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How long is a photon? · Daniel and Kelly’s Extraordinary Universe

• 0:08
  Speaker 1: Hey, Daniel, When you think about a photon, what image
• 0:12
  Speaker 1: comes to mind? Oh? Depends on what what you've been
• 0:16
  Speaker 1: smoking that day?
• 0:18
  Speaker 2: Yes, and also on the context. Are we talking about
• 0:21
  Speaker 2: light from distant stars or rainbows or single photon lasers
• 0:25
  Speaker 2: or what?
• 0:26
  Speaker 1: What? Aren't they all the same? Like a photon is
• 0:28
  Speaker 1: a photon, isn't it?
• 0:29
  Speaker 2: Nobody really knows what a photon is. There's something weird
• 0:32
  Speaker 2: and mysterious we might never fully understand.
• 0:35
  Speaker 1: So you're just gonna leave us in the dark. You're
• 0:37
  Speaker 1: not gonna shed any light on it.
• 0:39
  Speaker 2: That's as bright as I can be on the topic.
• 0:56
  Speaker 1: I am wore handmade cartoonists and the author of Oliver's
• 0:58
  Speaker 1: Great Big Universe.
• 0:59
  Speaker 2: Hi, I'm Daniel. I'm a particle physicist and a professor
• 1:02
  Speaker 2: at UC Irvine, and I'm still hunting for a brilliant
• 1:05
  Speaker 2: explanation about photons.
• 1:07
  Speaker 1: I thought brilliance was your job description. Isn't it your
• 1:09
  Speaker 1: job to provide that brilliance.
• 1:13
  Speaker 2: My job is to hunt for the brilliance, to try
• 1:15
  Speaker 2: to mine the truth from the firmament of reality. We
• 1:19
  Speaker 2: don't always find it.
• 1:21
  Speaker 1: M I guess it's hard to shine light on some
• 1:24
  Speaker 1: of the corners of the universe that are hard to see.
• 1:26
  Speaker 1: We just have to hope somebody out there is bright enough,
• 1:29
  Speaker 1: somebody has a light bulb moment there. But what do
• 1:32
  Speaker 1: you mean? Are you saying photons depend on where they
• 1:35
  Speaker 1: come from?
• 1:36
  Speaker 2: I'm saying that the language of physics we use to
• 1:38
  Speaker 2: explain things uses as the basic mental building blocks. Things
• 1:42
  Speaker 2: We do understand waves and bits of sand and tiny
• 1:46
  Speaker 2: little particulate stuff, and none of those things really completely
• 1:50
  Speaker 2: and fully describe the photon. It's those things, but also
• 1:53
  Speaker 2: something else.
• 1:54
  Speaker 1: I see. It's a language issue. Blame it on the
• 1:57
  Speaker 1: linguists if we don't understand the universe. Yes, it's not
• 2:01
  Speaker 1: the physicists fault.
• 2:02
  Speaker 2: It also turns out to be fundamental to how we
• 2:04
  Speaker 2: do science. We often tell different stories about the same
• 2:07
  Speaker 2: kind of stuff depending on the question we are asking.
• 2:10
  Speaker 2: None of our science is totally exact and complete. It's
• 2:13
  Speaker 2: always approximate. And which approximation, which idea we use, which
• 2:17
  Speaker 2: conceptualization is relevant, depends on the questions we're asking.
• 2:22
  Speaker 1: Sounds like it's a big relativity problem because it's all relative.
• 2:27
  Speaker 2: It's relatively complicated.
• 2:28
  Speaker 1: Yeah, indeed, But anyways, welcome to our podcast. Daniel and
• 2:33
  Speaker 1: Jorge Explain the Universe, a production of iHeartRadio.
• 2:36
  Speaker 2: In which we take the whole universe as the context
• 2:39
  Speaker 2: for our goal to understand things. We want to understand
• 2:42
  Speaker 2: how droplets form into hurricanes, how tiny little quarks make protons,
• 2:46
  Speaker 2: how enormous masses of stuff swirl into black holes. We
• 2:51
  Speaker 2: want to answers for everything, and we hope one day
• 2:53
  Speaker 2: to be able to stitch those answers together into a single, comprehensive,
• 2:57
  Speaker 2: complete understanding of the universe, even though that might actually
• 3:01
  Speaker 2: be impossible.
• 3:02
  Speaker 1: Yeah, we try to track the journey of humanity from
• 3:05
  Speaker 1: the shadows into the shining light of understanding and comprehension
• 3:09
  Speaker 1: about this amazing universe we live in. Yeah, even if
• 3:13
  Speaker 1: it sometimes takes a few stories or different stories along
• 3:17
  Speaker 1: the way.
• 3:17
  Speaker 2: The history of physics is seeing stuff we don't understand
• 3:20
  Speaker 2: and then cobbling together some sort of mathematical explanation for
• 3:24
  Speaker 2: what might be going on. And the bigger picture is
• 3:27
  Speaker 2: to then try to weave those explanations together into a
• 3:30
  Speaker 2: single coherent idea. But that task is still not finished,
• 3:33
  Speaker 2: and it leaves us sometimes in an awkward situation of
• 3:36
  Speaker 2: not being able to answer pretty basic questions about what's
• 3:40
  Speaker 2: going on out there?
• 3:41
  Speaker 1: Are you saying physicists can't get their story straight. It's
• 3:43
  Speaker 1: a little suspicious.
• 3:45
  Speaker 2: I'm saying the universe is a little bit lack Russiamon.
• 3:47
  Speaker 2: You know, the story you tell depends on your context.
• 3:50
  Speaker 2: But this is not something that only physicists do. You know,
• 3:53
  Speaker 2: if I ask you how the baseball game went yesterday,
• 3:56
  Speaker 2: you tell me a story about the teams and who
• 3:58
  Speaker 2: was playing well and who was struggling. You put it
• 4:00
  Speaker 2: in context to make it exciting. You don't just give
• 4:03
  Speaker 2: me a dry list of what happened to every single
• 4:05
  Speaker 2: particle in the vicinity of the stadium that day.
• 4:08
  Speaker 1: But there'd just be one story about who won and
• 4:09
  Speaker 1: who lost.
• 4:10
  Speaker 2: If you think that's the story, right, Maybe the story
• 4:13
  Speaker 2: is something else, the changing of the hot dogs, how
• 4:15
  Speaker 2: the mustard now tastes, you know, the weather. Everybody might
• 4:18
  Speaker 2: ask different questions about the same sets of events, and
• 4:21
  Speaker 2: then they might need to use different physical concepts, even
• 4:24
  Speaker 2: different mathematical formalisms to get those answers, which makes a
• 4:28
  Speaker 2: very complicated answer very basic sounding questions.
• 4:31
  Speaker 1: Right, right, Sometimes you need hot dog particles, sometimes you
• 4:33
  Speaker 1: need baseball particles.
• 4:36
  Speaker 2: Yeah, exactly. You can build a whole universe. On the
• 4:38
  Speaker 2: hot dog theory, hot dogs are the fundamental component and
• 4:41
  Speaker 2: what's inside them doesn't really.
• 4:42
  Speaker 1: Matter, right, right, is it the hot dog on or
• 4:44
  Speaker 1: the hot legino?
• 4:47
  Speaker 2: The whole brilliance of hot dogs is just enjoying them
• 4:50
  Speaker 2: and not even caring what they're made at it.
• 4:52
  Speaker 1: What's the shape of a hot dog on? And how
• 4:55
  Speaker 1: long is it?
• 4:57
  Speaker 2: That depends on which city you're in, you.
• 4:58
  Speaker 1: Know, yeah, yeah, or which country too.
• 5:01
  Speaker 2: That's right. And your relative velocity, because some of these
• 5:04
  Speaker 2: things can be length contracted.
• 5:05
  Speaker 1: That's right. If you eat it fast, then it's a
• 5:08
  Speaker 1: lot shorter than it is.
• 5:12
  Speaker 2: If you're at high velocity relative to your hot dog,
• 5:14
  Speaker 2: it will seem shorter. So, yeah, somebody shoots a hot
• 5:17
  Speaker 2: dog into your mouth near the speed of light, then
• 5:20
  Speaker 2: you're gonna have an interesting experience.
• 5:22
  Speaker 1: But then it depends on which direction it is subndy, Right,
• 5:25
  Speaker 1: if it's shutted on the side, it's still going to
• 5:28
  Speaker 1: be the same length.
• 5:29
  Speaker 2: Yeah, exactly, it's just going to be thinner.
• 5:31
  Speaker 1: Yeah, let's just spend the rest of the episode talked
• 5:33
  Speaker 1: about hot dog physics.
• 5:36
  Speaker 2: High velocity hot dog physics, relativistic hot dog physics, A
• 5:39
  Speaker 2: topic nobody has ever explored. We can be the first
• 5:42
  Speaker 2: to write a paper in the Journal of Hot Dog Physics.
• 5:45
  Speaker 1: Well, I think we're definitely the first to ever talk
• 5:47
  Speaker 1: about it in a physics podcast. I'm thinking, I don't know.
• 5:50
  Speaker 1: I haven't done the exhaustive literature search.
• 5:53
  Speaker 2: Somebody out there let us know if we need to
• 5:55
  Speaker 2: cite you.
• 5:56
  Speaker 1: Yeah, somebody else do the research for us. But anyways,
• 5:59
  Speaker 1: it is interesting to talk about how long things are,
• 6:01
  Speaker 1: you know, basic questions like that about everyday objects we
• 6:04
  Speaker 1: see every day.
• 6:06
  Speaker 2: It is often really fruitful, but sometimes frustrating to ask intuitive,
• 6:10
  Speaker 2: natural questions about the kind of things we think the
• 6:13
  Speaker 2: universe is made out of. We think everything out there
• 6:15
  Speaker 2: has certain properties, it has a size, a link, the mass,
• 6:18
  Speaker 2: et cetera. And so we try to apply those concepts,
• 6:21
  Speaker 2: these things we're familiar with from the kind of stuff
• 6:24
  Speaker 2: we're used to interacting with, and apply that to quantum objects.
• 6:27
  Speaker 2: But it doesn't always quite work.
• 6:29
  Speaker 1: Yes, we've found out the quantum world is very straying,
• 6:32
  Speaker 1: very mysterious, very uncertain, and very hard for our simple
• 6:36
  Speaker 1: brain sometimes to understand and capture and to get an
• 6:39
  Speaker 1: intuitive understanding of it.
• 6:40
  Speaker 2: But that doesn't mean it's impossible, and that doesn't mean
• 6:42
  Speaker 2: it's not useful. In fact, it's very helpful for shining
• 6:46
  Speaker 2: a light onto what we do understand and what we
• 6:48
  Speaker 2: don't understand, and it can help you make a better
• 6:51
  Speaker 2: mental picture for what's going on at the quantum level.
• 6:54
  Speaker 1: Right, But the question is can we shine a light
• 6:57
  Speaker 1: on light itself? And so to be on the podcast
• 7:00
  Speaker 1: jagging the question how long is a photon? Now it is?
• 7:09
  Speaker 1: Is that a photon coming off of a hot dog?
• 7:11
  Speaker 1: Or does it matter where it's bouncing off of.
• 7:14
  Speaker 2: A hot dog? Colored photon?
• 7:16
  Speaker 1: Wow? What is the color of a hot dog? What
• 7:18
  Speaker 1: is the color of a hot dog? Sounds like the
• 7:21
  Speaker 1: topic of a philosophy class here.
• 7:23
  Speaker 2: If you eat your hot dog with eyes closed, does
• 7:25
  Speaker 2: it have a color or not? I wonder if there's
• 7:30
  Speaker 2: a paint shade out there that's called hot dog.
• 7:34
  Speaker 1: I think people usually avoid having their runs painted hot dog.
• 7:39
  Speaker 2: There's probably more adjectives to it, like bright, summer hot dog.
• 7:42
  Speaker 1: Or something summer baseball hot dog, home run, the hot dog,
• 7:49
  Speaker 1: hot dog, vapor, wild mountain hot dog. There's so many
• 7:55
  Speaker 1: shades to a hot dog, isn't there?
• 7:57
  Speaker 2: But we're not here to talk about hot dogs, though
• 8:00
  Speaker 2: it seems like we're gonna We're here to try our
• 8:01
  Speaker 2: best to answer a very simple but very hard question
• 8:05
  Speaker 2: about the nature of light.
• 8:06
  Speaker 1: Mmm, Now, how long it's a photon? Is that a
• 8:09
  Speaker 1: question about its length or like how long it lasts?
• 8:12
  Speaker 2: Oh? I interpreted it as a question about its length,
• 8:14
  Speaker 2: like its physical extent. Photons can last forever, you know,
• 8:18
  Speaker 2: their lifetime is potentially infinite. You shoot a photon into
• 8:21
  Speaker 2: empty space, it'll just keep going forever.
• 8:23
  Speaker 1: But you can kill a photon, can it?
• 8:25
  Speaker 2: You can kill a photon, yes, absolutely, you can absorb it,
• 8:27
  Speaker 2: you can interact with it. But a photon on its
• 8:30
  Speaker 2: own will not like necessarily.
• 8:31
  Speaker 1: Decay, can it ever, like, is there a possibility for
• 8:34
  Speaker 1: it to, you know, have its energy convert into something else?
• 8:37
  Speaker 2: Absolutely? A photon flying through space can just fly through space.
• 8:40
  Speaker 2: But it can also turn into an electron and positron
• 8:43
  Speaker 2: and then back into a photon, or into a muon
• 8:46
  Speaker 2: or an anti muon and then back into a photon,
• 8:48
  Speaker 2: or all sorts of other stuff. So there's lots of
• 8:50
  Speaker 2: quantum possibilities constantly for photons.
• 8:53
  Speaker 1: Can it turn into a hot dog technically? Like you know,
• 8:57
  Speaker 1: in the infinity of infinities. Is there us light chance
• 9:00
  Speaker 1: it can turn it suddenly into hot dog?
• 9:02
  Speaker 2: Yes, there's a slight chance a very high energy photon
• 9:06
  Speaker 2: could turn into a mutually charged hot dog momentarily.
• 9:10
  Speaker 1: Hopefully it doesn't turn into hot dog inside your eye.
• 9:13
  Speaker 2: And that tells me exactly what I want to paint
• 9:15
  Speaker 2: my room next year, which is quantum hot dog.
• 9:18
  Speaker 1: Oh boy, it's like it's different shades at the same.
• 9:21
  Speaker 2: Time, exactly Shrewdinger's hot.
• 9:25
  Speaker 1: It's like yellow mustard red ketchup. But it depends on
• 9:29
  Speaker 1: how you look at it, kind of like the dress. Anyways,
• 9:31
  Speaker 1: that's a very spicy idea. All right, let's talk about
• 9:34
  Speaker 1: this question. But first we were wondering how many people
• 9:36
  Speaker 1: out there had thought about the length of a photon,
• 9:39
  Speaker 1: or even if photons have length.
• 9:42
  Speaker 2: Thanks very much to everybody who answered this question. I
• 9:44
  Speaker 2: only got one response online. So I walked around campus
• 9:47
  Speaker 2: at you see Irvine last week and I asked a
• 9:49
  Speaker 2: bunch of psych majors and other random people about photons.
• 9:53
  Speaker 1: All right, Well, if you spot a physicist with a
• 9:56
  Speaker 1: microphone on the ucroline campus, make sure to I don't know,
• 10:00
  Speaker 1: runaway or approach if you think you can answer physics
• 10:04
  Speaker 1: questions on the.
• 10:04
  Speaker 2: Spot, or even if you don't, I love to hear
• 10:07
  Speaker 2: your thoughts.
• 10:08
  Speaker 1: All right, So think about it for a second. How
• 10:10
  Speaker 1: long do you think a photon is? Here's what people
• 10:13
  Speaker 1: had to say.
• 10:15
  Speaker 2: I don't think.
• 10:15
  Speaker 1: We don't know about that yet because of the mathematics
• 10:20
  Speaker 1: going weird, since the photon is traveling in speed of
• 10:24
  Speaker 1: the light.
• 10:26
  Speaker 2: So that's my guess. Oh my gosh, I don't know.
• 10:30
  Speaker 1: I'm gonna say and like, uh like ten to the
• 10:36
  Speaker 1: power of negative twenty centimeters.
• 10:42
  Speaker 3: Let's say, so I guess photons they don't have a
• 10:46
  Speaker 3: mask or that have amentin right an, So it depends
• 10:50
  Speaker 3: on the wave length light.
• 10:54
  Speaker 4: No, I wouldn't even have like a guess of like
• 10:56
  Speaker 4: length yeah a long in science, yeah.
• 11:00
  Speaker 1: Phot I don't even know the pot I remember from biocam, okay,
• 11:05
  Speaker 1: like maybe in chips.
• 11:07
  Speaker 4: Yeah, I wouldn't even know I would photons. When I
• 11:10
  Speaker 4: think of physics, I think get like small like particles. Yeah,
• 11:13
  Speaker 4: and then I'm thinking like centimeters and like in minute
• 11:16
  Speaker 4: signs two millimeters.
• 11:19
  Speaker 2: I don't.
• 11:21
  Speaker 1: I don't even know what a photon is. I major
• 11:23
  Speaker 1: in a criminal justice, so completely outside of my major
• 11:27
  Speaker 1: there's a photon, a particle light.
• 11:29
  Speaker 2: Yes, how long is it?
• 11:31
  Speaker 1: I'm gonna say point zeros are one light years. I
• 11:39
  Speaker 1: don't know, like some ran like maybe like zero point
• 11:42
  Speaker 1: one microns.
• 11:43
  Speaker 4: I don't know.
• 11:44
  Speaker 1: All right, some pretty good answers. Some of them are
• 11:47
  Speaker 1: very specific. Zero point one microns ten to the power
• 11:52
  Speaker 1: of negative twenty centimeters.
• 11:55
  Speaker 2: There's a huge, huge range of answers here. I think
• 11:58
  Speaker 2: the biggest one is probably zero points zero zero one
• 12:02
  Speaker 2: light years. That turns out to be a very big number.
• 12:05
  Speaker 1: Well, I'm impressed that they even stuck to the metric system.
• 12:08
  Speaker 1: I mean, everyone nobody switched to inches or miles.
• 12:12
  Speaker 2: You think photons are metric? I don't know. Yeah, I
• 12:14
  Speaker 2: believe in imperial photons.
• 12:16
  Speaker 1: I believe photons are king, but you know, I think
• 12:19
  Speaker 1: they should stick to the more reasonable metric system.
• 12:22
  Speaker 2: That's why Darth Vader is all black, because there are
• 12:24
  Speaker 2: no imperial photons.
• 12:29
  Speaker 1: Wow, reach there took me three seconds there wait? Oh
• 12:36
  Speaker 1: imperial Yes, Yes, that was a very dark joke. I
• 12:40
  Speaker 1: thought you were going to go with a lightsaber response.
• 12:42
  Speaker 1: I totally set you up for that. What would be
• 12:43
  Speaker 1: the lightsaber joke?
• 12:44
  Speaker 2: Lightsabers only cut things in metric units. I don't know.
• 12:49
  Speaker 1: Lightsabers are about a meter long. There you go, all right,
• 12:51
  Speaker 1: well interesting azers. So Daniel to dig into it. For
• 12:55
  Speaker 1: first of all, what is a photon? How do we
• 12:57
  Speaker 1: define a photon? So, photon is like the minimum packet
• 13:01
  Speaker 1: of light. If you take a really bright source of light,
• 13:04
  Speaker 1: you might imagine it's just shooting out huge amounts of light.
• 13:07
  Speaker 1: As you dial it down, it'll get dimmer and dimmer
• 13:09
  Speaker 1: and dimmer, but it can't get infinitely dim. As you
• 13:12
  Speaker 1: dial that light source down, eventually you'll notice that the
• 13:15
  Speaker 1: light is actually discreete that it comes out in little
• 13:18
  Speaker 1: packets rather than just being dimmer and dimmer waves. So
• 13:21
  Speaker 1: photons are like the minimum unit of light. Wait are
• 13:25
  Speaker 1: you saying that photons don't have a minimum energy.
• 13:28
  Speaker 2: Photons do not have a minimum energy. That's true, but
• 13:31
  Speaker 2: photons of a specific frequency have a fixed energy. And
• 13:34
  Speaker 2: if you have, for example, a laser at his very
• 13:37
  Speaker 2: specific wavelength and you dial it down so it's dimmer
• 13:40
  Speaker 2: and dimmer and dimmer, eventually you're going to notice that
• 13:42
  Speaker 2: beam gets broken up and it comes out in pieces.
• 13:46
  Speaker 1: Like you lower the power to the laser, and eventually
• 13:48
  Speaker 1: you'll see it go down steps.
• 13:50
  Speaker 2: Yeah, exactly. It's just like everything else in our quantum world.
• 13:54
  Speaker 2: Matter is not continuous. You can't zoom in forever on
• 13:57
  Speaker 2: matter and have it always look the same way you
• 14:00
  Speaker 2: zoom in on matter. You notice that it has a
• 14:02
  Speaker 2: particular scale that at some point it breaks up into
• 14:04
  Speaker 2: discrete bits out of which everything is built, just like
• 14:07
  Speaker 2: the resolution on your screen. So light itself has a resolution.
• 14:11
  Speaker 2: It's made out of these little quantum bits, these discrete
• 14:14
  Speaker 2: building blocks. It's not perfectly smooth. And what is that
• 14:18
  Speaker 2: smallest bit for light? It's a photon. That's what we
• 14:21
  Speaker 2: call the photon. It's the smallest bit of light.
• 14:24
  Speaker 1: Like if I if I'm shooting lighters are in frequency,
• 14:26
  Speaker 1: the little steps that I see as I dial down
• 14:29
  Speaker 1: the power to it, that's what you would call a photon.
• 14:32
  Speaker 2: Exactly. Those are photons. And so if you have a
• 14:34
  Speaker 2: bunch of light, you can always ask how many photons
• 14:37
  Speaker 2: are there. There's a specific number. It has to be
• 14:39
  Speaker 2: an integer number of photons. You don't usually notice this
• 14:42
  Speaker 2: because the number of photons usually around hitting your eyeball
• 14:45
  Speaker 2: is enormous. It doesn't really matter that they're countable. But
• 14:48
  Speaker 2: as things get very very small. Then you can notice
• 14:51
  Speaker 2: that you can have zero or one or two photons,
• 14:54
  Speaker 2: you can't have one point seven photons.
• 14:56
  Speaker 1: Now, how do we think about light? Is it like
• 14:59
  Speaker 1: you say, it's like a packet, Like it's a discrete
• 15:01
  Speaker 1: little object.
• 15:03
  Speaker 2: Yeah, So this really gets at the heart of the
• 15:04
  Speaker 2: question because how you describe this object helps you answer
• 15:07
  Speaker 2: the question how big is it? And the answer is
• 15:10
  Speaker 2: that we think about light in lots of different contradictory ways,
• 15:14
  Speaker 2: depending on the context. Sometimes we think about light as
• 15:17
  Speaker 2: like a tiny little object, but we think about it
• 15:20
  Speaker 2: like a particle which has no extent, just like zero
• 15:23
  Speaker 2: volume particle. Sometimes we ignore the quantum nature of it
• 15:27
  Speaker 2: because it doesn't matter. We're thinking about really bright sources
• 15:29
  Speaker 2: where the quantum nature is irrelevant, So we just think
• 15:32
  Speaker 2: about it as classical waves of electromagnetism, the way people
• 15:35
  Speaker 2: did two hundred years ago. And sometimes we think about
• 15:39
  Speaker 2: light interacting with quantum particles, like light hitting an electron,
• 15:42
  Speaker 2: and then we think about it as a little quantum
• 15:45
  Speaker 2: packet and excitation in the electromagnetic field. So we have
• 15:48
  Speaker 2: lots of different pictures of what a photon is, and
• 15:51
  Speaker 2: the one that we use depends kind of on the question,
• 15:53
  Speaker 2: we're asking.
• 15:54
  Speaker 1: Well, so do you want to then tackle each one
• 15:57
  Speaker 1: of these different ways to look at it at a time?
• 15:59
  Speaker 2: Yeah? Sure. I think probably the most relevant in this
• 16:01
  Speaker 2: case is the quantum field theory one the last one
• 16:04
  Speaker 2: we talked about, But each one gives you a different answer.
• 16:07
  Speaker 1: All right, well, then let's maybe tackle each one of these.
• 16:10
  Speaker 1: But what does quantum field theory say about the nature
• 16:13
  Speaker 1: of light?
• 16:13
  Speaker 2: Quantum field theory is an updated version of classical field theory,
• 16:17
  Speaker 2: which sounds fancy, but it just says light is a
• 16:20
  Speaker 2: wave in the electromagnetic field. That's what Faraday and Maxwell
• 16:24
  Speaker 2: and those guys figured out a couple of hundred years ago,
• 16:27
  Speaker 2: that the universe is filled with this electromagnetic field and
• 16:30
  Speaker 2: that waves in it are what we call light, and
• 16:32
  Speaker 2: so you can shoot light from one planet to another,
• 16:35
  Speaker 2: and the medium for that is the electromagnetic field. Even
• 16:38
  Speaker 2: though space is empty, it has these fields in it.
• 16:40
  Speaker 2: So light is a ripple in those fields. And we
• 16:42
  Speaker 2: talk about that all the time on the podcast. And
• 16:44
  Speaker 2: you have electric fields and magnetic fields and there are
• 16:47
  Speaker 2: ninety degrees from each other and they're oscillating, and that's
• 16:50
  Speaker 2: what light is. From a classical point of view. That's
• 16:52
  Speaker 2: a traditional classical field theory. The quantum field theory version
• 16:56
  Speaker 2: of that is the same. It just says that there's
• 16:58
  Speaker 2: a minimum to how you can oscillate, so that as
• 17:01
  Speaker 2: you turn it down you discover that you can't turn
• 17:03
  Speaker 2: it to any intensity. There's certain steps. So the quantum
• 17:06
  Speaker 2: field theory version says the universe is filled with this
• 17:09
  Speaker 2: electromagnetic field which has certain steps in energy that it
• 17:13
  Speaker 2: can take.
• 17:13
  Speaker 1: Well, I guess, first of all, I wonder if listeners,
• 17:15
  Speaker 1: as sometimes you get confused by this like I do,
• 17:17
  Speaker 1: which is that you say light is a wave, but
• 17:20
  Speaker 1: like if I think that's rippling through a field, But
• 17:22
  Speaker 1: if I think of a wave like rippling through a
• 17:25
  Speaker 1: lake or my bathtub, it's something that ripples outwards in
• 17:29
  Speaker 1: all directions. Or if I think of it like a
• 17:31
  Speaker 1: wave in the ocean, it's like this broad thing that's
• 17:35
  Speaker 1: moving and undulating across kind of a wide area. But
• 17:38
  Speaker 1: in terms of light, it's not that right. It's not
• 17:42
  Speaker 1: spreading in all directions, and it's not broad like that.
• 17:45
  Speaker 2: It can be though, I mean, think about a star.
• 17:47
  Speaker 2: A star is emitting light, and it's emitting light in
• 17:50
  Speaker 2: all directions, and before you think about the quantum nature
• 17:53
  Speaker 2: of it, it is in fact spreading out. And that's
• 17:55
  Speaker 2: why stars seem more dim the further you are away
• 17:58
  Speaker 2: from them, right, because the intense see the light drops
• 18:01
  Speaker 2: with the distance squared, and so you have light waves
• 18:05
  Speaker 2: which start out very intense and then they spread out
• 18:07
  Speaker 2: and so they get dimmer and dimmer. The quantum version
• 18:10
  Speaker 2: of that is the same, except that now you have
• 18:12
  Speaker 2: individual photons being sent out and close to the star
• 18:16
  Speaker 2: you have a high intensity of those photons, and further
• 18:18
  Speaker 2: away you have a smaller intensity of those photons. And
• 18:20
  Speaker 2: you can understand why the intensity the photon drops as
• 18:24
  Speaker 2: you get further away because the space they're feeling is
• 18:27
  Speaker 2: getting bigger and bigger. And so if you have like
• 18:29
  Speaker 2: the same size eyeball and you're gonna have fewer number
• 18:32
  Speaker 2: of photons hit your eyeball when you're further away than
• 18:35
  Speaker 2: when you are close up to the star.
• 18:37
  Speaker 1: Right, you can sorder think about it that way. But
• 18:38
  Speaker 1: I guess what do you call the photon? Then? Is
• 18:41
  Speaker 1: the photon the ripple that's shooting in all directions or
• 18:45
  Speaker 1: just if you catch that ripple in a particular spot.
• 18:49
  Speaker 1: You know what I mean? Like you can imagine a
• 18:51
  Speaker 1: star and it's rippling light out. Is a photon that
• 18:55
  Speaker 1: a ring that emanates from the star or what?
• 18:58
  Speaker 2: Yeah, great question. Say you slow the star down so
• 19:01
  Speaker 2: it's only emitting one photon at a time somehow, right,
• 19:04
  Speaker 2: like a single photon star. Basically, we're just putting a
• 19:07
  Speaker 2: laser out there in space, but it's interesting to think
• 19:09
  Speaker 2: about how it could go in any direction. So then
• 19:12
  Speaker 2: any individual photon has the same probability to go in
• 19:14
  Speaker 2: any direction from the star if it's totally symmetric. And
• 19:18
  Speaker 2: so an individual photon has a ring of probability around
• 19:22
  Speaker 2: the star where it can go, and then when it
• 19:24
  Speaker 2: actually hits something, then the universe decides, Okay, this one's
• 19:27
  Speaker 2: over here or this one's over there. It's just like
• 19:29
  Speaker 2: when you shoot photons at a screen. They have a
• 19:31
  Speaker 2: range of possible locations where they can land, and then
• 19:34
  Speaker 2: when the photon actually hits, that's when the universe decides
• 19:37
  Speaker 2: this photon's over here and this photon's over there. So yeah,
• 19:40
  Speaker 2: individual photons come out in only one direction, but they
• 19:42
  Speaker 2: have a probability to come out in any direction. There's
• 19:45
  Speaker 2: a bit of a quantum.
• 19:46
  Speaker 1: Wrinkle there, So it's a little bit like the Schrodinger's cat.
• 19:49
  Speaker 1: I know you don't always like this analogy, but it's
• 19:51
  Speaker 1: sort of like the photon as it comes out of
• 19:53
  Speaker 1: the sun or the star. It's in all directions at
• 19:57
  Speaker 1: the same time.
• 19:58
  Speaker 2: It has the possibility the probability to be in all
• 20:00
  Speaker 2: directions at the same time. You can only ever observe
• 20:03
  Speaker 2: it in one. So it depends what you mean by
• 20:05
  Speaker 2: like it is in those places at the same time,
• 20:08
  Speaker 2: it has the possibility to be there can ever be
• 20:10
  Speaker 2: seen to be in more than one place at once.
• 20:13
  Speaker 1: So like it emanates like a bubble basically out of
• 20:18
  Speaker 1: the star, that ripple that's the photon technically, right until
• 20:22
  Speaker 1: something hits it, or until it hits something in my
• 20:24
  Speaker 1: role to die and say okay, yeah that's where I
• 20:26
  Speaker 1: was at.
• 20:26
  Speaker 2: Yeah that's right.
• 20:28
  Speaker 1: So then these ripples, these bubble ripples have a wavelength
• 20:32
  Speaker 1: to them.
• 20:32
  Speaker 2: Yeah, exactly, So these bubble ripples have a wavelength, right.
• 20:36
  Speaker 2: High energy photons have a very short wavelength, like blue
• 20:39
  Speaker 2: photons have a shorter wavelength a higher frequency than red photons,
• 20:43
  Speaker 2: which have a longer wavelength and a shorter frequency. And
• 20:46
  Speaker 2: so that immediately feels like ooh, that might be part
• 20:49
  Speaker 2: of the answer that tells us about the length of
• 20:52
  Speaker 2: these photons, because red photons have a longer wiggle than
• 20:55
  Speaker 2: blue photons, which have a shorter wiggle. And the answer
• 20:58
  Speaker 2: is sort of in that direction. But it's not the
• 21:01
  Speaker 2: answer a red photon with a very specific energy. It's
• 21:04
  Speaker 2: the length of the photon, is not the wavelength of
• 21:07
  Speaker 2: that ripple.
• 21:08
  Speaker 1: Well, let's talk a little bit about this wavelength. How
• 21:11
  Speaker 1: do you measure this wavelength? Like, it's the distance at
• 21:14
  Speaker 1: which the ripple repeats itself.
• 21:17
  Speaker 2: Yeah, remember we're talking about a ripple in the electromagnetic field.
• 21:21
  Speaker 2: What is the electromagnetic field. It's a vector in space,
• 21:24
  Speaker 2: which means every point in space has an arrow with
• 21:26
  Speaker 2: a direction in it. That's confusing to you. You can
• 21:29
  Speaker 2: just pretend it's just a number. Don't worry about the vector.
• 21:31
  Speaker 2: And the wavelength tells you when the electromagnetic field returns
• 21:35
  Speaker 2: to its original value. Right, So the electromagnetic field is
• 21:38
  Speaker 2: pointing up and then it oscillates down, and then it
• 21:40
  Speaker 2: oscillates back up again. And this is just like the
• 21:43
  Speaker 2: direction of the electric field.
• 21:44
  Speaker 1: Meaning like it increases in value, like if I put
• 21:47
  Speaker 1: my finger in front of me, that's a point in space,
• 21:50
  Speaker 1: and that point in space has an electromagnet field going
• 21:53
  Speaker 1: through it, and that field can certainly have a value
• 21:54
  Speaker 1: where I'm pointing my finger.
• 21:56
  Speaker 2: The electromagnetic field has a value at every point in space. Yes,
• 21:58
  Speaker 2: it has a vector value, which means has a direction
• 22:01
  Speaker 2: and a length.
• 22:03
  Speaker 1: Right, But we're just talking about value, and so like
• 22:05
  Speaker 1: where I'm pointing my finger can suddenly go up in value,
• 22:08
  Speaker 1: Like it can be zero right now, zero zero, but
• 22:10
  Speaker 1: suddenly it can go up to ten.
• 22:11
  Speaker 2: Yeah, exactly. It can change with time.
• 22:14
  Speaker 1: And then it can go back down to zero. And
• 22:16
  Speaker 1: that's a ripple.
• 22:17
  Speaker 2: And if you want to think about the wavelength, you know,
• 22:19
  Speaker 2: you have your finger at one point, and the electromagnetic
• 22:22
  Speaker 2: field has a value there. If there's a photon moving
• 22:25
  Speaker 2: through space there, then if you could put another finger
• 22:28
  Speaker 2: somewhere else, you can ask where do I have to
• 22:30
  Speaker 2: put my other finger so it has the same value
• 22:32
  Speaker 2: as my first finger. And that's what the wavelength is
• 22:34
  Speaker 2: telling us. Because the wavelength tells us the electromagnetic field
• 22:37
  Speaker 2: goes up and then down, where does it come back
• 22:39
  Speaker 2: to its original value? That's the wavelength for blue photons.
• 22:43
  Speaker 2: Your two fingers be closer together and for red photons,
• 22:45
  Speaker 2: your fingers would be further apart.
• 22:47
  Speaker 1: And so light is like the value going up in
• 22:50
  Speaker 1: one of my fingers and down and then go up
• 22:52
  Speaker 1: and down in my other finger. But it only happens
• 22:55
  Speaker 1: once for each photon, Like a photon passing through is
• 22:58
  Speaker 1: just a one.
• 23:00
  Speaker 2: I understand why that's confusing, but that's not actually what
• 23:03
  Speaker 2: one photon is. And this is going to sound like nonsense,
• 23:06
  Speaker 2: But a single photon of specific energy, like if you
• 23:08
  Speaker 2: say exactly what the wavelength is, that photon actually has
• 23:12
  Speaker 2: an infinite size in space, like that photon exists everywhere
• 23:17
  Speaker 2: in the universe. I told you it was going to
• 23:19
  Speaker 2: sound like nonsense. I tried to warn, Well.
• 23:25
  Speaker 1: It sounds like we're going to get a pretty deep
• 23:26
  Speaker 1: into this, So why don't we take a quick break,
• 23:29
  Speaker 1: and then when we come back, we'll dig into what
• 23:32
  Speaker 1: it means for light to be everywhere, all at once.
• 23:35
  Speaker 1: So let's do that, But first let's take a quick break.
• 23:50
  Speaker 1: All right, we're talking about light and how long light is,
• 23:54
  Speaker 1: and Daniel, you just kind of blew our minds here
• 23:57
  Speaker 1: and said that light can be everywhere, all at one,
• 24:00
  Speaker 1: which is the name of a great movie which coincidentally
• 24:03
  Speaker 1: involved hot dog fingers.
• 24:05
  Speaker 2: That's true, not coincidentally. Man, that was the long term
• 24:09
  Speaker 2: plan for this whole joke. I was going to bring
• 24:10
  Speaker 2: it back together.
• 24:12
  Speaker 1: Yes, it was just a giant plug.
• 24:14
  Speaker 2: For a movie A twenty four. Send us some free passes.
• 24:19
  Speaker 1: Yeah, there you go. So we're talking about like a
• 24:22
  Speaker 1: photon is a giant bubble that emanates from a light source.
• 24:26
  Speaker 1: It's everywhere, all at once, in all directions until something
• 24:29
  Speaker 1: hits it. But then if I'm the person that it hits,
• 24:33
  Speaker 1: you're saying, it's not something that just washes over me.
• 24:36
  Speaker 2: Yes, So we're going to talk about the length of
• 24:37
  Speaker 2: a photon, then we have to know something about the
• 24:40
  Speaker 2: energy of the photon. Might think, hold on, isn't he
• 24:43
  Speaker 2: changing the subject. Remember that for quantum objects, their location
• 24:47
  Speaker 2: and the uncertainty in their location how well you can
• 24:50
  Speaker 2: pin that down, is intimately connected with their energy. The
• 24:53
  Speaker 2: Heisenberg uncertainty principle tells us that you can't know perfectly
• 24:57
  Speaker 2: well the energy of an object or it's momentum nearly
• 25:00
  Speaker 2: equivalently and its location. And so for a photon, if
• 25:05
  Speaker 2: you know exactly its energy, if I have a laser,
• 25:07
  Speaker 2: for example, which always puts out photons at one wavelength,
• 25:11
  Speaker 2: and I know it exactly those photons because we specify
• 25:14
  Speaker 2: their energy precisely. That means we can't know anything about
• 25:17
  Speaker 2: their location. And so, like the quantum field theory version
• 25:20
  Speaker 2: says that the whole universe, the electromagnetic fields of the
• 25:23
  Speaker 2: whole universe, has that photon in it. It's oscillating simultaneously everywhere.
• 25:29
  Speaker 1: And now I guess it's getting kind of hairy because
• 25:31
  Speaker 1: we just talked about how like a photon is a ripple,
• 25:34
  Speaker 1: like a bubble that emanates from a star or a
• 25:36
  Speaker 1: light source, right, and so that bubble is getting bigger
• 25:39
  Speaker 1: and bigger until it hits something. But that bubble kind
• 25:42
  Speaker 1: of has a location, right, It's on the surface of
• 25:45
  Speaker 1: that bubble. So how can it be on the surface
• 25:47
  Speaker 1: of the bubble and also everywhere all at once?
• 25:50
  Speaker 2: Yeah, great question. The answer is in the uncertainty of
• 25:53
  Speaker 2: its energy. If a star really could produce photons of
• 25:57
  Speaker 2: exactly one energy, then they would be everywhere all at once.
• 26:01
  Speaker 2: But that's totally unphysical. You can't have something everywhere in
• 26:04
  Speaker 2: the universe all at once, right, That like violates all
• 26:07
  Speaker 2: sorts of principles of relativity. Quantum mechanics and relativity sometimes
• 26:10
  Speaker 2: take a little bit of conceptual glue to stick together.
• 26:13
  Speaker 2: The way to resolve it is to realize, well, there
• 26:15
  Speaker 2: are no such photons in the universe. Nothing is actually
• 26:18
  Speaker 2: made with that exact, super specific energy. In reality, photons
• 26:22
  Speaker 2: always have an uncertainty in their energy. A star is
• 26:25
  Speaker 2: never making exact energy photons. There's always a spread. Even
• 26:29
  Speaker 2: lasers that you think of as having one specific energy,
• 26:32
  Speaker 2: there's always an uncertainty. Even atoms when they're emitting photons
• 26:36
  Speaker 2: between energy levels, there's always a little bit of fuzziness there.
• 26:39
  Speaker 2: So there's an uncertainty in the photon's energy, and the
• 26:42
  Speaker 2: more uncertainty in the energy, the more constrained the photon
• 26:45
  Speaker 2: can be in space. So what's coming out of the
• 26:48
  Speaker 2: star is a ripple and it's localized in space because
• 26:51
  Speaker 2: there's an uncertainty in its energy.
• 26:53
  Speaker 1: Is the time at which it gets made also uncertain
• 26:56
  Speaker 1: or is that something we're allowed to know for sure,
• 26:59
  Speaker 1: because then you know, we know exactly when it was emanated,
• 27:01
  Speaker 1: and we know the speed of light never changes, and
• 27:03
  Speaker 1: we know sort of exactly where that bubble is.
• 27:06
  Speaker 2: Yeah, there's a Heisenberg and certainty relationship between uncertainty and
• 27:09
  Speaker 2: energy and uncertainty in time. So now you can't know
• 27:12
  Speaker 2: that exactly either either.
• 27:14
  Speaker 1: No, so there's three things or they're all tied together.
• 27:16
  Speaker 2: They're all tied together. There's location and momentum, and then
• 27:19
  Speaker 2: there's energy and time. Those are two separate Heisenberg and
• 27:22
  Speaker 2: certainty relationships. But for a photon, momentum and energy are
• 27:25
  Speaker 2: the same thing. They're only different from massive particles, and
• 27:28
  Speaker 2: so they really are all three things tied together by
• 27:31
  Speaker 2: this fuzziness. And so it's the uncertainty. The fact that
• 27:34
  Speaker 2: we can never have pure single energy photons means we
• 27:38
  Speaker 2: always get these packets, these blobs. It's like, well, maybe
• 27:41
  Speaker 2: this photon is this energy, maybe it has that energy,
• 27:43
  Speaker 2: maybe it has this other energy. And that's what defines
• 27:46
  Speaker 2: the length of a photon. It's really a packet of
• 27:49
  Speaker 2: this uncertainty, and the amount of energy uncertainty in that
• 27:52
  Speaker 2: packet gives us the length of the uncertainty in its location.
• 27:56
  Speaker 1: Meaning like the bubble that emanates from the star or
• 27:58
  Speaker 1: light soars is not like a hard bubble, like a
• 28:02
  Speaker 1: real like soap bubble, but it's actually more like an
• 28:05
  Speaker 1: expanding fuzzy cloud.
• 28:06
  Speaker 2: Way I think about it, it's sort of like a
• 28:07
  Speaker 2: little wave packet. You got lots of frequencies together, they
• 28:10
  Speaker 2: add a subject, did they interfere positively and negatively to
• 28:13
  Speaker 2: give you this wave packet that's moving through space. For
• 28:16
  Speaker 2: those of you out there who know like signal analysis
• 28:18
  Speaker 2: or Fourier analysis, you know that, like a single momentum
• 28:22
  Speaker 2: corresponds to an infinite extent in space. But if you
• 28:24
  Speaker 2: add up a bunch of different momentum and different frequencies,
• 28:26
  Speaker 2: you can make any sort of shape you want in space.
• 28:29
  Speaker 1: Right, But then I feel like this, all this uncertainty
• 28:31
  Speaker 1: comes from the fact that we don't know when it
• 28:34
  Speaker 1: was made, this photon. We don't know how it was made.
• 28:36
  Speaker 1: We don't know how energy it had when it was made.
• 28:39
  Speaker 1: But once we detect it, we sort of do know
• 28:41
  Speaker 1: all these things. Right, Well, we never measure an energy
• 28:43
  Speaker 1: of a photon exactly right. You can never precisely measure
• 28:46
  Speaker 1: the energy of a photon. How do you measure it?
• 28:48
  Speaker 2: Anyway? You have it impact some device, and that device
• 28:51
  Speaker 2: has some mechanism inside of it, and you read that
• 28:53
  Speaker 2: number off. There's always uncertainty, not just because the mechanism
• 28:57
  Speaker 2: is something cheap you bought off Amazon. But because there
• 28:59
  Speaker 2: is an hair and quantum uncertainty in the measurement itself.
• 29:03
  Speaker 1: There's a little bit of uncertainty, sure, but like when
• 29:05
  Speaker 1: I'm looking at a hot dog, it doesn't suddenly turn
• 29:07
  Speaker 1: yellow or purple, or hopefully it doesn't turn yellow and
• 29:10
  Speaker 1: purple as I look at it.
• 29:11
  Speaker 2: The hot dog is not a laser, and it's not
• 29:13
  Speaker 2: an idealized laser. It's emitting a spread of colors, and
• 29:17
  Speaker 2: so every photon that comes out of that hot dog
• 29:19
  Speaker 2: has the possibility to be a little greener or a
• 29:21
  Speaker 2: little redder, or a little bluer. There's the fussiness in
• 29:24
  Speaker 2: every single photon that comes out of the hot dog.
• 29:27
  Speaker 1: But once I measure it, don't I know exactly what
• 29:29
  Speaker 1: frequency it had.
• 29:30
  Speaker 2: There's still an uncertainty when you measure it. But yeah,
• 29:33
  Speaker 2: it does collapse some of that uncertainty. I mean, you
• 29:34
  Speaker 2: see a blue photon, or you see a red photon,
• 29:37
  Speaker 2: or you see a green photon, but again still never
• 29:39
  Speaker 2: super precisely.
• 29:40
  Speaker 1: What if we had a perfect measurement device and we
• 29:43
  Speaker 1: can collapse it perfectly, would we know it's exact frequency.
• 29:48
  Speaker 2: I think such a device would have to be the
• 29:49
  Speaker 2: size of the universe, and so then you would know
• 29:51
  Speaker 2: nothing about where it was.
• 29:53
  Speaker 1: Can you explain that?
• 29:54
  Speaker 2: First of all, a device that measures anything exactly is
• 29:56
  Speaker 2: just impossible. Right, You can take the limit of something,
• 30:00
  Speaker 2: can start with like, what's the most precise measurement device
• 30:02
  Speaker 2: I can have, and then try to think about taking
• 30:04
  Speaker 2: the limit of it to perfect precision. Or to measure
• 30:06
  Speaker 2: something very precisely that has a lot of energy, you
• 30:09
  Speaker 2: need to have an object which you can interact with
• 30:11
  Speaker 2: photons of very different wavelengths. Right, wavelengths can be very
• 30:14
  Speaker 2: very short for very high energy, or very very large
• 30:17
  Speaker 2: for very low energy, and so measuring things that are
• 30:19
  Speaker 2: very very large requires large objects. Like you want to
• 30:22
  Speaker 2: receive radio waves, you need a very big antenna. You
• 30:26
  Speaker 2: want to receive microwaves, you need very small antennas. So
• 30:29
  Speaker 2: you want to measure something super precisely that can be
• 30:32
  Speaker 2: of any wavelength, you're going to need essentially an antenna
• 30:35
  Speaker 2: the size of the universe.
• 30:36
  Speaker 1: Oh boy, that's a that would be a very big
• 30:39
  Speaker 1: hot dog.
• 30:40
  Speaker 2: It costs more than a hot dog, all right, But.
• 30:42
  Speaker 1: Maybe let's give up on perfection and say that you know,
• 30:45
  Speaker 1: I measure a photon coming for my hot dog, and
• 30:48
  Speaker 1: I see that it's red plus or minus point one, hurts.
• 30:51
  Speaker 1: That's a pretty good measurement of its wavelength. No, we
• 30:54
  Speaker 1: can get to that point, right.
• 30:55
  Speaker 2: Yeah, you can make fairly precise measurements of individual photons. Yes,
• 30:59
  Speaker 2: you can also sources of photons that are fairly pure,
• 31:03
  Speaker 2: that are very tight bands of energy ranges. Yeah.
• 31:06
  Speaker 1: So then if I know the wavelength of the photon,
• 31:08
  Speaker 1: doesn't that give me a sense of how long it is?
• 31:11
  Speaker 2: If you know the wavelength of the photon and you
• 31:13
  Speaker 2: know the uncertainty in that wavelength, then yes, that defines
• 31:16
  Speaker 2: the length of this wave packet, all these possible photons
• 31:20
  Speaker 2: that are flying through space together. It's a little unsatisfying
• 31:23
  Speaker 2: as an answer because it's not something inherent to the photon.
• 31:26
  Speaker 2: It's like you got a bunch of these blobs all
• 31:28
  Speaker 2: moving together through the universe. The answer how long is
• 31:31
  Speaker 2: the photon depends sort of like on your uncertainty in
• 31:35
  Speaker 2: your knowledge of its energy. So I think it's accurate
• 31:37
  Speaker 2: from a quantum mechanical point of view, but it's very
• 31:39
  Speaker 2: unsatisfying from a philosophical point of view because it feels
• 31:42
  Speaker 2: like the photon should have a length that's just inherent
• 31:45
  Speaker 2: to it. It shouldn't depend on your measurement of it
• 31:48
  Speaker 2: or your knowledge of it.
• 31:49
  Speaker 1: But it doesn't sort of depend on my knowledge or
• 31:52
  Speaker 1: measurement of it, right, Like if I measured it and
• 31:54
  Speaker 1: I measured the red pleasure minus point when it hurts,
• 31:56
  Speaker 1: and somebody else measured would have measured it, they would
• 31:58
  Speaker 1: have probably gone the same result.
• 32:00
  Speaker 2: Right, Yeah, it doesn't depend on your particular knowledge of it.
• 32:02
  Speaker 2: There is an inherent uncertainty in it because it's a
• 32:05
  Speaker 2: quantum state, and to me that's a little bit unsatisfying.
• 32:08
  Speaker 2: The idea that it doesn't have a fixed length or
• 32:11
  Speaker 2: that it's length somehow depends on that uncertainty. To answer
• 32:14
  Speaker 2: your specific question. If there's uncertainty, it means that no
• 32:17
  Speaker 2: two people would make exactly the same measurement. They'd be
• 32:19
  Speaker 2: probably consistent, you know, within the uncertainties, but they wouldn't
• 32:22
  Speaker 2: get exactly the same.
• 32:23
  Speaker 1: Answer, right Right. We would all see it as vapor hotdog, right,
• 32:28
  Speaker 1: And so couldn't you. I mean, I know we're not
• 32:30
  Speaker 1: we can ever get super preciped, but we can probably
• 32:32
  Speaker 1: say you and I can both agree that yeah, that's
• 32:35
  Speaker 1: vapor hotdog and not miss the hot dog.
• 32:37
  Speaker 2: Yeah, And I'm not saying photons don't have a length.
• 32:40
  Speaker 2: I'm just saying that the length depends not just on
• 32:42
  Speaker 2: the wavelength of light, but on the uncertainty on the wavelength,
• 32:45
  Speaker 2: because in the end, they're quantum objects.
• 32:47
  Speaker 1: Right, So then can we answer the question of how
• 32:49
  Speaker 1: long a photon is or was? Or is it that
• 32:52
  Speaker 1: we can only answer what the length of a pooton was.
• 32:55
  Speaker 2: We can answer the question if you know the energy
• 32:58
  Speaker 2: and the uncertainty on that energy that determines the length
• 33:02
  Speaker 2: of the photon in this sense of length.
• 33:04
  Speaker 1: So that's good, right, possible? Yeah, are you saying it's impossible.
• 33:09
  Speaker 2: No, No, I'm saying it's possible.
• 33:11
  Speaker 1: All right, So then that's the quantum field theory version
• 33:13
  Speaker 1: of a photon. You said that how long a ploton
• 33:16
  Speaker 1: is depends on how you look at it. So then
• 33:18
  Speaker 1: if we assume light is a particle, can we measure
• 33:21
  Speaker 1: the length of that particle?
• 33:23
  Speaker 2: Yeah, The answer does depend a little bit on how
• 33:25
  Speaker 2: you look at it, because in some cases you don't
• 33:27
  Speaker 2: care about the length of photon. You don't care about
• 33:29
  Speaker 2: these details, and you don't care about the size of
• 33:31
  Speaker 2: anything that's really really small. So you could just treat
• 33:34
  Speaker 2: them as zero point particles. And we talk on the
• 33:37
  Speaker 2: podcast a lot about how like electrons have no size
• 33:39
  Speaker 2: and quarks have no size, And the answer to that
• 33:42
  Speaker 2: really is they have no size that we measure or
• 33:44
  Speaker 2: in some cases that we care about, and so we
• 33:46
  Speaker 2: can treat them as if they're zero point particles with
• 33:49
  Speaker 2: no length to them. For some problems where it doesn't
• 33:51
  Speaker 2: really matter if they have length, you know, like when
• 33:54
  Speaker 2: they're hitting a screen, we didn't really care how long
• 33:56
  Speaker 2: it took to hit the screen or what their extent
• 33:58
  Speaker 2: was as they were flying through space. We can just
• 33:59
  Speaker 2: treat them as if they were tiny, zero point particles,
• 34:03
  Speaker 2: And so that picture is useful for answering some kinds
• 34:05
  Speaker 2: of questions, just the same way we can think about
• 34:07
  Speaker 2: classical waves moving through space.
• 34:10
  Speaker 1: Well, I feel like it's sort of useful, maybe, I
• 34:12
  Speaker 1: wonder in some applications, like for example, let's say photons
• 34:16
  Speaker 1: are super duper long, they're the size of a planet
• 34:19
  Speaker 1: sized hotdog. Then when that photon hits me, it's going
• 34:23
  Speaker 1: to take a long time, you know, minute for me
• 34:27
  Speaker 1: to feel the photon all the way, as opposed to
• 34:30
  Speaker 1: if a photon is just an infinitely small point particle,
• 34:33
  Speaker 1: then I'm going to feel the photon instantly. So is
• 34:36
  Speaker 1: there sort of a time at which I get to
• 34:39
  Speaker 1: feel photons or is it relevant or what are the
• 34:42
  Speaker 1: hot dog dynamics here?
• 34:43
  Speaker 2: Yeah, so that's a great question, and to answer that question,
• 34:45
  Speaker 2: you definitely need to use the quantum field theory version
• 34:48
  Speaker 2: of a hot dog. You need to think about the
• 34:49
  Speaker 2: probability of photon having various wavelengths and those wavelengths overlapping
• 34:53
  Speaker 2: with you. When that probability wave packet overlaps with you,
• 34:56
  Speaker 2: and when it doesn't overlap with you, when it does collapse,
• 34:59
  Speaker 2: though it collapses instantly across the entire photon, you can't
• 35:02
  Speaker 2: feel like part of a photon. There is no part
• 35:04
  Speaker 2: of a photon, right, Photons are quantized.
• 35:07
  Speaker 1: Like when I feel a photon, it's instantaneous, is what
• 35:10
  Speaker 1: you're saying?
• 35:10
  Speaker 2: Yeah, you feel the whole photon or no photons exactly
• 35:13
  Speaker 2: or so or seven photons.
• 35:15
  Speaker 1: What if it has like super duper big waves like
• 35:17
  Speaker 1: we've talked about light waves having a wavelength the size
• 35:21
  Speaker 1: of a galaxy for example, Like we feel those instantly
• 35:24
  Speaker 1: or do we need to wait a long time to
• 35:25
  Speaker 1: feel them?
• 35:26
  Speaker 2: Yeah? You either feel them or you don't. Right, there's
• 35:29
  Speaker 2: no time at which you're like crewing a photon, right.
• 35:33
  Speaker 1: But the ripple of it isn't the ripple of it
• 35:35
  Speaker 1: in space long to or what?
• 35:37
  Speaker 2: Yeah, so photons could be really really long, right, if
• 35:40
  Speaker 2: you have a photon with really long wavelengths and really
• 35:43
  Speaker 2: large uncertainty, those photons could be the size of a galaxy, absolutely,
• 35:48
  Speaker 2: and that photon could interact with something within the galaxy. Right,
• 35:51
  Speaker 2: But then the whole photon collapses all at once, just
• 35:54
  Speaker 2: the same way that a pair of entangled particles you
• 35:55
  Speaker 2: shoot off in opposite directions, they're really still part of
• 35:58
  Speaker 2: one big quantum state. You measure one on one side
• 36:01
  Speaker 2: of the galaxy, the whole quantum state collapses at once,
• 36:04
  Speaker 2: because it's really just one quantum state. Same way for
• 36:08
  Speaker 2: this galaxy size hot dog size photon. If it's really
• 36:11
  Speaker 2: as big as the galaxy, If it interacts anywhere, then
• 36:14
  Speaker 2: the whole quantum state collapses at once.
• 36:16
  Speaker 1: M So, like you can think of it as having
• 36:20
  Speaker 1: a giant photon the size of a galaxy. But once
• 36:22
  Speaker 1: I catch it, it's really just a little tiny point particle.
• 36:25
  Speaker 2: Yeah, exactly. It interacts in that one spot and you
• 36:27
  Speaker 2: might think, hold on a second, doesn't this violate special relativity?
• 36:31
  Speaker 2: And it feels like, you know, that might allow you
• 36:33
  Speaker 2: to send messages faster than time. And there is a
• 36:36
  Speaker 2: real subtlety there with how quantum theory and relativity interact.
• 36:40
  Speaker 2: We talked about in the podcast. It's the reason why
• 36:42
  Speaker 2: we have anti particles. Antiparticles patch all this up with
• 36:46
  Speaker 2: all these negative probabilities and make sure that everything is
• 36:49
  Speaker 2: following all the rules. Check out our episode on why
• 36:52
  Speaker 2: quantum mechanics and special relativity require antiparticles.
• 36:55
  Speaker 1: Well, I feel like you're kind of making a judgment
• 36:57
  Speaker 1: on the particle view of life. You're saying it's not
• 37:01
  Speaker 1: really a particle, or you ultimately have to kind of
• 37:05
  Speaker 1: go back to quantum theory to talk about light. We
• 37:08
  Speaker 1: can't stay in the particle view at for very long.
• 37:11
  Speaker 2: I'm definitely using this field picture here, thinking about light
• 37:15
  Speaker 2: as ripples in electromagnetic field, and that I think is
• 37:18
  Speaker 2: the most mainstream view. But there's definitely a chunk of
• 37:21
  Speaker 2: particle theorists who think in the particle picture, and you
• 37:24
  Speaker 2: absolutely can you can replace the field with an infinite
• 37:28
  Speaker 2: number of virtual particles and do all the same calculations
• 37:31
  Speaker 2: and it all works. So what I've described is the
• 37:34
  Speaker 2: field picture of light as a ripple in this electromagnetic field.
• 37:37
  Speaker 2: You can also think about these probabilities in terms of
• 37:39
  Speaker 2: like these virtual particles, which are conceptually kind of slippery
• 37:43
  Speaker 2: because they're not really particles or really just probabilities. But
• 37:46
  Speaker 2: you can think about all these kind of interactions and
• 37:48
  Speaker 2: these transmissions in terms of an infinite number of virtual particles,
• 37:51
  Speaker 2: if you like. Though I think it's a lot more awkward,
• 37:54
  Speaker 2: especially in this case.
• 37:55
  Speaker 1: Well awkward is a relative tern I know they might
• 37:57
  Speaker 1: say the same thing about you.
• 37:58
  Speaker 2: Yeah, absolutely, and a little bit subjective. Mathematically, both pictures work,
• 38:02
  Speaker 2: so I'm trying not to make a judgment on what
• 38:05
  Speaker 2: is the best picture of the quantum universe. There's a
• 38:08
  Speaker 2: particle people and the fields people, and both of them
• 38:10
  Speaker 2: have strong cases. Conceptually, For me, the fields picture is
• 38:13
  Speaker 2: more intuitive, though that doesn't mean that it's right.
• 38:16
  Speaker 1: Right. So then let's say we replace you, Daniel. We
• 38:19
  Speaker 1: call this podcast Mark and Jorge explain the universe, and
• 38:23
  Speaker 1: Mark happens to be a particle person that sees the
• 38:26
  Speaker 1: world as particles. How would they answer the question how
• 38:29
  Speaker 1: long is a particle?
• 38:30
  Speaker 2: Even in the particle picture of the universe where there
• 38:32
  Speaker 2: are no fields, there's just an infinite number of real
• 38:35
  Speaker 2: particles and an infinite number of virtual particles communicating between them.
• 38:38
  Speaker 2: There are still probabilities, you still have wave functions about
• 38:42
  Speaker 2: where these particles are, and uncertainties on where the particles
• 38:45
  Speaker 2: are and how much energy they have. So in the end,
• 38:47
  Speaker 2: the answer is very much the same. Right. A photon,
• 38:50
  Speaker 2: even if you think about it as a particle, has
• 38:52
  Speaker 2: an uncertainty in its location. A photon is infinitely well known,
• 38:56
  Speaker 2: but still have an infinite uncertainty in its location. And
• 39:00
  Speaker 2: so even if you think about in terms of particles,
• 39:01
  Speaker 2: you get the same answer. It's either a packet of
• 39:04
  Speaker 2: waves moving through the universe with a range of frequencies,
• 39:07
  Speaker 2: or it's a packet of possible particles moving through the
• 39:10
  Speaker 2: universe with a range of possible energies.
• 39:12
  Speaker 1: All right, thank you Mary for answering that question. Now,
• 39:15
  Speaker 1: I think what you're saying is that even if you
• 39:17
  Speaker 1: look at the lightest particles, a particle is a point particle,
• 39:21
  Speaker 1: so itself, it doesn't have any length. So it kind
• 39:24
  Speaker 1: of doesn't make sense to talk about the length of
• 39:25
  Speaker 1: a photon. But these point particles have a certain fuzziness
• 39:29
  Speaker 1: about where they can be in the universe, and maybe
• 39:31
  Speaker 1: you can talk about the length of that fuzzy cloud
• 39:33
  Speaker 1: of where it could be, but ultimately you kind of
• 39:38
  Speaker 1: have to make a call about where where you draw
• 39:41
  Speaker 1: those boundaries, Like these fuzzy clouds don't have a hard
• 39:44
  Speaker 1: edge to them, the kind of fuzzy out to infinity,
• 39:47
  Speaker 1: and so it's up to you to say, this is
• 39:50
  Speaker 1: what I would call the photon, this is what would
• 39:51
  Speaker 1: not call the photon.
• 39:52
  Speaker 2: Exactly, And the pure concept of a single photon isn't
• 39:55
  Speaker 2: really helpful number one because they never exist in the universe,
• 39:58
  Speaker 2: and number two because they have infinite uncertainty in their
• 40:01
  Speaker 2: location and so they're sort of everywhere.
• 40:03
  Speaker 1: Cool, Well, I like this new podcast host Mary. Does
• 40:07
  Speaker 1: Mary like white chocolate? Then, because she's.
• 40:10
  Speaker 2: The of you, No, she agrees with me and everything else?
• 40:14
  Speaker 1: Right, all right, Well, let's talk about how you might
• 40:18
  Speaker 1: actually measure what you might call the length of a photon,
• 40:21
  Speaker 1: or not measure it, or maybe it's impossible. So let's
• 40:25
  Speaker 1: dig into that question. But first let's take one more break.
• 40:40
  Speaker 1: All right, we're talking about the length of a photon,
• 40:42
  Speaker 1: and now is it Daniel's backwards. It's still Mary.
• 40:49
  Speaker 2: Let's go back to Dan.
• 40:52
  Speaker 1: All right, we've sort of concluded that, you know, light
• 40:55
  Speaker 1: is a fuzzy quantum thing, so to talk about its
• 40:58
  Speaker 1: length kind of doesn't make sense. But there's sort of
• 41:00
  Speaker 1: the other aspect of it, which is what I was
• 41:02
  Speaker 1: trying to get at, which is like, when you measure
• 41:04
  Speaker 1: a photon, maybe you can measure its length sort of
• 41:07
  Speaker 1: because maybe it depends on how big your eyeball is
• 41:09
  Speaker 1: or how you know, how long you're there waiting for
• 41:12
  Speaker 1: the hot dog to hit you. So let's talk about
• 41:13
  Speaker 1: measuring how do you measure a photon and how does
• 41:16
  Speaker 1: it change the length of it?
• 41:17
  Speaker 2: Yeah, this is really fun. I spent some time thinking
• 41:20
  Speaker 2: about this and starting with how you measure the size
• 41:23
  Speaker 2: of other particles. It's a little bit easier to think
• 41:25
  Speaker 2: about like measuring the size of a proton, because we've
• 41:28
  Speaker 2: done that, or try to measure the size of the electron,
• 41:31
  Speaker 2: because we've tried to do that.
• 41:33
  Speaker 1: Well, we have measured the length of a proton.
• 41:35
  Speaker 2: We have measured the width of a proton. Yes, absolutely
• 41:37
  Speaker 2: we know something about the size of a proton.
• 41:40
  Speaker 1: Wait, wait, I thought we just concluded that you can't
• 41:42
  Speaker 1: do that with quantum particles.
• 41:43
  Speaker 2: We decided you can. But protons are not like fundamental
• 41:46
  Speaker 2: objects in the universe, right, So really we're talking about
• 41:49
  Speaker 2: like a bound state of quarks and how close do
• 41:51
  Speaker 2: they stay to each other?
• 41:52
  Speaker 1: But even that has a sort of an uncertainty that
• 41:55
  Speaker 1: spills out to infinity, doesn't it. So where do you
• 41:57
  Speaker 1: define the bounds of a proton?
• 42:00
  Speaker 2: Yeah, it's a little bit fuzzy, and you have to
• 42:02
  Speaker 2: do a little bit of mental gymnastics and come up
• 42:04
  Speaker 2: with a concept of size that makes sense for these particles.
• 42:07
  Speaker 2: You have to think about like what can I actually
• 42:10
  Speaker 2: measure and what number does that give me? And is
• 42:12
  Speaker 2: that really measuring the size of the object.
• 42:15
  Speaker 1: All right, let me do some mental stretching here before
• 42:18
  Speaker 1: I do some mental gymnastics. Well, what do you mean?
• 42:21
  Speaker 1: So when you say the side, because you just said
• 42:23
  Speaker 1: the size of a proton pretty decisively, would then as
• 42:26
  Speaker 1: a particle physicist, what do you define as the edge
• 42:29
  Speaker 1: of a proton?
• 42:30
  Speaker 2: Yeah? So I will be totally upfront here. The physics
• 42:33
  Speaker 2: has redefined size and then answered the question. What we
• 42:36
  Speaker 2: really mean is that we do a specific kind of
• 42:38
  Speaker 2: experiment where we bounce stuff off the proton, and we
• 42:43
  Speaker 2: notice how that changes as we scan across a proton. So,
• 42:46
  Speaker 2: for example, you shoot electrons at the proton and they
• 42:49
  Speaker 2: mostly go through, and then you shoot them a little
• 42:51
  Speaker 2: bit to the right and oops, now they're bouncing back,
• 42:53
  Speaker 2: or now they're exploding the proton. And as you keep going,
• 42:56
  Speaker 2: you discover that as you sweep your beam over past
• 42:59
  Speaker 2: the other side of the proton, then now it's missing
• 43:01
  Speaker 2: the proton again. So there's like a size of the
• 43:04
  Speaker 2: proton there in the sense of like how it reacts
• 43:06
  Speaker 2: to the beam and electrons that you're sweeping over it.
• 43:10
  Speaker 1: It's sort of like searching for a stud on your.
• 43:13
  Speaker 2: Wall, right, Yeah, exactly. It's a little bit philosophical to
• 43:16
  Speaker 2: interpret this as size, because what do you mean anyway
• 43:19
  Speaker 2: by the size of a proton. A proton is an
• 43:21
  Speaker 2: easier thing to talk about than a photon, because at
• 43:24
  Speaker 2: least a proton has mass. You can like hold one,
• 43:26
  Speaker 2: you can capture one, you can say this is the
• 43:28
  Speaker 2: one I'm talking about. Photons are much harder, and we'll
• 43:31
  Speaker 2: talk in a minute about how you might be able
• 43:32
  Speaker 2: to measure their size. But this is the kind of
• 43:34
  Speaker 2: thing we do for a proton and This is one way,
• 43:36
  Speaker 2: for example, that we discovered that the atom had a
• 43:39
  Speaker 2: proton inside of it. Right. Rutherford's original experiment was basically this,
• 43:43
  Speaker 2: You shut alpha particles at gold foils and notice that
• 43:45
  Speaker 2: they bounce back sometimes and not other times. And he
• 43:47
  Speaker 2: used this to see like, oh, there's like hard little
• 43:50
  Speaker 2: nuggets inside the gold foil, and those were the nuclei.
• 43:53
  Speaker 2: And you can do the same kind of thing to
• 43:55
  Speaker 2: see the size or a proton. You can also do
• 43:57
  Speaker 2: the same kind of thing to see inside a proton,
• 43:59
  Speaker 2: to see like how often is it bouncing off of
• 44:01
  Speaker 2: a quark that's inside the proton?
• 44:03
  Speaker 1: Right, But like you said, it's sort of a fuzzy boundary,
• 44:05
  Speaker 1: isn't it. Like as you're scanning where the proton is
• 44:08
  Speaker 1: by shooting electrons at it, at some point like sometimes
• 44:11
  Speaker 1: it'll hit, sometimes it won't, even though you're shooting in
• 44:13
  Speaker 1: the same exact direction, And as you scan through the right,
• 44:16
  Speaker 1: for example, the frequency at which it might glance off
• 44:19
  Speaker 1: of the proton changes. So there's a bit of fuzziness.
• 44:22
  Speaker 1: So when do you make the call like okay, that's
• 44:24
  Speaker 1: the edge of the proton, or do you.
• 44:26
  Speaker 2: Know you're exactly right? There's a little bit of fuzziness there,
• 44:28
  Speaker 2: Like if you did this experiment with billiard balls, right,
• 44:31
  Speaker 2: there'd be a moment when they come into contact and
• 44:33
  Speaker 2: then a moment when they don't, and there's a precision there,
• 44:36
  Speaker 2: and we don't have the same thing with protons. There's
• 44:39
  Speaker 2: some point at which you shoot the electron and sometimes
• 44:41
  Speaker 2: it bounces back and sometimes it passes through, and so
• 44:44
  Speaker 2: like is that the edge of the proton, And so
• 44:46
  Speaker 2: we just make a sort of mathematical definition. We define
• 44:48
  Speaker 2: the width of this distribution, and we say that with
• 44:51
  Speaker 2: this distribution tells us the size of the proton.
• 44:54
  Speaker 1: Meaning like the width of a proton is the width
• 44:57
  Speaker 1: at which if you aim at an electron added beyond
• 45:00
  Speaker 1: that then only you know ten percent of them will hit.
• 45:03
  Speaker 2: It, exactly like if you know a Gaussian distribution, you
• 45:06
  Speaker 2: can characterize the width of it. It doesn't capture the
• 45:08
  Speaker 2: whole distribution. It's just like a characteristic number that tells
• 45:11
  Speaker 2: you roughly how wide it is. And there's a possibility
• 45:14
  Speaker 2: you go past that with and you still interact with
• 45:16
  Speaker 2: the proton. And there's a possibility you go below that
• 45:18
  Speaker 2: width and you don't interact with the proton. So it's
• 45:20
  Speaker 2: a quantum fuzzy definition of size. That's fuzzy in another
• 45:24
  Speaker 2: way too, because it depends on the thing you're touching
• 45:26
  Speaker 2: it with, Like protons will react to electrons differently than
• 45:30
  Speaker 2: they will react to muons or react to neutrinos. So
• 45:33
  Speaker 2: the whole concept of size is really about the interaction
• 45:37
  Speaker 2: of two things. It's not inherent property of the object anyway,
• 45:41
  Speaker 2: at least this quantum definition of size.
• 45:43
  Speaker 1: I see, like it depends on the experiment. The width
• 45:45
  Speaker 1: of a proton. You can't talk about the width of
• 45:47
  Speaker 1: a proton. You have to say what's the width of
• 45:49
  Speaker 1: a proton when it's interacting with electrons? Or what's the
• 45:52
  Speaker 1: width of a proton when it's interacting with hot dog ginos?
• 45:56
  Speaker 1: Even then it's fuzzy and you kind of have to
• 45:57
  Speaker 1: make a call and say, well, you know it's about
• 45:59
  Speaker 1: here that it starts to taper off.
• 46:02
  Speaker 2: Yeah, exactly, all.
• 46:03
  Speaker 1: Right, So then let not switch to photons. Do the
• 46:05
  Speaker 1: same thing apply to photons? Like does it depend on
• 46:07
  Speaker 1: how we measure it?
• 46:08
  Speaker 2: So this is tricky because photons don't like to interact
• 46:10
  Speaker 2: with each other. You can't just like shoot one photon
• 46:13
  Speaker 2: at another and say, like how often are they going
• 46:15
  Speaker 2: to touch each other this kind of stuff. Remember, photons
• 46:18
  Speaker 2: only interact with things that have electric charge. So you
• 46:20
  Speaker 2: can shoot photons at electrons, but you can't shoot photons
• 46:24
  Speaker 2: at photons and see them interact. Very often. When they do,
• 46:27
  Speaker 2: it's because they've actually spontaneously transformed into electrons and positrons
• 46:31
  Speaker 2: and then interacted. So I was thinking about it, and
• 46:33
  Speaker 2: there's another way you might be able to get a
• 46:35
  Speaker 2: sense for the length of a photon because photons don't
• 46:38
  Speaker 2: interact with each other the same way particles do, but
• 46:40
  Speaker 2: they can interfere with each other. If photons are at
• 46:43
  Speaker 2: the same place at the same time, they will interfere,
• 46:46
  Speaker 2: like the way we have interferometers. You know, we talk
• 46:49
  Speaker 2: about interference, you get like light patches and dark patches.
• 46:52
  Speaker 1: Wait, wait, let maybe take a step back. What is
• 46:54
  Speaker 1: it that you're trying to do. You're trying to measure
• 46:56
  Speaker 1: the size of this wave packet or the size of
• 46:59
  Speaker 1: the fuzziness an electron. Is that kind of what you're
• 47:01
  Speaker 1: trying to do.
• 47:02
  Speaker 2: I'm thinking about how to measure the length of that
• 47:04
  Speaker 2: wave packet of a photon, And I was thinking about
• 47:06
  Speaker 2: if you send two photons through an interference experiment, Like,
• 47:10
  Speaker 2: do the interfere with each other? They will if they're
• 47:12
  Speaker 2: right on top of each other. They won't if they're
• 47:14
  Speaker 2: really separated, Like if you wait ten seconds between shooting photons,
• 47:18
  Speaker 2: they won't interfere with each other. There's some point in
• 47:20
  Speaker 2: which if you send two photons through the experiment close
• 47:23
  Speaker 2: enough together in time that their wave packets are overlapping,
• 47:26
  Speaker 2: that they will interfere with each other. And so I'm
• 47:29
  Speaker 2: thinking that's like one way to define the width of
• 47:32
  Speaker 2: the wave packet of each photon is like how close
• 47:35
  Speaker 2: they have to be to each other in time, which
• 47:37
  Speaker 2: then gets translated to distance so that they start interfering
• 47:40
  Speaker 2: with each other.
• 47:41
  Speaker 1: Doesn't light interact with electrons? For example, So like we
• 47:44
  Speaker 1: use electrons like you you said, to measure the width
• 47:47
  Speaker 1: of a proton, couldn't we kind of flip it and
• 47:49
  Speaker 1: use an electron to measure the width of a light particle?
• 47:52
  Speaker 1: Like what if I sit an electron there on a
• 47:54
  Speaker 1: table and I just shoot photons at it? Wouldn't this
• 47:57
  Speaker 1: sort of tell me how whine my photon?
• 48:00
  Speaker 2: Yeah? But are we talking about the length of a
• 48:01
  Speaker 2: photon or the width of a photon?
• 48:03
  Speaker 1: Wait? Wait? Meaning like is it light shape like a
• 48:06
  Speaker 1: hot dog. Let's assume the light is shape like a
• 48:09
  Speaker 1: meat ball. Wouldn't the length also tell you the width.
• 48:12
  Speaker 2: The length or the width it depends on the uncertainty
• 48:14
  Speaker 2: of its production. Right, The entire length of the photon
• 48:16
  Speaker 2: comes from the uncertainty you have in how it was produced.
• 48:19
  Speaker 2: It's either infinitely long if it's perfectly well measured, or
• 48:23
  Speaker 2: it's very very tiny if it's very uncertain in its energy.
• 48:26
  Speaker 2: So the width might come from a different uncertainty. So, yeah,
• 48:29
  Speaker 2: if you want to talk about the width of the photon,
• 48:30
  Speaker 2: like which direction does it come out of the laser,
• 48:33
  Speaker 2: this uncertainty there in the photon's width as well as
• 48:36
  Speaker 2: in its length. That could be a different number.
• 48:38
  Speaker 1: But I feel like when you were talking about the proton,
• 48:40
  Speaker 1: you were using the word length to mean it's.
• 48:43
  Speaker 2: With Yeah, for proton, we really are measuring its width
• 48:46
  Speaker 2: in that case. You're right, So.
• 48:48
  Speaker 1: You're assuming protons are meat bull shaped. Well, I mean
• 48:53
  Speaker 1: that is important, right, No, you're right. Yeah, you're right,
• 48:56
  Speaker 1: you're assuming protons are meat bull shapes, But you're not
• 48:59
  Speaker 1: assuming that light is meatball shape. You're assuming it might
• 49:01
  Speaker 1: be hot dog shape or not. I don't know.
• 49:03
  Speaker 2: Yeah, absolutely, I'm using the meatbon model of a proton,
• 49:05
  Speaker 2: the hot dog model of a photon, and somebody else
• 49:08
  Speaker 2: might have a different you know, maybe a French version
• 49:12
  Speaker 2: of it, right whe there's a pastry version.
• 49:13
  Speaker 1: Donut, a French fry version, the pompfleet model.
• 49:17
  Speaker 3: Yes.
• 49:18
  Speaker 2: So to measure the width of a photon, you could
• 49:20
  Speaker 2: scan a beam across a bunch of electrons and see
• 49:22
  Speaker 2: when they interact and then will give you a sense
• 49:24
  Speaker 2: for like the width of your beam. And if you
• 49:26
  Speaker 2: slow it down to individual photons, if you go a
• 49:28
  Speaker 2: sense of the width of the wave packet of the photon.
• 49:30
  Speaker 2: I think to get a sense of the length of
• 49:32
  Speaker 2: a photon, you might want to see how the photons
• 49:34
  Speaker 2: overlap in an interference experiment, see when they start interfering.
• 49:38
  Speaker 2: That proves something we call coherence length of the photon.
• 49:41
  Speaker 1: I wonder if you can measure the length of a
• 49:43
  Speaker 1: hot dog photon by measuring by using time, Like, if
• 49:47
  Speaker 1: there's more uncertainty in when you receive the photon, would
• 49:50
  Speaker 1: that tell you that it's a really long it's a
• 49:52
  Speaker 1: foot long hot dog. Where I suppose if the uncertainty
• 49:56
  Speaker 1: and when you receive the photon is very short. It's like, oh,
• 49:59
  Speaker 1: it's a vienna.
• 50:00
  Speaker 2: Yeah, And principle, if you know the energy and uncertainty,
• 50:02
  Speaker 2: you can just define the length. I was trying to
• 50:04
  Speaker 2: think about a way to like experimentally measure another sense
• 50:07
  Speaker 2: of the length in terms of like when two photons
• 50:09
  Speaker 2: overlap with each other, rather than just thinking about that
• 50:11
  Speaker 2: length of an individual photon theoretically. But yeah, you can
• 50:15
  Speaker 2: definitely define the length of an individual photon theoretically from
• 50:18
  Speaker 2: its energy and the uncertainty, which again is coupled to
• 50:21
  Speaker 2: the uncertainty and its time measurement.
• 50:22
  Speaker 1: So I feel like maybe the headline from this podcast
• 50:25
  Speaker 1: episode is a physicists claim light is shaped like a
• 50:29
  Speaker 1: hot dog.
• 50:32
  Speaker 2: You know, one thing I love about this podcast is
• 50:34
  Speaker 2: I've never have any idea where it's going to end
• 50:36
  Speaker 2: up going. There's no way to prepare for this.
• 50:38
  Speaker 1: There's an uncertainty about its length.
• 50:40
  Speaker 2: Also, the topic, the concept, the analogies we end up using.
• 50:44
  Speaker 2: This is proof that this podcast is unscripted because nobody
• 50:47
  Speaker 2: could write this stuff.
• 50:51
  Speaker 1: Well we are, we're writing it right now, Daniel. It's happening.
• 50:54
  Speaker 1: It's happening, we're living it.
• 50:55
  Speaker 3: Man.
• 50:57
  Speaker 1: Well, I mean, would you I feel that that's the
• 50:59
  Speaker 1: biggest thing that I'm getting out of this is that
• 51:01
  Speaker 1: you know, you're in your thought point of view. A
• 51:04
  Speaker 1: photon is not spherical, it's maybe has different dimensions to it.
• 51:07
  Speaker 2: Yeah, I hadn't thought about the width of a photon,
• 51:09
  Speaker 2: but you're right. It has all the same theoretical questions
• 51:13
  Speaker 2: to it and experimental trickery to measure the width of it.
• 51:16
  Speaker 2: But the width and the length of a photon could
• 51:18
  Speaker 2: be very different. You could have a source of photons
• 51:20
  Speaker 2: that's very uncertain in length and very certain in width.
• 51:23
  Speaker 1: I think that you know, as you you gave a
• 51:26
  Speaker 1: proton of definite size, right like in physics you have
• 51:29
  Speaker 1: a size with plus or mind is a certain amount
• 51:31
  Speaker 1: of uncertainty. If you had to do that for a light,
• 51:34
  Speaker 1: for a photon, like maybe an everyday photon that we
• 51:36
  Speaker 1: see every day, what would you say it is its length?
• 51:39
  Speaker 2: Yeah, that's a great question. You know, a typical photon
• 51:42
  Speaker 2: that's like coming out of the light that's made from
• 51:45
  Speaker 2: a light bulb in your house, that's a glow of
• 51:47
  Speaker 2: like a little piece of metal. So there's a very
• 51:49
  Speaker 2: wide spread in the uncertainty of those photons.
• 51:52
  Speaker 1: Oh cool, Now, how would say it compares to its
• 51:56
  Speaker 1: width like our photons hot like the every day times
• 52:00
  Speaker 1: we see hot dyck shaped or are they football shaped
• 52:02
  Speaker 1: or are they more spherically? Tis like, what kind of fun?
• 52:06
  Speaker 1: What kind of bunch should I get to eat it?
• 52:09
  Speaker 2: I think it's probably curved, so you should get a croissant?
• 52:12
  Speaker 2: Oh no, I don't know the answer that it depends
• 52:15
  Speaker 2: a lot on the source for a typical filament from
• 52:18
  Speaker 2: like an incandescent bulb. There's again going to be a
• 52:20
  Speaker 2: lot of uncertainty in the direction, So these things are
• 52:23
  Speaker 2: going to be pretty fat. Maybe there's sausage paddies after all.
• 52:26
  Speaker 1: Oh yeah, oh man, I hadn't even thought about that
• 52:28
  Speaker 1: snag Like they could be like pancakes flying at you
• 52:31
  Speaker 1: face face.
• 52:32
  Speaker 2: Forward, Yeah, more sideways.
• 52:34
  Speaker 1: Yeah. Interesting. All right, So I guess we sort of
• 52:37
  Speaker 1: answered the question how long a photon is?
• 52:39
  Speaker 2: We know that these things are really hard to think about,
• 52:41
  Speaker 2: and that the answer depends a little bit on the
• 52:43
  Speaker 2: question you're asking and exactly how you want to answered,
• 52:46
  Speaker 2: And along the way you often have to redefine what
• 52:49
  Speaker 2: you mean by your question in order to get a
• 52:51
  Speaker 2: specific unsatisfying answer.
• 52:53
  Speaker 1: Yeah, and in the enda, I guess it's all a
• 52:55
  Speaker 1: little bit fuzzy due to the fuzzy nature of the universe.
• 52:59
  Speaker 2: But put enough mustard it it'll be delicious.
• 53:01
  Speaker 1: Yeah, it's a little fuzzy though. The Fuzzy hot Dog Podcast.
• 53:08
  Speaker 1: All right, well, another interesting dive into the quantum nature
• 53:12
  Speaker 1: of the universe and how even simple questions like how
• 53:15
  Speaker 1: big is a photon or what shape it has requires
• 53:18
  Speaker 1: a whole conversation about the nature of length and what
• 53:23
  Speaker 1: even means to be something in the universe.
• 53:25
  Speaker 2: That's right, the most basic questions are the hardest to answer.
• 53:28
  Speaker 1: All right, well, we hope you enjoyed that. Thanks for
• 53:30
  Speaker 1: joining us. See you next time.
• 53:36
  Speaker 2: For more science and curiosity, come find us on social
• 53:39
  Speaker 2: media where we answer questions and post videos. We're on Twitter,
• 53:43
  Speaker 2: disc Org, Instant, and now TikTok. Thanks for listening, and
• 53:47
  Speaker 2: remember that Daniel and Jorge Explain the Universe is a
• 53:50
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• 53:58
  Speaker 2: your favorite shows.