The physics of shadows

Daniel and Kelly shine a light on the mysterious every day physics of shadows, including whether they can move faster than light. See omnystudio.com/listener https://omnystudio.com/listener for privacy information.

Speaker names marked ? are inferred from speaking time and the show’s known hosts — they are not voice-verified.

• 0:00Daniel Whiteson?

  This is an iHeart Podcast. Guaranteed Human.

• 0:04Speaker 01

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• 1:21Speaker 04

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• 2:06Speaker 04

  Severe side effects may include inflamed pancreas or gallbladder problems. Tell your doctor if you experience vision changes before scheduled procedures with anesthesia, if you're nursing, pregnant, plant-to-be, or taking birth control pills. Taking Zepbound with a sulfonylurea or insulin may cause low blood sugar. Side effects include nausea, diarrhea, and vomiting, which can cause dehydration and worsen kidney problems.

• 2:28Speaker 03

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• 2:38Daniel Whiteson?

  Hey, everyone. It's Daniel. I have a new book out now called Do Aliens Speak Physics? About whether or not we have physics in common with aliens. I think it's a lot of fun, but here's a totally unbiased review from Phil Plait, author of Bad Astronomy. Quote, "Whitson makes us think about how we think about aliens and how we think about the universe at a fundamental level, and does it in a readable, understandable, and even funny way. If we ever meet aliens, the first thing we should do is give them this book." So go out and pick up a copy of Do Aliens Speak Physics. I think you'll enjoy it.

• 3:20Daniel Whiteson?

  When you see the universe through the lens of physics, you start to see physics everywhere. Why is glass transparent, but stone isn't? Why are bicycles stable? How do tornadoes start? The mysteries of physics aren't just out there in deep space, they're right here in front of us, raising questions and demanding answers every day. Today, we'll tackle a topic that I've wondered about since I was a little baby physicist, looking at the world and wondering how it worked. All you need for this inspiration is a light bulb and your hands, with which you can make something out of nothing. Shadows aren't really anything, they're a lack of something, they're negative objects. But our minds see them as positive, as a thing in the world, rather than just the

• 4:05Daniel Whiteson?

  absence of light. And shadows have been surprising and confusing people for thousands of years. Today, we'll dig into the physics of shadows and ask whether they can break one of physics' most hallowed principles. Welcome to Daniel and Kelly's Extraordinary Shadowy Universe.

• 4:36Kelly Weinersmith?

  Hello, I'm Kelly Weinersmith. I study parasites and space. And one of my absolute favorite things is when it's a clear night and it's a full moon and I'm out on the farm and you can see the shadows of the trees and your own shadow in the middle of the night. It feels sort of like spooky, but fantastic. And I just absolutely love it. I like shadows.

• 4:55Daniel Whiteson?

  Are those star shadows or moon shadows?

• 4:58Kelly Weinersmith?

  No, it's just that there's so much sunlight reflecting off the moon and making it down, you know, to earth that you can see, you know, the shadows of the trees. Wait, what did you ask me? Are they moon shadows? Yeah, they're moon shadows. I'll call them moon shadows.

• 5:11Daniel Whiteson?

  Yeah, moon shadows are awesome.

• 5:12Kelly Weinersmith?

  Yeah, I love them.

• 5:13Daniel Whiteson?

  That's cool.

• 5:15Kelly Weinersmith?

  As I was explaining it, I was like, there's no way Daniel doesn't understand where the light is coming from. Why am I explaining it to him? I must have missed the question.

• 5:26Daniel Whiteson?

  Hi, I'm Daniel. I'm a particle physicist and I do understand why the moon is bright at night.

• 5:32Kelly Weinersmith?

  Go ahead, Daniel. Why is the moon bright at night?

• 5:37Daniel Whiteson?

  Or you're doubting me? Or you're putting me on the spot?

• 5:41Daniel Whiteson?

  Let's call it bluff. No, obviously because the moon is being illuminated by the sun. Just because we can't see the sun doesn't mean that some parts of the moon can't see the sun.

• 5:49Kelly Weinersmith?

  And I've never lived in an area where there was so little light pollution that I could see the shadows cast by the moon. Another reason why Virginia is amazing.

• 6:06Kelly Weinersmith?

  So my question for you today, Daniel. So I was looking at the question asked by the listener and I realized that faster than light questions sometimes just like make my brain go, oh, this is really not making sense to me. What physics concept that you, when you think about, makes you think, ah, I just, my brain can't really, I have to accept that this is true because the science is good, but my brain is just having trouble wrapping itself around this fact? What confuses you about physics?

• 6:37Daniel Whiteson?

  Thermodynamics. Yeah, I've never really been a fan of statistical physics and thermodynamics because it verges on chemistry. It's like you start from all these micro states and these little particles and then you zoom out and you draw these conclusions about the macroscopic state. But there's so many particles involved and I don't know, it just feels like there's lots of approximations and considerations. And you never really feel like you're on solid footing. And when I showed up at Berkeley, they make you take this qualifying exam to see like, how good is your physics? And I remember my advisor at the time, he was like, good students pass this exam on the first try. And I was like gulp. So I spent that whole summer before grad school studying, studying, studying.

• 7:21Daniel Whiteson?

  And I spent most of it studying thermodynamics because I knew that was my weak spot. And this qualifying exam is terrifying. It's two eight hour days. Right? So it's like a serious ordeal. It's like a marathon.

• 7:32Kelly Weinersmith?

  This is right when you get there? This isn't like two years in or something?

• 7:35Daniel Whiteson?

  Well, you can wait and take it after you've taken the grad school classes. But my advisor was like, the best students show up and pass it on the first try. And I was like, yikes, that's pressure.

• 7:44Kelly Weinersmith?

  Yeah.

• 7:45Daniel Whiteson?

  But I lucked out in that year, no thermo questions, just zero. So I was like, yeah, all right, I'm gonna skate on through.

• 7:54Kelly Weinersmith?

  And did you? Did you pass it on the first time?

• 7:56Daniel Whiteson?

  I did pass it on the first try and then I had to take a thermodynamics class. And I skated just over the minimum threshold for that class, let me tell you.

• 8:05Kelly Weinersmith?

  Nice, but you did it and now it's behind you.

• 8:08Daniel Whiteson?

  I did it.

• 8:08Kelly Weinersmith?

  Congratulations.

• 8:09Daniel Whiteson?

  And I'm a certified physicist and I don't have to do thermodynamics.

• 8:14Kelly Weinersmith?

  You know, speaking of chemistry, last night I was giving a talk to a women in chemistry group over Zoom. And halfway through, I realized that I was describing a chemistry thing and I suddenly got really insecure because I was like, what if this is wrong? Cause I just pretend to know chemistry.

• 8:29Daniel Whiteson?

  Everybody pretends to know chemistry, even the chemists.

• 8:32Kelly Weinersmith?

  And then I was like, well, and what if any of them listen to the podcast and they know what I've said about chemistry?

• 8:39Kelly Weinersmith?

  But I, you know, I think it went okay in the end.

• 8:42Daniel Whiteson?

  I think it's important to admit our strengths and weaknesses. You know, that's part of being a scientist. But that's not the kind of stuff that just got me into physics. I wasn't always fascinated with concepts about temperature and pressure and this kind of stuff. I remember as a kid thinking about light, you know, wondering like if I block the sun, if I put my hand in front of the sun, how does light get to that tree over there? And just thinking about like the way light moves. This is like six year old Daniel, like doing little experiments. And like, obviously now it seems like a silly experiment that like can get to the tree, even if my hand is blocking the light from my face. But you don't know till you do the experiment, right? And so anyways, I've always been super fascinated by light and by shadows.

• 9:22Kelly Weinersmith?

  Well, let's not leave our listeners in the shadows. Let's jump into today's topic. See, I did it this time.

• 9:30Daniel Whiteson?

  I was trying to get us there. Today's episode is inspired by a question from a listener who asked if we could do a whole episode on shadows. And specifically, they were wondering about three scenarios. One, where you're standing in the shadow of something like a tree, but you can still see things around you. Why is it the photons are reaching his eyes even if the sun is obstructed? Number two, he was wondering about what are shadows like on the moon? Are they sharper or darker than on Earth? And then finally, of course, can shadows move faster than light?

• 10:02Kelly Weinersmith?

  Oh, that sounds like it could be a trick question. I think we should pass that on to our listeners.

• 10:09Daniel Whiteson?

  That's a great idea. So let's play a trick question on our listeners. And so I went out there and I asked them if they thought shadows could travel faster than light. Here's what our listeners had to say on this brilliant question.

• 10:21Speaker 06

  The obligatory answer is nothing can move faster than the speed of light. But I sense that there are multiple levels to this question.

• 10:30Speaker 07

  No, however, I would say that they can appear to travel faster than light as they tend to loom larger than the object that's being illuminated. But no, they can't travel faster than light according to Einstein. No, they can't because they are the absence of light.

• 10:43Speaker 03

  So they are related to light's speed. If I had a spherical projector screen at the orbit of Pluto and I had a bright light in front of me, and then all of a sudden I put my hand in front of the bright light, it would appear that a giant swath of shadow would go across the screen all at once to me. But I think to Pluto, they would see it different, like it would take this speed of light to go from one place to another. Since you asked, it means that there's probably some exception, so let's find out.

• 11:10Speaker 06

  I would say no. Shadows are the absence of light and light can only travel at the speed of light.

• 11:16Daniel Whiteson?

  That's a fascinating one. I'm gonna say no.

• 11:19Speaker 06

  Shadows aren't a thing, they're absence of a thing, but the answer, my answer would be no, they can't.

• 11:26Speaker 02

  Yes, but it cannot be used for communication. Yes, shadows can move faster than light with caveat that only their appearance can move faster than light. No actual photon is moving faster than light.

• 11:42Speaker 04

  I think so because nothing's actually moving. It's just a change in the pattern of light, but you're really breaking my brain here.

• 11:50Speaker 06

  I think shadows are dependent upon upon light. It's kind of like the absence of light. And so I think no. I think there's something like this about the way the point where two blades of scissors cross, if the scissors are closed extremely quickly. I don't see how shadows could travel faster than the speed of light. So I'm gonna say they travel at the same speed.

• 12:17Kelly Weinersmith?

  So I think our listeners know us pretty well. A couple of them are like, oh, this feels like a trick question.

• 12:26Daniel Whiteson?

  I love hearing them use their physics knowledge and try to work it out.

• 12:29Kelly Weinersmith?

  Yes.

• 12:30Daniel Whiteson?

  Some people got halfway there, you know, there nothing is actually moving. Other people are like, no, this is an absolute principle in physics. Nothing can move faster than the speed of light. But as usual, language is the culprit here because the principles of physics are very clear mathematically, but things get fuzzy when you express them in language. And so we're gonna end up splitting hairs between nothing and no thing.

• 12:53Kelly Weinersmith?

  So thanks for playing along, even though we set you up for failure over and over and over again. We appreciate you.

• 13:00Daniel Whiteson?

  We really do. We really do. And thanks to listeners like Eric who write in with their questions and inspire these episodes. We really want to hear what you are curious about because we want to scratch your itch about physics, not just ours.

• 13:12Kelly Weinersmith?

  Yes, we want to shine light on the darkness in your lives. And if you want to contact us, you can write us at [email protected]. And that's both to send us questions and to get on the list of people that we send these trick questions to.

• 13:25Daniel Whiteson?

  Or just to tell us about your day and send us cute pictures of your cat.

• 13:28Kelly Weinersmith?

  Yeah, that'd be great.

• 13:29Daniel Whiteson?

  It works for everything.

• 13:30Kelly Weinersmith?

  Yep, I would love that. All right, Daniel, let's start with the basics. Shadows sound like something where you're like, oh, yeah, I know what a shadow is. But physics always makes simple concepts much more complicated. So, Daniel, how would you define a shadow?

• 13:46Daniel Whiteson?

  A shadow is just the absence of light. And in the simplest sense, this is fairly straightforward. If you have a single source of light, and you have things with very crisp edges, then you can use the concepts of geometric optics where light travels in straight lines. And some regions of your experiment will be illuminated, and some regions will not. And those are the shadows. And so places that are obstructed from direct line with the light source will be in shadow, and places that are not will not be in shadow. So that's sort of the simplest, clearest setup where the shadows are very straightforward.

• 14:21Kelly Weinersmith?

  Okay, now give us some more complicated information, because I know that's where you're going.

• 14:26Daniel Whiteson?

  Yeah, because we don't live in that kind of situation. We never have a single source of light. Like, number one, our light sources are not points. They're extended, right? They're a little bit wide. And in physics, we treat a wide light source, you know, like a filament that's a centimeter across, as a bunch of different sources of light. You can treat it as like a set of pinpoint sources, right? So what happens if you have multiple sources of light? Well, imagine two sources of light. You're in a room and there's two light bulbs. Well, you can be in total shadow if you are blocked from both sources of light, right? And you can be in total illumination if you can see both sources of light, but there's also a Middle ground. What if you're blocked from one source of light and not the other? Then you're in like half shadow, right?

• 15:10Daniel Whiteson?

  And so now this is the fuzzy region. This is actually called the penumbra. Places where you are blocked partially from the full illumination, but not completely. And so any room that you're in, your shadows are going to have these fuzzy edges because of their penumbras. Like I'm in a room right now and I have like four banks of fluorescent lights, each of which is like a meter across. As if I hold up my hand above my desk, my shadows are very fuzzy. In fact, there's like four of them and they partially overlap. And so I don't have crisp shadows anywhere in my office.

• 15:42Kelly Weinersmith?

  Oh, how sad.

• 15:44Daniel Whiteson?

  It's good. It's good. Because otherwise the shadows would be very, very stark, right? But this causes really interesting effects that are sometimes hard to understand.

• 15:53Kelly Weinersmith?

  So could you still like, if you had one light in a room and it was like a broad light, like a foot wide or something? Could you still have like a shadowy shadow, a fuzzy shadow is what I meant? If the light is like reflecting off of the walls a lot and coming back underneath your hand?

• 16:11Daniel Whiteson?

  Absolutely. Yes. And so that's another contribution is that the things in your room do not perfectly absorb light. If everything in your room was a black body object where it just absorbed light and didn't reflect any, then you would have crisper shadows. But if you have a white wall, it's white because it's reflecting light. And so even in the scenario where you have one source of light, where you expect crisp shadows, if your walls are white, they're reflecting light. And so some of that light is going to reduce the shadow. So yeah, there's lots of ways that shadows get fuzzier because things are reflective and because there are multiple sources of light.

• 16:48Kelly Weinersmith?

  Can I tell you the story about the one time I came across the word penumbra while doing research?

• 16:52Daniel Whiteson?

  Is it gonna make me spit up my coffee or throw up?

• 16:55Kelly Weinersmith?

  No, it's not gross. It's, it's a little silly, but it's not gross. So.

• 17:00Daniel Whiteson?

  Let's do it.

• 17:01Kelly Weinersmith?

  We were reading about space settlement proposals and there was someone who was proposing that Mercury would be a great place to set up a space settlement.

• 17:10Daniel Whiteson?

  Really?

• 17:10Kelly Weinersmith?

  Really? Yes.

• 17:11Daniel Whiteson?

  Because of the wonderful outdoor temperatures.

• 17:13Kelly Weinersmith?

  Yes, right. So Mercury being the closest planet to the sun and with no atmosphere, it gets very hot on one side and very cold on the other.

• 17:22Daniel Whiteson?

  Cause it's tightly locked.

• 17:23Kelly Weinersmith?

  Yes, right. But at the penumbra, the temperature is pretty nice.

• 17:31Daniel Whiteson?

  How wide is that? It's like centimeters or meters?

• 17:34Kelly Weinersmith?

  It's it's wide enough that they thought you could like put a moving habitat that would have to constantly move to stay with the penumbra. And if you fall behind or get too far ahead, you die, you know, different ways, depending on if you're going too fast or too slow.

• 17:48Daniel Whiteson?

  I like that if my house breaks down, I die very quickly.

• 17:51Kelly Weinersmith?

  Yeah.

• 17:51Daniel Whiteson?

  That's very relaxing. I could definitely take a nap in that kind of house.

• 17:54Kelly Weinersmith?

  Yeah, I think I'll pass on this plan. The other plan was to bury yourself underground at the poles where the temperature was also not so lethal. But anyway, I'm happy where I am. Daniel, I've heard this phrase, shadow blister? And I have no idea what it means. What is a shadow blister?

• 18:11Daniel Whiteson?

  Shadow blister is a really cool effect where as two shadows get closer together, they seem to kind of merge. And even more than that, it seems like they leap out towards each other. They grow towards each other. So if you like stand next to a telephone pole, you have a shadow, telephone pole has a shadow. As you inch towards the telephone pole, you'll see your shadows merge, but they don't just like overlap geometrically. When you get close to the telephone pole, the two shadows grow out to meet each other. It looks really weird. You're like, what is going on? Are my shadows like hugging? Do they know about each other? Are they conscious? Is this, am I living in some weird science fiction novel? No, physics can't explain this.

• 18:51Kelly Weinersmith?

  Well, and so is it called a shadow blister because you're not supposed to pop them when they get together because then they might get infected? What is, why are they called shadow blisters?

• 18:58Daniel Whiteson?

  Way to make it gross, Kelly. Way to make it gross.

• 19:00Kelly Weinersmith?

  I had to get there.

• 19:01Daniel Whiteson?

  They call shadow blisters because they sort of grow out beyond the edge of the existing shadow. So you can like create a blister on the light pole shadow because of your shadow. Well, what's happening here is that you don't notice that the telephone pole shadow has many layers. There's the sort of major shadow, but then there's sort of minor shadows. There's the penumbras, right? And as your penumbras overlap with the penumbras of the shadow, then you start to notice these things. And Minute Physics, which is a great channel on YouTube, which is amazing, explainers about lots of stuff, has a great video on this. You really got to see this video to understand it. So check out that video. But the key concept there is the penumbra, the fact that shadows are almost never crisp because you don't have single sources of light.

• 19:43Kelly Weinersmith?

  That's awesome. I didn't know that at all.

• 19:45Daniel Whiteson?

  Yeah, exactly.

• 19:45Kelly Weinersmith?

  Daniel, I understand my world better now.

• 19:50Daniel Whiteson?

  And so, obviously, in our lives, most of the shadows come from the sun, right, or from interior lights. But like Kelly told us, you can also get shadows from any source of light, including the moon, which of course, originally comes from the sun. But it's still kind of cool to be walking around at night and see your shadow. And if the moon is dark, and it's an exceptionally clear night, and you're out very, very far from light pollution, you can see something really awesome, which is a star shadow.

• 20:18Kelly Weinersmith?

  What?

• 20:19Daniel Whiteson?

  You can see your shadow from the stars. Yeah.

• 20:22Kelly Weinersmith?

  How could you be sure that it was from the stars? I guess if you can't see the moon, then it's definitely not the moon that's causing it. This is crazy. I can't wrap my head around this.

• 20:30Daniel Whiteson?

  It's hard to identify because the stars come from all directions. But in principle, right, it's there. And it's kind of amazing poetic that those photons have crossed like billions and billions of kilometers of space, only to be like blocked from hitting the earth by your hand, or whatever.

• 20:47Kelly Weinersmith?

  By your dumb face.

• 20:53Daniel Whiteson?

  And it just reminds me of how frustrating it is that all these photons come from all over the universe, splash on the earth, and mostly, they're just ignored. Like, those photons carry so much information about what happened inside that star, what was going on, you know, its future, its history, its hopes and dreams. And they just like, get absorbed by some plant, or whatever. And that's just lost. We are tapping into like, the tiniest bit of this huge river of information that's coming at us from the universe. Anyway, and sometimes it makes cool shadows for you to go, ooh, nice.

• 21:29Kelly Weinersmith?

  So Daniel clearly is suffering from a massive case of FOMO.

• 21:32Daniel Whiteson?

  Yes.

• 21:33Kelly Weinersmith?

  Missing out on what those photons could tell us. Let's take a break, and hopefully, Daniel doesn't descend into despair. And when we get back, we'll talk about some more complicated features of shadows.

• 21:53Speaker 03

  Cash Flow Crunch? OnDeck's small business line of credit gives your business immediate access to funds, up to $200,000, right when you need it. Cover seasonal dips, manage payroll, restock inventory, or tackle unexpected expenses without missing a beat. With flexible draws, transparent pricing, and control over repayment, get funded quickly and confidently. Apply today at ondeck.com. Funds could be available as soon as tomorrow. Depending on certain loan attributes, your business loan may be issued by OnDeck or Celtic Bank. OnDeck does not lend in North Dakota. All loans and amounts subject to lender approval. Snoring, gasping for air during sleep, daytime sleepiness? I'm Shaquille O'Neal, and this shouldn't be anybody's experience. Ask your doctor about Zepbound Trisepotide, the first and only FDA-approved prescription

• 22:35Speaker 03

  medicine for moderate to severe obstructive sleep apnea in adults with obesity.

• 22:39Speaker 04

  Zepbound is a prescription medicine used with a reduced calorie diet and increased physical activity to help adults with moderate to severe obstructive sleep apnea (OSA) and obesity to improve their OSA. Zepbound is approved as a 2.5, 5, 7.5, 10, 12.5, or 15-milligram injection. Zepbound contains trisepotide and should not be used with other trisepotide-containing products or any GLP-1 receptor agonist medicines. It is not known if Zepbound is safe and effective for use in children. Don't share needles or pens, or reuse needles. Don't take if allergic to it, or if you or someone in your family had medullary thyroid cancer, or if you've had multiple endocrine neoplasia syndrome type 2. Tell your doctor if you get a lump or swelling in your neck. Stop Zepbound and call your doctor if you have severe stomach pain or a serious allergic

• 23:24Speaker 04

  reaction. Severe side effects may include inflamed pancreas or gallbladder problems. Tell your doctor if you experience vision changes before scheduled procedures with anesthesia, if you're nursing, pregnant, plant-to-be, or taking birth control pills. Taking Zepbound with a sulphonal urea or insulin may cause low blood sugar. Side effects include nausea, diarrhea, and vomiting, which can cause dehydration and worsen kidney problems.

• 23:46Speaker 03

  Talk to your doctor. Call 1-800-545-5979, or visit zepbound.lily.com.

• 23:54Speaker 02

  Hey, it's Niall Rogers, host of Before the Moment, a Lincoln original series. I'm driving across the country, meeting some of today's most fascinating artists. We're exploring the stories before the moment that changed everything. This week, I'm joined by rising star Sienna Spiro. My first ever In the Wild show was when I was 10.

• 24:16Speaker 06

  And I sang at a pub called The Georgian Devonshire. I was so nervous. I was tense, so they let me in only to sing my song and they kicked me out right after. And then I remember I wrote, I've been writing since I was 10, so I, I wrote this song called Lady in the Mirror, made this little track with it, but I just like pressed it. And I remember I sang this song and there was like five people in there. And I remember how myself I felt and I felt like I understood myself and understood what I needed to do.

• 24:40Daniel Whiteson?

  So does that take us back to, I sing just to know I'm alive?

• 24:44Speaker 06

  Hundred percent, hundred percent, in my whole life.

• 24:47Daniel Whiteson?

  Experience before the moment. A Lincoln original series on the iHeart Radio app, YouTube, or wherever you get your podcasts.

• 25:07Daniel Whiteson?

  All right, we're back and we're talking about light and shadows. And Kelly's comment reminds me that I actually have a solution to my existential angst about missing all those photons.

• 25:16Kelly Weinersmith?

  How are you going to collect all the photons, Daniel?

• 25:19Daniel Whiteson?

  Well, look, I figure if Dyson can come up with this concept of Dyson spheres, which are basically just solar panels that surround the sun, I'm just going to make a Whitson sphere, which is basically space telescopes that surround the earth.

• 25:32Kelly Weinersmith?

  Ah.

• 25:33Daniel Whiteson?

  Like, let's just give up our vision of the night sky and replace it with like a solid bank of telescopes that gobble all that information. Imagine what we could learn. I mean, every time we develop a space telescope that appears at one little corner of the sky, we have our minds blown by what we see out there. And so think about what we could learn if we had like a thousand, a billion times more capacity. It's never going to happen, but we're also never going to build the Dyson sphere. So I can put the Whitson sphere into the same category of fantastical concepts.

• 26:03Kelly Weinersmith?

  Yeah, if only we could collect all the photons and deprive the plants of them. I'm sure that would be great for all of us. I don't know who's, who's less realistic, you or Dyson, but it's a fun idea to think about.

• 26:15Daniel Whiteson?

  All right, maybe just half the earth then, you know, when the earth is in shadow. How about that?

• 26:19Kelly Weinersmith?

  Which hemisphere, oh, when it's in shadow. Okay, I was going to ask which hemisphere you're going to condemn to death.

• 26:24Daniel Whiteson?

  Let's have a vote. No, obviously at night, yeah. So yeah, this project gets more complicated and less realistic as we think about it, but it was never going to happen anyway.

• 26:33Kelly Weinersmith?

  Okay. I mean, people are upset enough about Starlink satellites going in orbit. I'm not sure. Anyway, let's move on.

• 26:40Daniel Whiteson?

  So, so far, we've been talking about shadows from a sort of geometrical optics point of view. Shadows just travel in straight lines and like at the edge of an object, either the light is absorbed or it passes. But the way light interacts with objects is more complicated than that, and it makes shadows fuzzier and weirdly surprising. So shadows played a really important role in the debate about the nature of light. Is it a particle or is it a wave?

• 27:03Kelly Weinersmith?

  Is this the two slit experiment?

• 27:05Daniel Whiteson?

  Yes, the two slit experiment was part of it, but that actually didn't settle the question in most people's minds about whether light was a particle or wave until they did this crazy shadow experiment. So what's the connection between shadows and the particle or wave nature of light? Well, if light is a particle, then shadows should be very sharp, right? Either the particle light passes the edge of the object or it's absorbed. But if light is a wave, then it's more complicated because you get things like diffraction and interference. So in the early 1800s, people had done this experiment, like the Young double slit experiment, to show that light had this wave-like behavior, that it was interfering with itself. And so light was very likely a wave. And you can see the same thing, not just in interference, but in diffraction.

• 27:49Daniel Whiteson?

  Diffraction is like the big sister of interference. The way it works is imagine you have like a circular object and you shine a light on it and you have a screen behind it. What do you expect? Well, you expect a circular shadow, right? If light is a particle, that shadow should be perfect and crisp if you have a single light source, because there's like an edge where the particles just barely make it around the object and a tiny bit further in, for example, they don't. So you get a very crisp shadow. You with me still?

• 28:16Kelly Weinersmith?

  Yep, I'm with you.

• 28:17Daniel Whiteson?

  All right, but if light is a wave, that's not how light interacts with matter. What you need to do instead is imagine a bunch of sources of light all around the object. The same way as in the interference experiment where you have two slits. The way you think about that mathematically is each slit is now a source of light. The light makes it through the slit. You imagine that as a point source and then those two point sources interfere. Diffraction is like that, but now you have lots and lots and lots of sources of light all around. The edge of this object, everywhere the light is making it around is a point source of light, and those are all going to interfere. And so if you look very carefully at the edge of shadows, you can see this diffraction effect. There is no perfectly crisp shadow,

• 28:59Daniel Whiteson?

  even in a room filled with black bodies and a single source of pinpoint light, you will still get these fringes at the edge of shadows. You'll have like a dark black center, and then you'll have a white band, and then a black ring, and then a white band. You get the zebra shadow effect from the diffraction edges.

• 29:17Kelly Weinersmith?

  So I've got a bright light, and I'm putting my hand under it, and I'm not seeing the like, so in our outline, you have this great picture with ripples, that of course, our audience can't see, but imagine like you drop a rock in a lake, and there's ripples, there's a black spot where the rock got dropped, and then there's ripples coming out from there. But that's not what I feel like I'm seeing when I put my hand under the light. And so what was different about the lab conditions that find these ripples, obviously relative to my office?

• 29:44Daniel Whiteson?

  Yeah, well, you have to have a single source of light, because otherwise you have all these perunbras which are overlapping, and it's hard to isolate this effect. And you have to have a room with no other reflections, because this is a subtle effect. It's not easy to see. But it was not so subtle that 200 years ago, they couldn't do it. And so the interference experiment, and this shadow diffraction experiment were very strong indicators that light was a wave, not a particle. And at the time, people really believed like Newton's idea that light was little corpuscles, right? It was this little bits of stuff. So these were hard to absorb, and there were lots of people who like really dug in, and they were like,

• 30:23Daniel Whiteson?

  this is absurd, light cannot be a wave. And one famous physicist Poisson of Poisson Statistics, and all sorts of stuff, he studied this theory in detail, and he was trying to prove it wrong.

• 30:34Kelly Weinersmith?

  I have a feeling I would have been on team Poisson. Light cannot be a wave. It doesn't make sense.

• 30:39Daniel Whiteson?

  That's just because Poisson means fish, and you're always on team fish, Kelly.

• 30:43Kelly Weinersmith?

  I am always on team fish. It's true. The number of jokes that fish people make about Poisson Statistics is maybe nauseating.

• 30:51Daniel Whiteson?

  Off the hook.

• 30:53Kelly Weinersmith?

  Off the hook.

• 30:57Kelly Weinersmith?

  Oh, thank you, Daniel. Okay, moving on.

• 31:00Daniel Whiteson?

  I had to dive into that one. Anyway, this is a great example of something that happens in physics all the time, that people look for a ridiculous prediction from a theory as a way to prove it wrong, but then it turns out to actually prove it right. So Poisson did the calculations and he discovered a weird prediction from the wave theory. He discovered that at the center of that shadow, the wave theory predicted a bright spot. So, you know, the particle theory was the shadow should be perfectly circular. And the wave theory predicted all these fringes, these zebra lines at the edge, but also at the very center, all of the waves add up and constructively interfere because they're all equidistant from the edge. And so all those photons should be in phase.

• 31:45Daniel Whiteson?

  And so Poisson was like, aha, this is a ridiculous prediction. You're telling me there should be a bright spot at the center of the shadow? Absurd. And so this obviously disproves the wave nature of light, right?

• 31:55Kelly Weinersmith?

  That would be fishy.

• 31:58Daniel Whiteson?

  You were just waiting for that joke.

• 31:59Kelly Weinersmith?

  I was. You could see it in my face.

• 32:03Daniel Whiteson?

  And so this was a very strong argument to reject the wave theory. But then a guy went out and actually did it. A guy called Arago. He went out and did this experiment and there is a bright spot at the center of the shadow.

• 32:15Kelly Weinersmith?

  What?

• 32:16Daniel Whiteson?

  If you Google this image, you can see there is a tiny white dot. This is now called the Arago dot. And it was very conclusive and people were like, okay, well, you know, we said that wave theory makes this absurd, nonsensical prediction, so therefore it can't be true. But then if the universe actually does it that way, that's a pretty clear indicator that the wave theory is correct. So while the Young's double slit experiment and these diffraction experiments were very strong evidence already of the wave nature of light, it wasn't until this shadow experiment, seeing a light at the center of the shadow, that people were like, okay, fine, light is a wave.

• 32:52Kelly Weinersmith?

  Was Poisson a good sport? And was he like, oh, you got me?

• 32:58Kelly Weinersmith?

  Now I'm hooked on the wave theory. I know I took your joke, but was Pusson convinced after this or was he long gone by then?

• 33:05Daniel Whiteson?

  Well, the historical summary I read suggests that he wasn't especially gracious about it.

• 33:10Kelly Weinersmith?

  Oh, no.

• 33:10Daniel Whiteson?

  You know, he didn't like on the spot, admit the wave theory. He was skeptical for a while, but you know, he went on to have a perfectly fine reputation. So he definitely survived scientifically.

• 33:20Kelly Weinersmith?

  Yeah, but, but you know, I still, I like my scientists to be gracious when they're wrong. But anyway, what are you gonna do? So first of all, you have blown my mind because when I first looked at this picture, I didn't see the little white dot in the center. I thought that it was a speck on my screen. And so while you were explaining it, I was moving the outline up and down and I was like, that dot is actually on the image.

• 33:39Daniel Whiteson?

  Yeah.

• 33:39Kelly Weinersmith?

  That's awesome.

• 33:40Daniel Whiteson?

  So shadows taught us something about the nature of light, right? The patterns of shadows are much more complex than you might imagine. And the wave nature of light really is revealed by the patterns of the shadows.

• 33:50Kelly Weinersmith?

  Yeah. So this is all a little hard to believe, but I'm with you. But like, what? Next year, you're gonna tell me that shadows have colors?

• 33:57Daniel Whiteson?

  Shadows do have colors. Yes.

• 33:59Kelly Weinersmith?

  No, they don't.

• 33:59Daniel Whiteson?

  Absolutely. Yes. So far, we've only been thinking about single sources of white light and complete shadows, right? But remember, we talked about penumbras, right? You can have multiple sources of light. And so you can have intermediate shadows. Well, now take those multiple sources of light, so you have three of them, and you make those colors. You have a filter for each one. So you have like a red source, a green source, and a blue source. Anywhere where you can see all three sources, you'll be seeing white light. And anywhere all three sources are blocked, you'll be in shadow. But what happens if you're in a place where you're only blocking the red light? Then you have green and blue light, which make a cyan shadow. Or if you block the blue light, the red and green merge,

• 34:42Daniel Whiteson?

  making a yellow shadow. Or if you block the green, the red and blue combine to form a magenta shadow.

• 34:49Kelly Weinersmith?

  Daniel.

• 34:49Daniel Whiteson?

  Right? So.

• 34:51Kelly Weinersmith?

  I don't feel like I've ever seen. Yes, this is awesome. But this is totally going against my intuition. Shadows are black, Daniel. Is there, is there a good video online?

• 34:58Daniel Whiteson?

  Yeah, I'm sure you can find a good video. But this sort of bends the definition of shadow, I think is the issue, because in these regions, is it really? A magenta shadow, well, you're shining a red and blue light on it. So people would say it's magenta because you're shining magenta light on it, not because you've blocked the green light, right? And so it really depends on how you define the things that are not fully illuminated or the things that are not fully blocked. Are they parts of the shadow? Are they the colored penumbras? Or are they partially illuminated?

• 35:28Kelly Weinersmith?

  This is a physicist trick.

• 35:32Daniel Whiteson?

  I'm retreating into philosophy to avoid being proven wrong.

• 35:38Kelly Weinersmith?

  All right, well, maybe I'll try this with my kids one day. Okay, so I have a son who loves swimming, constantly he's swimming. Is there anything interesting or different about how shadows are cast underwater? Are their penumbras bigger or something because of how water deflects? Deflects? Oh, what is the word? Defracts? Bends?

• 36:02Daniel Whiteson?

  Refracts.

• 36:03Kelly Weinersmith?

  Refracts, thank you.

• 36:06Daniel Whiteson?

  Shadows and water are fascinating because it's a little bit counterintuitive, but water casts a shadow. Like if you pour water into a glass and you shine a light on it and then have a screen on the other side, you will see the shadow of the water. And at first you're like, wait a second, why would water have a shadow? Water is transparent, right? It's like glass, light passes through it. Why is it making a shadow? What's going on? Did I find a glitch in the matrix? You have not found a glitch in the matrix. Water is transparent, but light does not pass through it without bending. And so what's happening here is that the light is acting like a lens and it's refracting a lot of that light away. And so the shadow there doesn't come from the object being completely opaque, but from it being darker behind the column of water

• 36:50Daniel Whiteson?

  because some of that light has been refracted away.

• 36:53Kelly Weinersmith?

  Ah, so if you were to measure the light on the sides of the water, it would be brighter after you put the cup there?

• 36:59Daniel Whiteson?

  It might be a tiny bit, but it gets refracted in many, many directions. So most of it won't even hit the screen. Yeah.

• 37:05Kelly Weinersmith?

  So when you are standing in the ocean, which I guess you get to do as a Californian, probably pretty often.

• 37:11Daniel Whiteson?

  Whenever I want.

• 37:17Kelly Weinersmith?

  All right, that is pretty solid. So you're standing in the ocean and it's a bright sunny day and you can see shadows cast on the sand by the water.

• 37:27Daniel Whiteson?

  Yes.

• 37:27Kelly Weinersmith?

  Is that just because like the way the waves build up, it's changing the patterns of how the light is bent?

• 37:33Daniel Whiteson?

  Yeah, exactly, because the surface is not flat. If you stood in a perfectly still body of water, you would see no shadows from the water on the bottom of the pool, for example. But as soon as you make a ripple on the surface, then you're going to see shadows of those ripples for the same reason that now when the light is hitting the surface of the water, it's not going straight down to the bottom anymore, it's getting bent away. And so when you have a pool that's just like sitting there and it's like gently fluctuating, you get these amazing patterns on the bottom of the pool, right? Brighter spots where the light is being concentrated and darker spots, shadows essentially, where the light is being bent away. And so again, this is a case, not a full obstruction, but of like a rearrangement

• 38:16Daniel Whiteson?

  of where the light is going, creating these patterns of light and shadow. Really beautiful. One of my favorite things about water is that you're essentially seeing the surface, right? You're seeing the shadows of the surface.

• 38:27Kelly Weinersmith?

  I like that it keeps me from dying.

• 38:31Daniel Whiteson?

  Water is good. Yes, we are definitely pro water on this podcast.

• 38:36Kelly Weinersmith?

  Another place where I like looking at shadows is on a foggy day. So like we on our farm, every once in a while, the fog will like come up from the bottom fields. And one, I like looking at my shadow in it. But two, I like imagining that a zombie movie is going to be filmed in my fields, uh, cause it's kind of creepy. But so is there anything interesting about shadows in fog?

• 38:57Daniel Whiteson?

  Yeah, fog is wonderful for studying shadows because it shows you where the light is, right? It's like having a bunch of lasers and throwing up dust particles in front of them. You can see where the lasers are. And so fog is just like a bunch of particles of water suspended in the air and they tell you where the light is and you can have a shadow on the fog. And so there's a well-known effect called the Brocton Spectre Effect, where you can have a shadow on a cloud, right? And so for example, if you stand in In front of a car with bright headlights in the fog, you'll see your shadow on the fog. And it can be this like huge looming shape, right? But also, if you stand in front of your headlights, you could see your shadow on a cloud in the sky.

• 39:38Kelly Weinersmith?

  What?

• 39:38Daniel Whiteson?

  If you do it right. Yeah.

• 39:39Kelly Weinersmith?

  Cool.

• 39:40Daniel Whiteson?

  And that's the incredible thing about shadows is that, you know, there's this projection effect where your shadow can be so much bigger than you are, right? And so you can like wave your arms and then like the huge sky version of you is also waving its arms.

• 39:56Kelly Weinersmith?

  So I want to imagine that the Brocken Specter effect is an effect from, you know, somebody who was like studying ghosts and got confused about what was happening with the fog. But how did this actually get its name? Cause Specter always sounds sort of ghostbusters to me.

• 40:13Daniel Whiteson?

  I don't know the exact history of it, but it shows up in like Lewis Carroll and Samuel Taylor Coleridge poems. So it's definitely a thing that's been around for quite a while.

• 40:21Kelly Weinersmith?

  Okay. Awesome.

• 40:23Daniel Whiteson?

  It's sometimes called the mountain specter or specter of the Brocken.

• 40:27Kelly Weinersmith?

  Oh, oh, that sounds even cooler. Specter of the Brocken. That sounds like it should be in like a Viking tale.

• 40:32Daniel Whiteson?

  It comes from this mountain in Germany, the Brocken. It was first observed and described by Johann Schilberschlag in 1780. We saw his shadow on the Brocken.

• 40:43Kelly Weinersmith?

  Ah, cool. All right. Learn something new every day. All right. So every once in a while you'll hear about like pressure fronts coming through. And is that like air that is more or less dense and could that change how shadows are made? I guess I'm trying to figure out if your neck's gonna tell me that air can also impact shadows.

• 41:02Daniel Whiteson?

  Yes, air can have shadows for the same reason that water can, right? Because air is not a fluid of constant density. When it is, light just passes through it. But if some pockets of air have higher density or lower density, then the light bends in exactly the same way as it does when it hits the surface of the water. And you can already see this effect, like when you look at heat rising above the road on a hot day, right? What you're seeing there are pressure waves in the air and that's changing how light goes through. That's why you're able to see it, right? And the similar consequences for How light moves through the air. This is why, for example, stars twinkle, right? Stars don't twinkle in space. They only twinkle through the atmosphere because their light is getting bent away from you

• 41:46Daniel Whiteson?

  in a sort of a chaotic, turbulent manner. This is why telescopes on the ground can't see as clearly as telescopes in the sky because light has passed through this complicated atmosphere. And they have these amazing adaptive optics to counteract for this to like in real time, bend the path of the light back to regather all that light into a crisper image. But effectively, it's like a shadow. If you looked at the ground as light passes through it, you would see regions that are darker and regions that are lighter because of these density fluctuations in the air.

• 42:18Kelly Weinersmith?

  That's amazing. And, you know, one of my favorite things about this time of year is that, so like I don't stay up late because I'm a total wimp. But in the winter and in the fall, you get, when I go out to do the animal chores, it's already dark. And so on clear days, I can see the stars twinkling. And the other day I was late to wake up my kids because the stars were twinkling and I was like totally enamored of it and forgot about what time it was and, and fell behind on my schedule.

• 42:45Daniel Whiteson?

  Well, this star twinkling effect is very cool. And it's, you know, the kind of physics you can enjoy any evening. But the same physics gives you a really weird effect during a lunar eclipse.

• 42:56Kelly Weinersmith?

  What? I am dying to hear about that. So let's take a break to increase the suspense. And when we get back, you'll tell us all about it.

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• 46:31Kelly Weinersmith?

  Okay, you were going to tell us about shadows and eclipses, Daniel.

• 46:35Daniel Whiteson?

  Yeah, so I'm lucky enough to have seen a total eclipse. This was in 2018. Amazing experience. And I went into it not preparing to be amazed. I was like, I know the physics. Yeah, it's cool. It's going to get darker, whatever. But there was something really moving about being in the totality, having the sun be so completely blocked momentarily. It just, everything felt so odd. And you know, I'm not a religious person, but I almost felt spiritual at that moment. I tried to imagine what it might be like to not understand the physics at all and go through that experience, feel like, whoa, something is happening today.

• 47:09Kelly Weinersmith?

  Yes, we are all gonna die. That's absolutely what I would think.

• 47:13Daniel Whiteson?

  Have you seen totality?

• 47:14Kelly Weinersmith?

  No. In Virginia, a year or two ago, we got a partial eclipse, but even that was pretty amazing. And but, you know, as a biologist, I was trying to listen to see if the nighttime animals were waking up and being like, oh, what's going on? And I think there was a little bit of that. I bet there was a lot more of that where you were.

• 47:29Daniel Whiteson?

  Actually, this was in Idaho, I believe it was, for the path of totality for that eclipse. Really amazing. Totally encourage everyone to see totality if they can. And there's a really cool effect, which I didn't see at the time, didn't even know about until preparing for this episode. But there are these things called shadow bands that happen during a total eclipse for the same reason as star twinkling. What's happening is that you have the sun now narrowed to a very, very, very narrow source of light, right? Instead of being a huge blob in the sky, you have like a very thin crescent. So now sunlight is very collimated. All the rays are very, very parallel. And what happens is it creates- It's these thin wavy lines of alternating dark and light that can be seen moving and undulating

• 48:13Daniel Whiteson?

  in parallel just before and just after the total solar eclipse. So there's these like shadow bands of the eclipse. Just the same way that a solid object will have these diffraction patterns around it, these zebra patterns. The moon has those patterns, a shadow on the earth, but not for diffraction reasons. It's for the same reason it's the star twinkling because now you have this column of light, which then gets bent randomly by the varying density of the air. But it all is moving in a column, so you get these bands. Really incredible. I want to see this in person.

• 48:46Kelly Weinersmith?

  I want to see that also, and I also, for the first time, want to see our podcast as a video episode because the extent to which your arms were moving to try to explain that was really fantastic.

• 48:57Daniel Whiteson?

  That's the Italian in me coming out, you know?

• 49:02Kelly Weinersmith?

  So the listener asked a question about what shadows would be like on the moon. And the context for the question was, you know, the moon has a very tiny, thin atmosphere, an exosphere. How does Earth's atmosphere impact the way shadows are made? And would it be different if you were on the moon? So like, what did the Apollo astronauts see when they looked at their shadows?

• 49:21Daniel Whiteson?

  All right, well, I'm going to give you a pop quiz. I've taught you now enough physics on this episode to answer this question. What do you think, Kelly? Do you think shadows are crisper on the moon or less crisp?

• 49:31Kelly Weinersmith?

  I think that it's probably about the same because you still have light reflecting from lots of different surfaces. And I bet the surface of the moon in particular is pretty reflective. I guess here, when the atmosphere changes in density, that bounces light around and makes it a little bit less crisp. And so maybe with no atmosphere, I'm gonna guess crisper.

• 49:53Daniel Whiteson?

  Crisper, yes, exactly. It's crisper. And the issue is the atmosphere. I mean, think about how if you're standing on the earth, you look up, the sky is blue, right? What's the color of the sky in the moon?

• 50:06Kelly Weinersmith?

  Black?

• 50:06Daniel Whiteson?

  It's black, right? Because there's no atmosphere there to reflect light. And so the atmosphere here is blue. That means that you're getting light from all directions, right? Yes, it's mostly from the sun and you can see shadows. But the answer to Eric's other question is the reason you can see still light when you're standing behind a tree and the sun is blocked is because it's light reflecting everywhere on the earth from all over the sky. And yes, and all the buildings and whatever. But the atmosphere itself is like bouncing light everywhere. And on the moon, you have no atmosphere. And so it's much more geometric. Yes, you do have rocks that are reflecting light. And of course, during the nighttime, we see the reflection of the moon. But the atmosphere is a big contributor to making shadows fuzzy. And of course, fluctuations in the atmosphere make that fuzzy.

• 50:51Daniel Whiteson?

  So yes, the reason you still see stuff when you're standing in shadow is because it's light coming from many sources, not just one. And on the moon, shadows are crisper. But shadows are also important on the moon for another reason that you might find interesting, which is they provide place for water ice to accumulate, right? Because shadows on the moon are very, very cold. The moon surface is crazy. It's super hot, it's super cold. It depends on are you in the blinding path of the sunlight? And if you're not, then water ice can survive. And isn't there a place like on the lunar pole where light never reaches?

• 51:24Kelly Weinersmith?

  Yes, that's right, on both poles and places like Shackleton Crater.

• 51:28Daniel Whiteson?

  It's like eternal shadow or something really dark?

• 51:31Kelly Weinersmith?

  Oh, craters of eternal darkness.

• 51:34Daniel Whiteson?

  Exactly. And shadows there are really important because they preserve water ice. And if we ever do live on the moon, that would be really valuable, right? So shadows could save our lives on the moon.

• 51:45Kelly Weinersmith?

  Yeah, although I'll note that there's not a lot of water in those shadows. But there is some, it could get us started. We'd have to be real careful about recycling it.

• 51:54Daniel Whiteson?

  Shadows have also really helped us understand the nature of our place in the cosmos. Famously, more than 2,000 years ago, the Greeks used shadows to measure the radius of the earth, right? Greeks, so clever, so geometrical. They realized that the earth is probably a sphere because as you move around. Around it, you can see different kinds of stars, right? Different constellations emerge as you move around the earth. So the Greeks, much smarter than like, you know, certain rappers and YouTube influencers who still deny that the earth is a sphere. But they went beyond just saying, oh, the earth is likely a sphere. They used the behavior of shadows and a little bit of geometry to measure

• 52:36Daniel Whiteson?

  the radius of the earth and got it pretty accurate, like more than 2,000 years ago.

• 52:41Kelly Weinersmith?

  Well, okay, so can you give us more details about how they did that?

• 52:44Daniel Whiteson?

  Yeah, so imagine you're in a city where the sun is directly overhead. It's like high noon. And so all the shadows point straight down, right? There's basically no shadows. Then you have another city that's like 100 kilometers away. And that city is not going to be at high noon. It's going to have some shadows, right? And you can measure the length of those shadows. And now make a triangle where you know the distance between the two cities. And you can measure the length of the shadow. The length of that shadow depends on the curvature of the earth. Because if the earth was very, very flat, that shadow would be small. And as the earth gets more and more curvature, that shadow would grow longer and longer. So by measuring the length of the shadow in Alexandria,

• 53:26Daniel Whiteson?

  at the time that the sun was directly overhead in another city, which I can't pronounce, a Greek dude named Erasthenes was able to measure the circumference of the earth just using like a stick and some geometry and measuring a shadow.

• 53:41Kelly Weinersmith?

  So like Eratosthenes, I'm not going to say it right. Called his friend down in Sain and was like, we're both taking the measurements right now. Go. How did, I guess, did they also have really good clocks? Or they just were like, that is, okay.

• 53:54Daniel Whiteson?

  Yeah, exactly.

• 53:55Kelly Weinersmith?

  That's amazing.

• 53:55Daniel Whiteson?

  So this is really cool. But the fascinating thing is that flat earthers have not let this point go. And they argue that this experiment doesn't actually prove that the earth is round. And they're kind of right.

• 54:07Kelly Weinersmith?

  Oh, no.

• 54:07Daniel Whiteson?

  Because even if the earth was flat, you would still have a shadow in one place when the sun is directly overhead in the other city. That's true. Essentially, it's like measuring the earth to have an infinite radius. But you would definitely get a shadow because this whole method assumes that the sun is Really, really far away and that the light is parallel essentially. But in the flat earth model, the sun is very, very close. And so you would still get a shadow in one place and not a shadow in the other. But there's a way around it. All you need to do is to add a couple more sticks. So instead of just having two points, you have like three or four. And the two models give different patterns of shadows. In the flat earth model, you get a linear relationship between the length of the shadows.

• 54:51Daniel Whiteson?

  And in the spherical earth, you get a nonlinear relationship as things move around the curve of the earth. So anyway, we're pretty sure that the earth is not flat. And you can actually prove it using shadow and rod experiments. It's true, the two point experiment doesn't refute the flat earth. But anyway, shadows do show us that the earth is round. And you allow us to measure the roundness of the earth, which is kind of amazing.

• 55:15Kelly Weinersmith?

  That is amazing. Way to go, shadows. So the last question the listener had was the trick question that we shared with our extraordinaries, which is, do shadows move faster than light? So, all right, now we have all of the background we need to understand the nuance to this question.

• 55:34Daniel Whiteson?

  Yeah.

• 55:35Kelly Weinersmith?

  So take it home, Daniel.

• 55:36Daniel Whiteson?

  The answer is, yes, shadows do move faster than light.

• 55:41Kelly Weinersmith?

  What?

• 55:42Daniel Whiteson?

  But, but the problem is that the rule says no thing can move faster than the speed of light relative to anything else, but shadows are not a thing. That's the problem. They're the absence of a thing. And as a shadow moves, it's not really the same object. So let's imagine a concrete scenario, right? Let's say you have a screen in the sky. Instead of the sky, you're like, you know, Daniel has built his telescopes that block the view of the world. And so you-

• 56:13Kelly Weinersmith?

  Death is impending.

• 56:14Daniel Whiteson?

  Exactly. Right? And you can imagine a scenario, you have a bright source of light and you can do like shadow puppets on the sky.

• 56:21Kelly Weinersmith?

  Yeah.

• 56:21Daniel Whiteson?

  Or imagine clouds or whatever. And you can move your hand a small amount and a shadow will move a very large amount, right? Because the screen is very, very far away. And so this projection is far away and you get this multiplier effect. And then you can ask- Well, if I move my hand really fast and the screen is really far away, could those shadows move faster than light? And the answer is yes, in the sense that like the image of your hand could be in one place. And then faster than light could go from that one image to another image. The image, the shadow could appear somewhere else, right? Does that making sense? Like imagine somebody in the sky shooting a laser from one shadow to the other. The second shadow would appear before the laser arrived. In that sense, the shadow is moving faster than light.

• 57:06Daniel Whiteson?

  And you're wondering like, how does that make sense physically? What's really going on? And the issue is that the second shadow is not the same thing as the first shadow. Right? Both of them are being created by the absence of light. Nothing is moving faster than light in this scenario. And there's no way to communicate between the shadow one and shadow two. There's no information passing. It's just like if I shown a laser in one direction and then I turned it off and shown it in another direction. The laser spot would appear to move faster than light, but it's not the same spot. Right? It's like I made a spot and then later I made another spot. I'm connecting them in my mind because they both came from the same laser. But it's not like anything moved from laser spot one to laser spot two.

• 57:46Kelly Weinersmith?

  Okay.

• 57:47Daniel Whiteson?

  So in the same way, nothing went from the shadows initial location to the shadows final location. You have a wave of light that's obstructed and not obstructed. It's creating the shadow and then a different wave of light. It's creating a different shadow somewhere else. And your mind is like shadows are a thing. And so it was here and it was there. And if I do distance divided by time, I get a number bigger than the speed of light. Yeah, that's true, but shadows aren't a thing. It's like comparing where one thing is and later something else is. And calculating the velocity between those two, it doesn't really work.

• 58:18Kelly Weinersmith?

  Is that like saying that the information that the photon has been stopped? Travels faster than a photon would travel?

• 58:26Daniel Whiteson?

  No, the photon, the information that the photon has been stopped isn't traveling from shadow one to shadow two. It's traveling from the source to shadow one and from the source to shadow two. Like you could signal somebody up there in the sky using shadows or not shadows. But then information obviously travels at the speed of light because you're either sending photons or you're stopping to send. But all that information travels at the speed of light. And you could do the same for person two, but you can't get information from shadow one to shadow two, right? There's no way for you to do that. You can send information. All the information is coming from the source of light to the shadows or the non-shadows, not between them.

• 59:02Kelly Weinersmith?

  Got it.

• 59:03Daniel Whiteson?

  So the appearance of shadows can move faster than light, but shadows are not really a thing. They don't carry information. And so in that sense, you know, they're breaking the rules, but they're not really limited by the rules because they're not a thing that don't have information.

• 59:17Kelly Weinersmith?

  Well, thank you, Daniel, for illuminating all of our understanding of this question. I learned a lot and had a lot of fun talking about shadows because they're neat.

• 59:26Daniel Whiteson?

  Well, I hope that we can help bring shadows out of the darkness. Shadows are a wonderful way to think about light and to think about physics and just to like, you know, wonder in an everyday sense, how everything works. There are fantastic mysteries of physics all around us.

• 59:39Kelly Weinersmith?

  There are. And please send us your questions about the mysteries of, you know, physics, I guess, if that's what keeps you up at night, but definitely the questions that you have about biology, which is a fascinating topic.

• 59:51Daniel Whiteson?

  And if you're the descendant of the famous physicist Poisson and you want to write in to defend his legacy, please do.

• 59:58Kelly Weinersmith?

  All right, until next time, Extraordinary's.

• 1:00:06Kelly Weinersmith?

  Daniel and Kelly's Extraordinary Universe is produced by iHeartRadio. We would love to hear from you.

• 1:00:12Daniel Whiteson?

  We really would. We want to know what questions you have about this extraordinary universe.

• 1:00:18Kelly Weinersmith?

  We want to know your thoughts on recent shows, suggestions for future shows. If you contact us, we will get back to you.

• 1:00:24Daniel Whiteson?

  We really mean it. We answer every message. Email us at [email protected].

• 1:00:31Kelly Weinersmith?

  Or you can find us on social media. We have accounts on X, Instagram, Blue Sky, and on all of those platforms, you can find us at D and K Universe.

• 1:00:40Daniel Whiteson?

  Don't be shy. Write to us.

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