The Science of Putting Sunglasses on The Sun

Why does the cartoon Sun always wear sunglasses? And how big would a real pair need to be? Hannah Fry and Michael Stevens investigate space mirrors, Reflect Orbital and solar eclipses to find out. Starting with a pencil held at arm’s length, they use the Sun’s angular diameter to calculate how big a pair of solar sunglasses would need to be for one eye, two eyes, everyone in London and eventually the entire planet. Their hypothetical answer is a strange structure more than a million kilometres wide, positioned deep in space. Entirely practical. They compare its extraordinary scale with NASA’s Parker Solar Probe, the real spacecraft flying closer to the Sun than anything ever built, before using Lagrange points to work out where their creation could possibly go. But could putting something between Earth and the Sun ever be useful? Real proposals for space sunshades and climate geo-engineering suggest that blocking a small fraction of sunlight could help cool the planet. Moving from shade to reflection, they revisit Soviet attempts to reflect sunlight onto Earth at night and explore how Reflect Orbital’s Earendil 1 space mirror experiment might one day extend solar energy beyond daylight hours. That leads them to the strange physics of light itself. Photons can push solar sails despite having no rest mass, revealing why E=mc² is only part of the full energy momentum equation from special relativity. Finally, there is the optical phenomenon known as the Arago spot to contend with. It helps explain why NASA’s starshade concepts, future technology designed for hunting exoplanets, look like enormous sunflowers. ------------------- For more information about Cancer Research UK, their research, breakthroughs and how you can support them, visit https://www.cancerresearchuk.org/our-research/rest-is-science Cancer Research UK is a registered charity in England and Wales (1089464), Scotland (SC041666), the Isle of Man (1103) and Jersey (247). A company limited by guarantee. Registered company in England and Wales (4325234) and the Isle of Man (5713F). Registered address: 2 Redman Place, London, E20 1JQ. ------------------- Find The Rest Is Science all over the internet by ⁠⁠clicking here.⁠⁠ ------------------- Video Producer: Adam Thornton + Oli Oakley + Jack Meek Animator: Sam Benson Video & Social: Bex Tyrrell Assistant Producer: Lucy Lipscombe Producer: Simona Rata Senior Producer: Lauren Armstrong-Carter Chief Digital Officer: Samuel Oakley Exec Producer: Neil Fearn Learn more about your ad choices. Visit podcastchoices.com/adchoices

• 0:01Speaker 00

  Welcome to The Rest Is Science. I am Michael Stevens.

• 0:03Speaker 01

  And I'm Hannah Fry.

• 0:04Speaker 00

  And today, Hannah, help me with this episode. We're going to be figuring out how to put sunglasses on the sun.

• 0:15Speaker 00

  A cartoonist's dream.

• 0:17Speaker 01

  It's the simple questions that we are here to answer.

• 0:21Speaker 00

  So let's get into the science. You're probably sitting there thinking, putting sunglasses on the sun, that's never gonna work. Why? Because the sun doesn't have ears or a nose that are gonna fall right off.

• 0:34Speaker 01

  Wasn't my first thought.

• 0:35Speaker 00

  Oh, it wasn't. Well, it was my first thought, so I spent hours trying to figure out, does the sun have anything on it that is kind of like an ear? There's no sound in space, but there is sound inside the sun. The vibrations of the gas moving around, all of that really does create pressure waves that are like low-frequency sound waves in the sun. But can it hear itself? No, I just kept coming back to the sun not alive. It doesn't have ears, it doesn't have a nose. And then I arrived at another problem, which I don't know if you thought of this one, but the sun can't really wear sunglasses because it's, well, it's the sun, it's too hot.

• 1:15Speaker 01

  That one, that one was, I would say it was still probably about fourth or fifth on my list of things, but that one didn't get to me.

• 1:23Speaker 00

  Okay, all right, then what was your, what was your number one?

• 1:25Speaker 01

  Why?

• 1:27Speaker 00

  Oh, yes. Okay, yeah, why, why should we do this? Funny enough, we began thinking that this was just a big old goof. And then, as it turns out, there is an answer to the why question. And it's a pretty serious one. I don't want to spill it all right now, but there could be a good reason to do this.

• 1:45Speaker 01

  You've done a hook and tease there, Michael, and I'm here for it.

• 1:47Speaker 00

  I've done a few hooks and teases, but there's one more, there's one more. Not only is occluding the sun in some way potentially a good idea by exploring how to put sunglasses on the sun to make it a really cool dude sun, I think we're also going to discover that E equals mc squared is incomplete. Yeah. As usual, we're gonna be absurdly deep today and deeply absurd.

• 2:21Speaker 01

  This episode is brought to you by Cancer Research UK.

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  For more information about Cancer Research UK, their research and breakthroughs, and how you can support them, visit cancerresearchuk.org/restisscience.

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• 4:31Speaker 00

  Okay, first things first. We want to put sunglasses on the sun. We can't get close enough to actually put a physical pair of sunglasses made of any known material actually on the surface of the sun because it's too hot.

• 4:45Speaker 01

  And this is because, this is not because necessarily of the gravitational pull of it, but this is because the radiation that it's giving off is so gargantuan. Essentially, every material that we reliably use on Earth would be obliterated.

• 5:02Speaker 00

  The Parker Solar Probe was the instrument that humans have brought closest to the sun, and it only approached about 6 million kilometers away. But it can stay there for a long time. If you get closer, you get burned up in a shorter and shorter amount of time.

• 5:19Speaker 01

  Hang on a second, how far away are we from the sun?

• 5:22Speaker 00

  We're 150 million kilometers.

• 5:24Speaker 01

  150 million, okay, so this was 6 million kilometers away. Wow, that is actually really close.

• 5:29Speaker 00

  That's really close, so it's about 4 million miles.

• 5:32Speaker 01

  Right.

• 5:33Speaker 00

  The Parker Solar Probe got, I mean, it got into like the, the very outer parts of the sun's atmosphere. So in a way, it visited the sun, even though it was still 6 million kilometers away.

• 5:45Speaker 01

  That's, that's closer than I was expecting.

• 5:48Speaker 00

  And that's not quite on the sun yet, obviously. But you bring up another good point, which is that anything you put on the sun is gonna fall right into it because it doesn't have a hard, solid surface like my noggin does. It can hold glasses really easily. So we're not out of luck, though, because we don't necessarily need to put the sunglasses on the sun itself. We just, I'm okay just making it look like the sun is wearing sunglasses.

• 6:14Speaker 01

  So almost like I'm holding up a pair of glasses in front of your face.

• 6:18Speaker 00

  Exactly.

• 6:19Speaker 01

  And from my perspective, it looks like you're wearing them.

• 6:22Speaker 00

  Exactly, so, so this brings us to angular diameter. This is how we're going to crack this nut, okay? So Hannah, you already know all of this, but for the listeners out there, we have talked about angular diameter before, but it describes how large something is, not in reality, but how large it appears to you, how wide across in your field of view it is. And so when you have an object, it's got a really large angular diameter when it's close to your eyes. But the further away it gets, the smaller it looks, right? From far away, I can crush people's heads. And the way, the way we measure this is by imagining 360 degrees all the way around our bodies. That's this whole field.

• 7:06Speaker 00

  And then we ask, what angle does the object subtend from one end to the other? And if an object is really big, then, you know, its angle when it's far away will be, you know, something large. But a smaller object would fill that angle if it was close. I don't think my visual here, if you're watching, really helped, but I'm trying to use my hands to show an angle.

• 7:26Speaker 01

  Holding up a tape measure.

• 7:28Speaker 00

  Yeah, and I'm showing, I'm holding a tape measure between my fingers, but the same object could fit between my palms and it would be a lot smaller and still take up the same angle of my field of view. So we've got degrees, and then the degrees of something's width can be cut into pieces. And they can be cut into sixtieths to give us what are called arc minutes. So something that's an arc minute wide in your field of view is a sixtieth of a degree of a circle all the way around you. That's pretty small. That's about the smallest width the human eye can differentiate as actually having width and not just being a point, one arc minute, a sixtieth of a degree.

• 8:10Speaker 00

  In the sky, the sun takes up about 32 arc minutes of room. All right, so that's about half a degree.

• 8:21Speaker 01

  Yeah, I came up with actually a very similar answer, but I did it a very different way.

• 8:26Speaker 00

  Oh.

• 8:27Speaker 01

  Well, maybe not very different. I don't think it deserves the very, but, but it does, a slightly different way, should we say? Okay, so here, here's what I was thinking. Imagine your eye is like a single point, you've got one eye, one eye open and you're looking at the sun, right? And now imagine that there is a triangle shooting out from your eye, okay?

• 8:45Speaker 00

  And please, I want to make sure that everyone's only imagining this.

• 8:49Speaker 01

  Yes. Important point, don't do this at home, okay? Yes, this is how the ancient Greeks thought the light worked, but for the purposes of this podcast, it's an imagination thing only. Okay, so you've got this triangle that is sort of shooting out of your eye. One side of the triangle meets up with one edge of the sun. The other side of the triangle meets up with the other edge of the sun. So that essentially, if you're kind of looking top-down at you viewing the sun, you see this flat triangle where the width of it, at the end of it, is the exact width of the sun. And then it kind of, it draws these lines backwards towards your eye to form this triangle. When it comes to the sun itself, we know exactly what that triangle looks like because we know that the width of the sun is 1.39 million kilometers.

• 9:34Speaker 01

  And we know that it's 149.6 million kilometers away. So you can work out this triangle to give you that exact angle. Essentially, that tells you the ratio of how far away something needs to be and how wide it is to cover that object. So when you work it out for the sun, it's basically 1/107. So any object that you have, that you hold up in front of the sun, as long as it is at least 1/107 as wide as it is far away, it will cover the sun. So if I make a disk, right, and I, and I, and I hold up and it's at least 1/107

• 10:16Speaker 01

  as wide as it is far away, then I can, I can be sure that it will cover the sun.

• 10:22Speaker 00

  Yeah, so here, here's a specific example. I think there's some overlap in our calculations. In order to, in order for an object held at arm's length, which I'm assuming is about 70 centimeters, in order for an object to just be exactly the same width as the sun in the sky from Earth, that object would need to be 0.65 centimeters wide. Okay? If you held such an object out in front of you at arm's length, it would block the sun. And that happens to be almost exactly how wide a pencil is. Okay, so at arm's length, a pencil will completely block out the disk of the sun, assuming your arms are 70 centimeters long. Don't try this at home, though, because it's

• 11:04Speaker 00

  only going to just cover the sun, and you will hurt your eyes.

• 11:08Speaker 01

  You will.

• 11:09Speaker 00

  By trying this. This leads to a really cool fact that we can, we can add on to that classic one about how solar eclipses happen, because we always know that there's this weird coincidence, which is that the sun is 400 times bigger than the moon. But the moon is 400 times closer to us. So they appear the same size. They take up the same angular diameter in the sky, so the moon can perfectly cover the sun. Where this is going is that if you made a tiny little pair of sunglasses that were only as wide as a pencil, you could hold them at arm's length and put them over the sun, and the sun would look

• 11:49Speaker 00

  like it was wearing sunglasses. But also, you would damage your eyes, because most of the sun would not be covered. Sunglasses only cover like 15% of, of the face that they're on. So don't do it that way.

• 12:03Speaker 01

  No, no, unless, unless you don't value your eyesight in that one eye that you're using.

• 12:07Speaker 00

  No, please, don't even give them ideas. As it turns out, you need to dim the sun a lot more than just covering like 15 to 30% of it. The ISO standard for safe solar viewing is that you need to decrease the sun's light by 100,000 times. It needs to be 100,000th as bright as it normally is to safely look at.

• 12:30Speaker 01

  And when we say safely, what, what we're talking about here is, I mean, it's literally sunburn on the inside of your eye.

• 12:38Speaker 00

  Yeah, your retina gets burned, and it gets burned fast, and, and, and usually permanently. I actually found some pictures of people's retinas that were damaged by looking at these most recent solar eclipse. And guess what? We won't show it because it's very gory, but it's basically exactly the same as another picture I saw of a soldier's retina that was damaged because he watched an atomic bomb go off.

• 13:01Speaker 01

  Ah.

• 13:02Speaker 00

  So don't do it.

• 13:04Speaker 01

  Don't do it.

• 13:05Speaker 00

  You know what else is almost exactly the same? People who have had laser pointers shined in their eyes.

• 13:10Speaker 01

  Oh.

• 13:12Speaker 00

  Same kind of retinal damage. This just hotspot where it's just burned, and that we'll never see again.

• 13:18Speaker 01

  That is one of, you know, I, I often think about this as one of the great collaborations of humanity. Bear with me for one moment, which is that when early lasers were being developed, people realized that it would be very possible to create a laser that could instantly blind people, and that it could be used as a weapon of war. You know, imagine how powerful that would be, that you go onto a battlefield, press a button and blind all of your opponents. And there was, I mean, essentially before the technology had even been fully developed, there was an international agreement that we would never do that, that

• 13:58Speaker 01

  nobody on this planet Earth wanted to live on an earth where that was, that was an option. Every now and then, we do actually manage to properly collaborate as an entire species.

• 14:09Speaker 00

  Yeah, and I'm glad when it happens. But the point is, don't look at the sun. I, I found a chair of the ophthalmology department at the University of Washington, pointed out that if you look at the sun, you're getting a million billion photons per second onto your retina. And 10 seconds of looking at the sun is the equivalent of dropping a AA battery right onto your retina from a foot up. Same amount of energy.

• 14:36Speaker 01

  Right.

• 14:37Speaker 00

  Okay, so we're gonna need to come up with a, a different way to put sunglasses on the sun. What I'm thinking, on a small scale, is that you get a big sheet of safe solar viewing film, which is essentially opaque. You cannot see anything else through this stuff, except the sun, an arc from, from welding, that's about it. But if you had that, and then you put little tiny, you drew little tiny sunglasses on it somehow that were completely opaque, and were only 0.65 centimeters across, you could then cover the sun with this sheet, you would see the sun as like a kind of a, a dimmer ball, and you could line up the sunglasses to appear to be on the sun, and we're done.

• 15:21Speaker 01

  Hey. The end.

• 15:24Speaker 00

  That's the end of the episode, but it shouldn't be, because I don't want to enjoy this alone. I want everyone around me to also go, it looks like the sun is wearing sunglasses. How silly. My day has been improved.

• 15:38Speaker 01

  I imagine you want to enjoy it with both of your eyes, even, right?

• 15:43Speaker 00

  Oh.

• 15:43Speaker 01

  The description that you've, you've just come up with is one eye peering through. Oh, but look at it, both eyes, no, there's no, there's no joy there to be had.

• 15:51Speaker 00

  Okay, so yeah, let's, let's take a one little baby step here. How do we make this work for two eyes?

• 15:57Speaker 01

  Okay, so, I mean, actually, technically, to even get it to work for one eye, we've sort of assumed that your one eye is a single point in space at the moment. But actually, your pupil is about four millimeters across, right? So really, if you, if you want it to fully work for, for the actual shape of your eye, you have to take that four millimeters into account. And so instead of being six and a half millimeters wide, it actually needs to be 10 and a half millimeters wide. You need it to be about a centimeter.

• 16:28Speaker 00

  So what would it look like if it was only 6.5 millimeters wide? Would it be?

• 16:35Speaker 01

  It would just be a bit, it'd be a bit janky. It would work, it would work for sort of one point of your retina, and not the rest of it. But your, your retina is going to be, you know, amalgamating the image, so it wouldn't really work.

• 16:47Speaker 00

  Okay, so that's really good to know. So we need, you're saying 10 millimeters.

• 16:52Speaker 01

  But your eyes are, on average, about six centimeters apart. So binocular viewing, you need to account for the fact that there are all of those points across. So you basically need to add on that six centimeters to where you, where you started. So in total, for both eyes, you actually need more like 7.05 centimeters.

• 17:12Speaker 00

  Whoa, that's big.

• 17:14Speaker 01

  It is, because, because you're so, you know, because, because the distance of your eyes is so big compared to the distance that you are, that you're sort of holding away.

• 17:23Speaker 00

  Right, so I've got a tape measure here. I'm gonna get seven centimeters. I mean, it's, that's actually not very big. So for those of you watching, there's seven centimeters. I would need a pair of sunglasses drawn on a piece of solar viewing film that wide, so that when I hold it up, that seems so wide, though.

• 17:40Speaker 01

  Unfortunately, your eyes are wide.

• 17:42Speaker 00

  But the sun isn't this wide, when I, if I were to look at it.

• 17:47Speaker 01

  No, but think about how much things jump. I mean, you've done the, the trick where you hold up a thumb, you close one eye, and you hold up a thumb, and you line it up with an object in the distance, and then change eyes and see just how much it jumps.

• 18:00Speaker 00

  So is this, is this calculation for making it so that when I switch my eyes, the sun is still within this boundary?

• 18:09Speaker 01

  Exactly.

• 18:10Speaker 00

  Okay, I see. So let me just.

• 18:12Speaker 01

  For those of you who are listening only.

• 18:15Speaker 00

  I'm looking at other, like, circles around my office that are about 32 arc minutes wide, and I'm covering them with seven centimeters of material. And I think, but if I, if I focus on the distant sun, then this is like, looks like this. I've got this huge thing.

• 18:34Speaker 01

  Look, look, Michael, I'm sorry, but it's not gonna be perfect, unless you can shrink your eye down to one point. It's a...

• 18:41Speaker 00

  Okay, I see, I see what you're saying, though, because this is actually really cool. I've got this baby head on my wall over there that's about 32 arc minutes wide from where I'm sitting. And if I put the baby head, so with one eye, I see it here, and then I open my other eye, it pops over to the other end. So this is exactly that shift because of my eyes. But the effect is not cool. It doesn't look like the baby is wearing a tape measure mask.

• 19:09Speaker 01

  And I think unfortunately, that's because you've got it at arm's length. I think the only way to make it work effectively at arm's length is because the distance of your eyes is such a dominant factor.

• 19:20Speaker 00

  Yeah.

• 19:20Speaker 01

  Essentially, at such a short distance, you know, it's, it's about 10% of the distance.

• 19:24Speaker 00

  It's about 10%.

• 19:25Speaker 01

  That you're, you're holding. I think the only way to get it to work at such a short distance would be closing one eye.

• 19:31Speaker 00

  That's so cool. I, see, this is why we're a good team, because I just assumed one eye, that's what we're going to do.

• 19:37Speaker 01

  Everyone's gonna be fine.

• 19:38Speaker 00

  And you're like, no, Michael, we can do better. If you want both of your eyes to do it, I just assumed it would still be fine. But no, you're right. It jumps seven centimeters across the thing I'm holding.

• 19:49Speaker 01

  Exactly.

• 19:50Speaker 00

  Okay, so this gets better as we get bigger.

• 19:53Speaker 01

  Right, because, because you said about sharing this effect with other people, and in a lot of ways, that's the same problem, right? You jump from one eye to the other, you jump from one human to the next.

• 20:03Speaker 00

  Yeah, so how, how big do the glasses have to be? Like, obviously, just to, to put a, a, an end on this spectrum, if the sun somehow was truly wearing a pair of sunglasses, it would look like the sun had them on from no matter where you were on Earth, no matter where you were in the universe. But that, again, as we've said, is not possible. The sun is just too hot, and, and it's too gaseous. Or plasmatic or whatever, but the point is, how how big of a pair of glasses do we have to get such that two eyed viewing is a pleasant, funny scene?

• 20:40Speaker 01

  Okay, so I think basically you just need to get it far enough away that the six centimeters of your eyes is like a rounding error, essentially. So I think even if you get up to, let's say, 10 meters, right? I think that would work. I think that would work, right?

• 20:55Speaker 00

  Yeah, I I'm imagining this now, like I'm imagining a large pair of sunglasses at the top of like a skyscraper. And if I line myself up just right, they appear to be on the sun. Yeah, 10 meters is kind of not that much.

• 21:07Speaker 01

  Yeah, right. Let's let's let's say 100 meters. Okay, so 100 meters, right? 10,000 centimeters. Goodness me, centimeters are a stupid unit, aren't they? Divided by 107, tells you how wide it needs to be, which is 93 centimeters. But then you need to add the distance between your eyes, which is six centimeters. So it would be about 100 centimeters, so it'd be about a meter. I mean, look, you can basically divide it by 100, about a meter.

• 21:33Speaker 00

  About a meter?

• 21:33Speaker 01

  Yeah, 100 meters away. That's very doable.

• 21:36Speaker 00

  That's very doable. We can make a meter wide pair of sunglasses. Now, keep in mind, we can't just put them up at the top of a building, like on a flagpole, and then ask people to, you know, stand where they appear to be on the sun. Because the sun will still, 85% of it will still be shining right into their eye, destroying their retina. So we've got to actually put this on a large solar viewing sheet, like a big pane of solar viewing film that dims the sun 100,000 times. And then the glasses are painted on or stuck on there, and they're completely opaque. Then the illusion works for both eyes. And how many people can see this?

• 22:18Speaker 00

  Is there is there like one little spot you're gonna have to stand at where the illusion works, but someone next to you is not gonna like it?

• 22:25Speaker 01

  Correct. I mean, you probably even have to put your head into a vice to make sure that you're, I mean, this has only got like six centimeters of tolerance, this one. So you could watch it with somebody else if you both used only one eye and you put your heads together.

• 22:38Speaker 00

  Okay, but I I am I am falling in love with this, Hannah. I didn't realize what these numbers would sound like.

• 22:44Speaker 01

  That this was so easy.

• 22:45Speaker 00

  Have I ever told you one of my dreams for like a legacy? When I get really old, I want to make sure this happens. I want to build a sculpture garden, like a free sculpture garden for people, but all the sculptures are illusions, right? They look like impossible shapes from certain angles, or on on a certain day of the year, the way the sun hits them causes a really weird, funny shadow. And I want there to be like 365 of these, 366 for the leap day years. So every day that you come, there's some new thing happening, like right at noon, a cool shadow. And I think that there should be one of these solar viewing films with sunglasses drawn on it, so you can finally see the cool sun the cartoons promised us our whole lives.

• 23:25Speaker 01

  I like this.

• 23:26Speaker 00

  And I need to know how to build it. Okay, so now I know that I can please one person at a time with a meter wide pair of sunglasses that's 100 meters away from them.

• 23:33Speaker 01

  Exactly.

• 23:34Speaker 00

  Okay, but let's go up.

• 23:35Speaker 01

  Can we also, in in this sculpture garden, can we, just to backref for previous episodes, can we also make sure that this is where your skeleton resides, picking its own nose?

• 23:45Speaker 00

  Oh, yeah. Yeah, and people can, I can be dressed up for different holidays.

• 23:50Speaker 01

  Yeah, the Halloween version's gonna be easy.

• 23:53Speaker 00

  That would be easy, just naked.

• 23:55Speaker 01

  I agree, though. Let's make this bigger. Let's be more ambitious. We don't want one person at a time, we want more.

• 24:00Speaker 00

  Should we jump all the way to outer space? Do we need to go that far for, say, everyone in London to see the sun wearing sunglasses?

• 24:13Speaker 01

  You you want to go, you want to go higher? Let's go, let's go to the ISS, which is not that high. I mean, it's outer space, but it's not, it's not crazy high.

• 24:20Speaker 00

  No, it's not.

• 24:21Speaker 01

  400 kilometers?

• 24:22Speaker 00

  That's nothing.

• 24:23Speaker 01

  I feel like we, every time that the height of the ISS comes up, I forget what the number is and then have to look it up again. But I think it's 400.

• 24:29Speaker 00

  And and and every time we bring up the ISS height, we say the same thing, which is, is that all? That's puny. That's nothing. We gotta up those numbers, ISS.

• 24:39Speaker 01

  Guys, pathetic.

• 24:40Speaker 00

  But it's true. I mean, if the if the Earth was the size of an apple, the International Space Station would be like on the surface, basically. Like, it's compared to the diameter of the Earth, its altitude is nothing.

• 24:51Speaker 01

  It's nothing. It's nothing. Okay, so 400 kilometers up, divided by 107, will tell you how wide it needs to be. So if it's just you, it's gonna be 3.7 kilometers big. That's how big it needs to be.

• 25:02Speaker 00

  Okay, whoa, whoa, we've gone up by order of magnitude here. That's big.

• 25:05Speaker 01

  That's big, that's big, right? But it's further away, so it's gonna take up, it needs to be much bigger in order to take up the same angle of your field of view. One, one benefit that you do get here, though, is that 3.7 kilometers, the six centimeters of your eyes, doesn't make any difference anymore.

• 25:21Speaker 00

  Right, so two eyed viewing is fine. We don't need to even worry about that anymore.

• 25:25Speaker 01

  Absolutely fine.

• 25:26Speaker 00

  But that that's only going to look good from one person's perspective?

• 25:30Speaker 01

  From one person's perspective, and that's just, I mean, pathetic, frankly. So let's say all of London, being a bit selfish about this, I'm gonna choose my hometown.

• 25:38Speaker 00

  Yeah.

• 25:38Speaker 01

  But if you want to include all of London, which is, let's say, 40 kilometers wide, I mean, changes in different places, but let's say, you know, about 40 kilometers wide, you need to sort of add that 40 kilometers to the distance so that it works all the way across. So you're talking 44 kilometers at that point.

• 25:57Speaker 00

  That's, so that's how wide this pair of sunglasses would need to be, so that when it transited the sun, it looked like to everyone in London that the sun was wearing sunglasses.

• 26:08Speaker 01

  Exactly.

• 26:09Speaker 00

  44 kilometers is big, but it that's doable.

• 26:13Speaker 01

  It's doable.

• 26:13Speaker 00

  We could, we could build such a thing.

• 26:16Speaker 01

  I think if we really put the effort in, one tiny problem, it will also pass over in about six seconds because of how fast the ISS is moving. So you're gonna have to be quick, gonna have to be quick.

• 26:27Speaker 00

  Oh, shoot. Yeah, it's gonna, so it's gonna be passing through the sky, and then it will transit the sun, which means it'll go in between the sun and us, but not cover the sun completely. It's gonna just transit the sun. It's gonna take six seconds to completely start covering the sun and then leave. But there will only be like a moment where it really appears to be on the sun like it should be as a pair of sunglasses. So there'll be this like moment where everyone goes, whoa, yeah, oh.

• 27:02Speaker 01

  Yeah, yeah, yeah. This is the problem. If you want, if you want binocular vision, you know, you're still too close at 400 kilometers away, you're still way too close to get it to work really properly. You're gonna need to go much, much further out.

• 27:15Speaker 00

  I know.

• 27:16Speaker 01

  So that the distance of between people in London, for instance, is like a mi, like tiny, tiny, tiny rounding area for how wide the sunglasses are.

• 27:25Speaker 00

  Yeah. Now, if instead of orbiting, we put these 44 kilometer wide sunglasses at the top of a of a structure that was 400 kilometers tall, then it wouldn't be, the the the limiting factor wouldn't be the speed of the glasses orbit, it would be the speed of Earth turning us away from the sun. So you'd probably have a longer moment. I don't know how how long, though. We should calculate this. How many seconds, or even maybe like a couple of minutes, it would, the illusion would hold? And it would look, if you stood in the in London, like the sun had sunglasses on. Okay, so I just looked it up. The sun, the sun moves across the sky at a speed of about a quarter of a degree a minute.

• 28:08Speaker 00

  But the sun is only half a degree wide in our field of view. So, so you're gonna have like, what, a minute where the illusion kind of works? Your minute will begin when the glasses are a little bit like off to the side of the sun. And then a minute later, they will be like right on the sun perfectly. And then a minute after that, they'll be only, yeah, they'll be halfway off the sun. Is that right?

• 28:38Speaker 01

  Yeah, that's right. But that's only if they were perfectly set up for a small number of people to see them. If you're trying to do it for all of London at once, I think the glasses are always gonna look too big.

• 28:50Speaker 00

  Yeah, so I'm just thinking, is this gonna be fun? Like, if you're watching at home, I'm holding up my own glasses in front of my face, and I've got the the nose bridge part at the very edge of one side of my face, and that's how it could start. And you might say, oh, the sun's putting them on. And then a minute passes and they're here, and it looks perfect. And then another minute passes and they're half off. So you've got like a solid.

• 29:14Speaker 01

  I just want to add, for anyone who's not watching these visuals, it's the most delightful thing because of the the effect of the lens on Michael's glasses. For a moment there, when it was perfectly lined up, he looked like a normal headed man, with teeny, teeny, tiny eyes.

• 29:29Speaker 00

  You've got, I'd say, maybe a minute where it's gonna look good, and that's it. But that's a minute every day.

• 29:37Speaker 01

  That's a minute every day.

• 29:38Speaker 00

  And everyone's gonna have to.

• 29:39Speaker 01

  Well, you have to worry about north and south as well, though, don't you?

• 29:42Speaker 00

  I was just thinking about that. How do we fix that? Because this, this works for everyone in London, but the sun is going to trace a different path every day.

• 29:53Speaker 01

  You're gonna have to move it north and south. This is gonna have to be a moving flag.

• 29:56Speaker 00

  Oh, I see. Yes, we don't have to make it the tower taller, we just have to move it north and south every day.

• 30:02Speaker 01

  Look, I think the best solution here is is to go further out. I think, I think 400 kilometers is nowhere near enough, you know?

• 30:08Speaker 00

  Okay, let's go further out.

• 30:09Speaker 01

  I think if you push it out to geostationary orbit, for example, then then we're talking, which, as I learned approximately one week ago, is is really, really far away.

• 30:19Speaker 00

  It's really far away.

• 30:20Speaker 01

  35,786 kilometers.

• 30:23Speaker 00

  Far enough away that, one, we solve the problem of motion, because everyone on one side of the Earth, could, could they see the sun appear to wear sunglasses all day? Well, no, because the sun does move. But a geostationary satellite stays in one place above them. If you put a pair of sunglasses up in geosynchronous orbit, then how many people could all enjoy a sun wearing sunglasses illusion at the same time?

• 30:54Speaker 01

  I mean, definitely one.

• 30:58Speaker 00

  What do you mean one?

• 30:59Speaker 01

  I mean, look, you do it for yourself, a geostationary orbit, and then it would need to be 334 kilometers wide. And that would be fine. That would be great.

• 31:08Speaker 00

  How lo- how wide?

• 31:09Speaker 01

  334 kilometers.

• 31:11Speaker 00

  Oh, so we went from 44 kilometers to over 300. That's not a huge jump.

• 31:16Speaker 01

  No, it's fine. It's easy. Just to make it a slightly bigger flag.

• 31:19Speaker 00

  But but only one person would enjoy it?

• 31:20Speaker 01

  I mean, look, that's the number for one person. But you could have people face spaced, you know, five kilometers apart, and they'd still still be be able to see largely the illusion. I mean, what what kind of tolerance do you do you want here? Within 1%, then it's then you then you could only be within a a space of three kilometers.

• 31:37Speaker 00

  Oh, I see what you're saying. Yeah. We've got two things to figure out. We've got how far away the thing is. It's going to look smaller, so it needs to be built bigger. But then also, we need to consider how wide a swath of surface on the earth we want the viewing platform to be.

• 31:54Speaker 01

  And that viewing platform gets added on to the total. It's like, from your eye is a triangle, but you and the next person is a rectangle, essentially, right?

• 32:04Speaker 00

  Ah, this gets terrible, because if you wanted the entire illuminated part of the earth, like anywhere on earth where you could see the sun, to see sunglasses on the sun. The sunglasses are going to have to be so big, they don't fit on the sun.

• 32:19Speaker 01

  Exactly, that it will just ruin the illusion. Because because at that distance, even at 35,000 kilometers away, the radius of the earth is not a rounding error. It's so big.

• 32:29Speaker 00

  Really, what we would be building are glasses that would in some way occlude the sun for everyone who was looking at the sun. But they would only look like they were being worn properly by the sun for a small group, or maybe no one, because they would just be too big.

• 32:46Speaker 01

  I think for no one. They would be too big. Yeah, they would be too big.

• 32:48Speaker 00

  So is there some magic point where this solves itself? And suddenly, obviously, when when they're on the sun itself, then everyone gets to enjoy it.

• 32:59Speaker 01

  Then it's fine. But this is it. The the closer that you get to the sun, the smaller the earth gets in the distance.

• 33:05Speaker 00

  Yeah.

• 33:06Speaker 01

  And the smaller the distance between people on the earth becomes as part of the overall picture.

• 33:12Speaker 00

  True.

• 33:13Speaker 01

  So, you know, as you said, if you are at the sun, the distance between you and me doesn't make a difference. The distance between my eyeballs doesn't make a difference. We're basically all at the same point. So you want to get to the to the stage where the triangle, as it were, between the sunglasses and the observer, where the distance between observers is basically, it doesn't, it it makes no difference. So we are talking way, way, way, way, way closer to the sun than than geostationary orbit. For it to actually probably work for more people than just you and your your mate who's standing right next to you.

• 33:45Speaker 00

  That it's I didn't even think of this until now, but you're right. We started by trying to fix the distance between one person's two eyes. And then we said, but what if your eyes were as wide apart from each other as the edges of London? Now, we're looking at the edges of the earth. Do you have figures for that? How big and how far away do these glasses have to be so that everyone who can see the sun sees the sun wearing sunglasses?

• 34:09Speaker 01

  So, look, the the final point that I that I figured, I mean, I was like, look, let's just pick sensible places, right? So the final point that I thought about was the Lagrange point.

• 34:20Speaker 00

  Yeah.

• 34:20Speaker 01

  Do you know about the Lagrange point?

• 34:22Speaker 00

  I do. Do you want to talk about it?

• 34:23Speaker 01

  No, you go you go ahead about what the Lagrange point is.

• 34:26Speaker 00

  Yes, the Lagrange point, specifically Lagrange point one, was what I started to think about. So it's different than a geosynchronous orbit in that it's it's much further from the earth. But you reach a point where you're also falling towards the sun. It's not stable, though, at at L1. That's the point in between the sun and the earth. Because you move a little closer to earth, now the earth's pull becomes stronger. It winds out and you fall to earth. You get closer to the sun, you fall to the sun. But you can you can just manipulate with a little amount of energy where you are, and you can stay there, and it's kind of a nice place. And you would you would always be right in between the earth and the sun. Earth's rotation wouldn't get into the way.

• 35:11Speaker 00

  So you'd always be right there in front of anyone's view of the sun.

• 35:16Speaker 01

  Yes, exactly. Now, the thing about this point is that it is, uh, the the L1 sits at one and a half million kilometers from earth.

• 35:27Speaker 00

  Okay.

• 35:28Speaker 01

  All right, so if it's at the Lagrange point, it needs to be 14,000 kilometers wide.

• 35:32Speaker 00

  14,000 kilometers wide. Again, I think we could do it.

• 35:37Speaker 01

  You think we could do it?

• 35:38Speaker 00

  And this would allow everyone in London to see sunglasses on the sun. What's the benefit of doing the Lagrange point?

• 35:45Speaker 01

  More than London for that. I reckon if you're willing to accept sort of 10% tolerance.

• 35:51Speaker 00

  One way or the other.

• 35:52Speaker 01

  Then you, you know, you can have 1,000 kilometers easy.

• 35:56Speaker 00

  Oh, sweet. Okay. So that's that's the advantage to putting them further away. We gain a little bit more, uh, radius of our where the illusion works. If we allow the glasses to be 10% off to the right or off to the left, then Lagrange point one, there are five, by the way. We could talk more in depth about them, because there's there's other ones that don't really help us with today's illusion. But L1 would require a 14,000-kilometer-wide pair of sunglasses.

• 36:27Speaker 01

  I think that's honestly, I think that's our best bet so far.

• 36:29Speaker 00

  Can we do better?

• 36:34Speaker 01

  How big do you want to go?

• 36:35Speaker 00

  Well, I want as big, I I what I want is for everyone who can see the sun to see sunglasses on the sun all day.

• 36:44Speaker 01

  I think you only need 140 million kilometers.

• 36:46Speaker 00

  140 million kilometers wide?

• 36:50Speaker 01

  For everyone on earth, yeah.

• 36:51Speaker 00

  So how how big do the sunglasses need to be?

• 36:54Speaker 01

  Sunglasses need to be 1.3 million.

• 36:56Speaker 00

  Okay, 1.3 million kilometers wide. That's how big we need to build this pair of sunglasses. And then we put it.

• 37:04Speaker 01

  140 million kilometers.

• 37:06Speaker 00

  140 million kilometers away from earth, in between the earth and the sun. And then, for everyone who can see the sun, the sun will appear to have sunglasses on.

• 37:19Speaker 01

  Yeah, they will. They will. And they'll only be 1% too big, and that'd be great.

• 37:24Speaker 00

  Right. Okay, so the the sunglasses are 1% bigger than they need to be.

• 37:28Speaker 01

  Yeah.

• 37:29Speaker 00

  That'll still look fine. It'll still look cool.

• 37:32Speaker 01

  Still look great. Still look great. And everybody will have this experience. I mean, depending on how how you're doing the propulsion system, but everybody can have this experience of the sun kind of moving through the sunglasses.

• 37:45Speaker 00

  So with a propulsion system, keeping this pair of sunglasses exactly between the earth and the sun, a pair of sunglasses that's 140 million kilometers away, but 1.3 million kilometers wide, the sun will appear to be wearing sunglasses all day for everyone who can see it.

• 38:08Speaker 01

  And what better monument to your life would there be than that?

• 38:12Speaker 00

  It's not a monument to my life. It's a monument to life itself. It's a monument to not just human achievement, but the human concept of being too cool for school.

• 38:28Speaker 00

  And a little bit absurd, because at the end of the day, why is the sun wearing sunglasses? Well, we're going to take a quick break, and we're going to talk about why we might actually want the sun to wear sunglasses. Because, I'll give you a hint, if you put sunglasses on the sun, you're also kind of putting them on earth.

• 38:55Speaker 00

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• 40:58Speaker 00

  Okay, welcome back. So, uh, we have just, and by we, I mean Hannah, has just calculated the exact blueprint needed for us to do the most amazing thing ever. Make it look like the sun is wearing sunglasses all the time. And as it turns out, you're gonna need to build a disk. I'm just gonna, I'm gonna give our final answer here for those of you that joined us halfway through, which is nobody, but I love this.

• 41:20Speaker 01

  Know how YouTube works. Go on.

• 41:23Speaker 00

  You're gonna need a disk, a disk of safe solar viewing film that reduces the sun's brightness by a hundred thousand times. So we can all look at it and look up in the sky and see a safe to look at ball, okay? Bright ball. And then in the middle of that disk, you're going to draw sunglasses. And this disk needs to be 1.3 million kilometers across, and it needs to be 140 million kilometers away from Earth.

• 41:48Speaker 01

  No big deal.

• 41:49Speaker 00

  Except it is a huge deal, because it would be hilarious and awesome and totally rad for the sun to wear sunglasses, but life on Earth would not enjoy it, because we would be blocking an enormous amount of the sun's light.

• 42:03Speaker 01

  Yeah, yeah. And I think life on Earth quite likes the sun's light.

• 42:07Speaker 00

  Yes, it would be as dark on Earth as it is during a total solar eclipse. Okay, it would, it would, it would really mess up photosynthesis, it would mess up.

• 42:17Speaker 01

  Birds wouldn't know what hit them.

• 42:18Speaker 00

  Birds would, birds would not know, they would be very unhappy. And I don't think birds would go, yeah, but it was worth it, dude. Look, the sun's got sunglasses on. It doesn't make any sense.

• 42:28Speaker 01

  You'd have owls hooting in the middle of the day.

• 42:30Speaker 00

  It's just like the sun on the Hot Tamale's box.

• 42:34Speaker 00

  However, there are some realistic proposals that are less silly that have said, yeah, but you know, if you, if you dim the sun only 1% or 2%, that would actually cool the Earth down by the same amount that all of our carbon emissions are increasing the Earth's temperature. So rather than cleaning up and polluting less, what if we just blocked the sun? Again, 1% to 2%, we could stop at least one consequence of climate change. Eh?

• 43:09Speaker 01

  I mean, guys, it feels like there's easier ways to do this, but sure, go on.

• 43:14Speaker 00

  Anyway, this idea is a real idea, and it's called a space sunshade. If we put something up there that covered some of the sun, we don't need to cover the whole sun, even just part of it. It doesn't need to be covered by a solid disk thing. It could just be a scattering of, of dust. One idea was that we could literally put a colony on the moon that just mined the moon and then flung that dust out into space between the Earth and the sun. We'd have to constantly do this, because the, the dust wouldn't stay there. But it would just dim the sun by, you know, 1% to 2%, and then we would counteract the warming effect of climate change. We would not, of course, stop toxic chemicals in the air,

• 43:55Speaker 00

  the acidification of the oceans, um, the, the increased CO2 in the air, which our brains don't like. That would all keep happening. So of course, many organizations like Greenpeace have said, oh, great, yeah, let's build a, a sunshade in space and then give ourselves more license to just keep polluting or pollute more.

• 44:14Speaker 01

  Yeah. You know that, you know that story about the woman who swallowed a fly?

• 44:19Speaker 00

  Yes, exactly.

• 44:20Speaker 01

  Sort of feels a little bit like that. Guys, why don't we just go and mine the moon and then spray out loads of moon dust all over? Like that we have no control over and it may end up like falling as rocks on Earth, but no big deal. Guys, guys, guys, don't swallow the fly.

• 44:37Speaker 00

  Yeah, don't take the fly out of your stomach, just swallow a frog. And then you can still have the fly. Oh, shoot, but now you got a frog. Well, just swallow a dog.

• 44:47Speaker 01

  Yeah.

• 44:47Speaker 00

  Exactly.

• 44:48Speaker 01

  Right.

• 44:48Speaker 00

  Exactly. But then there's the opposite of a space sunshade, which is a space mirror. And it's the opposite, because instead of shadowing the Earth, it actually catches sunlight and reflects it back down to Earth. And it can do this during the nighttime to produce light during the night.

• 45:06Speaker 01

  So the energy at nighttime.

• 45:08Speaker 00

  This idea is, is actually more real than even I knew. Um, I'd heard of this in sci-fi, but it's less sci-fi than I thought. In 1993, the Soviet Union attempted to do just this. They built like a 60, 65-foot diameter Mylar mirror that they launched up and deployed to be so high up that even when it was nighttime, this thing could reflect sunlight down onto Earth. And they thought this could be the secret we needed. Because what we can do now is during dark winter months, we can shine light on cities. And it's, it's gonna be like two, three times as bright as the full moon if we do this right.

• 45:47Speaker 01

  Right.

• 45:48Speaker 00

  So we'll increase productivity in the cities, we'll increase productivity of farms, because photosynthesis can go on for longer.

• 45:55Speaker 01

  I mean, it's sort of unlimited energy. If you can get it right, you sort of got unlimited energy. There's plenty of sun going around.

• 46:02Speaker 00

  Exactly. And so, believe it or not, uh, this was actually done. And the Soviet Union launched this in 1993. It produced a few miles wide patch of light during the night on the ground that was brighter than the full moon, about two to three times brighter, that moved at about eight miles a second across the surface of the Atlantic Ocean, then Europe, and then into Russia, before it didn't work. They, they couldn't, they couldn't control this mirror, um, to like keep a spot of light focused on one particular place on Earth. But this happened. I cannot find any good observer statements. There's no photographs of this happening.

• 46:43Speaker 00

  Apparently, the observers felt like they just noticed a quick flash of light and that was it. So it wasn't like, whoa, they just lit up my town at night.

• 46:55Speaker 01

  Where is this mirror now? Has it just floated off?

• 46:57Speaker 00

  Uh, it, it burned up in the atmosphere, like right after, uh, it, it, it was used once. And then they built a second one, and it got caught on one of mirror's antennas when it was deployed and ripped, and it didn't work, and they never tried again. But there is a company today that is trying to do this again.

• 47:18Speaker 01

  Uh, sorry, what?

• 47:19Speaker 00

  So imagine that someone goes missing in the woods, or a boat is missing. It's too dark, you have to turn, turn the rescue mission off until day. Not anymore. You get one of these space mirrors to just catch the sunlight and reflect it right down to where you need to look. Boom, you've got bright light.

• 47:36Speaker 01

  Hang on, there's actual funding for this?

• 47:39Speaker 00

  Yeah.

• 47:39Speaker 01

  Right. Wh-where?

• 47:42Speaker 00

  They're planning an array of 50,000 mirror-bearing satellites to orbit the Earth.

• 47:48Speaker 01

  Guys, we need less energy, okay? Less energy pointed back at Earth. Please don't let rich, crazy people do this stuff without getting the buy-in from the rest of us. This, I think this is a really bad idea.

• 48:03Speaker 00

  But Hannah, it's for search and rescue.

• 48:05Speaker 01

  Use a torch, okay?

• 48:07Speaker 00

  Well, okay, I'll tell you, here's the details. The startup is called Reflect Orbital. They're out of Hawthorne, California. And

• 48:13Speaker 01

  Of course, they're in California.

• 48:14Speaker 00

  The United States government just approved a mission to launch this giant mirror to test this out. Their, their, their plan eventually is to put 50,000 mirrors into orbit by 2035, which will allow for full noon brightness in a select spot on Earth at night. Noon brightness.

• 48:33Speaker 01

  Right. I'm bothered by this deeply. I'm deeply bothered by this, because, look, what you're describing here is essentially a Dyson sphere, right? Which is, it's this sci-fi idea that you could do this, and if you connect up the mirrors in the right way, you can harvest more energy from the sun, and you end up with sort of more energy than your planet currently has. Sort of like an unlimited energy thing. And when you have unlimited free energy, you can do all kinds of crazy things, right? So, I mean, for example, stripping salt out of water, which is something that would actually be quite a good thing for people who are alive on the planet right now, but takes a lot of energy. And we can't really do it, because actually, energy is expensive.

• 49:16Speaker 01

  But actually, if you had free unlimited energy, you would, it could end water being a problem. You could turn the Sahara back into a rainforest, right? It's sort of like a big, grand-scale idea. I'm fine with that. But if we do it, it needs to be as an entire planet collectively. We need to like be really careful, get people's buy-in, make sure that we are considering all of the options, make sure that that is what we actually want to do collectively. It cannot be some rich dudes who've got more money than makes sense for any human to ever have, that just decide on their own back that they're gonna do some startup and then ruin the planet for everybody else. I may have got slightly angry.

• 50:01Speaker 01

  I don't like it. I don't like it at all. Make it go away. I feel like, I feel like there's a lot of stuff. I feel like, I feel like there's sort of like rich dudes who kind of ruin the world in a lot of ways without asking permission. And this feels like another example of that.

• 50:17Speaker 00

  In my defense, I want to say a couple of things. My idea of putting sunglasses on the sun is not going to happen because I'm not a billionaire. I've only got YouTuber money. Which means the, the, the 65 millimeter version, I could do that one. Also, um, I was joking.

• 50:37Speaker 00

  But Reflect Orbital is not-

• 50:38Speaker 01

  Oh, this was never aimed at you. I'm, I'm fully supportive of the sunglasses idea. Is this, uh, is this such a rusty one that's bothering me? Go on.

• 50:48Speaker 00

  Reflect Orbital is not joking. They got approval on the ninth of July of 2026. And they've got a satellite, uh, it's called Erendil One. That sounds like a Lord of the Rings name, by the way. Erendil. Look at that. They're even stealing humanity's characters. It's going to launch, uh, later this year is the plan. 625 kilometers above Earth's surface. The satellite will then deploy a mirror and it's going to be able to illuminate a patch that's 24 square kilometers on Earth's surface. Now, it won't be noon brightness, this first test.

• 51:19Speaker 01

  Right.

• 51:19Speaker 00

  I mean, is there a world where this can be okay if like they only focus the light down on like solar panels?

• 51:27Speaker 01

  Yes, of course. Of course. I like, if, if you, I mean, I sort of think that there's different things you could do, but sure. Yes, I agree. You know what, though? This, this reminds me of, I made this documentary once about disabilities. And I went to go meet loads of tech bros in Silicon Valley and they were like, oh, we've come up with this amazing design that are glasses that can create subtitles for people who are deaf and that there's this amazing like robotics that blah, blah, blah, blah, blah, la, la, la, la. And then I went to go and talk to some disability advocates and they were like, you know what we really need? Ramps. It's boring, but that's what we actually need. And like, you know what the world needs? Like renewable energy and good batteries. You know what it doesn't need? Fricking mirrors in space.

• 52:10Speaker 00

  Yeah, the, the sexiness factor matters so much when it comes to getting funding. And at the moment, and especially, uh, even a few years ago, space was such a sexy thing to invest in, but there just weren't enough options. And so these, these investment firms were like, I need something space related. And someone would say, oh, I've got this thing called spin launch. We're going to spin things around and then let them go like David and Goliath and they'll go into space. And they raised millions, millions, millions. Here's someone that's like, I'm going to put mirrors in space. Think about how cool that would be. We could like bright, make the night bright. Cool. Here's a bunch of money. Here's a satellite. Do it.

• 52:47Speaker 01

  If they want to get in touch and convince us that we're wrong, I'm fine with that, by the way. I'm open to being persuaded, but right now I hate the world.

• 52:55Speaker 00

  Oh, Hannah, before we forget, remember how in the beginning I teased about E equals mc squared being incomplete? It is. And this becomes relevant when it comes to building any kind of, uh, uh, sun shade or sun sunglasses out in space, or even honestly, even a solar mirror that's just around orbit in Earth. The problem is these things have, have a big surface area and the sun is shooting out a lot of light and light can push things.

• 53:28Speaker 01

  Solar sails.

• 53:30Speaker 00

  Exactly. Solar sails. This can be a cool way to propel a spaceship. You just put a big sail on it. That's a reflective material. Sunlight shines on it, bounces off and the, the darn thing literally gets pushed by the light. So whatever giant structure we put out there, whether it be to light up the night, uh, or, or put sunglasses on the sun, is going to have to deal with the fact that it's going to need to, um, deal with and maybe try to, try to mitigate the pressure of light pushing it. And this might bring up a question in your mind, which is, hold on, how can light push anything when it literally has no mass?

• 54:09Speaker 01

  Got momentum though, doesn't it?

• 54:11Speaker 00

  It does. It has momentum, but that doesn't really answer the question because how can you have momentum without mass? You know, physics 101 has told us that momentum is your mass times your velocity. Okay. Well, we know the velocity of light, but do we know the mass of light? It's zero and zero times anything is zero. So it should have no momentum. Well, as it turns out, that's not exactly true. Light can have momentum because E equals mc squared isn't the whole equation. All right? E equals mc squared is the famous mass energy equivalence formula. It tells us how much energy exists in, it tells us how much energy could be liberated completely from an object

• 54:52Speaker 00

  based on its mass. However, the answer must be different for an object at rest versus that same object moving. A moving object has more energy. So, but equals mc squared doesn't include how fast the thing is moving. The full equation is E squared equals mc squared squared plus pc squared, the quantity pc where p is momentum. And so looking at that equation in its full form, you can see that the momentum a piece of light has is equal to its energy divided by the speed of light. And we just don't need to worry about this kind of thing when we're making calculations

• 55:36Speaker 00

  in our day to day lives about how much momentum like a bowling ball has. We don't need-

• 55:40Speaker 01

  I mean, it's nothing in comparison to the energy that's involved in the mass.

• 55:43Speaker 00

  That's right. That's right. And, and when it comes to a photon which has no mass, that doesn't mean that it has, uh, no momentum. It means in fact that it has an amount of momentum equal to its amount of energy divided by the speed of light. And so it can push things.

• 56:00Speaker 01

  Basically, E equals mc squared is not the equation that Einstein wrote down. It's just the cute version that fits on t-shirts.

• 56:07Speaker 00

  It's the cute version that fits on t-shirts and describes things like baseballs and bowling balls and uranium, but it does not describe light. The full version does. So we'll have to keep this in mind as we build our-

• 56:22Speaker 01

  How are you going to do that though? What are we going to do? Put holes in it? What are you going to do? Make it a mesh?

• 56:27Speaker 00

  Yeah, you can, you can make it a mesh. Um, you're never going to get rid of the, the push from light altogether. Um, but, but you can make, make a mesh. You can try to redirect so it reflects in different directions. Uh, I mean, all of this has, has been thought about a lot by people who were trying to design sun shades and it's going to usually-

• 56:48Speaker 01

  Which people have for real?

• 56:50Speaker 00

  Which people have for real. Yeah. Um, I don't think it'll ever happen because I think there are much better ideas to, to do what, what a sun shade attempts to do.

• 57:01Speaker 01

  Okay. I've got one, one final thing to say about your, your, I mean, all the other ideas are hocus, right? All the other ideas are like complete crazy junk, but your idea, your idea, Michael, I'm absolutely behind. I've just got one tiny little thing to add to it, which makes it even better, I think, which is that at the moment, we've been talking about having a disc, right? A sort of disc, a kind of opaque disc. The only tiny problem about this is that, um, light that's sort of bending around a smooth edged opaque disc, it will, um, there'll be like a bright spot right at the dead center of its shadow. It's called the Arago spot. Um, don't know if you thought about this, which is-

• 57:38Speaker 00

  I did not think about this.

• 57:40Speaker 01

  This is going to be difficult, but there's a way around it. We can, we can easily get around this, um, which is, uh, people have been talking about this since the 1800s, by the way. The way around this is that we can, um, put petals around the outside and then it won't happen. Basically, I think the final solution for us is that the sunglasses that the sun is wearing need to be like petal ones.

• 58:02Speaker 00

  Wait, yeah, what is the, okay, what is the deal with the petals? Because NASA is building a, a star shade, which we hadn't talked about because, uh, the stars are so far away, they're too small. They appear too small to put sunglasses on them. But for real, star shades are being made by NASA to cover up the light of a star so that we can see exoplanets potentially orbiting around it. But every picture I've seen of a star shade has had petals on the outside.

• 58:32Speaker 01

  This is why.

• 58:32Speaker 00

  Because I was only worried about our sun. I didn't look deeper into why it had petals. Why does it have petals?

• 58:38Speaker 01

  So it's to prevent, because essentially if you imagine that you've got a disc and then you shine a really bright light on it.

• 58:44Speaker 00

  Yeah.

• 58:44Speaker 01

  Then, then what happens is that you've got the kind of light going all around, right? And the, the light is essentially, because you're talking about such large distances, it's going to be bending around this smooth edged opaque disc and then you're going to get this central point in the middle.

• 58:58Speaker 00

  Oh, wow. Okay. So I'm loving this idea even more now. We're going to put sunglasses on the sun with a flower.

• 59:09Speaker 01

  Exactly.

• 59:10Speaker 00

  Okay. So what have we learned?

• 59:13Speaker 01

  Uh, that some people have too much money. That's my main takeaway.

• 59:16Speaker 00

  What I've learned is that sometimes a really silly question like that leads to a lot of learning. I mean, you've taught me so much in this. I didn't even consider the Arago spot and now I've got all this new, all these new things to research.

• 59:29Speaker 01

  Hey, you're welcome. You're welcome. Maybe we'll do another episode about that because frankly, I only understand it very superficially, just enough to know that petals are important.

• 59:38Speaker 00

  Yeah, exactly. And so, so, so if you out there have a question that's equally absurd, please do not be afraid to send it to us because a lot of knowledge can be gained even by looking into the silliest things. You can reach out to us at [email protected]. Send us an email.

• 59:54Speaker 01

  Absolutely. Or leave us a comment wherever you are watching or listening to this podcast or hop on over to our Reddit, r/the rest of science and we will see you next time.

• 1:00:04Speaker 00

  Yep. See you next time. Bye bye.

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