Is light made of waves or particles? Is matter made of waves or particles? What does it mean for matter to “wave”, anyway? Why is this so hard to understand? I discuss these questions and more in today’s Ask a Spaceman!
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EPISODE TRANSCRIPTION (AUTO-GENERATED)
If something seems weird, don't throw it away. No, it's an opportunity to expand your understanding. Right? This is this means your brain is breaking and sometimes it's like if you break a bone and then keep it set, a new bone will appear in the gap. If you break your brain and then keep it open, sometimes new brain stuff can fill in and you can have a new understanding.
Don't try that with real physical brains. This is just a metaphor. Just a metaphor. The the reason I bring this up is the nature of the subatomic world is weird. Deeply, fundamentally weird.
So weird that it's almost impossible to describe with language. But we don't understand the subatomic world through language, through through English or German or French or Thai or whatever. We understand the subatomic world through math. Mathematics. We use mathematics to understand the subatomic the quantum world.
Quantum mechanics is our mathematical description of really tiny stuff. And this, I think, highlights it's don't worry. I'm gonna try to explain all this. But this highlights the power of mathematics. Right?
And this is why science uses mathematics because math in science is a tool. Like, mathematicians use math and develop math just for for the fun of it, which is great. Love them. Physicists pick and choose from that buffet of things, of logical structures and ways of organizing and etcetera etcetera to to solve problems, to describe nature, to make predictions. We have a set of mathematics, several sets of mathematics that we use to describe the quantum world, and they're great.
They accurately describe nature as best we can. They make predictions that are verified by experiment. They do this. They do that. Like, it's a powerful, powerful way of describing the subatomic world.
But then we start talking about it, like, on a podcast, where we try to wrap our heads around it. Like the math is crystal clear when it comes to quantum mechanics, and then you start to to talk about it because, you know, you want to put some words to it and words come up short. One of the biggest highlights of this is something we call the wave particle duality. The wave particle duality is a fundamental concept of the subatomic world, a fundamental tenet it's a cornerstone of quantum mechanics. Good luck describing it.
Good luck to me because I'm about to try. To me, this also highlights how there's so many human biases. Like, evolution did not give us a mind so that we could eventually understand the quantum world using language. No. Our mind is is evolved to hunt mammoths and make fires.
Right? If there's any evolutionary biologists in the audience, I'm sorry. But then we and so we develop these these biases, these intuitions, these these assumptions about the way the world ought to work, and then, bam, we look in the microscope and in the particle collider and things don't behave that way. And we're stuck because we're layering on thousands, hundreds of thousands of years of the way we think, and then we're trying to describe something that is totally outside of our ability to conceptualize. But But that's why we have math.
Math is a tool that allows us to do things we wouldn't normally be able to do. Sometimes we just have a problem describing it. Like with the wave particle duality. What is a wave? A wave is a wiggling thing.
It's an oscillation. It's a disturbance. It spreads out. It moves. It transports.
It takes up a lot of space. Right? And you can stand in one spot, and you can feel the wave rolling over you, and then going away, and then rolling over you again as another wave comes by. Yeah. And and waves can do all sorts of cool things.
They can interfere with each other. Right? If you if you send one wave towards another wave, sometimes the peaks add together and you get a double wave. Sometimes the peaks meet a trough and they cancel out and you get no wave at all. Waves can wrap around corners.
They can spread out. They can fan out. Waves do all sorts of cool stuff, and they they mostly, they wiggle. What is a particle? It's chunk.
It's a finite object. It's a it's a thing. Right? It's a it's a thing you can hold in your hand. Imagine trying to hold a wave in your hand, like, oh, there's a wave.
I think I'll describe it. No. That's not how waves work. But a particle, yeah. I can grab a particle and hold onto a particle.
I could throw a particle. I can catch a particle. It's a finite object. It can bounce off things. It's travel it's localized, like a bullet.
Bullet is a particle. An ocean wave is obviously a wave. And it seems like based on, you know, all of human evolution, the way we think, the way we fit look at the world, that waves and particles are two entirely separate, totally different things. That you can organize, you can classify the entire world into two camps, the chunks and the wiggles. It seems like a nice handy useful division, like, okay.
Look at that. Okay. That is a chunk. That's a particle. Oh, look at that.
That is a wave. It's a wiggle. And and so on and so on and so on for everything in the world. It's pretty handy useful division. It's away because particles have certain ways of behaving.
We have certain mathematics for describing that, and waves have certain ways of behaving. We have mathematics to describe that. So it seems pretty handy except when it isn't like in the quantum world. And quantum mechanics was born in the early nineteen hundreds right after we had spent a couple centuries really solidifying this concept of waves and particles and stuff that acts like waves and stuff like that acts like particles and all the cool mathematics that describe them and predict their behavior, like, we we were really really solid on waves and particles, on chunks and wiggles, and then we get to the quantum world and it's just nothing makes sense. For example, light.
Is light a wave or a particle? Is light a wiggle or a chunk? My favorite answer, and I got this answer in undergraduate optics course, it was a course on light. I remember the professor standing up. He said, is light a wave a particle?
Well, the answer is it depends. Let's say I blast you with a certain kind of light called radio. Radio is a kind of light. It's a piece of the electromagnetic spectrum. And imagine you were sensitive to radio light.
You could feel it. What would it feel like? Well, you'd feel your electrons waving up and down, up and down as this radio wave washes over you. I mean, look at a radio tower. What's happening in a radio tower is a bunch of electrons are racing up and down and up and down and up and down.
It's like holding one end of a string on one end of a rope and wiggling it up and down generating a wave. That's what's happening in a radio tower. It's very, very wavy, very wiggly, very sloshy. Let's say it blasts you with some x rays or some gamma rays. Will you feel your electrons wiggling up and down?
No. You're gonna get hit. You're gonna get smacked. You're gonna get punched with tiny, tiny little bullets. Like, gamma rays can sometimes even slice up our DNA.
They act like they're both they're chunks. They're particles. Like a gamma ray acts like a particle, but a radio wave acts like, well, a wave. But fundamentally, there's no difference between radio and x rays. They're both on the electromagnetic spectrum.
They're both part of it. It's the exact same thing, just one's a higher energy than the other. Why should light that's high energy act like a particle and light that's low energy act like a wave? It gets even worse. Because you think, okay, maybe there's like some sliding scale.
I can go from radio up in the microwave and infrared and visible and then ultraviolet on in it. Like, I slowly transition from, like, wavy light to particle light, but, no, it gets way worse. For example, we have black body radiation, which was studied all through the eighteen hundreds, and I did a whole episode on this. Max Planck cracked it, was able to figure out how hot glowy things emit light, and he realized that the light is emitted in chunks, that light is fundamentally a particle. It's a chunky thing called a photon.
Same thing, just a few years later, actually. Einstein was able to explain the photoelectric effect where you shine light on a piece of metal and electrons bounce off. The only way to explain that behavior is if light is made of chunks called photons. So it looks like light's a particle. Everyone, that's it.
No matter the wavelength, no matter, like, even look at the do you realize the word I just used? I'm sitting here talking about light being a particle, but I literally just use the word wavelength as in wave. How confusing is this? But according to the results from black body radiation, the photoelectric effect, no matter what kind of light you're talking about, it's made of little chunks called photons. But then we have Maxwell's equations.
Maxwell's equations, these are equations of electricity and magnetism, describe light as waves of electricity and magnetism. That's what a what that's what light is. What's going on? We have Maxwell's equations that tell us all kinds of light acts like a wave, and then we have, like, black body radiation photoelectric effect telling us that all kinds of light act like a particle. And then sometimes I blast you with radio, and it it feels like a wave.
Sometimes I blast you with X rays or gamma rays. It feels like a particle. What's going on? It gets even weirder because there's something called the double slit experiment. The double slit experiment very, very, very, very much makes it look like light is a wave.
To explain the double slit experiment, I need to talk about diffraction, which is the word of the day. Diffraction. Diffraction is what happens when a wave, when it goes through a narrow opening, starts spreading out at the opposite end of that opening. Very, very much a wavy wiggling thing to do. You can see and ocean waves do that.
If they pass through a narrow channel, when they come out, they fan out. This is called diffraction. So if you shine light through a very narrow slit and a little narrow opening, and then let that light shine onto a screen behind it, you get diffraction. What you see is a pattern. It's not just, like a blob of light on the back screen.
No. There'll be stripes. It's a very wavy looking pattern. Some bright stripes, some dark stripes. And this is caused by as the light goes through that opening or the wave goes through that opening, and it starts spreading out, it can start running into itself.
So some of the waves add up, some of the waves cancel out, and then you end up with this pattern. That's great. But notice I used a very specific word in describing this opening or slit. I used the word narrow. When the slit is about the same width as the wavelength of the light, you get diffraction.
If it's too small, it doesn't pass through at all, and if it's too big, you get no diffraction at all. Weird. Weird. If I send light through a wide opening, I just get a splotch of light at the end. When I narrow it down so that the the width of that slit is about the same as the wavelength of the light, I get diffraction.
And when it's too wide or when it's too small, it doesn't pass through at all. When I open this up to two slits and let light pass through two slits and then let them overlap those wavy things, I get, interference pattern. I get a stripy pattern at the end. This was Young's experiment. Thomas Young in the early eighteen hundreds, he did this.
Very clearly showed that light is a wave. And so through all the eighteen hundreds, everyone's like, yep, light's a wave. And then we get things like blackbody radiation and photoelectric effect. What's going on? It seems like two contradictory ideas that light can sometimes act like a particle, sometimes act like a wave, Sometimes a little bit of column a and a little bit of column b can be perfectly totally described by electromagnetic waves, but also totally completely described by photons.
What is it? It's both. It has a dual identity. And which identity comes out more depends on what you're studying. If you're studying black body radiation, then the the particle nature will come out more.
If you're studying emission from a radio tower, the wave nature will come out more. If you if you run the light through a double slit, the wave nature will come out more. But if you're looking at light bouncing off of metal, the particle nature will come out more. It has both. It'll act like one or the other whenever it feels like it.
But it gets weirder than this, folks. Yeah. If you just thought this was weird, this is just the tip of the weird iceberg. We are not even close to the amount of weird when it comes to the wave particle duality. And the weirdness comes from a lovely guy by the name of Louis de Broglie.
It's spelled b r o g l I e. So if you wanna if you want a little physics in joke here, you call him de Broglie, Louis de Broglie. Just if you wanna pass yourself off as a legit physicist, just drop that. Everyone will chuckle and accept you in to the little circle of conversation. So he realized something.
He was working off of Planck and Einstein's work. And through this work, the early stages of quantum mechanics, looking at light as made up of tiny little particles called photons, there was a relationship between the frequency of the light, the wavelength of the light, and how much energy it had. This was a fundamental relationship. What tied the two together is Planck's constant. Planck's constant tells you for a given wavelength of light how much energy is in each photon.
Louis de Broccoli made a very very interesting connection. It was a hypothesis. It was just a guess. Like, hey folks, check this out. There's a connection in light between energy and wavelength.
Light has energy, light has momentum, and it has a wavelength. Okay? So far so good. A moving particle has energy. A moving particle has momentum.
Maybe hear me out here. Maybe moving particles have a wavelength. Maybe for a given energy of particle, give how fast is moving, how much mass it has, just like with light, if there's a relationship between energy and wavelength, maybe with matter, there's a relationship between energy and wavelength. But what would it mean for, like, a bullet, which is a chunk, it's a thing. What does it mean to have a wavelength?
Well, your guess is as good as mine. The wave nature of matter, of particles, really comes out in this two slit experiment. The same experiment that Young did in the early eighteen hundreds to show that, look, light's a wave. It's acting like a wave. So check this out.
When you have two slits, two narrow little openings, and you start shooting bullets at it, shooting electrons at it, just blasting it with electrons, What would you expect? You would naively expect, well, the electrons either hit the wall or they pass through the opening, and if they pass through the opening, they're just gonna go straight on through and hit the screen behind it. So you expect on that back screen that's, like, recording where all the bullets, where all the electrons, where all the chunks are going, there's gonna be, like, two narrow patterns where the openings of the slits were. Yeah. That that doesn't happen.
You get stripes. You get a diffraction pattern. You get a lot of electrons here, and then next to it, not a lot of electrons, and then next to that, a lot of electrons, and then next to that, not repeated. Exactly like what the light did. Exactly.
So Young's experiment in the early eighteen hundreds very, very clearly showed light's a wave cause it's doing wavy things. Exactly if you put water waves through two narrow openings, this is exactly what you get. Then a hundred years later, we started doing it to electrons and we got waves. What's the conclusion? Electrons are waves.
But wait a minute. I can grab an electron. I can throw an electron. I can look. I can stare.
It's right there. I can point to it. And it's not moving. It's not wiggling around, but then it ends up with a wave pattern. This is weird.
Took a while to come to grips with this idea. The answer came through the mathematics of quantum mechanics. If matter has a wave nature, which it obviously does because you shoot electrons through two slits, you end up with a wave pattern on the back, just like light does. If matter has a wave nature, what are these waves of? What does it mean for an electron to have a wave?
The mathematics describes it. The mathematics of quantum mechanics describes what's happening just fine, but then we gotta talk about it. We wanna answer this question. What does it mean for matter out of way? Yes.
Yes. You can just point to the broccoli's to Roy's equation and say that's what it means. That's not really helpful, I guess, if you're trying to build a mental picture of reality. So welcome to the wonderful world of interpretations of quantum mechanics. Interpretations of quantum mechanics are trying to wrap the mathematics with a natural language, which is difficult, but not nearly as difficult as contributing to Patreon.
Go to patreon.com/pmsutter to learn how you can keep this show going. That's right. All of my education, outreach, everything I do, like, this is my life, folks. This is my job, and I got bills to pay just like you. Don't worry.
Your bills come first. If you got a little bit of extra, $1 a month, five dollars a month, doesn't matter. Go to patreon.com/pmcenter. I really appreciate it. I'm not gonna get into all the gory details of the various interpretations of quantum case, but feel free to ask, and I'll I'll talk about it, and how they're different and what they mean.
But I'm gonna give you what, is the so called Copenhagen interpretation because it was developed at a conference in Copenhagen. Imagine that. In this in, quote, unquote, interpretation of quantum mechanics, in this way of describing quantum mechanics, the waves of matter, or the waves that we associate with matter, are waves of probability. You see, if you have an electron and it's just floating around, flying along, doing its own thing, no one's looking, it's not interacting with anything, it's just doing another thing, it's not localized. It's not a little chunk.
It's not a tiny little thing that you can hold in your hand. No. What's actually traveling is a cloud, a cloud of where the electron could be as it travels. This cloud moves along, and it represents all the places the electron might be. Then when you go looking at it, or it interacts with something, or something happens to disturb it, this cloud disappears, and the electron chooses one of its options.
It's like, oh, I could be over here. Okay. I'll be over here today. Or I could have been over there or I could have been over there. I hadn't decided yet until I interact with some, that's when I'm gonna decide.
This cloud, that's what we call an electron, but is really just a cloud of probability of where the electron might be the next time we go looking for it, is described by a particular mathematical equation. We call it the Schrodinger equation developed by Erwin Schrodinger. Congratulations, Nobel Prize. This equation, the mathematics of the equation, are the exact same mathematics that describe waves. The exact same structure, the exact same form.
It's a it's an equation of waves. Just like if you were to write down equations to describe ocean waves, or write down equations to describe electromagnetic waves, or write down equations to describe waves on a slinky, it's the exact same structure. The the exact same bones here in the Schrodinger equation. It's an equation that describes waves, But these are waves of probability. These are waves of where the electron might be.
Where you're really likely, like, where the equation peaks, where that cloud is really dense, that's where you'll likely define the electron. But, you know, you're not guaranteed. It might be over here, it might be down there, it might be over there, it might be behind you. You never know until you go looking for it. And the mathematics that describes that is the exact same mathematics that describe waves.
So in this picture, in this Copenhagen interpretation, the electrons are being shot through the slits, but since no one's looking, they're really clouds. They're really fuzzy. They're represented by equations that, you know, could tell you where that electron might be. Then the these waves of probability pass through the slits and interfere with each other exactly like water waves do, exactly like electromagnetic waves do. Sometimes they add together, sometimes they cancel out.
And then the electron hits the screen, and it actually has to pick somewhere to be, and it's gonna pick somewhere to be based on the wave that hit the screen. So it's almost like instead instead of thinking of shooting electrons, you're shooting waves, the waves pass through the slits, interfere with each other, then when the wave hits the screen, it actually has to do an interaction. It has to pick. Like, okay. Fine.
The electron will be right here. But because some parts of the waves have interfered with each other and canceled each other out, there are some places where you'll never find the electron. Just like there are some places where you'll never find the light because the light canceled each other out, the waves of light canceled each other out. Or you would never find the ocean wave washing up on the shore because the waves there canceled each other out. Because that's a wavy thing to do is the ability to add together or cancel each other out.
Now that's the Copenhagen Interpretation. If that doesn't sit well with you, don't worry about it. The mathematics works just fine. The ultimate lesson here is that both matter and light have some sort of wavy nature and also some sort of particle nature. Sometimes light acts like a chunk.
Sometimes it acts like a wiggle. Sometimes matter acts like a chunk, sometimes it acts like a wiggle. What's the best way to phrase it? Is everything both a particle and a wave? Is it sometimes a particle or sometimes a wave?
Is it neither particle nor a wave? Is it a pave, a whortical? No. I think we can all agree we're not gonna do those. The answer is it's weird, And it could be that we're just not evolutionarily equipped to understand it unless we, you know, break our brains a little.
Thank you so much to everyone who asked questions to get this episode going. Rowan h on Facebook, Ethan l on email, Brock p on YouTube, Matt Abb d on Facebook, Grisham j on email, Jeff g on email, Courtney h on email, Joshua z on email, at Shrennick Sean Twitter, Mike d on email, lynn r on email, a c on YouTube, break s on email, Robert p on email, and at shawnfosmark on Twitter. And remember, you can go to patreon.com/pm starting to support this show and all of my education and outreach. I'd like to thank my top Patreon contributors this month, John, Matthew K, How Good B, Justin G, Matt W, Justin G, Kevin O, Duck and M, Corey D, Kirk B, Barbara K, Nuder Dude, Chris C, Eric M, Steve C, and Digital Neo is you and everyone else. What a great community.
And if you don't want to contribute to Patreon, that's cool. Could you at least go on iTunes and leave a review? That'd really help. Or you can go to askaspaceman.com, send me questions there, email at askaspacemangmail dot com, use the hashtag askaspaceman on all social channels. And I'll see you next time for more complete knowledge of time and space.