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Speaker 1: Hey everyone, it's Daniel. I have a new book out

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Speaker 1: now called Do Aliens Speak Physics? About whether or not

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Speaker 1: we have physics in common with aliens. I think it's

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Speaker 1: a lot of fun. But here's a totally unbiased review

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Speaker 1: from Phil Plate, author of Bad Astronomy, Quote whites and

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Speaker 1: makes us think about how we think about aliens and

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Speaker 1: how we think about the universe at a fundamental level,

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Speaker 1: and does it in a readable, understandable, and even funny way.

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Speaker 1: If we ever meet aliens, the first thing we should

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Speaker 1: do is give them this book. So go out and

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Speaker 1: pick up a copy of Do Aliens Speak Physics? I

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Speaker 1: think you'll enjoy it. When you see the universe through

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Speaker 1: the lens of physics, you start to see physics everywhere.

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Speaker 1: Why is glass transparent but stone isn't? Why or bicycle stable?

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Speaker 1: How do tornadoes start? The mysteries of physics aren't just

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Speaker 1: out there in deep space. They're right here in front

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Speaker 1: of us, raising questions and demanding answers every day. Today

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Speaker 1: we'll tackle the topic that I've wondered about since I

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Speaker 1: was a little baby physicist, looking at the world and

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Speaker 1: wondering how it worked. All you need for this inspiration

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Speaker 1: is a light bulb and your hands with which you

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Speaker 1: can make something out of nothing. Shadows aren't really anything.

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Speaker 1: They're a lack of something. They're negative objects, but our

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Speaker 1: minds see them as positive, as a thing in the

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Speaker 1: world rather than just the absence of light, and shadows

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Speaker 1: have been surprising and confusing people for thousands of years.

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Speaker 1: Today we'll dig into the physics of shadows and ask

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Speaker 1: whether they can break one of physics' most hallowed principles.

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Speaker 1: Welcome to Daniel and Kelly's Extraordinary Shadowy Universe.

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Speaker 2: Hello, I'm Kelly Wadersmith. I study parasites and space and

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Speaker 2: one of my absolute favorite things is when it's a

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Speaker 2: clear night and it's a full moon and I'm out

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Speaker 2: on the farm and you can see the shadows of

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Speaker 2: the trees and your own shadow in the middle of

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Speaker 2: the night. It feels sort of like spooky but fantastic,

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Speaker 2: and I just absolutely love it. I like, shadows.

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Speaker 1: Are those star shadows or moonshadows?

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Speaker 2: No, it's just that there's so much sunlight reflecting off

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Speaker 2: the moon and making it down, you know, to Earth

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Speaker 2: that you can see, you know, the shadows of the trees. Wait,

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Speaker 2: what did you ask me? Are they moonshadows. Yeah, they're moonshadows.

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Speaker 2: I'll call them moonshadows.

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Speaker 1: Yeah, moonshadows are awesome. Yeah, I love it cool.

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Speaker 2: As I was explaining it, I was like, there's no

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Speaker 2: way Daniel doesn't understand where the light is coming from.

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Speaker 2: Why am I explaining it to him? I must have

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Speaker 2: missed the question.

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Speaker 1: Hi, I'm Daniel. I'm a particle physicist, and I do

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Speaker 1: understand why the moon is bright at night.

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Speaker 2: Go ahead, Daniel, why's the moon bright at night?

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Speaker 1: Are you doubting me? Or you're putting me on the spot?

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Speaker 1: Good's College left No, obviously, because the moon is being

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Speaker 1: illuminated by the sun. Just because we can't see the

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Speaker 1: sun doesn't mean that some parts of the moon can't

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Speaker 1: see the sun.

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Speaker 2: And I've never lived in an area where there was

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Speaker 2: so little light pollution that I could see the shadows

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Speaker 2: cast by the moon. Another reason why Virginia is amazing.

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Speaker 2: So my question for you today, Daniel. So I was

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Speaker 2: looking at the question asked by the listener, and I

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Speaker 2: realized that faster than light questions sometimes just like make

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Speaker 2: my brain go, oh, this is really not making sense

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Speaker 2: to me. What physics concept that when you think about

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Speaker 2: makes you think, I just my brain can't really. I

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Speaker 2: have to accept that this is true because the science

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Speaker 2: is good, but my brain is just having trouble wrapping

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Speaker 2: itself around this fact confuses you about physics thermodynamics.

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Speaker 1: Yeah, I've never really been a fan of statistical physics

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Speaker 1: and thermodynamics because it verges on chemistry. It's like you

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Speaker 1: start from all these micro states and these little particles,

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Speaker 1: and then you zoom out and you draw these conclusions

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Speaker 1: about the macroscopic state, but there's so many particles involved,

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Speaker 1: and I don't know, it just feels like there's lots

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Speaker 1: of approximations and considerations and you never really feel like

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Speaker 1: you're on solid footing. And when I showed up at Berkeley,

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Speaker 1: they make you take this qualifying exam to see like

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Speaker 1: how good is your physics. And I remember my advisor

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Speaker 1: at the time. He was like, good students pass this

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Speaker 1: exam on the first try, and I was like gold yep.

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Speaker 1: So I spent that whole summer before grad school studying, studying, studying,

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Speaker 1: and I spent most of it studying thermodynamics because I

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Speaker 1: knew that was my weak spot. And this qualifying exam

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Speaker 1: is terrifying. It's two eight hour days, right, so it's

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Speaker 1: like a serious ordeal. It's like a marathon.

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Speaker 2: This is right when you get there. This isn't like

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Speaker 2: two years in or something.

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Speaker 1: Well, you can wait and take it after you've taken

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Speaker 1: the grad school classes. But my advisor was like, the

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Speaker 1: best students show up and pass it on the first try.

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Speaker 1: And I was like, that'scish. Yeah, but I lucked out

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Speaker 1: and that year no THERMO questions, just zero. I was like, yeah,

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Speaker 1: all right, I'm gonna skate on through.

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Speaker 2: And did you did you pass it on the first time.

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Speaker 1: I did pass it on the first try, And then

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Speaker 1: I had to take up through my dnemics class and

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Speaker 1: I skated just over the minimum threshold for that class.

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Speaker 2: Let me tell you, nice, but you did it, and

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Speaker 2: now it's behind you.

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Speaker 1: Congratulations, And I'm a certified physicist and I don't have

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Speaker 1: to do themo dynamics, you know.

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Speaker 2: Speaking of chemistry, last night, I was giving a talk

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Speaker 2: to a women in chemistry group over Zoom, and halfway

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Speaker 2: through I realized that I was describing a chemistry thing

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Speaker 2: and I suddenly got really insecure because I was like,

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Speaker 2: what if this is wrong? Because I just pretend to

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Speaker 2: know chemistry.

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Speaker 1: Everybody pretends to know chemistry, even the chemists.

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Speaker 2: And I was like, well, what if any of them

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Speaker 2: listen to the podcast and they know what I've said

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Speaker 2: about chemistry. But you know, I think it went okay

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Speaker 2: in the end.

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Speaker 1: I think it's important to admit our strengths and weaknesses,

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Speaker 1: you know, that's part of being a scientist. But that's

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Speaker 1: not the kind of stuff that just got me into physics.

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Speaker 1: I wasn't always fascinated with concepts, but temperature and pressure

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Speaker 1: and this kind of stuff. I remember as a kid

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Speaker 1: thinking about light, you know, wondering like if I block

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Speaker 1: the sun, if I put my hand in front of

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Speaker 1: the sun, how does light get to that tree over there,

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Speaker 1: And just thinking about like the way light moves. This

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Speaker 1: is like six year old Daniel, like doing little experiments

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Speaker 1: and like, obviously now it seems like a silly experiment.

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Speaker 1: The light can get to the tree even if my

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Speaker 1: hand is blocking the light from my face. But you

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Speaker 1: don't know until you do the experiment, right, And so anyways,

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Speaker 1: I've always been super fascinated by light and by shadows.

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Speaker 2: Well let's not leave our listeners in the shadows. Let's

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Speaker 2: jump into today's topic. See, I did.

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Speaker 1: Very nice. I was trying to get us there. Today's

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Speaker 1: episode is inspired by a question from a listener who

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Speaker 1: asked we could do a whole episode on shadows, and specifically,

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Speaker 1: they were wondering about three scenarios. One where you're standing

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Speaker 1: in the shadow of something like a tree, but you

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Speaker 1: can still see things around you. Why is it the

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Speaker 1: photons are reaching his eyes even if the sun is obstructed.

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Speaker 1: Number Two, he was wondering about what are shadows like

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Speaker 1: on the Moon? Are they sharper or darker than on Earth?

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Speaker 1: And then finally, of course, can shadows move faster than light?

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Speaker 3: Oh?

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Speaker 2: That sounds like it could be a trick question. I

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Speaker 2: think we should pass that on to our listeners.

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Speaker 1: That's a great idea. So let's play a trick question

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Speaker 1: on our listeners. And so I went out there and

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Speaker 1: I asked them if they thought shadows could travel faster

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Speaker 1: than light. Here's what our listeners had to say on

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Speaker 1: this brilliant question. The obligatory answers, nothing can move faster

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Speaker 1: than the speed of light, but a sense that there

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Speaker 1: are multiple levels to this question. No, however, I would

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Speaker 1: say that they can appear to travel faster than light,

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Speaker 1: as they tend to loom larger than the object that's

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Speaker 1: being illuminated. But no, they can't travel fast and light.

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Speaker 2: According to Einstein, No, they can't because they are the

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Speaker 2: absence of light, so they are related to light speed.

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Speaker 3: If I had a spherical projector's screen at the orbit

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Speaker 3: of Pluto, and I had a bright light in front

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Speaker 3: of me, and then all of a sudden, I put

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Speaker 3: my hand in front of the bright light, it would

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Speaker 3: appear that a giant swath of shadow would go across

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Speaker 3: the screen all at once to me. But I think

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Speaker 3: to Pluto they would see it different, like it would

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Speaker 3: take this speed of light to go from one place

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Speaker 3: to another.

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Speaker 2: Since you asked, it means that there's probably some exception,

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Speaker 2: so let's find out.

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Speaker 3: I would say, No, shadows are the absence of light,

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Speaker 3: and light can only travel at the speed of light.

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Speaker 1: That's a fascinating one. I'm gonna say, No, shadows aren't

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Speaker 1: a thing. There are absence of a thing. But the

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Speaker 1: answer Boyans would be doll I can't. Yes, but it

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Speaker 1: cannot be used for communication. Yes, shadows can move faster

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Speaker 1: than light, with caveat that only their appearance can move faster.

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Speaker 1: Than light. No, actual photon is moving faster than light.

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Speaker 1: I think so, because nothing's actually moving. It's just a

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Speaker 1: change in the pattern of light. But you're really breaking

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Speaker 1: my brain here. I think shadows are dependent upon the light.

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Speaker 1: It's kind of like the absence of light, and so

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Speaker 1: I think no. I think there's something like this about

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Speaker 1: the way the point where two blades of a scissors cross.

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Speaker 1: If the scissors are closed extremely quickly, I'll start see

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Speaker 1: how shadows could travel false with than the spate of light.

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Speaker 1: So I'm gonna say I travel at the same spade.

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Speaker 2: So I think our listeners know us pretty well. A

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Speaker 2: couple of them are like, uh, this feels like a

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Speaker 2: trick question.

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Speaker 1: I love hearing them use their physics knowledge and try

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Speaker 1: to work it out. Yes, some people got halfway there.

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Speaker 1: You know that nothing is actually moving. Other people are like, no,

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Speaker 1: this is an absolute principle and physics nothing can move

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Speaker 1: faster than the speed of light. But as usual, language

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Speaker 1: is the culprit here, because the principles of physics are

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Speaker 1: very clear mathematically, but things get fuzzy when you express

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Speaker 1: them in language, and so we're gonna end up splitting

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Speaker 1: the hairs between nothing and no thing, So.

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Speaker 2: Thanks for playing along, even though we set you up

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Speaker 2: for failure over and over and over again. We appreciate you.

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Speaker 1: We really do, We really do, And thanks to listeners

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Speaker 1: like Eric who write in with their questions and inspire

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Speaker 1: these episodes. We really want to hear what you are

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Speaker 1: curious about, because we want to scratch your itch about physics,

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Speaker 1: not just ours.

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Speaker 2: Yes, we want to shine light on the darkness in

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Speaker 2: your lives. And if you want to contact us, you

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Speaker 2: can write us at questions at Danielankelly dot org. And

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Speaker 2: that's both to send us questions and to get on

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Speaker 2: the list of people that we send these trick questions to.

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Speaker 1: Or just to tell us about your day and send

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Speaker 1: us cute pictures of your cat.

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Speaker 2: Yeah, that'd be great.

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Speaker 1: It works for everything, yep, I would love that.

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Speaker 2: All right, Daniel, let's start with the basics. Shadows sound

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Speaker 2: like something where you're like, oh, yeah, I know what

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Speaker 2: a shadow is, But physics always makes simple concepts much

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Speaker 2: more complicated. So, Daniel, how would you define a shadow?

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Speaker 1: A shadow is just the absence of light, And in

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Speaker 1: the simplest sense, this is fairly straightforward. If you have

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Speaker 1: a single source of light and you have things with

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Speaker 1: very crisp edges. Then you can use the concepts of

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Speaker 1: geometric optics where light travels and straight lines, and some

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Speaker 1: regions of your experiment will be illuminated and some regions

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Speaker 1: will not, and those are the shadows. And so places

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Speaker 1: that are obstructed from direct line with the light source

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Speaker 1: will be in shadow, and places that are not will

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Speaker 1: not be in shadow. So that's sort of the simplest,

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Speaker 1: clearest setup where the shadows are very straightforward.

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Speaker 2: Okay, now give us some more complicated information, because I

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Speaker 2: know that's where you're going.

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Speaker 1: Yeah, because we don't live in that kind of situation.

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Speaker 1: We never have a single source of light. Like number one,

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Speaker 1: Our light sources are not points. They're extended, right, They're

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Speaker 1: a little bit wide. And in physics we treat a

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Speaker 1: wide light source, you know, like a filament that's the

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Speaker 1: centimeter across, as a bunch of different sources of light.

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Speaker 1: You can treat it as like a set of pinpoint sources. Right,

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Speaker 1: So what happens if you have multiple sources of light, Well,

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Speaker 1: imagine two sources of light. You're in a room and

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Speaker 1: there's two light bulbs. Well, you can be in total

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Speaker 1: shadow if you are blocked from both sources of light, right,

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Speaker 1: And you can be in total illumination if you can

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Speaker 1: see both sources of light. But there's also a middle ground.

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Speaker 1: What if you're blocked from one source of light and

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Speaker 1: not the other, then you're in like half shadow, right,

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Speaker 1: And so now this is the fuzzy region. This is

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Speaker 1: actually called the penumber, places where you are blocked partially

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Speaker 1: from the full illumination but not completely. And so any

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Speaker 1: room that you're in, your shadows are going to have

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Speaker 1: these fuzzy edges because of their per numbers. Like I'm

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Speaker 1: in a room right now, and I have like four

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Speaker 1: banks of fluorescent lights, each of which is like a

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Speaker 1: meter across. So if I hold on my hand above

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Speaker 1: my desk, my shadows are very fuzzy. In fact, there's

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Speaker 1: like four of them and they partially overlap, and so

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Speaker 1: I don't have crisp shadows anywhere in my office.

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Speaker 2: Oh, how sad.

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Speaker 1: It's good good because otherwise the shadows would be very

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Speaker 1: very stark. Right, But this cost is really interesting effects

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Speaker 1: that are sometimes hard to understand.

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Speaker 2: So could you still like if you had one light

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Speaker 2: in a room and it was like a broad light,

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Speaker 2: like a foot wide or something, could you still have

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Speaker 2: like a shadowy shadow? Hey, a fuzzy shadow is what

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Speaker 2: I meant. If the light is like reflecting off of

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Speaker 2: the walls a lot and coming back underneath your hand.

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Speaker 1: Absolutely yes. And so that's another contribution is that the

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Speaker 1: things in your room do not perfectly absorb light. If

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Speaker 1: everything in your room was a black body object where

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Speaker 1: it just absorbed light and didn't reflect any then you

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Speaker 1: would have crisper shadows. But if you have a white wall,

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Speaker 1: it's white because it's reflecting light. And so even in

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Speaker 1: this scenario where you have one source of light where

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Speaker 1: you expect crisp shadow, if your walls are white, they're

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Speaker 1: reflecting light, and so some of that light is going

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Speaker 1: to reduce the shadow. So yeah, there's lots of ways

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Speaker 1: that shadows get fuzzier because things are reflective and because

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Speaker 1: there are multiple sources of light.

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Speaker 2: Can I tell you the story about the one time

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Speaker 2: I came across the word penumbra while doing research.

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Speaker 1: Is it gonna make me spit out my coffee or

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Speaker 1: throw up?

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Speaker 2: No, it's not gross. It's a little silly, but it's

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Speaker 2: not gross.

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Speaker 1: So let's do it.

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Speaker 2: We were reading about space settlement proposals, and there was

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Speaker 2: someone who was proposing that Mercury would be a great

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Speaker 2: place to set up a space settlement. Really really, yeah.

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Speaker 1: Because the wonderful outdoor temperatures.

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Speaker 2: Yes, right, So Mercury being the closest planet to the

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Speaker 2: Sun and with no atmosphere, it gets very hot on

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Speaker 2: one side and very cold on the.

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Speaker 1: Other because it's tidally locked.

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Speaker 2: Yes, right, but at the penumbra the temperature is pretty nice.

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Speaker 1: How wide is that? It's like centimeters or meters.

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Speaker 2: It's whe enough that they thought you could like put

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Speaker 2: a moving habitat that would have to constantly move to

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Speaker 2: stay with the p number, and if you fall behind

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Speaker 2: or get too far ahead, you die, you know, different ways,

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Speaker 2: depending on if you're going too fast or too slowly.

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Speaker 1: I like that. If my house breaks down, I die

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Speaker 1: very quickly. Yeah, that's very relaxing. I could definitely take

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Speaker 1: a nap in that.

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Speaker 2: Yeah. Yeah, I think I'll pass on this plan. The

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Speaker 2: other plan was to bury yourself underground at the poles,

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Speaker 2: where the temperature was also not so lethal. But anyway,

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Speaker 2: I'm happy where I am. Daniel. I've heard this phrase

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Speaker 2: shadow blister, and I have no idea what it means

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Speaker 2: what is a shadow blister?

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Speaker 1: Shadow blister is a really cool effect whereas two shadows

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Speaker 1: get closer together, they seem to kind of merge, and

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Speaker 1: even more than that, it seems like they leap out

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Speaker 1: towards each other. They grow towards each other. So if

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Speaker 1: you like stand next to a telephone pole, you have

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Speaker 1: a shadow. Telephone pole has a shadow. As you inch

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Speaker 1: towards a telephone pole, you'll see your shadows merge, but

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Speaker 1: they don't just like overlap geometrically. When you get close

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Speaker 1: to the telephone pole, the two shadows grow out to

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Speaker 1: meet each other. It looks really weird. You're like, what

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Speaker 1: is going on in my shadows? Like hugging? Do they

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Speaker 1: know about each other? Are they conscious? Is this am

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Speaker 1: I living in some weird science fiction novel? No physics

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Speaker 1: can't explain.

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Speaker 2: This well, So is it called a shadow blister because

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Speaker 2: you're not supposed to pop them when they get together,

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Speaker 2: because then they might get infected? Is why are they

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Speaker 2: called shadow?

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Speaker 1: Way? To make it grow scalty way? To make it crisal?

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Speaker 2: I had to get it.

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Speaker 1: They called shadow blisters because they sort of grow out

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Speaker 1: beyond the edge of the existing shadow, So you can

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Speaker 1: create a blister on the light pole shadow because of

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Speaker 1: your shadow. Well, what's happening here is that you don't

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Speaker 1: notice that the telephone pole shadow has many layers. There's

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Speaker 1: the sort of major shadow, but then there's sort of

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Speaker 1: minor shadows. There's the panenumbers, right, and as your numbers

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Speaker 1: overlap with the numbers of the shadow, then you start

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Speaker 1: to notice these things. And Minute Physics, which is a

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Speaker 1: great channel on YouTube which is amazing explainers about lots

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Speaker 1: of stuff, has a great video on this. You really

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Speaker 1: got to see this video to understand it, so check

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Speaker 1: got that video. But the key concept there is the number,

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Speaker 1: the fact that shadows are almost never crisp because you

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Speaker 1: don't have single sources of light.

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Speaker 2: That's awesome. I didn't know that at all. Yeah, exactly, Daniel,

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Speaker 2: I understand my world better.

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Speaker 1: Now, And so obviously in our lives, most of the

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Speaker 1: shadows come from the sun, right or from interior lights,

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Speaker 1: but like Kelly told us, you can also get shadows

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Speaker 1: from any source of light, including the moon, which of

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Speaker 1: course originally comes from the sun. But it's still kind

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Speaker 1: of cool to be walking around at night and see

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Speaker 1: your shadow, and if the moon is dark and it's

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Speaker 1: an exceptionally clear night and you're out very very far

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Speaker 1: from light pollution. You can see something really awesome, which

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Speaker 1: is a star shadow. What you can see your shadow

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Speaker 1: from the stars.

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Speaker 2: Yeah, how could you be sure that it was from

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Speaker 2: the stars? I get if you can't see the moon,

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Speaker 2: then it's definitely not the moon that's causing it. This

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Speaker 2: is crazy. I can't wrap my head around this.

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Speaker 1: It's hard to identify because the stars come from all directions,

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Speaker 1: but in principle, right, it's there. And it's kind of

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Speaker 1: amazing poetic that those photons have crossed like billions and

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Speaker 1: billions of kilometers of space only to be like blocked

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Speaker 1: from hitting the Earth by your hand.

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Speaker 2: Or whatever, by your dumb face.

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Speaker 1: And it just reminds me of how frustrating it is

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Speaker 1: that all these photons come from all over the universe

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Speaker 1: splash on the Earth and mostly they're just ignored. Like

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Speaker 1: those photons carry so much information about what happened inside

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Speaker 1: that star, what was going on. You know, it's future,

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Speaker 1: it's history, it's hopes and dreams, and they just like

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Speaker 1: get absorbed by some plant or whatever, and that's just lost.

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Speaker 1: We are tapping into, like the tiniest bit of this

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Speaker 1: huge river of information that's coming at us from the

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Speaker 1: universe anyway, and sometimes it makes cool shadows for you

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Speaker 1: to go ooh nice.

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Speaker 2: So Daniel clearly is suffering from a massive case of

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Speaker 2: fomo yes, missing out on what those photons could tell.

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Speaker 2: Let's take a break and hopefully Daniel doesn't descend into despair,

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Speaker 2: and when we get back, we'll talk about some more

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Speaker 2: complicated features of shadows.

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Speaker 1: All Right, we're back and we're talking about light and shadows.

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Speaker 1: And Kelly's comment reminds me that I actually have a

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Speaker 1: solution to my existential angst about missing all those photons.

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Speaker 2: How are you going to collect all the photons? Daniel?

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Speaker 1: Well, look, I figure if Dyson can come up with

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Speaker 1: its concept of Dyson spheres, which are basically just solar

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Speaker 1: panels that's around the Sun, I'm just going to make

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Speaker 1: a white sn sphere, which is basically space telescopes that's

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Speaker 1: around the Earth.

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Speaker 2: Ah.

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Speaker 1: Like, let's just give up our vision of the night

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Speaker 1: sky and replace it with like a solid bank of

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Speaker 1: telescope that gobble all that information. Imagine what we could learn.

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Speaker 1: I mean, every time we develop a space telescope that

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Speaker 1: appears at one little corner of the sky. We have

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Speaker 1: our minds blown by what we see out there, and

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Speaker 1: so think about what we could learn if we had

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Speaker 1: like a thousand, a billion times more capacity. It's never

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Speaker 1: gonna happen. But we're also never going to build the

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Speaker 1: Dyson Sphere, so I can put the White Sin sphere

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Speaker 1: into the same category of fantastical concepts.

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Speaker 2: Yeah, if only we could collect all the photons and

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Speaker 2: deprive the plants of them, I'm sure that would be

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Speaker 2: great for all of us. I don't know who's who's

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Speaker 2: less realistic, you or Dyson, but it's a fun idea

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Speaker 2: to think about.

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Speaker 1: All right, maybe just half the Earth then you know

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Speaker 1: when the Earth is in shadow?

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Speaker 2: Which, oh, when it's in shadow? Okay, I was going

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Speaker 2: to ask which hemisphere you're going to condemn to.

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Speaker 1: Let's have a vote. No, obviously at night. Yeah. So yeah,

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Speaker 1: this project gets more complicated and less realistic as we

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Speaker 1: think about it, but it was never gonna happen anyway.

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Speaker 2: So, I mean, people are upset enough about starlink satellites

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Speaker 2: going in orbit. I'm not sure anyway, let's move on.

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Speaker 1: So so far we've been talking about shadows from a

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Speaker 1: sort of geometrical optics point of view. Shadows just travel

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Speaker 1: in straight lines and like at the edge of an object,

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Speaker 1: either the light is absorbed or it passes. But the

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Speaker 1: wave light interacts with objects is more complicated than that,

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Speaker 1: and it makes shadows fuzzier and weirdly surprising. So shadows

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Speaker 1: played a really important role in the debate about the

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Speaker 1: nature of light. Is it a particle or is it

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Speaker 1: a wave?

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Speaker 2: Is this the two slit experiment.

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Speaker 1: Yes, the two slit experiment was part of it, but

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Speaker 1: that actually didn't settle the question in most people's minds

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Speaker 1: about whether light was a particle or wave until they

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Speaker 1: did this crazy shadow experiment. So what's the connection between

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Speaker 1: shadows and the particle or wave nature of light. Well,

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Speaker 1: if light is a particle, then shadows should be very sharp, right,

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Speaker 1: either the particle of light passes the edge of the

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Speaker 1: object or it's absorbed. But if light is a wave,

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Speaker 1: then it's more complicated because you get things like diffraction

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Speaker 1: and interference. So in the early eighteen hundreds, people had

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Speaker 1: done this experiment, like the Young double slit experiment, to

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Speaker 1: show that light had this like behavior, that it was

437
00:22:02,080 --> 00:22:05,480
Speaker 1: interfering with itself and so light was very likely a wave,

438
00:22:05,840 --> 00:22:08,480
Speaker 1: and you can see the same thing not just in interference,

439
00:22:08,680 --> 00:22:12,720
Speaker 1: but in diffraction. Diffraction is like the big sister of interference.

440
00:22:13,359 --> 00:22:15,840
Speaker 1: The way it works is, imagine you have like a

441
00:22:15,880 --> 00:22:18,600
Speaker 1: circular object and you shine a light on it, and

442
00:22:18,640 --> 00:22:20,920
Speaker 1: you have a screen behind it. What do you expect, Well,

443
00:22:21,000 --> 00:22:24,560
Speaker 1: you expect a circular shadow, right. If light is a particle,

444
00:22:24,640 --> 00:22:26,919
Speaker 1: that shadow should be perfect and crisp if you have

445
00:22:26,960 --> 00:22:30,000
Speaker 1: a single light source, because there's like an edge where

446
00:22:30,040 --> 00:22:32,399
Speaker 1: the particles just barely make it around the object, and

447
00:22:32,440 --> 00:22:34,679
Speaker 1: a tiny bit further in, for example, they don't, so

448
00:22:34,680 --> 00:22:37,360
Speaker 1: you get a very crisp shadow. With me still, yep,

449
00:22:37,359 --> 00:22:39,480
Speaker 1: I'm with you, all right, But if light is a wave,

450
00:22:39,760 --> 00:22:42,760
Speaker 1: that's not how light interacts with matter. What you need

451
00:22:42,800 --> 00:22:46,080
Speaker 1: to do instead is imagine a bunch of sources of

452
00:22:46,160 --> 00:22:49,359
Speaker 1: light all around the object, the same way as in

453
00:22:49,400 --> 00:22:51,960
Speaker 1: the interference experiment where you have two slits. The way

454
00:22:51,960 --> 00:22:54,360
Speaker 1: you think about that mathematically is each slit is now

455
00:22:54,440 --> 00:22:56,960
Speaker 1: a source of light. The light makes it through the slit.

456
00:22:57,000 --> 00:22:59,080
Speaker 1: You imagine that as a point source, and then those

457
00:22:59,080 --> 00:23:02,399
Speaker 1: two point sources interfere. Diffraction is like that, but now

458
00:23:02,480 --> 00:23:04,520
Speaker 1: you have lots and lots and lots of sources of

459
00:23:04,600 --> 00:23:08,199
Speaker 1: light all around the edge of this object. Everywhere the

460
00:23:08,280 --> 00:23:10,640
Speaker 1: light is making it around is a point source of light,

461
00:23:10,960 --> 00:23:13,920
Speaker 1: and those are all going to interfere. And so if

462
00:23:13,960 --> 00:23:17,119
Speaker 1: you look very carefully at the edge of shadows, you

463
00:23:17,160 --> 00:23:20,800
Speaker 1: can see this diffraction effect. There is no perfectly crisp shadow.

464
00:23:21,040 --> 00:23:23,320
Speaker 1: Even in a room filled with black bodies and a

465
00:23:23,359 --> 00:23:27,040
Speaker 1: single source of pinpoint light, you will still get these

466
00:23:27,200 --> 00:23:30,280
Speaker 1: fringes at the edge of shadows. You'll have like a

467
00:23:30,440 --> 00:23:33,000
Speaker 1: dark black center, and then you'll have a white band,

468
00:23:33,119 --> 00:23:35,520
Speaker 1: and then a black ring and then a white band.

469
00:23:35,800 --> 00:23:38,399
Speaker 1: You get this zebra shadow effect from the diffraction edges.

470
00:23:38,720 --> 00:23:40,600
Speaker 2: So I've got a bright light and I'm putting my

471
00:23:40,640 --> 00:23:43,359
Speaker 2: hand under it and I'm not seeing the like. So

472
00:23:43,440 --> 00:23:46,000
Speaker 2: in our outline you have this great picture with ripples

473
00:23:46,320 --> 00:23:48,919
Speaker 2: that of course our audience can't see. But imagine like

474
00:23:48,960 --> 00:23:52,040
Speaker 2: you drop a rock in a lake and there's ripples.

475
00:23:52,280 --> 00:23:54,280
Speaker 2: There's a black spot where the rock got dropped, and

476
00:23:54,280 --> 00:23:56,439
Speaker 2: then there's ripples coming out from there. But that's not

477
00:23:56,480 --> 00:23:58,240
Speaker 2: what I feel like I'm seeing when I put my

478
00:23:58,359 --> 00:24:00,920
Speaker 2: hand under the light. And what was different about the

479
00:24:01,000 --> 00:24:05,720
Speaker 2: lab conditions that find these ripples obviously relative to my office.

480
00:24:05,920 --> 00:24:08,560
Speaker 1: Yeah, well, you have to have a single source of light,

481
00:24:08,600 --> 00:24:11,639
Speaker 1: because otherwise you have all these panumbers which are overlapping,

482
00:24:12,040 --> 00:24:14,600
Speaker 1: and it's hard to isolate this effect. And you have

483
00:24:14,680 --> 00:24:17,520
Speaker 1: to have a room with no other reflections because this

484
00:24:17,560 --> 00:24:20,240
Speaker 1: is a subtle effect. It's not easy to see. But

485
00:24:20,400 --> 00:24:22,520
Speaker 1: it was not so subtle that two hundred years ago

486
00:24:22,560 --> 00:24:25,720
Speaker 1: they couldn't do it. And so the interference experiment and

487
00:24:26,000 --> 00:24:30,000
Speaker 1: this shadow diffraction experiment were very strong indicators that light

488
00:24:30,280 --> 00:24:32,760
Speaker 1: was a wave, not a particle. And at the time

489
00:24:32,840 --> 00:24:37,159
Speaker 1: people really believed, like Newton's idea that light was little corpuscules, right,

490
00:24:37,200 --> 00:24:40,400
Speaker 1: there was this little bits of stuff. So these were

491
00:24:40,440 --> 00:24:42,720
Speaker 1: hard to absorb, and there were lots of people who

492
00:24:42,800 --> 00:24:45,480
Speaker 1: like really dug in and they were like, this is absurd.

493
00:24:45,640 --> 00:24:49,080
Speaker 1: Light cannot be a wave. And one famous physicist Poisson

494
00:24:49,600 --> 00:24:52,600
Speaker 1: of Poisson statistics and all sorts of stuff. He studied

495
00:24:52,600 --> 00:24:54,560
Speaker 1: the theory in detail and he was trying to prove

496
00:24:54,600 --> 00:24:55,000
Speaker 1: it wrong.

497
00:24:55,160 --> 00:24:57,200
Speaker 2: I have a feeling I would have been on team Poissan.

498
00:24:57,720 --> 00:24:59,920
Speaker 2: Light cannot be a wave. It doesn't.

499
00:25:00,000 --> 00:25:03,080
Speaker 1: That's just because Poisson means fish, and you're always on

500
00:25:03,119 --> 00:25:03,600
Speaker 1: team fish.

501
00:25:03,680 --> 00:25:06,960
Speaker 2: Kelly, I am always on team Fish. It's true. The

502
00:25:07,080 --> 00:25:10,760
Speaker 2: number of jokes that Fish people make about Poissan's statistics

503
00:25:10,960 --> 00:25:12,639
Speaker 2: is maybe nauseating.

504
00:25:12,800 --> 00:25:15,040
Speaker 1: Off the hook, off the hock.

505
00:25:18,640 --> 00:25:21,680
Speaker 2: Oh, thank you, Daniel, Okay, moving on, I had to.

506
00:25:21,640 --> 00:25:25,280
Speaker 1: Dive into that one anyway. This is a great example

507
00:25:25,280 --> 00:25:27,239
Speaker 1: of something that happens in physics all the time that

508
00:25:27,359 --> 00:25:30,879
Speaker 1: people look for a ridiculous prediction from a theory as

509
00:25:30,880 --> 00:25:32,720
Speaker 1: a way to prove it wrong, but then it turns

510
00:25:32,720 --> 00:25:35,480
Speaker 1: out to actually prove it right. So Poissan did the

511
00:25:35,560 --> 00:25:39,399
Speaker 1: calculations and he discovered a weird prediction from the wave theory.

512
00:25:39,480 --> 00:25:42,520
Speaker 1: He discovered that at the center of that shadow, the

513
00:25:42,560 --> 00:25:46,600
Speaker 1: wave theory predicted a bright spot. Oh so you know,

514
00:25:46,680 --> 00:25:49,440
Speaker 1: the particle theory was the shadow should be perfectly circular,

515
00:25:50,000 --> 00:25:53,160
Speaker 1: and the wave theory predicted all these fringes, the zebra

516
00:25:53,240 --> 00:25:55,760
Speaker 1: lines at the edge, but also at the very center,

517
00:25:56,359 --> 00:26:00,000
Speaker 1: all of the waves add up and constructively interfere because

518
00:26:00,000 --> 00:26:02,800
Speaker 1: they are all equidistant from the edge, and so all

519
00:26:02,840 --> 00:26:07,679
Speaker 1: those photons should be in phase. And so Poissan was like, Aha,

520
00:26:07,760 --> 00:26:10,640
Speaker 1: this is a ridiculous prediction. You're telling me there should

521
00:26:10,640 --> 00:26:12,520
Speaker 1: be a bright spot at the center of the shadow

522
00:26:12,920 --> 00:26:16,480
Speaker 1: absurd and so this obviously disproves the wave nature of light, right.

523
00:26:16,480 --> 00:26:17,360
Speaker 2: That would be fishy.

524
00:26:19,560 --> 00:26:20,840
Speaker 1: You're just waiting with that jail.

525
00:26:21,960 --> 00:26:23,080
Speaker 2: You can see it in my face.

526
00:26:24,359 --> 00:26:26,960
Speaker 1: And so this was a very strong argument to reject

527
00:26:26,960 --> 00:26:29,040
Speaker 1: the wave theory. But then a guy went out and

528
00:26:29,080 --> 00:26:31,640
Speaker 1: actually did it, a guy called Arago. He went out

529
00:26:31,680 --> 00:26:34,679
Speaker 1: and did this experiment and there is a bright spot

530
00:26:34,680 --> 00:26:37,600
Speaker 1: at the center of the shadow. Do you google this image,

531
00:26:37,640 --> 00:26:39,679
Speaker 1: you can see there is a tiny white dot. This

532
00:26:39,760 --> 00:26:44,560
Speaker 1: is now called the Arago dot. And it was very conclusive,

533
00:26:44,600 --> 00:26:48,240
Speaker 1: and people were like, okay, well, you know, we said

534
00:26:48,359 --> 00:26:52,560
Speaker 1: that wave theory makes this absurd, nonsensical prediction, so therefore

535
00:26:52,560 --> 00:26:55,119
Speaker 1: it can't be true. But then if the universe actually

536
00:26:55,119 --> 00:26:57,480
Speaker 1: does it that way, that's a pretty clear indicator that

537
00:26:57,520 --> 00:27:00,240
Speaker 1: the wave theory is correct. So while the youngs double

538
00:27:00,240 --> 00:27:03,920
Speaker 1: slit experiment and these diffraction experiments were very strong evidence

539
00:27:03,960 --> 00:27:06,919
Speaker 1: already of the wave nature of light, it wasn't until

540
00:27:06,960 --> 00:27:09,960
Speaker 1: this shadow experiment, seeing a light at the center of

541
00:27:10,000 --> 00:27:13,159
Speaker 1: the shadow that people were like, okay, fine, light is

542
00:27:13,160 --> 00:27:13,520
Speaker 1: a wave?

543
00:27:13,720 --> 00:27:15,840
Speaker 2: What was poissan a good sport? And was he like,

544
00:27:15,920 --> 00:27:21,160
Speaker 2: oh you got me now I'm hooked on the wave theory.

545
00:27:21,680 --> 00:27:24,639
Speaker 2: I know I took your joke. But was Poissan convinced

546
00:27:24,680 --> 00:27:26,360
Speaker 2: after this or was he long gone by then?

547
00:27:26,560 --> 00:27:29,000
Speaker 1: Well, the historical summary I read suggests that it wasn't

548
00:27:29,119 --> 00:27:32,680
Speaker 1: especially gracious about it. You know, he didn't like on

549
00:27:32,720 --> 00:27:35,520
Speaker 1: the spot admit the wave theory. He was skeptical for

550
00:27:35,560 --> 00:27:37,840
Speaker 1: a while, but you know, he went on to have

551
00:27:37,880 --> 00:27:41,240
Speaker 1: a perfectly fine reputation, so he definitely survived scientifically.

552
00:27:41,600 --> 00:27:43,840
Speaker 2: Yeah, but you know, I still I like my scientists

553
00:27:43,880 --> 00:27:46,760
Speaker 2: to be gracious when they're wrong. But anyway, what are

554
00:27:46,800 --> 00:27:48,439
Speaker 2: you going to do? So first of all, you have

555
00:27:48,520 --> 00:27:50,440
Speaker 2: blown my mind because when I first looked at this picture,

556
00:27:50,480 --> 00:27:52,240
Speaker 2: I didn't see the little white dot in the center.

557
00:27:52,359 --> 00:27:54,240
Speaker 2: I thought that it was a speck on my screen.

558
00:27:55,000 --> 00:27:56,920
Speaker 2: And so while you were explaining it, I was moving

559
00:27:56,920 --> 00:27:58,520
Speaker 2: the outline up and down, and I was like, that

560
00:27:58,600 --> 00:28:01,200
Speaker 2: dot is actually on the image. Yeah, that's awesome.

561
00:28:01,480 --> 00:28:04,560
Speaker 1: So shadows taught us something about the nature of light, right.

562
00:28:04,560 --> 00:28:06,920
Speaker 1: The patterns of shadows are much more complex than you

563
00:28:07,000 --> 00:28:09,600
Speaker 1: might imagine, and the wave nature of light really is

564
00:28:09,680 --> 00:28:11,560
Speaker 1: revealed by the patterns of the shadows.

565
00:28:11,720 --> 00:28:13,760
Speaker 2: Yeah, so this is all a little hard to believe,

566
00:28:13,760 --> 00:28:15,439
Speaker 2: but I'm with you. But like, what next? Are you're

567
00:28:15,440 --> 00:28:17,000
Speaker 2: going to tell me that shadows have colors?

568
00:28:18,280 --> 00:28:19,960
Speaker 1: Shadows do have colors? Yes?

569
00:28:19,960 --> 00:28:20,080
Speaker 2: Oh?

570
00:28:20,280 --> 00:28:24,320
Speaker 1: Absolutely yes. So far we've only been thinking about single

571
00:28:24,400 --> 00:28:28,439
Speaker 1: sources of white light and complete shadows. Right, but remember

572
00:28:28,480 --> 00:28:31,240
Speaker 1: we talked about panumbers, right, you can have multiple sources

573
00:28:31,280 --> 00:28:34,640
Speaker 1: of light, and so you can have intermediate shadows. Well,

574
00:28:34,680 --> 00:28:36,639
Speaker 1: now take those multiple sources of light, so you have

575
00:28:36,720 --> 00:28:39,520
Speaker 1: three of them, and you make those colors. You have

576
00:28:39,560 --> 00:28:41,520
Speaker 1: a filter for each one, so you have like a

577
00:28:41,560 --> 00:28:44,520
Speaker 1: red source, a green source, and a blue source. Anywhere

578
00:28:44,520 --> 00:28:46,680
Speaker 1: where you can see all three sources, you'll be seeing

579
00:28:46,720 --> 00:28:50,520
Speaker 1: white light. And anywhere all three sources are blocked, you'll

580
00:28:50,520 --> 00:28:53,320
Speaker 1: be in shadow. But what happens if you're in a

581
00:28:53,360 --> 00:28:56,080
Speaker 1: place where you're only blocking the red light, then you

582
00:28:56,120 --> 00:28:59,840
Speaker 1: have green and blue light, which make a cyan shadow.

583
00:29:00,160 --> 00:29:02,120
Speaker 1: Or if you block the blue light, the red and

584
00:29:02,160 --> 00:29:05,440
Speaker 1: green merge, making a yellow shadow, or if you block

585
00:29:05,480 --> 00:29:07,920
Speaker 1: the green, the red and blue combined to form a

586
00:29:08,160 --> 00:29:09,560
Speaker 1: magenta shadow.

587
00:29:10,240 --> 00:29:14,600
Speaker 2: Daniel, Right, so I don't feel a good Yes, but

588
00:29:14,680 --> 00:29:17,760
Speaker 2: this is totally going against my into a shadows are black? Daniel?

589
00:29:18,080 --> 00:29:19,680
Speaker 2: Is there is there a good video online?

590
00:29:19,760 --> 00:29:21,640
Speaker 1: Yeah? I'm sure you can find a good video. But

591
00:29:21,760 --> 00:29:24,440
Speaker 1: this sort of bends the definition of shadow. I think

592
00:29:24,560 --> 00:29:28,040
Speaker 1: is the issue because in these regions, is it really

593
00:29:28,080 --> 00:29:30,760
Speaker 1: a magenta shadow? Well, you're shining a red and blue

594
00:29:30,840 --> 00:29:33,800
Speaker 1: light on it, so people would say it's magenta because

595
00:29:33,800 --> 00:29:37,280
Speaker 1: you're shining magenta light on it, not because you've blocked

596
00:29:37,360 --> 00:29:40,200
Speaker 1: the green light. Right, And so it really depends on

597
00:29:40,240 --> 00:29:42,480
Speaker 1: how you define the things that are not fully illuminated

598
00:29:42,680 --> 00:29:44,719
Speaker 1: or the things that are not fully blocked. Are they

599
00:29:44,760 --> 00:29:47,520
Speaker 1: parts of the shadow? Are they the colored panumbers? Or

600
00:29:47,560 --> 00:29:49,080
Speaker 1: are they partially illuminated.

601
00:29:49,520 --> 00:29:51,080
Speaker 2: This is a physicist trick.

602
00:29:53,480 --> 00:29:56,200
Speaker 1: I'm retreating to philosophy to avoid being proven wrong.

603
00:29:59,320 --> 00:30:01,040
Speaker 2: All right, well, maybe I'll try this with my kids

604
00:30:01,040 --> 00:30:05,160
Speaker 2: one day. Okay, so I have a son who loves swimming.

605
00:30:05,480 --> 00:30:09,880
Speaker 2: Constantly he's swimming. Is there anything interesting or different about

606
00:30:09,880 --> 00:30:13,640
Speaker 2: how shadows are cast underwater? Are their p numbers bigger something?

607
00:30:13,720 --> 00:30:19,040
Speaker 2: Because of how water defrects, deflex Oh? What is the

608
00:30:19,040 --> 00:30:25,480
Speaker 2: word defracts, bends, refracts, refracts. Thank you.

609
00:30:27,400 --> 00:30:31,080
Speaker 1: Shadows and water are fascinating because it's a little bit counterintuitive,

610
00:30:31,360 --> 00:30:35,160
Speaker 1: but water casts a shadow, like if you pour water

611
00:30:35,240 --> 00:30:37,680
Speaker 1: into a glass and you shine a light on it.

612
00:30:37,760 --> 00:30:39,600
Speaker 1: And then have a screen on the other side. You

613
00:30:39,640 --> 00:30:42,360
Speaker 1: will see the shadow of the water. And at first

614
00:30:42,360 --> 00:30:44,840
Speaker 1: you're like, wait a second, why would water have a shadow?

615
00:30:44,920 --> 00:30:48,360
Speaker 1: Water is transparent, right, It's like glass. Light passes through it.

616
00:30:48,560 --> 00:30:50,680
Speaker 1: Why is it making a shadow? What's going on? Did

617
00:30:50,720 --> 00:30:52,880
Speaker 1: I find a glitch in the matrix? You have not

618
00:30:52,960 --> 00:30:56,120
Speaker 1: found a glitch in the matrix. Water is transparent, but

619
00:30:56,280 --> 00:30:59,200
Speaker 1: light does not pass through it without bending. And so

620
00:30:59,240 --> 00:31:01,440
Speaker 1: it's happening here is that the light is acting like

621
00:31:01,480 --> 00:31:04,360
Speaker 1: a lens and it's refracting a lot of that light away.

622
00:31:04,920 --> 00:31:07,120
Speaker 1: And so the shadow there doesn't come from the object

623
00:31:07,160 --> 00:31:10,640
Speaker 1: being completely opaque, but from it being darker behind the

624
00:31:10,720 --> 00:31:13,040
Speaker 1: column of water because some of that light has been

625
00:31:13,040 --> 00:31:13,959
Speaker 1: refracted away.

626
00:31:14,520 --> 00:31:16,560
Speaker 2: Ah. So if you were to measure the light on

627
00:31:16,640 --> 00:31:19,160
Speaker 2: the sides of the water, it would be brighter after

628
00:31:19,200 --> 00:31:20,120
Speaker 2: you put the cup there.

629
00:31:20,280 --> 00:31:22,480
Speaker 1: It might be a tiny bit, but it gets refracted

630
00:31:22,560 --> 00:31:24,600
Speaker 1: in many, many directions, so most of it are even

631
00:31:24,680 --> 00:31:26,080
Speaker 1: hit the screen. Yeah.

632
00:31:26,120 --> 00:31:29,560
Speaker 2: So when you are standing in the ocean, which I

633
00:31:29,560 --> 00:31:31,520
Speaker 2: guess you get to do as a California and probably

634
00:31:31,560 --> 00:31:40,200
Speaker 2: pretty often. All right, that is pretty solid. So you're

635
00:31:40,240 --> 00:31:43,520
Speaker 2: standing in the ocean and it's a bright sunny day

636
00:31:44,080 --> 00:31:47,480
Speaker 2: and you can see shadows cast on the sand by

637
00:31:47,560 --> 00:31:50,080
Speaker 2: the water. Is that just because like the way the

638
00:31:50,120 --> 00:31:53,040
Speaker 2: waves build up, it's changing the patterns of how the

639
00:31:53,160 --> 00:31:53,800
Speaker 2: light is bent.

640
00:31:54,240 --> 00:31:57,000
Speaker 1: Yeah, exactly, because the surface is not flat. If you

641
00:31:57,040 --> 00:32:00,840
Speaker 1: stood in a perfectly still body of water, you would

642
00:32:00,840 --> 00:32:03,400
Speaker 1: see no shadows from the water on the bottom of

643
00:32:03,400 --> 00:32:05,920
Speaker 1: the pool, for example. But as soon as you make

644
00:32:05,920 --> 00:32:08,920
Speaker 1: a ripple on the surface, then you're gonna see shadows

645
00:32:08,960 --> 00:32:11,440
Speaker 1: of those ripples. For the same reason that now when

646
00:32:11,480 --> 00:32:13,720
Speaker 1: the light is hitting the surface of the water, it's

647
00:32:13,720 --> 00:32:16,400
Speaker 1: not going straight down to the bottom anymore. It's getting

648
00:32:16,440 --> 00:32:19,680
Speaker 1: bent away. And so when you have a pool that's

649
00:32:19,720 --> 00:32:22,560
Speaker 1: just like sitting there and it's like gently fluctuating, you

650
00:32:22,640 --> 00:32:25,400
Speaker 1: get these amazing patterns on the bottom of the pool, right,

651
00:32:25,720 --> 00:32:29,120
Speaker 1: brighter spots where the light is being concentrated, in darker

652
00:32:29,160 --> 00:32:32,479
Speaker 1: spots shadows essentially where the light is being bent away.

653
00:32:32,960 --> 00:32:35,400
Speaker 1: And so again this is a case not a full obstruction,

654
00:32:35,720 --> 00:32:38,400
Speaker 1: but of like a rearrangement of where the light is going,

655
00:32:38,600 --> 00:32:41,600
Speaker 1: creating these patterns of light and shadow. Really beautiful. One

656
00:32:41,600 --> 00:32:44,480
Speaker 1: of my favorite things about water is that you're essentially

657
00:32:44,520 --> 00:32:47,920
Speaker 1: seeing the surface right You're seeing the shadows of the surface.

658
00:32:48,280 --> 00:32:49,680
Speaker 2: I like that it keeps me from dying.

659
00:32:52,520 --> 00:32:54,840
Speaker 1: Water is good. Yes, we are definitely pro water on

660
00:32:54,920 --> 00:32:55,640
Speaker 1: this podcast.

661
00:32:57,680 --> 00:33:00,200
Speaker 2: Another place where I like looking at shadows is on

662
00:33:00,280 --> 00:33:02,720
Speaker 2: a foggy day. So like we on our farm, every

663
00:33:02,720 --> 00:33:04,320
Speaker 2: once in a while, the fog will like come up

664
00:33:04,360 --> 00:33:07,200
Speaker 2: from the bottom fields. And one, I like looking at

665
00:33:07,200 --> 00:33:10,000
Speaker 2: my shadow in it. But two, I like imagining that

666
00:33:10,120 --> 00:33:12,480
Speaker 2: a zombie movie is going to be filmed in my

667
00:33:12,640 --> 00:33:15,520
Speaker 2: fields because it's kind of creepy. But so is there

668
00:33:15,560 --> 00:33:17,840
Speaker 2: anything interesting about shadows in fog?

669
00:33:18,160 --> 00:33:21,880
Speaker 1: Yeah, fog is wonderful for studying shadows because it shows

670
00:33:21,920 --> 00:33:24,120
Speaker 1: you where the light is. Right, It's like having a

671
00:33:24,120 --> 00:33:27,200
Speaker 1: bunch of lasers and throwing up dust particles in front

672
00:33:27,200 --> 00:33:29,200
Speaker 1: of them. You can see where the lasers are. And

673
00:33:29,280 --> 00:33:31,520
Speaker 1: so fog is just like a bunch of particles of

674
00:33:31,600 --> 00:33:34,320
Speaker 1: water suspended in the air, and they tell you where

675
00:33:34,320 --> 00:33:36,240
Speaker 1: the light is. And you can have a shadow on

676
00:33:36,280 --> 00:33:39,080
Speaker 1: the fog. And so there's a well known effect called

677
00:33:39,080 --> 00:33:41,720
Speaker 1: the brocked in specter effect where you can have a

678
00:33:41,760 --> 00:33:44,920
Speaker 1: shadow on a cloud. Right, And so, for example, if

679
00:33:44,960 --> 00:33:47,200
Speaker 1: you stand in front of a car with bright headlights

680
00:33:47,480 --> 00:33:49,600
Speaker 1: in the fog, you'll see your shadow on the fog

681
00:33:49,640 --> 00:33:52,720
Speaker 1: and it can be this like huge looming shape. Right.

682
00:33:53,240 --> 00:33:55,720
Speaker 1: But also if you stand in front of your headlights,

683
00:33:55,800 --> 00:33:58,040
Speaker 1: you could see your shadow on a cloud in the

684
00:33:58,080 --> 00:34:01,360
Speaker 1: sky if you do it right. Yeah. Cool, And that's

685
00:34:01,400 --> 00:34:04,080
Speaker 1: the incredible thing about shadows is that you know, there's

686
00:34:04,120 --> 00:34:07,080
Speaker 1: this projection effect where your shadow can be so much

687
00:34:07,120 --> 00:34:09,279
Speaker 1: bigger than you are, right, and so you can like

688
00:34:09,640 --> 00:34:13,240
Speaker 1: wave your arms and then like the huge sky version

689
00:34:13,280 --> 00:34:15,600
Speaker 1: of you is also waving its arms.

690
00:34:17,920 --> 00:34:22,360
Speaker 2: So I want to imagine that the Brocken spec effect

691
00:34:22,840 --> 00:34:24,920
Speaker 2: is an effect from you know, somebody who was like

692
00:34:24,960 --> 00:34:27,719
Speaker 2: studying ghosts and got confused about what was happening with

693
00:34:27,800 --> 00:34:29,879
Speaker 2: the fog. But how did this actually get its name

694
00:34:30,320 --> 00:34:32,920
Speaker 2: because specter always sounds sort of ghostbustery to me.

695
00:34:34,040 --> 00:34:35,799
Speaker 1: I don't know the exact history of it, but it

696
00:34:35,840 --> 00:34:39,880
Speaker 1: shows up in like Lewis Carroll and Samuel Taylor Coleridge poems,

697
00:34:40,280 --> 00:34:42,080
Speaker 1: so it's definitely a thing that's been around for quite

698
00:34:42,080 --> 00:34:42,480
Speaker 1: a while.

699
00:34:42,680 --> 00:34:43,880
Speaker 2: Okay, awesome.

700
00:34:44,040 --> 00:34:48,080
Speaker 1: It's sometimes called the Mountain Spector or Specter of the Brocken.

701
00:34:48,520 --> 00:34:51,360
Speaker 2: Oh, oh, that sounds even cooler. Specter of the Brocken.

702
00:34:51,440 --> 00:34:53,360
Speaker 2: That sounds like it should be in like a Viking tale.

703
00:34:53,520 --> 00:34:56,759
Speaker 1: It comes from this mountain in Germany, the Brocken. It

704
00:34:56,880 --> 00:35:01,120
Speaker 1: was first observed and described by Johann Schilberslog in seventeen eighty.

705
00:35:01,560 --> 00:35:03,720
Speaker 1: We saw his shadow on the Brocken.

706
00:35:03,960 --> 00:35:07,440
Speaker 2: Ah cool, all right, learn something new every day, all right.

707
00:35:07,560 --> 00:35:09,640
Speaker 2: So every once in a while you'll hear about like

708
00:35:09,960 --> 00:35:14,480
Speaker 2: pressure fronts coming through and is that like air that

709
00:35:14,640 --> 00:35:17,600
Speaker 2: is more or less dense? And could that change how

710
00:35:17,680 --> 00:35:19,960
Speaker 2: shadows are made? I guess I'm trying to figure out

711
00:35:20,000 --> 00:35:21,520
Speaker 2: if your NeXT's going to tell me that air can

712
00:35:21,560 --> 00:35:23,120
Speaker 2: also impact shadows.

713
00:35:23,400 --> 00:35:26,200
Speaker 1: Yes, air can have shadows for the same reason that

714
00:35:26,280 --> 00:35:28,840
Speaker 1: water can, right, because air is not a fluid of

715
00:35:28,880 --> 00:35:32,160
Speaker 1: constant density. When it is, light just passes through it.

716
00:35:32,480 --> 00:35:35,279
Speaker 1: But if some pockets of air have higher density or

717
00:35:35,320 --> 00:35:38,200
Speaker 1: lower density, then the light bends in exactly the same

718
00:35:38,200 --> 00:35:40,239
Speaker 1: way as it does when it hits the surface of

719
00:35:40,280 --> 00:35:42,799
Speaker 1: the water. And you can already see this effect like

720
00:35:43,000 --> 00:35:45,840
Speaker 1: when you look at heat rising above the road on

721
00:35:45,880 --> 00:35:48,600
Speaker 1: a hot day. Right, what you're seeing there are pressure

722
00:35:48,640 --> 00:35:51,799
Speaker 1: waves in the air, and that's changing how light goes through.

723
00:35:51,880 --> 00:35:54,360
Speaker 1: That's why you're able to see it, right, And the

724
00:35:54,440 --> 00:35:57,920
Speaker 1: similar consequences for how light moves through the air. This

725
00:35:57,960 --> 00:36:01,880
Speaker 1: is why, for example, stars twinkle right. Stars don't twinkle

726
00:36:01,880 --> 00:36:05,160
Speaker 1: in space, They only twinkle through the atmosphere because their

727
00:36:05,200 --> 00:36:07,279
Speaker 1: light is getting bent away from you in a sort

728
00:36:07,320 --> 00:36:11,440
Speaker 1: of a chaotic, turbulent manner. This is why telescopes on

729
00:36:11,480 --> 00:36:14,040
Speaker 1: the ground can't see as clearly as telescopes in the sky,

730
00:36:14,239 --> 00:36:17,799
Speaker 1: because light has passed through this complicated atmosphere. And they

731
00:36:17,840 --> 00:36:21,440
Speaker 1: have these amazing adaptive optics to counteract for this to

732
00:36:21,600 --> 00:36:24,640
Speaker 1: like in real time, bend the path of the light

733
00:36:24,840 --> 00:36:28,000
Speaker 1: back to regather all that light into a crisper image.

734
00:36:28,120 --> 00:36:30,200
Speaker 1: But effectively it's like a shadow. I mean, if you

735
00:36:30,280 --> 00:36:32,840
Speaker 1: looked at the ground as life passes through it, you

736
00:36:32,840 --> 00:36:35,200
Speaker 1: would see regions that are darker and regions that are

737
00:36:35,280 --> 00:36:38,920
Speaker 1: lighter because of these density fluctuations in the air.

738
00:36:39,200 --> 00:36:41,120
Speaker 2: That's amazing. And you know, one of my favorite things

739
00:36:41,160 --> 00:36:43,360
Speaker 2: about this time of year is that so like, I

740
00:36:43,400 --> 00:36:45,719
Speaker 2: don't stay up late because I'm a total wimp, but

741
00:36:46,120 --> 00:36:48,480
Speaker 2: in the winter and in the fall, you get when

742
00:36:48,520 --> 00:36:50,879
Speaker 2: I go out to do the animal chores, it's already dark,

743
00:36:51,080 --> 00:36:53,080
Speaker 2: and so on clear days I can see the stars

744
00:36:53,160 --> 00:36:55,960
Speaker 2: twinkling and the other day, I was late to wake

745
00:36:56,040 --> 00:36:59,000
Speaker 2: up my kids because the stars were twinkling, and I

746
00:36:59,040 --> 00:37:02,000
Speaker 2: was like totally enamored of it and forgot about what

747
00:37:02,120 --> 00:37:05,400
Speaker 2: time it was and fell behind on my schedule.

748
00:37:06,560 --> 00:37:09,239
Speaker 1: Well, this start twinkling effect is very cool, and it's

749
00:37:09,280 --> 00:37:11,360
Speaker 1: you know, the kind of physics you can enjoy any evening,

750
00:37:11,840 --> 00:37:14,880
Speaker 1: But the same physics gives you a really weird effect

751
00:37:15,120 --> 00:37:16,600
Speaker 1: during a lunar eclipse.

752
00:37:17,040 --> 00:37:19,840
Speaker 2: What I am dying to hear about that. So let's

753
00:37:20,080 --> 00:37:23,400
Speaker 2: take a break to increase the suspense, and when we

754
00:37:23,440 --> 00:37:45,080
Speaker 2: get back, you'll tell us all about it. Okay, you

755
00:37:45,120 --> 00:37:48,040
Speaker 2: were going to tell us about shadows and eclipses, Daniel.

756
00:37:48,239 --> 00:37:51,400
Speaker 1: Yeah, so I'm lucky enough to have seen a total eclipse.

757
00:37:51,480 --> 00:37:55,040
Speaker 1: This was in twenty eighteen. Amazing experience, and I went

758
00:37:55,080 --> 00:37:57,719
Speaker 1: into it not preparing to be amazed. I was like,

759
00:37:57,760 --> 00:37:59,560
Speaker 1: I know the physics, Yeah, it's cool, it's going to

760
00:37:59,600 --> 00:38:03,000
Speaker 1: get dark or whatever. But there was something really moving

761
00:38:03,239 --> 00:38:06,319
Speaker 1: about being in the totality having the sun be so

762
00:38:06,400 --> 00:38:10,880
Speaker 1: completely blocked. Momentarily, it just everything felt so odd. And

763
00:38:10,920 --> 00:38:13,400
Speaker 1: you know, I'm not a religious person, but I almost

764
00:38:13,480 --> 00:38:16,120
Speaker 1: felt spiritual at that moment. I tried to imagine what

765
00:38:16,200 --> 00:38:18,400
Speaker 1: it might be like to not understand the physics at

766
00:38:18,440 --> 00:38:21,520
Speaker 1: all and go through that experience feel like, WHOA, something

767
00:38:21,560 --> 00:38:22,480
Speaker 1: is happening today.

768
00:38:22,760 --> 00:38:25,360
Speaker 2: Yes, we are all gonna die. That's absolutely what I

769
00:38:25,360 --> 00:38:25,839
Speaker 2: would think.

770
00:38:26,160 --> 00:38:27,360
Speaker 1: Have you seen totality?

771
00:38:27,600 --> 00:38:30,359
Speaker 2: No, In Virginia a year or two ago we got

772
00:38:30,400 --> 00:38:33,200
Speaker 2: a partial eclipse, but even that was pretty amazing. But

773
00:38:33,280 --> 00:38:35,320
Speaker 2: you know, as a biologist, I was trying to listen

774
00:38:35,360 --> 00:38:37,440
Speaker 2: to see if the nighttime animals were waking up and

775
00:38:37,480 --> 00:38:39,440
Speaker 2: being like, oh, what's going on? And I think there

776
00:38:39,480 --> 00:38:40,680
Speaker 2: was a little bit of that. I bet there was

777
00:38:40,680 --> 00:38:41,839
Speaker 2: a lot more that where you were.

778
00:38:42,080 --> 00:38:44,520
Speaker 1: Actually this was in Idaho, I believe it was for

779
00:38:44,560 --> 00:38:48,200
Speaker 1: the path of totality for that eclipse really amazing. Totally

780
00:38:48,280 --> 00:38:51,439
Speaker 1: encourage everyone to see totality if they can. And there's

781
00:38:51,440 --> 00:38:53,200
Speaker 1: a really cool effect which I didn't see at the time,

782
00:38:53,280 --> 00:38:56,279
Speaker 1: didn't even know about until preparing for this episode, but

783
00:38:56,400 --> 00:38:59,680
Speaker 1: there are these things called shadow bands that happen during

784
00:38:59,719 --> 00:39:03,000
Speaker 1: a total eclipse for the same reason as star twinkling.

785
00:39:03,440 --> 00:39:06,400
Speaker 1: What's happening is that you have the sun now narrowed

786
00:39:06,400 --> 00:39:09,440
Speaker 1: to a very very very narrow source of light, right

787
00:39:09,480 --> 00:39:11,880
Speaker 1: instead of being a huge blob in the sky, you

788
00:39:11,920 --> 00:39:14,640
Speaker 1: have like a very thin crescent. So now sunlight is

789
00:39:14,760 --> 00:39:18,160
Speaker 1: very columnated. All the rays are very very parallel, and

790
00:39:18,239 --> 00:39:21,920
Speaker 1: what happens is it creates these thin, wavy lines of

791
00:39:21,960 --> 00:39:25,239
Speaker 1: alternating dark and light that can be seen moving and

792
00:39:25,360 --> 00:39:29,280
Speaker 1: undulating in parallel just before and just after the total

793
00:39:29,280 --> 00:39:34,040
Speaker 1: solar eclipse. So there's these like shadow bands of the eclipse.

794
00:39:34,560 --> 00:39:37,319
Speaker 1: Just the same way that a solid object will have

795
00:39:37,360 --> 00:39:41,680
Speaker 1: these diffraction patterns around, these zebra patterns. The moon has

796
00:39:41,719 --> 00:39:44,239
Speaker 1: those patterns a shadow on the Earth, but not for

797
00:39:44,320 --> 00:39:47,400
Speaker 1: diffraction reasons. It's for the same reason it's the star twinkling,

798
00:39:47,640 --> 00:39:50,160
Speaker 1: because now you have this column of light which then

799
00:39:50,200 --> 00:39:53,640
Speaker 1: gets bent randomly by the varying density of the air,

800
00:39:53,920 --> 00:39:55,719
Speaker 1: but it all is moving in a column, so you

801
00:39:55,800 --> 00:39:58,400
Speaker 1: get these bands. Really incredible. I want to see this

802
00:39:58,440 --> 00:39:58,920
Speaker 1: in person.

803
00:39:59,120 --> 00:40:01,360
Speaker 2: I want to see that also, and I also, for

804
00:40:01,400 --> 00:40:04,400
Speaker 2: the first time, want to see our podcast as a

805
00:40:04,480 --> 00:40:07,680
Speaker 2: video episode, because the extent to which your arms were

806
00:40:07,719 --> 00:40:10,520
Speaker 2: moving to try to explain that was really fantastic.

807
00:40:10,680 --> 00:40:12,520
Speaker 1: That's the Italian in me coming out, you know.

808
00:40:15,600 --> 00:40:18,520
Speaker 2: So the listener asked a question about what shadows would

809
00:40:18,520 --> 00:40:20,600
Speaker 2: be like on the moon, and the context for the

810
00:40:20,680 --> 00:40:24,080
Speaker 2: question was you know the Moon has a very tiny,

811
00:40:24,160 --> 00:40:28,799
Speaker 2: thin atmosphere and exosphere. How does atmosphere impact the way

812
00:40:28,800 --> 00:40:30,840
Speaker 2: shadows are made? And would it be different if you

813
00:40:30,840 --> 00:40:32,280
Speaker 2: were on the moon. So like, what did the Apollo

814
00:40:32,320 --> 00:40:34,560
Speaker 2: astronauts see when they looked at their shadows?

815
00:40:34,640 --> 00:40:36,200
Speaker 1: All right, well, I'm going to give you a pop quiz.

816
00:40:36,280 --> 00:40:38,680
Speaker 1: I've taught to you now enough physics on this episode

817
00:40:38,719 --> 00:40:41,680
Speaker 1: to answer this question. What do you think, Kelly? Do

818
00:40:41,760 --> 00:40:44,400
Speaker 1: you think shadows are crisper on the Moon or less crisp?

819
00:40:44,719 --> 00:40:48,880
Speaker 2: I think that it's probably about the same because you

820
00:40:48,920 --> 00:40:51,600
Speaker 2: still have light reflecting from lots of different surfaces, and

821
00:40:51,640 --> 00:40:54,480
Speaker 2: I bet the surface of the Moon in particular is

822
00:40:54,520 --> 00:40:59,400
Speaker 2: pretty reflective. I guess here when the atmosphere changes in density,

823
00:40:59,480 --> 00:41:01,840
Speaker 2: that bounces light around and makes it a little bit

824
00:41:01,920 --> 00:41:04,800
Speaker 2: less crisp. And so maybe with no atmosphere, I'm gonna

825
00:41:04,880 --> 00:41:06,920
Speaker 2: guess crisper crisper.

826
00:41:07,040 --> 00:41:11,000
Speaker 1: Yes, exactly, it's crisper. And the issue is the atmosphere.

827
00:41:11,320 --> 00:41:14,000
Speaker 1: I mean, think about how if you're standing on the Earth,

828
00:41:14,040 --> 00:41:15,880
Speaker 1: you look up, the sky is blue, right, what's the

829
00:41:15,880 --> 00:41:17,320
Speaker 1: color of the sky and the moon.

830
00:41:18,960 --> 00:41:19,360
Speaker 2: Black?

831
00:41:19,560 --> 00:41:23,719
Speaker 1: It's black right, because there's no atmosphere there to reflect light,

832
00:41:23,840 --> 00:41:26,400
Speaker 1: and so the atmosphere here is blue. That means that

833
00:41:26,400 --> 00:41:29,320
Speaker 1: you're getting light from all directions. Right, Yes, it's mostly

834
00:41:29,360 --> 00:41:31,680
Speaker 1: from the sun, and you can see shadows. But the

835
00:41:31,760 --> 00:41:34,600
Speaker 1: answer to Eric's other question is the reason you can

836
00:41:34,640 --> 00:41:37,279
Speaker 1: see still light when you're standing behind a tree and

837
00:41:37,320 --> 00:41:40,080
Speaker 1: the sun is blocked is because it's light reflecting everywhere

838
00:41:40,080 --> 00:41:42,759
Speaker 1: on the Earth, from all over the sky. And yes,

839
00:41:42,800 --> 00:41:45,800
Speaker 1: and all the buildings and whatever. But the atmosphere itself

840
00:41:46,200 --> 00:41:49,080
Speaker 1: is like bouncing light everywhere, and on the Moon you

841
00:41:49,120 --> 00:41:52,239
Speaker 1: have no atmosphere, and so it's much more geometric. Yes,

842
00:41:52,280 --> 00:41:55,440
Speaker 1: you do have rocks that are reflecting light, and of

843
00:41:55,440 --> 00:41:57,879
Speaker 1: course during the nighttime we see the reflection of the moon.

844
00:41:58,320 --> 00:42:01,400
Speaker 1: But the atmosphere is a big contributor to making shadows fuzzy,

845
00:42:01,640 --> 00:42:04,520
Speaker 1: and of course fluctuations in the atmosphere make that fuzzy.

846
00:42:04,560 --> 00:42:06,960
Speaker 1: So yes, the reason you still see stuff when you're

847
00:42:07,040 --> 00:42:10,240
Speaker 1: standing in shadow is because it's light coming from many sources,

848
00:42:10,280 --> 00:42:13,320
Speaker 1: not just one, and on the moon, shadows are crisper.

849
00:42:13,440 --> 00:42:15,560
Speaker 1: But shadows are also important on the Moon for another

850
00:42:15,600 --> 00:42:18,480
Speaker 1: reason that you might find interesting, which is they provide

851
00:42:18,520 --> 00:42:22,080
Speaker 1: place for water ice to accumulate. Right, because shadows on

852
00:42:22,120 --> 00:42:25,200
Speaker 1: the Moon are very very cold, right, The moon surface

853
00:42:25,280 --> 00:42:27,840
Speaker 1: is crazy, it's super hot, it's super cold. It depends

854
00:42:27,880 --> 00:42:30,840
Speaker 1: on are you in the blinding path of the sunlight,

855
00:42:30,880 --> 00:42:33,480
Speaker 1: and if you're not, then the water ice can survive.

856
00:42:33,560 --> 00:42:36,120
Speaker 1: And isn't there a place like on the lunar pole

857
00:42:36,280 --> 00:42:37,520
Speaker 1: where light never reaches?

858
00:42:37,800 --> 00:42:41,280
Speaker 2: Yes, that's right, on both poles and places like Shackleton Crater.

859
00:42:41,640 --> 00:42:44,160
Speaker 1: It's like eternal shadow or something really dark.

860
00:42:44,280 --> 00:42:47,880
Speaker 2: Oh, craters of eternal darkness exactly.

861
00:42:48,160 --> 00:42:51,520
Speaker 1: And shadows there are really important because they preserve water

862
00:42:51,600 --> 00:42:53,480
Speaker 1: eyes and if we ever do live on the Moon,

863
00:42:53,480 --> 00:42:56,560
Speaker 1: that would be really valuable. Right, So shadows could save

864
00:42:56,600 --> 00:42:57,640
Speaker 1: our lives on the Moon.

865
00:42:57,880 --> 00:43:00,080
Speaker 2: Yeah, although I'll note that there's not a lot of

866
00:43:00,120 --> 00:43:02,160
Speaker 2: water in those shadows, but there is some. It could

867
00:43:02,160 --> 00:43:04,080
Speaker 2: get us started. We'd have to be real careful about

868
00:43:04,120 --> 00:43:07,279
Speaker 2: recycling it.

869
00:43:07,360 --> 00:43:10,520
Speaker 1: Shadows have also really helped us understand the nature of

870
00:43:10,560 --> 00:43:14,560
Speaker 1: our place and the cosmos. Famously, more than two thousand

871
00:43:14,680 --> 00:43:18,840
Speaker 1: years ago, the Greeks used shadows to measure the radius

872
00:43:18,960 --> 00:43:23,879
Speaker 1: of the Earth. Right, Greeks so clever, so geometrical. They

873
00:43:23,920 --> 00:43:27,440
Speaker 1: realize that the Earth is probably a sphere because as

874
00:43:27,480 --> 00:43:30,680
Speaker 1: you move around it you can see different kinds of stars, right,

875
00:43:30,719 --> 00:43:33,839
Speaker 1: Different constellations emerge as you move around the Earth. So

876
00:43:34,160 --> 00:43:37,640
Speaker 1: the Greeks much smarter than like, you know, certain rappers

877
00:43:37,800 --> 00:43:41,080
Speaker 1: and YouTube influencers who still deny that the Earth is

878
00:43:41,120 --> 00:43:43,759
Speaker 1: a sphere. But they went beyond just saying, oh, the

879
00:43:43,800 --> 00:43:46,400
Speaker 1: Earth is likely a sphere. They use the behavior of

880
00:43:46,400 --> 00:43:49,319
Speaker 1: shadows and a little bit of geometry to measure the

881
00:43:49,480 --> 00:43:52,759
Speaker 1: radius of the Earth and got it pretty accurate, like

882
00:43:52,880 --> 00:43:54,480
Speaker 1: more than two thousand years ago.

883
00:43:54,719 --> 00:43:56,640
Speaker 2: Well okay, so can you give us more details about

884
00:43:56,640 --> 00:43:57,200
Speaker 2: how they did that.

885
00:43:57,400 --> 00:43:59,200
Speaker 1: Yeah. So, imagine you're in a city where the sun

886
00:43:59,280 --> 00:44:02,960
Speaker 1: is directly it's like high noon, and so all the

887
00:44:03,000 --> 00:44:07,160
Speaker 1: shadows point straight down, right, there's basically no shadows. Then

888
00:44:07,200 --> 00:44:10,799
Speaker 1: you have another city that's like hundred kilometers away, and

889
00:44:10,960 --> 00:44:12,799
Speaker 1: that city is not going to be at high noon.

890
00:44:12,880 --> 00:44:15,759
Speaker 1: It's going to have some shadows, right, And you can

891
00:44:15,800 --> 00:44:18,120
Speaker 1: measure the length of those shadows, and now make a

892
00:44:18,160 --> 00:44:20,760
Speaker 1: triangle where you know the distance between the two cities,

893
00:44:20,800 --> 00:44:23,680
Speaker 1: and you can measure the length of the shadow. The

894
00:44:23,800 --> 00:44:26,799
Speaker 1: length of that shadow depends on the curvature of the Earth,

895
00:44:26,920 --> 00:44:29,520
Speaker 1: because if the Earth was very, very flat, that shadow

896
00:44:29,520 --> 00:44:31,759
Speaker 1: would be small. And as the Earth gets more and

897
00:44:31,800 --> 00:44:35,359
Speaker 1: more curvature, that shadow would grow longer and longer. So

898
00:44:35,480 --> 00:44:39,520
Speaker 1: by measuring the length of the shadow in Alexandria at

899
00:44:39,520 --> 00:44:42,640
Speaker 1: the time that the sun was directly overhead in another

900
00:44:42,800 --> 00:44:47,040
Speaker 1: city which I can't pronounce, a Greek dude named Erasthenes

901
00:44:47,480 --> 00:44:50,120
Speaker 1: was able to measure the circumference of the Earth just

902
00:44:50,280 --> 00:44:54,440
Speaker 1: using like a stick and some geometry and measuring a shadow.

903
00:44:54,800 --> 00:44:58,680
Speaker 2: So like Eritosthenes, I'm not gonna say it right uh,

904
00:44:58,920 --> 00:45:01,160
Speaker 2: called his friend down in Syena, was like, we're both

905
00:45:01,200 --> 00:45:03,920
Speaker 2: taking the measurements right now, though, how did I guess that?

906
00:45:04,000 --> 00:45:06,000
Speaker 2: Did they also have really good clocks or they just

907
00:45:06,080 --> 00:45:08,440
Speaker 2: were like that is okay, yeah, exactly, that's amazing.

908
00:45:08,560 --> 00:45:10,680
Speaker 1: So this is really cool. But the fascinating thing is

909
00:45:10,760 --> 00:45:13,719
Speaker 1: that flat Earthers have not let this point go and

910
00:45:13,920 --> 00:45:17,239
Speaker 1: they argue that this experiment doesn't actually prove that the

911
00:45:17,360 --> 00:45:20,080
Speaker 1: Earth is round, and they're kind of right, oh no,

912
00:45:20,280 --> 00:45:23,400
Speaker 1: because even if the Earth was flat, you would still

913
00:45:23,600 --> 00:45:26,920
Speaker 1: have a shadow in one place when the sun is

914
00:45:26,960 --> 00:45:30,480
Speaker 1: directly overhead in the other city. That's true. Essentially, it's

915
00:45:30,520 --> 00:45:33,400
Speaker 1: like measuring the earth to have an infinite radius, but

916
00:45:33,520 --> 00:45:36,759
Speaker 1: you would definitely get a shadow, because this whole method

917
00:45:36,840 --> 00:45:39,520
Speaker 1: assumes that the sun is really really far away and

918
00:45:39,719 --> 00:45:43,200
Speaker 1: that the light is parallel essentially, But in the flat

919
00:45:43,280 --> 00:45:45,960
Speaker 1: earth model, the sun is very very close, and so

920
00:45:46,040 --> 00:45:47,960
Speaker 1: you would still get a shadow in one place and

921
00:45:48,080 --> 00:45:50,080
Speaker 1: not a shadow in the other. But there's a way

922
00:45:50,160 --> 00:45:51,920
Speaker 1: around it. All you need to do is add a

923
00:45:51,960 --> 00:45:54,360
Speaker 1: couple more sticks, so instead of just having two points,

924
00:45:54,600 --> 00:45:57,720
Speaker 1: you have like three or four, and the two models

925
00:45:57,800 --> 00:46:01,600
Speaker 1: give different patterns of shadows. In the flat earth model

926
00:46:01,640 --> 00:46:04,200
Speaker 1: you get a linear relationship between the length of the shadows,

927
00:46:04,640 --> 00:46:07,240
Speaker 1: and in the spherical Earth you get a non linear

928
00:46:07,280 --> 00:46:09,880
Speaker 1: relationship as things move around the curve of the Earth.

929
00:46:10,000 --> 00:46:12,960
Speaker 1: So anyway, we're pretty sure that the Earth is not flat,

930
00:46:13,239 --> 00:46:17,239
Speaker 1: and you can actually prove it using shadow and rod experiments.

931
00:46:17,560 --> 00:46:20,359
Speaker 1: It's true the two point experiment doesn't refute the flat earth,

932
00:46:20,640 --> 00:46:23,640
Speaker 1: but anyway, shadows do show us that the Earth is round,

933
00:46:23,960 --> 00:46:27,279
Speaker 1: and you allow us to measure the roundness of the Earth,

934
00:46:27,320 --> 00:46:28,320
Speaker 1: which is kind of amazing.

935
00:46:28,520 --> 00:46:31,800
Speaker 2: That is amazing way to go shadows. So the last

936
00:46:31,960 --> 00:46:35,440
Speaker 2: question the listener had was the trick question that we

937
00:46:35,560 --> 00:46:40,240
Speaker 2: shared with our extraordinaries, which is do shadows move faster

938
00:46:40,400 --> 00:46:43,520
Speaker 2: than light. Yeah, so all right, now we have all

939
00:46:43,600 --> 00:46:46,480
Speaker 2: of the background, we need to understand the nuance to

940
00:46:46,560 --> 00:46:49,280
Speaker 2: this question. Yes, so take it home, Daniel.

941
00:46:49,560 --> 00:46:54,040
Speaker 1: The answer is, yes, shadows do move faster than light.

942
00:46:54,440 --> 00:46:59,960
Speaker 1: What But but the problem is that the rule says

943
00:47:00,239 --> 00:47:03,200
Speaker 1: no thing can move faster than the speed of light

944
00:47:03,280 --> 00:47:06,040
Speaker 1: relative to anything else. But shadows are not a thing,

945
00:47:06,520 --> 00:47:10,160
Speaker 1: that's the problem. They're the absence of a thing, and

946
00:47:10,920 --> 00:47:13,840
Speaker 1: as a shadow moves, it's not really the same object.

947
00:47:14,440 --> 00:47:16,280
Speaker 1: So let's imagine a concrete scenario.

948
00:47:16,480 --> 00:47:16,640
Speaker 2: Right.

949
00:47:17,080 --> 00:47:21,040
Speaker 1: Let's say you have a screen in the sky instead

950
00:47:21,080 --> 00:47:22,919
Speaker 1: of the sky. You're like, you know, Daniel has built

951
00:47:23,000 --> 00:47:26,120
Speaker 1: his telescopes that block the view of the world, and

952
00:47:26,239 --> 00:47:30,360
Speaker 1: so you have exactly right, and you can imagine a scenario.

953
00:47:30,400 --> 00:47:32,200
Speaker 1: We have a bright source of light and you can

954
00:47:32,239 --> 00:47:35,080
Speaker 1: do like shadow puppets on the sky, right, or imagine

955
00:47:35,120 --> 00:47:38,320
Speaker 1: clouds or whatever, and you can move your hand a

956
00:47:38,440 --> 00:47:41,600
Speaker 1: small amount and the shadow will move a very large amount, right,

957
00:47:41,600 --> 00:47:43,560
Speaker 1: because the screen is very very far away, and so

958
00:47:43,680 --> 00:47:46,840
Speaker 1: this projection is far away, and you get this multiplier effect.

959
00:47:47,239 --> 00:47:48,600
Speaker 1: And then you can ask, well, if I move my

960
00:47:48,640 --> 00:47:51,000
Speaker 1: hand really fast and the screen is really far away,

961
00:47:51,400 --> 00:47:54,120
Speaker 1: could those shadows move faster than light, And the answer

962
00:47:54,239 --> 00:47:57,160
Speaker 1: is yes, in the sense that like the image of

963
00:47:57,200 --> 00:47:59,919
Speaker 1: your hand could be in one place and then fast

964
00:48:00,239 --> 00:48:02,400
Speaker 1: and then light could go from that one image to

965
00:48:02,719 --> 00:48:06,759
Speaker 1: another image the image the shadow could appear somewhere else, Right,

966
00:48:07,280 --> 00:48:09,600
Speaker 1: does that make sense? Like imagine somebody in the sky

967
00:48:09,760 --> 00:48:12,600
Speaker 1: shooting a laser from one shadow to the other. The

968
00:48:12,680 --> 00:48:16,879
Speaker 1: second shadow would appear before the laser arrived. In that sense,

969
00:48:16,960 --> 00:48:20,040
Speaker 1: the shadow is moving faster than light, and you're wondering, like,

970
00:48:20,200 --> 00:48:23,640
Speaker 1: how does that make sense physically? What's really going on?

971
00:48:23,800 --> 00:48:25,600
Speaker 1: And the issue is that the second shadow is not

972
00:48:25,680 --> 00:48:29,200
Speaker 1: the same thing as the first shadow, right, both of

973
00:48:29,280 --> 00:48:31,799
Speaker 1: them are being created by the absence of light. Nothing

974
00:48:31,920 --> 00:48:34,279
Speaker 1: is moving faster than light in the scenario, and there's

975
00:48:34,320 --> 00:48:37,920
Speaker 1: no way to communicate between the shadow one and shadow two.

976
00:48:37,960 --> 00:48:41,000
Speaker 1: There's no information passing. It's just like if I shown

977
00:48:41,080 --> 00:48:43,120
Speaker 1: a laser in one direction and then I turned it

978
00:48:43,160 --> 00:48:46,120
Speaker 1: off and shown it in another direction, the laser spot

979
00:48:46,360 --> 00:48:48,360
Speaker 1: would appear to move faster than light, but it's not

980
00:48:48,480 --> 00:48:50,479
Speaker 1: the same spot. Right, It's like I made a spot

981
00:48:50,560 --> 00:48:52,719
Speaker 1: and then later I made another spot. I'm connecting them

982
00:48:52,760 --> 00:48:54,800
Speaker 1: in my mind because they both came from the same laser.

983
00:48:55,200 --> 00:48:58,120
Speaker 1: But it's not like anything moved from laser spot one

984
00:48:58,160 --> 00:49:01,919
Speaker 1: to laser spot two the same way nothing went from

985
00:49:01,960 --> 00:49:05,160
Speaker 1: the shadows initial location of the shadow's final location. You

986
00:49:05,239 --> 00:49:08,040
Speaker 1: have a wave of light that's obstructed and not obstructed

987
00:49:08,080 --> 00:49:10,480
Speaker 1: that's creating the shadow, and then a different wave of

988
00:49:10,560 --> 00:49:13,200
Speaker 1: light that's creating a different shadow somewhere else. And your

989
00:49:13,280 --> 00:49:16,160
Speaker 1: mind is like, shadows are a thing, and so it

990
00:49:16,280 --> 00:49:17,880
Speaker 1: was here and it was there, and if I do

991
00:49:18,040 --> 00:49:20,160
Speaker 1: distance divided by time, I get a number bigger than

992
00:49:20,200 --> 00:49:23,640
Speaker 1: the speed of light. Yeah, that's true, but shadows aren't

993
00:49:23,680 --> 00:49:26,160
Speaker 1: a thing. It's like comparing where one thing is and

994
00:49:26,320 --> 00:49:30,040
Speaker 1: later something else is and calculating the velocity between those two.

995
00:49:30,120 --> 00:49:31,120
Speaker 1: It doesn't really work.

996
00:49:31,520 --> 00:49:34,400
Speaker 2: Is that like saying that the information that the photon

997
00:49:34,480 --> 00:49:38,720
Speaker 2: has been stopped travels faster than a photon would travel.

998
00:49:38,880 --> 00:49:41,080
Speaker 1: No, the photon. The information that the photon has been

999
00:49:41,160 --> 00:49:44,480
Speaker 1: stopped isn't traveling from shadow one to shadow two. It's

1000
00:49:44,560 --> 00:49:47,000
Speaker 1: traveling from the source to shadow one, and from the

1001
00:49:47,120 --> 00:49:50,279
Speaker 1: source to shadow too. Like you could signal somebody up

1002
00:49:50,320 --> 00:49:53,120
Speaker 1: there in the sky using shadows or not shadows, but

1003
00:49:53,200 --> 00:49:55,799
Speaker 1: that information obviously travels at the speed of light because

1004
00:49:55,800 --> 00:49:58,480
Speaker 1: you're either sending photons or you're stopping to send photons.

1005
00:49:58,880 --> 00:50:00,920
Speaker 1: But all that information novels at the speed of light.

1006
00:50:00,960 --> 00:50:03,360
Speaker 1: And you could do the same for person two. But

1007
00:50:03,520 --> 00:50:06,000
Speaker 1: you can't get information from shadow one to shadow too,

1008
00:50:06,160 --> 00:50:07,759
Speaker 1: or there's no way for you to do that. You

1009
00:50:07,880 --> 00:50:10,600
Speaker 1: can send information. All the information is coming from the

1010
00:50:10,719 --> 00:50:14,040
Speaker 1: source of light to the shadows or the non shadows,

1011
00:50:14,280 --> 00:50:15,160
Speaker 1: not between them.

1012
00:50:15,520 --> 00:50:15,800
Speaker 2: Got it.

1013
00:50:16,120 --> 00:50:18,680
Speaker 1: So the appearance of shadows can move faster than light.

1014
00:50:18,719 --> 00:50:21,400
Speaker 1: But shadows are not really a thing. They don't carry information,

1015
00:50:22,080 --> 00:50:24,760
Speaker 1: and so in that sense, you know they're breaking the rules,

1016
00:50:24,800 --> 00:50:27,600
Speaker 1: but they're not really limited by the rules because they're

1017
00:50:27,640 --> 00:50:29,640
Speaker 1: not a thing. They don't have information.

1018
00:50:30,160 --> 00:50:33,520
Speaker 2: Well, thank you Daniel for illuminating all of our understanding

1019
00:50:34,400 --> 00:50:37,040
Speaker 2: of this question. I learned a lot and had a

1020
00:50:37,080 --> 00:50:38,680
Speaker 2: lot of fun talking about shadows because.

1021
00:50:38,480 --> 00:50:40,920
Speaker 1: They're neat Well, I'm hoping we can help bring shadows

1022
00:50:40,960 --> 00:50:43,279
Speaker 1: out of the darkness. Shadows are a wonderful way to

1023
00:50:43,400 --> 00:50:45,480
Speaker 1: think about light and to think about physics and just

1024
00:50:45,560 --> 00:50:48,200
Speaker 1: to like, you know, wonder in an everyday sense how

1025
00:50:48,360 --> 00:50:51,640
Speaker 1: everything works. They're fantastic mysteries of physics all around us.

1026
00:50:51,880 --> 00:50:54,600
Speaker 2: There are and please send us your questions about the

1027
00:50:54,680 --> 00:50:57,000
Speaker 2: mysteries of you know, physics, I guess if that's what

1028
00:50:57,160 --> 00:51:00,120
Speaker 2: keeps you up at night, but definitely the questions that

1029
00:51:00,160 --> 00:51:03,720
Speaker 2: you have about biology, which is a fascinating topic.

1030
00:51:04,000 --> 00:51:06,480
Speaker 1: And if you're the descendant of the famous physicist Poisson

1031
00:51:06,520 --> 00:51:08,759
Speaker 1: and you want to write into defend his legacy, please do.

1032
00:51:10,719 --> 00:51:20,880
Speaker 2: All right until next time, Extraordinaries Daniel and Kelly's Extraordinary

1033
00:51:21,000 --> 00:51:24,200
Speaker 2: Universe is produced by iHeartRadio. We would love to hear

1034
00:51:24,280 --> 00:51:24,560
Speaker 2: from you.

1035
00:51:24,840 --> 00:51:27,719
Speaker 1: We really would. We want to know what questions you

1036
00:51:28,000 --> 00:51:30,600
Speaker 1: have about this extraordinary universe.

1037
00:51:30,719 --> 00:51:33,600
Speaker 2: We want to know your thoughts on recent shows, suggestions

1038
00:51:33,680 --> 00:51:36,640
Speaker 2: for future shows. If you contact us, we will get

1039
00:51:36,719 --> 00:51:37,040
Speaker 2: back to you.

1040
00:51:37,400 --> 00:51:40,840
Speaker 1: We really mean it. We answer every message. Email us

1041
00:51:40,960 --> 00:51:44,040
Speaker 1: at questions at Danielankelly dot org, or.

1042
00:51:44,040 --> 00:51:46,120
Speaker 2: You can find us on social media. We have accounts

1043
00:51:46,239 --> 00:51:50,200
Speaker 2: on x, Instagram, Blue Sky and on all of those platforms.

1044
00:51:50,239 --> 00:51:53,200
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1045
00:51:53,400 --> 00:51:54,920
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