Speaker 1: Hey, they're listeners. You're into physics. So here's a tribua
Speaker 1: question for you. Do you know who won a Nobel
Speaker 1: Prize for relativity? That might feel like it's your question
Speaker 1: because you want to say Einstein, because they think relativity
Speaker 1: and Einstein will I'll tell you it's not Einstein. Now,
Speaker 1: maybe you're scrambling through your mind to think about the
Speaker 1: names of other physicists. You might know. How many physicists
Speaker 1: can you name? Anyway, you got Einstein, you got me. Well,
Speaker 1: I'll give you a clue. It's neither Einstein nor me.
Speaker 1: So who was it? Right? Well, some folks wanted for
Speaker 1: proving that relativity was correct. There were Nobel Prizes for
Speaker 1: gravitational waves and for binary pulsars. But the answer is
Speaker 1: that nobody wanted for relativity. Nobody who came up with
Speaker 1: this incredible earth shattering idea that now frames all of
Speaker 1: modern physics won the Nobel Prize for it. But you
Speaker 1: might be thinking, hold on, didn't Einstein win a Nobel Prize?
Speaker 1: And he did, but he wanted essentially for quantum mechanics.
Speaker 1: Hello everyone, I'm Daniel. I'm a particle physicist, and I'm
Speaker 1: the co host of this podcast together with Korge cham
Speaker 1: who can't be here this week, So you're just listening
Speaker 1: to me talking about the joys of physics and trying
Speaker 1: to simulate Jorge in my mind. Every time i'm talking,
Speaker 1: I'm thinking, here's what Jorge would say. At this moment,
Speaker 1: I'm trying to interject a little Jorge Ism for you,
Speaker 1: since we all miss him and you were listening to
Speaker 1: our podcast, Daniel and Jorge Explain the Universe, a production
Speaker 1: of I Heart Radio in which we zoom all around
Speaker 1: the universe and try to find interesting, fascinating, cool little
Speaker 1: nuggets of physics that would blow your mind, but take
Speaker 1: them apart so they don't actually explode your head and
Speaker 1: cause your brains displatter anywhere. Instead, we want to smoothly
Speaker 1: and calmly insert them into your mind so you understand them,
Speaker 1: so you can talk to your friends about them, so
Speaker 1: you can actually comprehend these amazing, wonderful facts that we
Speaker 1: have learned about the universe, and also understand all the
Speaker 1: things we don't know about the universe, which is my
Speaker 1: favorite part of physics, and that's why Jorge and I
Speaker 1: wrote the book We Have No Idea, A Guide to
Speaker 1: the Unknown universe, which takes you on an amazing tour
Speaker 1: of all the big and basic questions about the universe
Speaker 1: that we still have no idea what the answers are.
Speaker 1: And on the podcast, we've been doing something fun, which
Speaker 1: is taking a little tour of how we know what
Speaker 1: we know, and specifically how we know anything about particle physics.
Speaker 1: It's still incredible to me when I look around at
Speaker 1: the world that everything is made out of these tiny
Speaker 1: microscopic object so we can't see that we've taken thousands
Speaker 1: of years to even discover that they exist. Yet we
Speaker 1: have this really complex, really elaborate, really amazingly effective model
Speaker 1: of what's happening down there at the microscopic scale, all
Speaker 1: these tiny quantum particles interacting and zooming around. Physicists can
Speaker 1: do calculations to tell you exactly what's going to happen
Speaker 1: when this particle hits that particle. It's really incredibly complex
Speaker 1: and mature. Though of course we have lots of questions,
Speaker 1: but I think a lot of times people think of
Speaker 1: this as sort of like an idea, something people came
Speaker 1: up with a description of the universe. But it's critical
Speaker 1: that everybody understand that this isn't just an idea that
Speaker 1: came out of our heads. This is something born out
Speaker 1: of desperation. This is our attempt to grapple with the
Speaker 1: weird and bizarre and counterintuitive and frankly mind blowing experiments
Speaker 1: that have shattered our perceptions of reality. We thought the
Speaker 1: universe worked a certain way. We thought everything was smooth,
Speaker 1: You could cut objects as many times as you wanted
Speaker 1: to infinitely small pieces, but you can't. We thought the
Speaker 1: universe was deterministic, that if you did the same experiment
Speaker 1: twice you would get the same outcome. Right, that would
Speaker 1: make sense, But it's not. It's fundamentally random. And the
Speaker 1: core of that is particle physics, because it attempts to
Speaker 1: describe the entire universe in terms of these tiny, weird,
Speaker 1: nondeterministic little particles, in terms of these tiny, little, weird,
Speaker 1: nondeterministic particles that seem to follow rules that just do
Speaker 1: not describe the world that we are familiar with. So
Speaker 1: my goal is to take you on a tour of
Speaker 1: those experiments, the ones that change the way we think
Speaker 1: about the universe, that showed us that the universe is
Speaker 1: different from what we imagined, because it's not just the
Speaker 1: final idea that you want that I want you to understand.
Speaker 1: I want you to know what the evidence is. How
Speaker 1: do we know what we know? Now? Recently we talked
Speaker 1: about the discovery of the first particle, the first experiment
Speaker 1: that revealed this incredible revelation that the Uni verse is
Speaker 1: made out of time to little dots. And so today
Speaker 1: we are continuing that tour. We are talking about how
Speaker 1: do we know the photon is a thing? You're familiar
Speaker 1: with photons? To you, photon is a very normal word
Speaker 1: you hear bandied about here, talked about but how do
Speaker 1: we know that photons are there? How do we know
Speaker 1: that light is made out of photons that is chopped
Speaker 1: up into these little pieces that can't be cut down
Speaker 1: even further. What is the actual experiment that proves to
Speaker 1: us that photons are a thing, that light is not
Speaker 1: just electromagnetic waves, but it does these other weird things
Speaker 1: that you have to give it particle status to explain. So,
Speaker 1: as usual, I was wondering how many people out there
Speaker 1: know why we think the photon is a thing, Why
Speaker 1: we don't just think about light as electromagnetic waves. So
Speaker 1: I walked around the campus of U see Irvine and
Speaker 1: accosted a bunch of friendly and unsuspecting students, and I
Speaker 1: asked them, do you know how the photon was discovered?
Speaker 1: Do you have an idea of why we think the
Speaker 1: photon is a thing. So before you listen to these answers,
Speaker 1: think to yourself, or pause the podcast, or just take
Speaker 1: a moment. How do you know photons are a thing?
Speaker 1: Are you just believing physicists when they tell you, or
Speaker 1: do you know what the data says? I'm not entirely sure.
Speaker 1: I feel I shouldn't have that. I I'm sorry, I
Speaker 1: probably should know. But it was the slit experiments, wasn't it.
Speaker 1: And they projected a laser beam onto a single slit
Speaker 1: or double slits and it diffracted the beam and that's
Speaker 1: how they discovered it. Particle wave duality. Yeah, the photo
Speaker 1: electrical facts. You shone a light on a metal and
Speaker 1: then the metal you cross. Uh, you start for Einstein ninth.
Speaker 1: I don't remember the year. Yeah, I don't remember who
Speaker 1: did it, but I remember that you shine a light
Speaker 1: on a metal, do you give the electron enough energy
Speaker 1: to start conducting it's particles? Well, we know it's a
Speaker 1: wave because it travels through vacuum, and we know that
Speaker 1: it's a particle because you can transfer energy from it. Right, Yeah,
Speaker 1: it has it hasn't defined momentum, even though it has
Speaker 1: no mass. But the slit experiment double slip one showed
Speaker 1: that it was a wave, like a single slit showed
Speaker 1: that a particle. Well, it's not necessarily a particle. It's
Speaker 1: both a particle and a wave. And for a really
Speaker 1: long time we thought it was just a wave. But
Speaker 1: I believe the first time we figured out that it
Speaker 1: was a particle had to do um exciting metals to
Speaker 1: release photons and realized that the distributions were discreet. So
Speaker 1: I was really impressed with these answers. A lot of
Speaker 1: understanding here that photons are particles and that they're part
Speaker 1: of this larger idea of light being a wave and
Speaker 1: a particle. Even some discussion of the double slit experiment,
Speaker 1: which I'm dying to get into in a future podcast
Speaker 1: and talk all about the amazing facts of quantum mechanics.
Speaker 1: But the double slit experiment actually shows you that the
Speaker 1: photon is a wave. But there was somebody out there
Speaker 1: who talked about the photoelectric effect, and that's the key
Speaker 1: that was the experiment that showed us that photons were
Speaker 1: a thing. But before we talk about the crazy experiment
Speaker 1: to prove that quantum mechanics is our reality, that showed
Speaker 1: us that the universe is probably sliced up into little
Speaker 1: bits and not infinitely smooth. Let's set the stage. Okay,
Speaker 1: let's you remember how people thought about light, and to
Speaker 1: get the context of the story, you have to rewind
Speaker 1: all the way back to Isaac Newton. Isaac Newton, of
Speaker 1: course very famous not just for the cookies, but also
Speaker 1: for his discovery of his theory of gravity, which unified
Speaker 1: motion of objects here on Earth with motion of objects
Speaker 1: in the heavens. Really gave us access to the whole
Speaker 1: universe to imagine, Wow, maybe physics can actually describe things
Speaker 1: not just here in front of us, but out there
Speaker 1: in the universe. Those are things out there that follow
Speaker 1: laws of physics. Incredible accomplishments. But Newton also also made
Speaker 1: amazing discoveries in the field of optics. He spent a
Speaker 1: lot of time with lenses and with prisms, and he
Speaker 1: was convinced that light was a particle, and he thought
Speaker 1: a lot about how light traveled. He saw it moving
Speaker 1: in straight lines, except when he was bent by these
Speaker 1: lenses and he was convinced that light was a particle.
Speaker 1: And because he was a genius and he's a staggering
Speaker 1: influence on the field of physics, people listen to him
Speaker 1: and rightly so. And for hundreds of years people were
Speaker 1: convinced that it was a particle, even though other folks
Speaker 1: had really nice theories of light as a wave. And
Speaker 1: it wasn't until the eighteen hundreds when people started observing,
Speaker 1: like doing things that particles couldn't do, that they had
Speaker 1: to adapt their mindset. And that's the key. There you
Speaker 1: see experiment rearing its uncomfortable head again saying, oh no, no,
Speaker 1: you thought you understood the universe. You have an idea
Speaker 1: in your mind, you have a mental model of how
Speaker 1: this is working, but it can't describe what's actually happening.
Speaker 1: And that's why I'm an experimentalist. That's why I think
Speaker 1: experiment is the place to be, because experimentalists are the
Speaker 1: ones who make the discoveries. They are on the forefront
Speaker 1: of knowledge. They're out there exploring the universe, discovering things
Speaker 1: that don't make sense. Theorists, of course, do an incredible job.
Speaker 1: They tied all together they understand they predict future phenomenon.
Speaker 1: But for me, the bit about physics that's wonderful is
Speaker 1: the experimental side, is making those discoveries, is asking nature
Speaker 1: a question and demanding an answer, pinning nature in a
Speaker 1: corner so that nature has to tell you is the
Speaker 1: universe this way or that way? And so the thing
Speaker 1: that told people that photons couldn't just be a particle
Speaker 1: were wave like effects, things like interference. And you're familiar
Speaker 1: with interference, maybe you have noise canceling headphones. Now is
Speaker 1: canceling headphones work via interference? Sound is a wave. It's
Speaker 1: a shaking of air, and the air comes towards your head,
Speaker 1: and if you can create waves shake in the other
Speaker 1: direction at the same time, they basically cancel out those
Speaker 1: waves that are coming in your head. So sound canceling
Speaker 1: headphones are proof that sound is a wave because they
Speaker 1: can do this wave like thing that particles just cannot do.
Speaker 1: In the same way, people saw light behaving in a
Speaker 1: way that could only be described by a wave, and
Speaker 1: so you had interference effects, and you had all sorts
Speaker 1: of theories sort of built momentum until you get to
Speaker 1: James Clerk Maxwell his incredible genius pulled together lots of
Speaker 1: ideas about electricity and magnetism into his unified theory of
Speaker 1: electromagnetism that described light as oscillations of electromagnetic fields. And
Speaker 1: when he pulled all these equations together, he saw the
Speaker 1: equations fit together in a way to describe the oscillations
Speaker 1: of electromagnetic fields moving at a certain speed, a speed
Speaker 1: he could calculate, and that speed came out to be boom,
Speaker 1: exactly the speed of light. What a moment of epiphany
Speaker 1: that must have been for him. He pulls together all
Speaker 1: of this knowledge, he gets new insight, he looks at
Speaker 1: the world in a new way, and then it pops
Speaker 1: out this obvious, amazing prediction that light moves at this
Speaker 1: speed of light, this number that we had already known.
Speaker 1: So what amazing confirmation for him. So that was dominant,
Speaker 1: and people thought, okay, well, light's definitely a wave, right,
Speaker 1: does all these wave like things we have this beautiful theory,
Speaker 1: it's got to be a wave. Okay. So if light
Speaker 1: is a wave, right, we think about it in terms
Speaker 1: of electromagnetic radiation. It's just the waving of the field,
Speaker 1: just the same way sound is waving of the air.
Speaker 1: Different kinds of waves, but that doesn't really matter. And
Speaker 1: the key thing to understand if light is just electromagnetic radiation,
Speaker 1: it's just oscillations of electromagnetic fields. That means they can
Speaker 1: have any value. You can just turn up the intensity
Speaker 1: of the light right to make the light brighter. What
Speaker 1: happens when you make light brighter in the wave theory
Speaker 1: is to just increase how much the waves are shaking, right,
Speaker 1: They're just shaking more so they have more energy. So
Speaker 1: that's sort of the classical theory of electromagnetic radiation of
Speaker 1: light as just these wiggling of the waves that can
Speaker 1: have any value at all. You can turn it up,
Speaker 1: you can turn it down, just the same way you
Speaker 1: can make music louder or softer, and you can have
Speaker 1: essentially any value to that volume. So that was the
Speaker 1: sort of prevailing thinking at the time before the photon
Speaker 1: was discovered. But then, of course, an experiment came along
Speaker 1: that couldn't be explained, and experiment came along that just
Speaker 1: had answers that did not make sense in the wave
Speaker 1: theory of the universe. So we'll dig into what that
Speaker 1: experiment was and how it worked. But first let's take
Speaker 1: a quick break. So we're back and we're talking about
Speaker 1: why we think the photon is a thing. What experiment
Speaker 1: back there in history convinced people that photons had to
Speaker 1: be a particle. And remember that in the context of
Speaker 1: this experiment, light was thought to be a wave. It
Speaker 1: was thought to be electromagnetic radiation, just this oscillation of
Speaker 1: the fields. Somebody essentially shouting in the electromagnetic spectrum, and
Speaker 1: then came along this crazy experiment. The name of the
Speaker 1: experiment is not critical, but what it studied with something
Speaker 1: called the photoelectric effect. Essentially, what you're doing here is
Speaker 1: you're shining a really powerful beam of light at some surface.
Speaker 1: And a surface, of course we know now is made
Speaker 1: out of atoms. And what they observed is that if
Speaker 1: you shone light at a surface, electrons would boil off
Speaker 1: of it. You could pull them off by putting them
Speaker 1: in an electric field, and then you can measure their energy.
Speaker 1: People thought, oh, that's cool, we can boil particles off
Speaker 1: of the surface by shooting light beams at it. What
Speaker 1: would a physicist do in this scenario? She would probably think, Oh,
Speaker 1: let me see what I can do and what happens
Speaker 1: that I turn it up? What happens if I turn
Speaker 1: it down? What happens? So I made the light purple?
Speaker 1: What happens if I make the light green? Right, a
Speaker 1: physicist would want to know if the results make sense
Speaker 1: under all conditions. Sure, maybe we can understand how this
Speaker 1: works in this scenario. But can we push our limits
Speaker 1: of knowledge? Can we find some wrinkle, some corner of
Speaker 1: the space in which it doesn't make sense? That's right,
Speaker 1: experimentalists are always just trying to spoil everything for theorists.
Speaker 1: That's not true at all. Actually, as Jorge would say,
Speaker 1: because every time experimentalists do something and find a result
Speaker 1: that doesn't make sense, that's an amazing clue. That's the
Speaker 1: clue the theorists need to come up with a new
Speaker 1: theory of the universe. Anyway, back to the photoelectric effect.
Speaker 1: What happens when you shine light at the surface Electrons
Speaker 1: come off. Now, if you're thinking of light as electromagnetic waves,
Speaker 1: then what should happen if you turn up the intensity.
Speaker 1: If you turn up the intensity, then electron should shoot
Speaker 1: off with more energy. Because under the classical idea, the
Speaker 1: original idea of light is a wave. Then if you
Speaker 1: turn up the intensity of the light, the strength of
Speaker 1: the light beam, then you're putting more energy. Is just
Speaker 1: electromagnetic waves oscillating with more energy, and so there should
Speaker 1: be more energy there to dump into the electrons, and
Speaker 1: so the electron should boil off with more energy, and
Speaker 1: there should be no dependence on the frequency. You can
Speaker 1: just get the energy out of the electromagnetic waves. It
Speaker 1: doesn't matter how fast they're shaking, as long as the
Speaker 1: energy is there. The energy they're depending just on the intensity.
Speaker 1: So that's the idea. They thought, if we turn up
Speaker 1: the intensity of the light, we make the light brighter,
Speaker 1: then you should get electrons coming off with more energy,
Speaker 1: and there should be no dependence on the color. All right,
Speaker 1: So that's what they thought makes perfect sense. And then
Speaker 1: because their experimentalist, because they actually want to go out
Speaker 1: and explore the universe, not just do thought experiments in
Speaker 1: their head the way the old Greeks did, they went
Speaker 1: out and they actually tried this, and what they found,
Speaker 1: of course, blew their mind. Where they found is two
Speaker 1: things that didn't make any sense at all. First of all,
Speaker 1: the energy the electrons that came off the surface didn't
Speaker 1: depend on the intensity at all. You could turn up
Speaker 1: the intensity and the energy the electrons wouldn't change. You
Speaker 1: could turn down the intensity and the energy the electrons
Speaker 1: wouldn't change. Weirdly, if you turned up the intensity got
Speaker 1: more electrons. You didn't get any electrons with more energy,
Speaker 1: but you got more electrons boiling off. And if you
Speaker 1: made the light dimmer, if you turned down the intensity again,
Speaker 1: the energy didn't change, but the number of electrons dropped.
Speaker 1: And this didn't make any sense at all in the
Speaker 1: classical idea, if light is just a wave, if it's
Speaker 1: just oscillation of the electromagnetic field, then it should depend
Speaker 1: on the intensity, but there was no dependence on the
Speaker 1: intensity at all. Instead, changing the intensity didn't change the
Speaker 1: energy the electrons coming off. It only changed the number
Speaker 1: of electrons we saw. So then they said, ah, that's weird,
Speaker 1: So let's try changing the frequency of the light. So
Speaker 1: they go from blue light down to red light and
Speaker 1: back to purple light and just to see, and they
Speaker 1: found that the energy to electrons, weirdly, did depend on
Speaker 1: the frequency of the light. At higher frequencies, the electrons
Speaker 1: had more energy, and at low enough frequencies you wouldn't
Speaker 1: get any electrons at all. So this made no sense
Speaker 1: to anybody. People who are thinking, who are confident that
Speaker 1: light with just electromagnetic radiation could not explain either of
Speaker 1: these effects. One the fact that the energy to electrons
Speaker 1: didn't depend on the intensity of the radiation, which made
Speaker 1: no sense because they thought these are just classical waves
Speaker 1: and the intensity means more energy, so why aren't we
Speaker 1: getting more energy out of the electrons? And number two
Speaker 1: that the energy the electrons coming off did depend on
Speaker 1: the color of the light. But it made no sense
Speaker 1: to people because people were thinking about light as waves.
Speaker 1: Now there was somebody thinking about light in other terms,
Speaker 1: and that was Plunk. Plunk was studying a totally different problem,
Speaker 1: another unsolved question in physics, which had to do with
Speaker 1: black body radiation, which we'll talk about in another episode,
Speaker 1: and he was trying to solve that problem and he
Speaker 1: just couldn't. He was trying to explain why we didn't
Speaker 1: see in the lab what we expected to see based
Speaker 1: on the theory, and to solve his problem he had
Speaker 1: to come up with a crazy idea. He said, well,
Speaker 1: I don't know why, and I can justify this at all,
Speaker 1: But if I assume whom that light comes in little
Speaker 1: packets of energy that you can have like zero or
Speaker 1: one or two little bits of energy, but you can't
Speaker 1: have into your numbers in between. Then it solves my problem.
Speaker 1: And for him it was sort of a mathematical thing
Speaker 1: because like, I'm trying to do this calculation, it's not working.
Speaker 1: Nobody can figure it out. Oh look if I make
Speaker 1: this totally unjustified assumption that my calculation works and explains
Speaker 1: the data, and that's cool. That's a totally valid way
Speaker 1: to do theory and to do physics. And then you
Speaker 1: got to go back and say, well, what does that mean? Right?
Speaker 1: And it was Einstein who put it together. Einstein heard
Speaker 1: about Plunk's idea, he said, that's fascinating, and he heard
Speaker 1: about the photo electric effect and said, oh, interesting puzzle,
Speaker 1: and he put them together. And so Einstein, who never
Speaker 1: actually won the Nobel Prize for relativity, did win the
Speaker 1: Nobel Prize later for putting these two ideas together. And
Speaker 1: though he didn't do the experiments for the photo electric effect,
Speaker 1: and he also didn't have the original idea to break
Speaker 1: light down into little pieces, he just put the idea
Speaker 1: in the right place to solve the problem and explain
Speaker 1: this experiment. All right, So let's talk about how the
Speaker 1: idea that photons might be little particles, little packets of
Speaker 1: energy explains this experiment. But first, let's take another break.
Speaker 1: All right, we're back and we're talking about why photons
Speaker 1: are a thing. We reminded ourselves why people originally thought
Speaker 1: that photons were waves, and then we talked about the
Speaker 1: photo electric effect. This experiment with a weird result and
Speaker 1: a result that could not be explained using classical theory
Speaker 1: that could not be understood if you thought about light
Speaker 1: as a wave. So how do we explain the photo
Speaker 1: electric effect? How do we understand the weird results of
Speaker 1: this experiment just by saying that light comes in little packets?
Speaker 1: All right, Well, Einstein said, I'm gonna assume that likes
Speaker 1: in these little packets, and that the energy inside one
Speaker 1: packet is proportional to the frequency. That means that higher
Speaker 1: frequencies things like blue, have more energy than photons at
Speaker 1: lower frequencies, things like red. What that means is, if
Speaker 1: you want more energy in your photon, you need purple
Speaker 1: er photons. If you want less energy in your photons,
Speaker 1: you need redder photons. His microscopic understanding, what's happening is
Speaker 1: you have this surface of metal and it's got electrons
Speaker 1: in it, and electrons need a certain amount of energy
Speaker 1: in order to escape. They're bound to their atoms. They're happy,
Speaker 1: they're they're circling the nuclei right, They don't necessarily want
Speaker 1: to leave. In order for them to leave, they have
Speaker 1: to get a certain minimum of energy. So what happens
Speaker 1: when a photon comes and hits the surface. While photon
Speaker 1: hits the electron and either it has enough energy to
Speaker 1: kick the electron off or it doesn't. If it doesn't know,
Speaker 1: electron is kicked off. And what that means is that
Speaker 1: the frequency of the light has to be right high
Speaker 1: enough frequency to have a high enough energy to kick
Speaker 1: off any electrons. And that explains why when they turned
Speaker 1: the frequency down on the light, no matter how bright
Speaker 1: it was, if they turned the color down to deep
Speaker 1: deep red, they just didn't see any electrons coming off.
Speaker 1: And they couldn't explain that with their classical theory. With
Speaker 1: their classical theory, they thought, well, lights a wave, the
Speaker 1: color doesn't matter. We can make it red. As long
Speaker 1: as we make it really really bright, electron should still
Speaker 1: come off. But they didn't. And this theory explains why.
Speaker 1: Because the photons in little chunks, and each electron can
Speaker 1: only absorb energy from one photon at a time, and
Speaker 1: that's the critical idea. You can only interact with one
Speaker 1: photon at a time, so you if the photon doesn't
Speaker 1: have enough energy because it's too low frequency, it's too red,
Speaker 1: then it just can't get you out of your atom trap.
Speaker 1: And you have there are other photons coming down the
Speaker 1: pike if you have a really really intense beam, but
Speaker 1: those don't help because once that first photon has failed
Speaker 1: to get you out of the atom, then your back
Speaker 1: on the atom again, and the next one is also
Speaker 1: going to fail. The photons can't work together. So that's
Speaker 1: the key idea, the fact that the beam of light
Speaker 1: is not just one wave that's shaking the electrons so
Speaker 1: that if you turn it up, you're shaking them more
Speaker 1: and getting them enough energy to get out of those atoms.
Speaker 1: But it's broken up into pieces, and each piece needs
Speaker 1: enough energy on its own to get those electrons out
Speaker 1: of the atom. So the way you do it, the
Speaker 1: way you can get the electrons out of the atom
Speaker 1: is by changing the frequency because that gets more energy
Speaker 1: into each photon. And so if a purple one comes,
Speaker 1: remember purple being very high frequency, it has enough energy
Speaker 1: to get the electrons out of the atom and a
Speaker 1: little bit left over. So as you increase the frequency
Speaker 1: of the light, you're increasing the energy per photon, essentially
Speaker 1: the energy that each electron has access to, and then
Speaker 1: it has enough energy get out of the atom and
Speaker 1: to zoom off with a good amount of speed. So
Speaker 1: the higher the frequency of the light, the more energy
Speaker 1: in each photon, the more energy these electrons come out at.
Speaker 1: And that is exactly what they saw in the experiment,
Speaker 1: and that can only be explained if electrons can only
Speaker 1: interact with one particle of light at a time, and
Speaker 1: the light is in fact a particle. It also explains
Speaker 1: why the energy of electrons does not depend on the
Speaker 1: intensity of the beam. You can have a really powerful
Speaker 1: red beam, but it's too low frequency. All those photons
Speaker 1: are wasted because none of them have enough energy to
Speaker 1: get the electrons out. It doesn't matter how high you
Speaker 1: turn it up. And even if you're turn it up
Speaker 1: to green and you have enough energy to get the
Speaker 1: electrons out of there, you don't get more energetic electrons
Speaker 1: by increasing the intensity. Again, you have to change the
Speaker 1: energy in each photon. That's hitting the electron. You can
Speaker 1: only do that by changing the frequency. And this assumes
Speaker 1: again that electrons can only interact with one photon at
Speaker 1: a time, which is pretty solid assumption. So the amazing
Speaker 1: thing is that this idea, which really came from Plank,
Speaker 1: explain these experiments which really were done by other people.
Speaker 1: But the unification of it, the bringing together the idea,
Speaker 1: the moment of inside the explanation of this weird experiment,
Speaker 1: was done by Einstein. And that's what Einstein won the
Speaker 1: Nobel Prize for, not for doing the experiment, not for
Speaker 1: having the idea, but for being sort of in the
Speaker 1: right place at the right time to bring that idea
Speaker 1: to solve this open problem. Now, the photon was not
Speaker 1: named as a particle for decades later. All of this
Speaker 1: happened just around the turn of the nineteenth century, and
Speaker 1: Einstein won the Nobel Prize later for it, but it
Speaker 1: wasn't until nineteen twenty six that people started calling these
Speaker 1: things photons. And it comes from the Greek word for light.
Speaker 1: But it also touches on something I think is really interesting,
Speaker 1: which is the sort of concept of a particle. I
Speaker 1: like to imagine what we're physicists thinking back then, what
Speaker 1: did they think that the universe looked like at a
Speaker 1: microscopic scale, Because to us, the notion of a particle
Speaker 1: is kind of familiar. I mean, they're weird, they do
Speaker 1: things that we don't understand. They follow rules and make
Speaker 1: no sense to us. But we're comfortable with the idea
Speaker 1: that the universe is atomic, meaning that's made up of
Speaker 1: little bit it and all we have to do is
Speaker 1: sort of figure out what those bids do. But at
Speaker 1: the time, this whole concept of a particle was kind
Speaker 1: of new. Remember where they had discovered the electron. That
Speaker 1: was only recently. That was the first piece of evidence
Speaker 1: that there was something as a particle. Sort of the
Speaker 1: invention of the concept of a particle was the discovery
Speaker 1: of the electron. And all he really did there was
Speaker 1: identify something tiny that had both mass and charge, and
Speaker 1: so he said, oh, look there's a thing there as
Speaker 1: these two attributes. I'm going to call it a particle.
Speaker 1: Actually he called it a corpus skule. But the concept
Speaker 1: of intellectual groundwork was laid then for a particle. So
Speaker 1: then you get to the photon. Now the photon has energy,
Speaker 1: it has direction, but it doesn't have mass. It's not
Speaker 1: a thing in that sense, there's no stuff to it,
Speaker 1: So that immediately sort of bends your mind around what
Speaker 1: is this concept of a particle. Anyway, we've created this
Speaker 1: idea to accommodate the discovery the electron. We hope, oh,
Speaker 1: maybe there are other particles, And later on the podcasts
Speaker 1: will take a tour of the discoveries of other particles,
Speaker 1: which have hilarious and amazing and dramatic stories to them.
Speaker 1: But very early in the history of particles we had
Speaker 1: to already bend the rules and say, oh, well, we
Speaker 1: were talking about particles is a little bits of stuff.
Speaker 1: But they can also be not stuff, right, they can
Speaker 1: also just be energy. And so to me, it's amazing
Speaker 1: that this field of particle physics was founded on such
Speaker 1: crazy discoveries. So to me, it's wonderful that the field
Speaker 1: of particle physics is founded on such crazy discoveries. And
Speaker 1: you've got to give a lot of credit to the theorists,
Speaker 1: of course, who put these ideas together and helped us
Speaker 1: understand what we were seeing. But to me, the most
Speaker 1: exciting moments are those moments of experimental surprise when the
Speaker 1: universe does something that we don't understand when the unit,
Speaker 1: when we predict the universe will do a and instead
Speaker 1: it does be because that's the universe talking to us,
Speaker 1: or that's the universe answering our questions, that's the universe
Speaker 1: being the subject of our interrogation when we say we
Speaker 1: want to know how this works, prove it to us,
Speaker 1: or reveal to us the underlying mechanism. And that's what
Speaker 1: experimental physics is about. Is about cornering the universe and
Speaker 1: forcing it to reveal something new to you. And a
Speaker 1: lot of times that revelation happens when you didn't expect that.
Speaker 1: You thought, oh, we're just double checking this over here.
Speaker 1: We're pretty sure we understand it. Just dotting the eyes
Speaker 1: and crossing the teas and all of a sudden, oops,
Speaker 1: you get something totally surprising. But those are the moments
Speaker 1: that we learned something new about the universe, And those
Speaker 1: are the moments I'm striving for my own personal research.
Speaker 1: When I'm smashing particles together at the LHC. We think
Speaker 1: we understand what's going to happen, but I'm always secretly
Speaker 1: hoping that a student will come to me and say, hey, Daniel,
Speaker 1: what's this. I found this weird thing in our data
Speaker 1: that just doesn't make any sense, and that's only happened
Speaker 1: once or twice in my entire career, and I look
Speaker 1: forward to it happening again. So maybe one day we'll
Speaker 1: be hearing about a crazy discovery we made at the
Speaker 1: Large Adeon Collider. Until then, thanks for listening to this
Speaker 1: description of how we know the photon is a thing,
Speaker 1: and please, if you're interested in learning more about the
Speaker 1: history of physics or understanding how we know how the
Speaker 1: universe works and what we don't know, please send me
Speaker 1: a suggestion to feedback at Daniel and Jorge dot com.
Speaker 1: Thanks for tuning in. If you still have a question
Speaker 1: after listening to all these explanations, please drop us a line.
Speaker 1: We'd love to hear from you. You can find us
Speaker 1: at Facebook, Twitter, and Instagram at Daniel and Jorge That's
Speaker 1: one word, or email us at Feedback at Daniel and
Speaker 1: Jorge dot com. Thanks for listening, and remember that Daniel
Speaker 1: and Jorge Explain the Universe is a production of I
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