 1: Yeah, and so you have to invent some stuff that we can't see. Right. To explain light being a wave, you have to invent this universe filled with stuff, or there has to be that stuff between us and the sun for example, right, which is a huge amount of this new stuff you're inventing. And if you're looking at the stars, there has to be that stuff between you and the stars. Right, So now we're talking about billions of miles of this new stuff, and Dick Cart, you know, didn't know, so he just gave it a name. He's called I don't even know how to pronounce it, but he called it plenum. And he thought, well, there must be if light is a wave, there must be some stuff that's doing the waving, and we'll just give it a name and maybe we'll be right, and then we'll be famous forever.

00:15:31
Speaker 2: Isn't it is that different than the ether?

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Speaker 1: It's similar in concept, right, It's a different idea, but it's similar in concept that like, if light is a wave, it must be waving through something, and we don't know what it is, which is invents something to and give it a name as a placeholder so when later people do the hard work of actually discovering it, we'll still get credit.

00:15:49
Speaker 2: Okay, So it was a particle. Light was a particle, then was a wave, and then what happened?

00:15:53
Speaker 1: Well, then Newton came along, right, And Newton's a really smart guy, and everybody knows that he's famous for thinking about gravity, but he also like to think about optics and lenses and he thought for sure that light was a particle because he saw it moving in straight lines and he saw distinct shadows. But you know, Newton also did a lot of experiments with optics. He studied prisms, and he saw light bending, and he saw light splitting into colors. And you can't explain that if light is a particle, but he tried. You know, he's like, well, maybe when a particle hits the glass, it gets some sort of weird sideways force and that makes it bend. But you know, that's not really an explanation. That's just sort of like a I don't really understand it. But maybe it's something like this.

00:16:36
Speaker 2: Like, if light is a particle, why does it split into the rainbow kind of thing?

00:16:40
Speaker 1: Yeah, exactly. And you know this is again back in the day when empirical studies of science weren't the main way to answer questions. It was mostly thinking in your head about things that made sense to you, and then they would argue about them. Right. A lot of the way scientific disputes used to be resolved was people would argue about it and then say, well, that makes no sense, so it can't be true. And we know now, of course that the universe doesn't always make sense to us. What's real isn't necessarily the things that we would have accepted as true or accepted as a reasonable way to describe the universe. But you know, if that's the way nature works, that's the way nature works, you have to accept it. But that's the sort of primacy of experimental results came later on. So back in the day, people just sort of used to argue for an explanation that made sense to them.

00:17:25
Speaker 2: Right, Well, it was kind of hard for them to build a particle collider, right.

00:17:29
Speaker 1: That's right, Yeah, exactly, They didn't have the massive government funding to do that. These were men of leisure studying science and the spare time.

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Speaker 2: In fact, it was called like natural philosophy, right, It wasn't called science at the time, was it.

00:17:43
Speaker 1: Yeah, that's right exactly. All science grew out of philosophy. It was called these folks were natural philosophers. Okay, But you know, later on then people started doing experiments, and there were a bunch of French guys who did a bunch of experiments and some of some English folks, and they they were studying how light behaved and refraction and reflection, and they saw it doing these things, and they thought, there's no way Newton's right, this has to be a wave, you know. They saw things like interference patterns, right. Interference patterns is when you have two waves and sometimes one is rippling up at the same time another one is rippling down. Right. So imagine, for example, you have a bathtub of water in front of you, and you slap it with two hands at once, right, each one is going to send waves out, and then when those end, those waves are either rippling up or rippling down. And when they reach each other, if they're both rippling up at the same time, then they constructively interfere to get a double wave. If they're both rippling down at the same time, they constructively interfere to get a double down wave. If one is rippling up and the other's rippling down, then then they cancel each other.

00:18:51
Speaker 2: Out right, And so you would see no light.

00:18:54
Speaker 1: Yeah, exactly, And so you can do this kind of stuff, you know, in your bathtub. You can see interference patterns. And what happens if you have two sources like that, like one from each of your hands, is you get some areas where the waves are high, in some areas where the waves are low in some areas where there are no waves, and so as you say, if you do it with light, then you see these patterns of dark and light, these stripes.

00:19:16
Speaker 2: And you couldn't do that with particles, right, like a particle wouldn't cancel another particle.

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Speaker 1: Yeah, there's no way to explain that with particles. People thought, well, look, this is something that waves do and light is doing it, and there's no way to explain it with particles, so light must be a wave. Right. In fact, there's even famous cases where they said, well, you know, if light is a wave, then you know, if you set up this various experiment, you would get this crazy effect and so that's absurd and so it definitely can't be true. And then they went and did the experiment and saw the crazy wave effect and they're like, oh, that is true. You know this is a I love that because it's the primacy of experiment experimentalism, right, like, go and check the data, Go and actually get some data and see what the universe tells you.

00:19:59
Speaker 2: Yeah, Like you're like, a doughnut can't possibly be a croissant at the same time, But it turns out that you can bake something called a cronat.

00:20:08
Speaker 1: Yeah exactly. I think that's a big debate in pastry science still though. Yeah, is it from a donut like that's like a croissant or is it a croissant that's like a donut.

00:20:16
Speaker 2: Yeah. I'm getting my degree in a particle baking.

00:20:21
Speaker 1: Yeah, the large Pastry Collider. I'm looking forward to the construction of that project.

00:20:29
Speaker 2: But that's kind of what you mean. It's like you, people don't think it's possible until they actually see it. And waves and light has been doing this to people for hundreds of years, or they're like, they can't possibly be doing this, or they can't it can't possibly be doing that, but it just keeps doing all these weird things.

00:20:43
Speaker 1: Yeah, exactly. And that was the experiment it's called the double slit experiment, the one that really convinced people that light is a wave because they've shone a strong light and they had just two little narrow slits which act like as sources like slapping your hands in the bath to water, and then on a screen behind it. They saw these interference patterns, right, is that you could definitely only get if light was a wave. And so that was the early eighteen hundreds, and everybody was absolutely certain light was totally a wave. The question was settled. We knew forever light was a wave, and we still didn't know what was it waving through.

00:21:19
Speaker 2: But how did they explain all those particle experiments?

00:21:21
Speaker 1: Well, this was before we even really knew about particles, right, No real particles had been discovered at this point. With this idea from the Greeks of thousands of years ago that maybe things were made out of particles, and chemistry was getting warmed up, and you know, people are starting to think about atoms and molecules and stuff, but they hadn't really seen any actual particles yet. And it was decades later when the electron was discovered that people started to think about the particle model again. But you know, the wave theory was definitely ascended, right, Everybody definitely looked at these double slit experiments and saw light doing all this wavy stuff, and they were sure that light was a wave.

00:21:57
Speaker 2: Now did people extend that to other things, like, you know, they thought, oh, light is this weird wavy thing, but surely us, we're made out of little tiny atoms.

00:22:05
Speaker 1: Yeah, that's a good question. I wonder if people thought, hm light's a wave. Maybe we're a wave too, right.

00:22:11
Speaker 2: Yeah, or like everything is just like a wave.

00:22:14
Speaker 1: Yeah, probably not, because nobody thought that light had any mass to it, right, whereas we definitely know that we have mass. Right, we feel pretty heavy sometimes after a big meal. Even before the discovery particles, though, there was a huge advance in the theory of light, which was a Scottish guy named Maxwell. He was working on electricity and magnetism, and he put together all these equations to describe electricity magnetism, and he just sort of wrote them down in a new way. This is like the way you could do theoretical physics back in the days. You just take existing ideas and you find a new way to write them down. But he wrote them down in this way that looked like the mathematics of a wave. We have this equation, it's called a wave equation, and it describes how waves moved through a medium.

00:23:00
Speaker 2: Meaning like it could be described by equations that looked like sine waves and cosine waves, right, I mean, just in case anyone remembers high school math, that's kind of what we mean by mathematical equations. So you can describe it as a sine wave ors cosine wave.

00:23:15
Speaker 1: Right, that's right. Yeah, The solution to these equations are sine waves and cosine waves. These are differential equations to describe how things move through the medium, and if things follow these equations, then they're waves, right, okay. And so he looked at the equations for electricity and for magnetism and he rewrote them and he realized, you can rewrite them in a way that looks just like the wave equation. Right, So he said, oh, electricity magnetism has the same equation as waves moving through water or waves moving through air. And in fact, if you write it in terms of this wave equation, you can pull out what the speed of those waves must be. And the speed that he pulled out from this from these equations was the speed of light. So he had this moment of epift. He must have been like in his office late one night, rearranged these equations and realized, oh, my gosh, light is a wave and it's a wave of electromagnetism.

00:24:10
Speaker 2: So like a light bulb turned on on top of his head, emitting waves.

00:24:14
Speaker 1: Exactly the first appropriate light bulb ever.

00:24:17
Speaker 2: Yeah, so then that seems pretty definitive. The double slit experiment shows it light interferes with itself. And also this guy figured out that it's mathematically describable by sine ways and cosine waves right.

00:24:35
Speaker 1: Right right, That light is waves of electromagnetism. Yeah, exactly. So then it all seems really nice and tidy. But then the particle revolution comes, right. People discover the electron, people discover the neutron, people discovering all these particles. But then they were doing experiments where they were shining light onto materials and trying to get it to kick off electrons. So you shine a really bright light at something and you hope that some of the electrons in the material absorb that light and get enough energy to be free to run away. And so this is called the photoelectric effect. You shine light at something and you measure the electrons that come off. So what they saw in this experiment only made sense if the energy of the light comes in little packets rather than a continuous stream like waves. So they turned up the intensity of the light and they made it brighter, but that didn't increase the energy the electrons that were coming off, which doesn't make sense if it's a wave. It only makes sense if photons come in little packets, so that increasing the intensity of the light means more photons, but it doesn't give more energy to any one electron because each electron can only absorb one photon. And nobody understood this at all. This made no sense to anybody. It was a huge puzzle. We totally believe that it acted like a wave. We had the double slit experiment told us it was a wave. Maxwell's equations told us it was a wave. But then we had the photoelectric effect, which didn't quite make sense to anybody. Okay, And then Einstein said, well, what if light comes in these little packets like you were saying before, What if light is not this continuous stream of energy like a wave is right, a wave is continuous stream of energy. What if it comes in these little bits? And that explained everything if you if you thought that light was came in these little packets, it explained the photo electric effect, explained these all these other mysteries in physics, and that was the birth of quantum mechanics.

00:26:27
Speaker 2: Did they think that maybe it was little packets of waves? Do you know what I mean? Like little short bursts of ripples, you know, do you know what I mean? Like could that explain how it's both things that run through his brain.

00:26:38
Speaker 1: Yes, absolutely, I think that's probably the first way he thought about it. Is like a little localized ripple, right, like a little Yeah, that's the best way to put it, a little localized ripple, like the way you can send a little ripple of water, Yeah, through a swimming pool, or something.

00:26:54
Speaker 2: Like a chirp or like a little sound burst.

00:26:57
Speaker 1: Yeah, exactly, like a little chirp. But it's strange because you know, you can make a chirp of any size. You can make a big one, a little one, a long one, a fat one. But light, for some reason, wanted to come only these in these little distinct chirps of a specific size, and the size of those chirps was controlled by their color or their frequency. And so that was the birth of quantum mechanics, which we could spend a whole other podcast talking about, but it was the first clue that maybe light did come in these distinct little packages.

00:27:26
Speaker 2: Yeah, let's talk about that, but first let's take a quick break.

00:27:41
Speaker 1: And that's what we talked about, Like, what is a particle. It's a distinct little package. And then here's the part that blew my mind. Is that. Then they went back and they did that double slit experiment again, but they slowed it down. Instead of shining a really big beam of light, they just shown one photon at a time, right, Okay, because they wanted to see what's going to happen. Right, if light comes in these little packets, how does that explain the interference effect? How can light interfere if it's a particle?

00:28:08
Speaker 2: So like, instead of like pointing the hose of water at these two little holes and just seeing what happens on the other side, they were throwing one droplet of water at a.

00:28:18
Speaker 1: Time, yes, exactly. And what they expected to see was that there'ld be no interference pattern, right, because the interference comes from having two sources. Right. You have interference when you have two waves that are either adding up or canceling out.

00:28:31
Speaker 2: Meaning like a huge stream of light is going through these two little slits. Then the two little slits act like little sources, like little sorts of ripples, which can cancel az exactly. But if you throw one drop at time, it's either going to go in one slit or it's going to go on the other slid, right.

00:28:47
Speaker 1: That's right. Yeah, And so there should be nothing to interfere, right, So that's what they expected. But what they what they saw blew their minds. Right. What happens if you slow the experiment down, you send one photon at a time, is that you'll get an interference pattern. It's just that it builds up piece by piece. So you used to throw one photon through and it lands someplace on the screen. Throw another photon through, it lands somewhere else on the screen. After you add up a million photons, you rebuild the original interference pattern you saw.

00:29:16
Speaker 2: That's crazy.

00:29:17
Speaker 1: What light is a particle, but it's acting like a wave? Right? How is that? How can that even be? Right?

00:29:23
Speaker 2: It's not just that it's like it's a particle that's acting like a wave, as if it was in a huge stream of other particles.

00:29:31
Speaker 1: Right, that's right, And this blew everybody's mind. And the answer, of course, is that light is a particle. But like every kind of matter, like every particle, how it moves is governed by mathematics of wave equations. So every particle carries with it some quantum mechanical wave that determines where it goes. But what was happening in that experiment was that a particle photon was approaching the experiment and then it could either go through the left hand side or the right hand side slit right, And because it's quantum mechanical, it did both. It had a chance to do both, and what was interfering was the probability to go through the left slit or the right slit.

00:30:15
Speaker 2: So that's interesting. I don't think i've heard that explanation before, that it's a particle and a wave in the sense that it is a particle, but it moves according to wave equations.

00:30:26
Speaker 1: Yes, everything moves according to wave equations. It's just that the wavelength for things depends on how much energy they have. So that was this guy, Tobroguely. He came up with this equation. And maybe you've heard the expression to broguely wavelength.

00:30:39
Speaker 2: I've heard the expression wavelength. That seems to be a.

00:30:41
Speaker 1: Yeah, everything is wavelengths. We were making fun of that guy. Turns out he was right, oh twist ending, No, everything has a wavelength. Right, you can describe the motion of anything in terms of a wave. Now, the wavelength depends on the mass and the momentum, and for most things like me or you or a cantalope. The wavelength of its quantum mechanical wave function is tiny, and so you can't even notice, right, the wave effects of you and your son walking down the hallway and interfering with each other are basically negligible. But on the scale of particles, these wave functions interfere with each other.

00:31:23
Speaker 2: Yeah, that's a crazy thodd that, you know. We I think people think quantum is something that doesn't affect their lies, but quantum ideas and concepts are everywhere, right, Like you have a sort of like a quantum superposition, or you you're not really there. You sort of there's a cloud of you that is.

00:31:43
Speaker 1: I'm not really here, I'm just an AI on the internet. But that's that's differently cloud. Yeah, there is this quantum mechanical certainly and everything.

00:31:53
Speaker 2: Guess yeah, yeah, it's just that you can't notice.

00:31:55
Speaker 1: That really blew people's minds. This concept that like, okay, light is a particle, but it's sort of acts like a wave. We can use these wave equations to describe it. And you know, there's another layer to that experiment which is even crazier, right, which is if what's interfering is the probability to go through the left slit or the right slit right. When the photon approaches the experiment, it can go through one or the other. The interference pattern comes from the uncertainty of which it's going to go through. So what you can do is you can add a little detector to one slit that like gives you a ping if it goes through that slit right, so you know for sure if it goes through one slit or the other. If you do that, the interference pattern disappears. WHOA why does it disappear? It disappears because the interference only came from the interference of the possibility of the particles to go through the left slit or the right slit, our lack of knowledge. Once you know it goes through the right slit of left slit, there's no more uncertainty. There's nothing to interfere. It just goes through the left or it goes through the right.

00:32:56
Speaker 2: It's like you're throwing a box is full of cats that are either dead or alive, and you see what happens on the other side. It's different if you take a peek inside the box before it gets.

00:33:08
Speaker 1: There, exactly exactly, and no cats were harmed in the making of this podcast, I now feel energe to point out that's sort of where we are today, is that we know that light is a particle and then it comes in these little discrete packets become photons, right, Yeah, But we also know that, like everything else, light is determined by how its wave function moves. Every particle and every object has this wave function, and how it moves is controlled by wave equations.

00:33:40
Speaker 2: It's not like it's both a particle and a wave and people don't really know which one it is, or people are still confused about that. But it sort of sounds like you're not that confused about it, right, It sort of sounds like you know it's a particle, but it moves around like a wave.

00:33:54
Speaker 1: Yeah, but it's still confusing. I mean, I think you could. It's totally reasonable to say it's both. It's a particle but it acts like a wave. Right. It's also totally reasonable to say it's neither. It's not a particle, it's not a wave, it's something else. It's something weird, something totally strange, we've never seen before. It's a war we can.

00:34:17
Speaker 2: Or a pave.

00:34:18
Speaker 1: You are on fire.

00:34:20
Speaker 2: I am simple spelling.

00:34:25
Speaker 1: But that's that's a joke. But it's also serious because sometimes we discover things which are unlike anything else we've seen, and how do you describe them?

00:34:33
Speaker 2: I mean, we should stop using these words. We should maybe come up with a new word to describe what it is, because it's not, yes, not described by either word particle.

00:34:42
Speaker 1: That's right, it's a chapel, it's a cherry apple combination.

00:34:45
Speaker 2: Yeah, let's not call it a particle or a wave. Let's just make up anyw word that embodies these two ways to behave that's right.

00:34:52
Speaker 1: But here we've discovered something which is different from anything in our microscopic world. There's nothing in our world particles, wave, little puppies that is a good analogy for what light is. So we have to try to sort of describe it in terms of sometimes it's like this, sometimes like this. My personal belief is that it's it's not like anything else, and that these are approximations. But you know, like we were talking about earlier, you can be different contradictory things like how would you describe yourself? You know, sometimes your husband, sometimes your father, Sometimes you're a cartoonist, sometimes you're just asleep. You know, like all these things describe you, they're contradictory. There are different facets of who you are at your core, and none of them define you.

00:35:32
Speaker 2: Right right, But if you don't have to have the right label, you make up and you know that's right.

00:35:37
Speaker 1: Yes, we need a new thing, right. Light is definitely its own weird kind of thing.

00:35:44
Speaker 2: All right, Well, until next time.

00:35:54
Speaker 1: If you still have a question after listening to all these explanations, please drop us a line. We'd love to hear from you. You can find us at Facebook, Twitter, and Instagram at Daniel and Jorge That's one word, or email us at Feedback at Danielanorge dot com