Latest / Daniel and Kelly’s Extraordinary Universe / How do we know the photon is real?

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How do we know the photon is real? · Daniel and Kelly’s Extraordinary Universe

0:09
Speaker 1: Hey, they're listeners. You're into physics. So here's a tribua
0:13
Speaker 1: question for you. Do you know who won a Nobel
0:16
Speaker 1: Prize for relativity? That might feel like it's your question
0:19
Speaker 1: because you want to say Einstein, because they think relativity
0:22
Speaker 1: and Einstein will I'll tell you it's not Einstein. Now,
0:26
Speaker 1: maybe you're scrambling through your mind to think about the
0:28
Speaker 1: names of other physicists. You might know. How many physicists
0:31
Speaker 1: can you name? Anyway, you got Einstein, you got me. Well,
0:36
Speaker 1: I'll give you a clue. It's neither Einstein nor me.
0:39
Speaker 1: So who was it? Right? Well, some folks wanted for
0:43
Speaker 1: proving that relativity was correct. There were Nobel Prizes for
0:47
Speaker 1: gravitational waves and for binary pulsars. But the answer is
0:51
Speaker 1: that nobody wanted for relativity. Nobody who came up with
0:55
Speaker 1: this incredible earth shattering idea that now frames all of
0:59
Speaker 1: modern physics won the Nobel Prize for it. But you
1:03
Speaker 1: might be thinking, hold on, didn't Einstein win a Nobel Prize?
1:06
Speaker 1: And he did, but he wanted essentially for quantum mechanics.
1:26
Speaker 1: Hello everyone, I'm Daniel. I'm a particle physicist, and I'm
1:30
Speaker 1: the co host of this podcast together with Korge cham
1:34
Speaker 1: who can't be here this week, So you're just listening
1:37
Speaker 1: to me talking about the joys of physics and trying
1:40
Speaker 1: to simulate Jorge in my mind. Every time i'm talking,
1:43
Speaker 1: I'm thinking, here's what Jorge would say. At this moment,
1:46
Speaker 1: I'm trying to interject a little Jorge Ism for you,
1:49
Speaker 1: since we all miss him and you were listening to
1:52
Speaker 1: our podcast, Daniel and Jorge Explain the Universe, a production
1:57
Speaker 1: of I Heart Radio in which we zoom all around
2:00
Speaker 1: the universe and try to find interesting, fascinating, cool little
2:03
Speaker 1: nuggets of physics that would blow your mind, but take
2:06
Speaker 1: them apart so they don't actually explode your head and
2:09
Speaker 1: cause your brains displatter anywhere. Instead, we want to smoothly
2:13
Speaker 1: and calmly insert them into your mind so you understand them,
2:16
Speaker 1: so you can talk to your friends about them, so
2:18
Speaker 1: you can actually comprehend these amazing, wonderful facts that we
2:22
Speaker 1: have learned about the universe, and also understand all the
2:26
Speaker 1: things we don't know about the universe, which is my
2:29
Speaker 1: favorite part of physics, and that's why Jorge and I
2:32
Speaker 1: wrote the book We Have No Idea, A Guide to
2:35
Speaker 1: the Unknown universe, which takes you on an amazing tour
2:37
Speaker 1: of all the big and basic questions about the universe
2:41
Speaker 1: that we still have no idea what the answers are.
2:43
Speaker 1: And on the podcast, we've been doing something fun, which
2:46
Speaker 1: is taking a little tour of how we know what
2:49
Speaker 1: we know, and specifically how we know anything about particle physics.
2:53
Speaker 1: It's still incredible to me when I look around at
2:56
Speaker 1: the world that everything is made out of these tiny
2:58
Speaker 1: microscopic object so we can't see that we've taken thousands
3:02
Speaker 1: of years to even discover that they exist. Yet we
3:05
Speaker 1: have this really complex, really elaborate, really amazingly effective model
3:11
Speaker 1: of what's happening down there at the microscopic scale, all
3:14
Speaker 1: these tiny quantum particles interacting and zooming around. Physicists can
3:18
Speaker 1: do calculations to tell you exactly what's going to happen
3:21
Speaker 1: when this particle hits that particle. It's really incredibly complex
3:25
Speaker 1: and mature. Though of course we have lots of questions,
3:28
Speaker 1: but I think a lot of times people think of
3:30
Speaker 1: this as sort of like an idea, something people came
3:33
Speaker 1: up with a description of the universe. But it's critical
3:37
Speaker 1: that everybody understand that this isn't just an idea that
3:40
Speaker 1: came out of our heads. This is something born out
3:43
Speaker 1: of desperation. This is our attempt to grapple with the
3:47
Speaker 1: weird and bizarre and counterintuitive and frankly mind blowing experiments
3:53
Speaker 1: that have shattered our perceptions of reality. We thought the
3:56
Speaker 1: universe worked a certain way. We thought everything was smooth,
4:00
Speaker 1: You could cut objects as many times as you wanted
4:03
Speaker 1: to infinitely small pieces, but you can't. We thought the
4:06
Speaker 1: universe was deterministic, that if you did the same experiment
4:09
Speaker 1: twice you would get the same outcome. Right, that would
4:12
Speaker 1: make sense, But it's not. It's fundamentally random. And the
4:15
Speaker 1: core of that is particle physics, because it attempts to
4:18
Speaker 1: describe the entire universe in terms of these tiny, weird,
4:22
Speaker 1: nondeterministic little particles, in terms of these tiny, little, weird,
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Speaker 1: nondeterministic particles that seem to follow rules that just do
4:30
Speaker 1: not describe the world that we are familiar with. So
4:34
Speaker 1: my goal is to take you on a tour of
4:36
Speaker 1: those experiments, the ones that change the way we think
4:38
Speaker 1: about the universe, that showed us that the universe is
4:41
Speaker 1: different from what we imagined, because it's not just the
4:44
Speaker 1: final idea that you want that I want you to understand.
4:47
Speaker 1: I want you to know what the evidence is. How
4:49
Speaker 1: do we know what we know? Now? Recently we talked
4:53
Speaker 1: about the discovery of the first particle, the first experiment
4:56
Speaker 1: that revealed this incredible revelation that the Uni verse is
5:00
Speaker 1: made out of time to little dots. And so today
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Speaker 1: we are continuing that tour. We are talking about how
5:12
Speaker 1: do we know the photon is a thing? You're familiar
5:15
Speaker 1: with photons? To you, photon is a very normal word
5:18
Speaker 1: you hear bandied about here, talked about but how do
5:21
Speaker 1: we know that photons are there? How do we know
5:24
Speaker 1: that light is made out of photons that is chopped
5:26
Speaker 1: up into these little pieces that can't be cut down
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Speaker 1: even further. What is the actual experiment that proves to
5:32
Speaker 1: us that photons are a thing, that light is not
5:35
Speaker 1: just electromagnetic waves, but it does these other weird things
5:39
Speaker 1: that you have to give it particle status to explain. So,
5:42
Speaker 1: as usual, I was wondering how many people out there
5:44
Speaker 1: know why we think the photon is a thing, Why
5:47
Speaker 1: we don't just think about light as electromagnetic waves. So
5:50
Speaker 1: I walked around the campus of U see Irvine and
5:53
Speaker 1: accosted a bunch of friendly and unsuspecting students, and I
5:57
Speaker 1: asked them, do you know how the photon was discovered?
6:00
Speaker 1: Do you have an idea of why we think the
6:02
Speaker 1: photon is a thing. So before you listen to these answers,
6:06
Speaker 1: think to yourself, or pause the podcast, or just take
6:08
Speaker 1: a moment. How do you know photons are a thing?
6:11
Speaker 1: Are you just believing physicists when they tell you, or
6:14
Speaker 1: do you know what the data says? I'm not entirely sure.
6:18
Speaker 1: I feel I shouldn't have that. I I'm sorry, I
6:21
Speaker 1: probably should know. But it was the slit experiments, wasn't it.
6:27
Speaker 1: And they projected a laser beam onto a single slit
6:32
Speaker 1: or double slits and it diffracted the beam and that's
6:35
Speaker 1: how they discovered it. Particle wave duality. Yeah, the photo
6:38
Speaker 1: electrical facts. You shone a light on a metal and
6:41
Speaker 1: then the metal you cross. Uh, you start for Einstein ninth.
6:46
Speaker 1: I don't remember the year. Yeah, I don't remember who
6:48
Speaker 1: did it, but I remember that you shine a light
6:51
Speaker 1: on a metal, do you give the electron enough energy
6:54
Speaker 1: to start conducting it's particles? Well, we know it's a
6:59
Speaker 1: wave because it travels through vacuum, and we know that
7:02
Speaker 1: it's a particle because you can transfer energy from it. Right, Yeah,
7:07
Speaker 1: it has it hasn't defined momentum, even though it has
7:12
Speaker 1: no mass. But the slit experiment double slip one showed
7:16
Speaker 1: that it was a wave, like a single slit showed
7:19
Speaker 1: that a particle. Well, it's not necessarily a particle. It's
7:25
Speaker 1: both a particle and a wave. And for a really
7:28
Speaker 1: long time we thought it was just a wave. But
7:31
Speaker 1: I believe the first time we figured out that it
7:36
Speaker 1: was a particle had to do um exciting metals to
7:43
Speaker 1: release photons and realized that the distributions were discreet. So
7:49
Speaker 1: I was really impressed with these answers. A lot of
7:52
Speaker 1: understanding here that photons are particles and that they're part
7:56
Speaker 1: of this larger idea of light being a wave and
7:59
Speaker 1: a particle. Even some discussion of the double slit experiment,
8:02
Speaker 1: which I'm dying to get into in a future podcast
8:05
Speaker 1: and talk all about the amazing facts of quantum mechanics.
8:08
Speaker 1: But the double slit experiment actually shows you that the
8:10
Speaker 1: photon is a wave. But there was somebody out there
8:13
Speaker 1: who talked about the photoelectric effect, and that's the key
8:17
Speaker 1: that was the experiment that showed us that photons were
8:20
Speaker 1: a thing. But before we talk about the crazy experiment
8:23
Speaker 1: to prove that quantum mechanics is our reality, that showed
8:27
Speaker 1: us that the universe is probably sliced up into little
8:29
Speaker 1: bits and not infinitely smooth. Let's set the stage. Okay,
8:33
Speaker 1: let's you remember how people thought about light, and to
8:36
Speaker 1: get the context of the story, you have to rewind
8:38
Speaker 1: all the way back to Isaac Newton. Isaac Newton, of
8:41
Speaker 1: course very famous not just for the cookies, but also
8:44
Speaker 1: for his discovery of his theory of gravity, which unified
8:48
Speaker 1: motion of objects here on Earth with motion of objects
8:51
Speaker 1: in the heavens. Really gave us access to the whole
8:53
Speaker 1: universe to imagine, Wow, maybe physics can actually describe things
8:58
Speaker 1: not just here in front of us, but out there
9:00
Speaker 1: in the universe. Those are things out there that follow
9:02
Speaker 1: laws of physics. Incredible accomplishments. But Newton also also made
9:07
Speaker 1: amazing discoveries in the field of optics. He spent a
9:11
Speaker 1: lot of time with lenses and with prisms, and he
9:14
Speaker 1: was convinced that light was a particle, and he thought
9:17
Speaker 1: a lot about how light traveled. He saw it moving
9:19
Speaker 1: in straight lines, except when he was bent by these
9:21
Speaker 1: lenses and he was convinced that light was a particle.
9:23
Speaker 1: And because he was a genius and he's a staggering
9:27
Speaker 1: influence on the field of physics, people listen to him
9:30
Speaker 1: and rightly so. And for hundreds of years people were
9:33
Speaker 1: convinced that it was a particle, even though other folks
9:36
Speaker 1: had really nice theories of light as a wave. And
9:39
Speaker 1: it wasn't until the eighteen hundreds when people started observing,
9:43
Speaker 1: like doing things that particles couldn't do, that they had
9:46
Speaker 1: to adapt their mindset. And that's the key. There you
9:49
Speaker 1: see experiment rearing its uncomfortable head again saying, oh no, no,
9:54
Speaker 1: you thought you understood the universe. You have an idea
9:57
Speaker 1: in your mind, you have a mental model of how
9:59
Speaker 1: this is working, but it can't describe what's actually happening.
10:03
Speaker 1: And that's why I'm an experimentalist. That's why I think
10:06
Speaker 1: experiment is the place to be, because experimentalists are the
10:09
Speaker 1: ones who make the discoveries. They are on the forefront
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Speaker 1: of knowledge. They're out there exploring the universe, discovering things
10:16
Speaker 1: that don't make sense. Theorists, of course, do an incredible job.
10:19
Speaker 1: They tied all together they understand they predict future phenomenon.
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Speaker 1: But for me, the bit about physics that's wonderful is
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Speaker 1: the experimental side, is making those discoveries, is asking nature
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Speaker 1: a question and demanding an answer, pinning nature in a
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Speaker 1: corner so that nature has to tell you is the
10:35
Speaker 1: universe this way or that way? And so the thing
10:38
Speaker 1: that told people that photons couldn't just be a particle
10:42
Speaker 1: were wave like effects, things like interference. And you're familiar
10:46
Speaker 1: with interference, maybe you have noise canceling headphones. Now is
10:50
Speaker 1: canceling headphones work via interference? Sound is a wave. It's
10:54
Speaker 1: a shaking of air, and the air comes towards your head,
10:58
Speaker 1: and if you can create waves shake in the other
11:00
Speaker 1: direction at the same time, they basically cancel out those
11:04
Speaker 1: waves that are coming in your head. So sound canceling
11:06
Speaker 1: headphones are proof that sound is a wave because they
11:09
Speaker 1: can do this wave like thing that particles just cannot do.
11:12
Speaker 1: In the same way, people saw light behaving in a
11:15
Speaker 1: way that could only be described by a wave, and
11:18
Speaker 1: so you had interference effects, and you had all sorts
11:20
Speaker 1: of theories sort of built momentum until you get to
11:23
Speaker 1: James Clerk Maxwell his incredible genius pulled together lots of
11:27
Speaker 1: ideas about electricity and magnetism into his unified theory of
11:31
Speaker 1: electromagnetism that described light as oscillations of electromagnetic fields. And
11:37
Speaker 1: when he pulled all these equations together, he saw the
11:39
Speaker 1: equations fit together in a way to describe the oscillations
11:42
Speaker 1: of electromagnetic fields moving at a certain speed, a speed
11:45
Speaker 1: he could calculate, and that speed came out to be boom,
11:48
Speaker 1: exactly the speed of light. What a moment of epiphany
11:52
Speaker 1: that must have been for him. He pulls together all
11:54
Speaker 1: of this knowledge, he gets new insight, he looks at
11:56
Speaker 1: the world in a new way, and then it pops
11:59
Speaker 1: out this obvious, amazing prediction that light moves at this
12:02
Speaker 1: speed of light, this number that we had already known.
12:06
Speaker 1: So what amazing confirmation for him. So that was dominant,
12:09
Speaker 1: and people thought, okay, well, light's definitely a wave, right,
12:12
Speaker 1: does all these wave like things we have this beautiful theory,
12:14
Speaker 1: it's got to be a wave. Okay. So if light
12:18
Speaker 1: is a wave, right, we think about it in terms
12:20
Speaker 1: of electromagnetic radiation. It's just the waving of the field,
12:23
Speaker 1: just the same way sound is waving of the air.
12:26
Speaker 1: Different kinds of waves, but that doesn't really matter. And
12:29
Speaker 1: the key thing to understand if light is just electromagnetic radiation,
12:33
Speaker 1: it's just oscillations of electromagnetic fields. That means they can
12:38
Speaker 1: have any value. You can just turn up the intensity
12:42
Speaker 1: of the light right to make the light brighter. What
12:44
Speaker 1: happens when you make light brighter in the wave theory
12:47
Speaker 1: is to just increase how much the waves are shaking, right,
12:49
Speaker 1: They're just shaking more so they have more energy. So
12:52
Speaker 1: that's sort of the classical theory of electromagnetic radiation of
12:55
Speaker 1: light as just these wiggling of the waves that can
12:58
Speaker 1: have any value at all. You can turn it up,
13:00
Speaker 1: you can turn it down, just the same way you
13:02
Speaker 1: can make music louder or softer, and you can have
13:05
Speaker 1: essentially any value to that volume. So that was the
13:08
Speaker 1: sort of prevailing thinking at the time before the photon
13:10
Speaker 1: was discovered. But then, of course, an experiment came along
13:15
Speaker 1: that couldn't be explained, and experiment came along that just
13:18
Speaker 1: had answers that did not make sense in the wave
13:21
Speaker 1: theory of the universe. So we'll dig into what that
13:24
Speaker 1: experiment was and how it worked. But first let's take
13:27
Speaker 1: a quick break. So we're back and we're talking about
13:43
Speaker 1: why we think the photon is a thing. What experiment
13:46
Speaker 1: back there in history convinced people that photons had to
13:49
Speaker 1: be a particle. And remember that in the context of
13:52
Speaker 1: this experiment, light was thought to be a wave. It
13:55
Speaker 1: was thought to be electromagnetic radiation, just this oscillation of
13:58
Speaker 1: the fields. Somebody essentially shouting in the electromagnetic spectrum, and
14:02
Speaker 1: then came along this crazy experiment. The name of the
14:05
Speaker 1: experiment is not critical, but what it studied with something
14:08
Speaker 1: called the photoelectric effect. Essentially, what you're doing here is
14:12
Speaker 1: you're shining a really powerful beam of light at some surface.
14:17
Speaker 1: And a surface, of course we know now is made
14:19
Speaker 1: out of atoms. And what they observed is that if
14:21
Speaker 1: you shone light at a surface, electrons would boil off
14:24
Speaker 1: of it. You could pull them off by putting them
14:27
Speaker 1: in an electric field, and then you can measure their energy.
14:30
Speaker 1: People thought, oh, that's cool, we can boil particles off
14:33
Speaker 1: of the surface by shooting light beams at it. What
14:36
Speaker 1: would a physicist do in this scenario? She would probably think, Oh,
14:39
Speaker 1: let me see what I can do and what happens
14:41
Speaker 1: that I turn it up? What happens if I turn
14:42
Speaker 1: it down? What happens? So I made the light purple?
14:44
Speaker 1: What happens if I make the light green? Right, a
14:47
Speaker 1: physicist would want to know if the results make sense
14:50
Speaker 1: under all conditions. Sure, maybe we can understand how this
14:53
Speaker 1: works in this scenario. But can we push our limits
14:56
Speaker 1: of knowledge? Can we find some wrinkle, some corner of
14:59
Speaker 1: the space in which it doesn't make sense? That's right,
15:02
Speaker 1: experimentalists are always just trying to spoil everything for theorists.
15:05
Speaker 1: That's not true at all. Actually, as Jorge would say,
15:08
Speaker 1: because every time experimentalists do something and find a result
15:12
Speaker 1: that doesn't make sense, that's an amazing clue. That's the
15:15
Speaker 1: clue the theorists need to come up with a new
15:18
Speaker 1: theory of the universe. Anyway, back to the photoelectric effect.
15:22
Speaker 1: What happens when you shine light at the surface Electrons
15:25
Speaker 1: come off. Now, if you're thinking of light as electromagnetic waves,
15:29
Speaker 1: then what should happen if you turn up the intensity.
15:33
Speaker 1: If you turn up the intensity, then electron should shoot
15:36
Speaker 1: off with more energy. Because under the classical idea, the
15:39
Speaker 1: original idea of light is a wave. Then if you
15:42
Speaker 1: turn up the intensity of the light, the strength of
15:44
Speaker 1: the light beam, then you're putting more energy. Is just
15:47
Speaker 1: electromagnetic waves oscillating with more energy, and so there should
15:51
Speaker 1: be more energy there to dump into the electrons, and
15:54
Speaker 1: so the electron should boil off with more energy, and
15:57
Speaker 1: there should be no dependence on the frequency. You can
16:00
Speaker 1: just get the energy out of the electromagnetic waves. It
16:03
Speaker 1: doesn't matter how fast they're shaking, as long as the
16:05
Speaker 1: energy is there. The energy they're depending just on the intensity.
16:09
Speaker 1: So that's the idea. They thought, if we turn up
16:12
Speaker 1: the intensity of the light, we make the light brighter,
16:15
Speaker 1: then you should get electrons coming off with more energy,
16:18
Speaker 1: and there should be no dependence on the color. All right,
16:21
Speaker 1: So that's what they thought makes perfect sense. And then
16:24
Speaker 1: because their experimentalist, because they actually want to go out
16:26
Speaker 1: and explore the universe, not just do thought experiments in
16:29
Speaker 1: their head the way the old Greeks did, they went
16:32
Speaker 1: out and they actually tried this, and what they found,
16:35
Speaker 1: of course, blew their mind. Where they found is two
16:38
Speaker 1: things that didn't make any sense at all. First of all,
16:43
Speaker 1: the energy the electrons that came off the surface didn't
16:46
Speaker 1: depend on the intensity at all. You could turn up
16:49
Speaker 1: the intensity and the energy the electrons wouldn't change. You
16:52
Speaker 1: could turn down the intensity and the energy the electrons
16:55
Speaker 1: wouldn't change. Weirdly, if you turned up the intensity got
17:00
Speaker 1: more electrons. You didn't get any electrons with more energy,
17:03
Speaker 1: but you got more electrons boiling off. And if you
17:07
Speaker 1: made the light dimmer, if you turned down the intensity again,
17:10
Speaker 1: the energy didn't change, but the number of electrons dropped.
17:13
Speaker 1: And this didn't make any sense at all in the
17:15
Speaker 1: classical idea, if light is just a wave, if it's
17:18
Speaker 1: just oscillation of the electromagnetic field, then it should depend
17:22
Speaker 1: on the intensity, but there was no dependence on the
17:24
Speaker 1: intensity at all. Instead, changing the intensity didn't change the
17:28
Speaker 1: energy the electrons coming off. It only changed the number
17:31
Speaker 1: of electrons we saw. So then they said, ah, that's weird,
17:35
Speaker 1: So let's try changing the frequency of the light. So
17:38
Speaker 1: they go from blue light down to red light and
17:40
Speaker 1: back to purple light and just to see, and they
17:42
Speaker 1: found that the energy to electrons, weirdly, did depend on
17:46
Speaker 1: the frequency of the light. At higher frequencies, the electrons
17:49
Speaker 1: had more energy, and at low enough frequencies you wouldn't
17:53
Speaker 1: get any electrons at all. So this made no sense
17:56
Speaker 1: to anybody. People who are thinking, who are confident that
17:59
Speaker 1: light with just electromagnetic radiation could not explain either of
18:03
Speaker 1: these effects. One the fact that the energy to electrons
18:07
Speaker 1: didn't depend on the intensity of the radiation, which made
18:10
Speaker 1: no sense because they thought these are just classical waves
18:13
Speaker 1: and the intensity means more energy, so why aren't we
18:15
Speaker 1: getting more energy out of the electrons? And number two
18:18
Speaker 1: that the energy the electrons coming off did depend on
18:21
Speaker 1: the color of the light. But it made no sense
18:24
Speaker 1: to people because people were thinking about light as waves.
18:28
Speaker 1: Now there was somebody thinking about light in other terms,
18:31
Speaker 1: and that was Plunk. Plunk was studying a totally different problem,
18:35
Speaker 1: another unsolved question in physics, which had to do with
18:38
Speaker 1: black body radiation, which we'll talk about in another episode,
18:42
Speaker 1: and he was trying to solve that problem and he
18:44
Speaker 1: just couldn't. He was trying to explain why we didn't
18:46
Speaker 1: see in the lab what we expected to see based
18:49
Speaker 1: on the theory, and to solve his problem he had
18:52
Speaker 1: to come up with a crazy idea. He said, well,
18:54
Speaker 1: I don't know why, and I can justify this at all,
18:58
Speaker 1: But if I assume whom that light comes in little
19:03
Speaker 1: packets of energy that you can have like zero or
19:06
Speaker 1: one or two little bits of energy, but you can't
19:09
Speaker 1: have into your numbers in between. Then it solves my problem.
19:12
Speaker 1: And for him it was sort of a mathematical thing
19:14
Speaker 1: because like, I'm trying to do this calculation, it's not working.
19:17
Speaker 1: Nobody can figure it out. Oh look if I make
19:20
Speaker 1: this totally unjustified assumption that my calculation works and explains
19:24
Speaker 1: the data, and that's cool. That's a totally valid way
19:26
Speaker 1: to do theory and to do physics. And then you
19:29
Speaker 1: got to go back and say, well, what does that mean? Right?
19:32
Speaker 1: And it was Einstein who put it together. Einstein heard
19:35
Speaker 1: about Plunk's idea, he said, that's fascinating, and he heard
19:38
Speaker 1: about the photo electric effect and said, oh, interesting puzzle,
19:41
Speaker 1: and he put them together. And so Einstein, who never
19:45
Speaker 1: actually won the Nobel Prize for relativity, did win the
19:48
Speaker 1: Nobel Prize later for putting these two ideas together. And
19:52
Speaker 1: though he didn't do the experiments for the photo electric effect,
19:55
Speaker 1: and he also didn't have the original idea to break
19:58
Speaker 1: light down into little pieces, he just put the idea
20:01
Speaker 1: in the right place to solve the problem and explain
20:04
Speaker 1: this experiment. All right, So let's talk about how the
20:07
Speaker 1: idea that photons might be little particles, little packets of
20:11
Speaker 1: energy explains this experiment. But first, let's take another break.
20:28
Speaker 1: All right, we're back and we're talking about why photons
20:30
Speaker 1: are a thing. We reminded ourselves why people originally thought
20:35
Speaker 1: that photons were waves, and then we talked about the
20:37
Speaker 1: photo electric effect. This experiment with a weird result and
20:40
Speaker 1: a result that could not be explained using classical theory
20:44
Speaker 1: that could not be understood if you thought about light
20:48
Speaker 1: as a wave. So how do we explain the photo
20:50
Speaker 1: electric effect? How do we understand the weird results of
20:53
Speaker 1: this experiment just by saying that light comes in little packets?
20:56
Speaker 1: All right, Well, Einstein said, I'm gonna assume that likes
21:00
Speaker 1: in these little packets, and that the energy inside one
21:03
Speaker 1: packet is proportional to the frequency. That means that higher
21:07
Speaker 1: frequencies things like blue, have more energy than photons at
21:11
Speaker 1: lower frequencies, things like red. What that means is, if
21:15
Speaker 1: you want more energy in your photon, you need purple
21:18
Speaker 1: er photons. If you want less energy in your photons,
21:20
Speaker 1: you need redder photons. His microscopic understanding, what's happening is
21:24
Speaker 1: you have this surface of metal and it's got electrons
21:27
Speaker 1: in it, and electrons need a certain amount of energy
21:30
Speaker 1: in order to escape. They're bound to their atoms. They're happy,
21:33
Speaker 1: they're they're circling the nuclei right, They don't necessarily want
21:36
Speaker 1: to leave. In order for them to leave, they have
21:38
Speaker 1: to get a certain minimum of energy. So what happens
21:41
Speaker 1: when a photon comes and hits the surface. While photon
21:45
Speaker 1: hits the electron and either it has enough energy to
21:49
Speaker 1: kick the electron off or it doesn't. If it doesn't know,
21:51
Speaker 1: electron is kicked off. And what that means is that
21:54
Speaker 1: the frequency of the light has to be right high
21:57
Speaker 1: enough frequency to have a high enough energy to kick
22:00
Speaker 1: off any electrons. And that explains why when they turned
22:03
Speaker 1: the frequency down on the light, no matter how bright
22:06
Speaker 1: it was, if they turned the color down to deep
22:08
Speaker 1: deep red, they just didn't see any electrons coming off.
22:12
Speaker 1: And they couldn't explain that with their classical theory. With
22:14
Speaker 1: their classical theory, they thought, well, lights a wave, the
22:16
Speaker 1: color doesn't matter. We can make it red. As long
22:19
Speaker 1: as we make it really really bright, electron should still
22:22
Speaker 1: come off. But they didn't. And this theory explains why.
22:26
Speaker 1: Because the photons in little chunks, and each electron can
22:29
Speaker 1: only absorb energy from one photon at a time, and
22:33
Speaker 1: that's the critical idea. You can only interact with one
22:36
Speaker 1: photon at a time, so you if the photon doesn't
22:39
Speaker 1: have enough energy because it's too low frequency, it's too red,
22:43
Speaker 1: then it just can't get you out of your atom trap.
22:45
Speaker 1: And you have there are other photons coming down the
22:47
Speaker 1: pike if you have a really really intense beam, but
22:50
Speaker 1: those don't help because once that first photon has failed
22:53
Speaker 1: to get you out of the atom, then your back
22:55
Speaker 1: on the atom again, and the next one is also
22:57
Speaker 1: going to fail. The photons can't work together. So that's
23:01
Speaker 1: the key idea, the fact that the beam of light
23:03
Speaker 1: is not just one wave that's shaking the electrons so
23:06
Speaker 1: that if you turn it up, you're shaking them more
23:08
Speaker 1: and getting them enough energy to get out of those atoms.
23:11
Speaker 1: But it's broken up into pieces, and each piece needs
23:14
Speaker 1: enough energy on its own to get those electrons out
23:17
Speaker 1: of the atom. So the way you do it, the
23:20
Speaker 1: way you can get the electrons out of the atom
23:22
Speaker 1: is by changing the frequency because that gets more energy
23:25
Speaker 1: into each photon. And so if a purple one comes,
23:29
Speaker 1: remember purple being very high frequency, it has enough energy
23:33
Speaker 1: to get the electrons out of the atom and a
23:35
Speaker 1: little bit left over. So as you increase the frequency
23:38
Speaker 1: of the light, you're increasing the energy per photon, essentially
23:42
Speaker 1: the energy that each electron has access to, and then
23:45
Speaker 1: it has enough energy get out of the atom and
23:47
Speaker 1: to zoom off with a good amount of speed. So
23:49
Speaker 1: the higher the frequency of the light, the more energy
23:52
Speaker 1: in each photon, the more energy these electrons come out at.
23:55
Speaker 1: And that is exactly what they saw in the experiment,
23:58
Speaker 1: and that can only be explained if electrons can only
24:01
Speaker 1: interact with one particle of light at a time, and
24:05
Speaker 1: the light is in fact a particle. It also explains
24:08
Speaker 1: why the energy of electrons does not depend on the
24:11
Speaker 1: intensity of the beam. You can have a really powerful
24:14
Speaker 1: red beam, but it's too low frequency. All those photons
24:17
Speaker 1: are wasted because none of them have enough energy to
24:20
Speaker 1: get the electrons out. It doesn't matter how high you
24:23
Speaker 1: turn it up. And even if you're turn it up
24:25
Speaker 1: to green and you have enough energy to get the
24:28
Speaker 1: electrons out of there, you don't get more energetic electrons
24:31
Speaker 1: by increasing the intensity. Again, you have to change the
24:35
Speaker 1: energy in each photon. That's hitting the electron. You can
24:38
Speaker 1: only do that by changing the frequency. And this assumes
24:41
Speaker 1: again that electrons can only interact with one photon at
24:45
Speaker 1: a time, which is pretty solid assumption. So the amazing
24:48
Speaker 1: thing is that this idea, which really came from Plank,
24:51
Speaker 1: explain these experiments which really were done by other people.
24:55
Speaker 1: But the unification of it, the bringing together the idea,
24:58
Speaker 1: the moment of inside the explanation of this weird experiment,
25:03
Speaker 1: was done by Einstein. And that's what Einstein won the
25:05
Speaker 1: Nobel Prize for, not for doing the experiment, not for
25:08
Speaker 1: having the idea, but for being sort of in the
25:11
Speaker 1: right place at the right time to bring that idea
25:14
Speaker 1: to solve this open problem. Now, the photon was not
25:17
Speaker 1: named as a particle for decades later. All of this
25:20
Speaker 1: happened just around the turn of the nineteenth century, and
25:23
Speaker 1: Einstein won the Nobel Prize later for it, but it
25:25
Speaker 1: wasn't until nineteen twenty six that people started calling these
25:29
Speaker 1: things photons. And it comes from the Greek word for light.
25:32
Speaker 1: But it also touches on something I think is really interesting,
25:35
Speaker 1: which is the sort of concept of a particle. I
25:38
Speaker 1: like to imagine what we're physicists thinking back then, what
25:41
Speaker 1: did they think that the universe looked like at a
25:44
Speaker 1: microscopic scale, Because to us, the notion of a particle
25:48
Speaker 1: is kind of familiar. I mean, they're weird, they do
25:50
Speaker 1: things that we don't understand. They follow rules and make
25:52
Speaker 1: no sense to us. But we're comfortable with the idea
25:56
Speaker 1: that the universe is atomic, meaning that's made up of
25:59
Speaker 1: little bit it and all we have to do is
26:00
Speaker 1: sort of figure out what those bids do. But at
26:03
Speaker 1: the time, this whole concept of a particle was kind
26:06
Speaker 1: of new. Remember where they had discovered the electron. That
26:09
Speaker 1: was only recently. That was the first piece of evidence
26:12
Speaker 1: that there was something as a particle. Sort of the
26:14
Speaker 1: invention of the concept of a particle was the discovery
26:17
Speaker 1: of the electron. And all he really did there was
26:20
Speaker 1: identify something tiny that had both mass and charge, and
26:24
Speaker 1: so he said, oh, look there's a thing there as
26:26
Speaker 1: these two attributes. I'm going to call it a particle.
26:29
Speaker 1: Actually he called it a corpus skule. But the concept
26:31
Speaker 1: of intellectual groundwork was laid then for a particle. So
26:36
Speaker 1: then you get to the photon. Now the photon has energy,
26:39
Speaker 1: it has direction, but it doesn't have mass. It's not
26:43
Speaker 1: a thing in that sense, there's no stuff to it,
26:47
Speaker 1: So that immediately sort of bends your mind around what
26:50
Speaker 1: is this concept of a particle. Anyway, we've created this
26:54
Speaker 1: idea to accommodate the discovery the electron. We hope, oh,
26:57
Speaker 1: maybe there are other particles, And later on the podcasts
27:00
Speaker 1: will take a tour of the discoveries of other particles,
27:02
Speaker 1: which have hilarious and amazing and dramatic stories to them.
27:06
Speaker 1: But very early in the history of particles we had
27:09
Speaker 1: to already bend the rules and say, oh, well, we
27:11
Speaker 1: were talking about particles is a little bits of stuff.
27:13
Speaker 1: But they can also be not stuff, right, they can
27:16
Speaker 1: also just be energy. And so to me, it's amazing
27:20
Speaker 1: that this field of particle physics was founded on such
27:22
Speaker 1: crazy discoveries. So to me, it's wonderful that the field
27:25
Speaker 1: of particle physics is founded on such crazy discoveries. And
27:29
Speaker 1: you've got to give a lot of credit to the theorists,
27:31
Speaker 1: of course, who put these ideas together and helped us
27:34
Speaker 1: understand what we were seeing. But to me, the most
27:37
Speaker 1: exciting moments are those moments of experimental surprise when the
27:41
Speaker 1: universe does something that we don't understand when the unit,
27:45
Speaker 1: when we predict the universe will do a and instead
27:47
Speaker 1: it does be because that's the universe talking to us,
27:50
Speaker 1: or that's the universe answering our questions, that's the universe
27:53
Speaker 1: being the subject of our interrogation when we say we
27:56
Speaker 1: want to know how this works, prove it to us,
27:58
Speaker 1: or reveal to us the underlying mechanism. And that's what
28:02
Speaker 1: experimental physics is about. Is about cornering the universe and
28:05
Speaker 1: forcing it to reveal something new to you. And a
28:08
Speaker 1: lot of times that revelation happens when you didn't expect that.
28:11
Speaker 1: You thought, oh, we're just double checking this over here.
28:13
Speaker 1: We're pretty sure we understand it. Just dotting the eyes
28:15
Speaker 1: and crossing the teas and all of a sudden, oops,
28:17
Speaker 1: you get something totally surprising. But those are the moments
28:21
Speaker 1: that we learned something new about the universe, And those
28:23
Speaker 1: are the moments I'm striving for my own personal research.
28:26
Speaker 1: When I'm smashing particles together at the LHC. We think
28:29
Speaker 1: we understand what's going to happen, but I'm always secretly
28:32
Speaker 1: hoping that a student will come to me and say, hey, Daniel,
28:35
Speaker 1: what's this. I found this weird thing in our data
28:37
Speaker 1: that just doesn't make any sense, and that's only happened
28:41
Speaker 1: once or twice in my entire career, and I look
28:43
Speaker 1: forward to it happening again. So maybe one day we'll
28:46
Speaker 1: be hearing about a crazy discovery we made at the
28:48
Speaker 1: Large Adeon Collider. Until then, thanks for listening to this
28:52
Speaker 1: description of how we know the photon is a thing,
28:55
Speaker 1: and please, if you're interested in learning more about the
28:57
Speaker 1: history of physics or understanding how we know how the
29:00
Speaker 1: universe works and what we don't know, please send me
29:03
Speaker 1: a suggestion to feedback at Daniel and Jorge dot com.
29:07
Speaker 1: Thanks for tuning in. If you still have a question
29:17
Speaker 1: after listening to all these explanations, please drop us a line.
29:21
Speaker 1: We'd love to hear from you. You can find us
29:23
Speaker 1: at Facebook, Twitter, and Instagram at Daniel and Jorge That's
29:27
Speaker 1: one word, or email us at Feedback at Daniel and
29:30
Speaker 1: Jorge dot com. Thanks for listening, and remember that Daniel
29:34
Speaker 1: and Jorge Explain the Universe is a production of I
29:37
Speaker 1: Heart Radio. For more podcast from my Heart Radio, visit
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Speaker 1: the I heart Radio app, Apple Podcasts, or wherever you
29:44
Speaker 1: listen to your favorite shows.