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