The Science of Putting Sunglasses on The Sun

12:03Hannah Fry:No, no, unless you don't value your eyesight in that one eye that you're using.

12:07Michael Stevens: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.

12:30Hannah Fry:And when we say safely, what we're talking about here is, I mean, it's literally sunburn on the inside of your eye.

12:38Michael Stevens:Yeah, your retina gets burned, and it gets burned fast, and usually permanently. I actually found some pictures of people's retinas that were damaged by looking at the 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. So don't do it. Don't do it. You know what else is almost exactly the same? People who have had laser pointers shined in their eyes. Same kind of retinal damage. This hot spot where it's just burned and that we'll never see again.

13:17Hannah Fry:That is one of, you know, 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 realised 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 nobody on this planet Earth wanted to live on an Earth where that was an option.

14:04Hannah Fry:Every now and then we do actually manage to properly collaborate as an entire species.

14:09Michael Stevens:Yeah, and I'm glad when it happens. But the point is, don't look at the sun. 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. Right. Right. Okay, so we're going to need to come up with 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.

14:50Michael Stevens:You cannot see anything else through this stuff except the sun, an arc 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 dimmer ball, and you could line up the sunglasses to appear to be on the sun, and we're done. Except - Hey, the end. 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.

15:36Michael Stevens:How silly. My day has been improved.

15:38Hannah Fry:I imagine you want to enjoy it with both of your eyes, even, right? The description that you've 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. Okay.

15:52Michael Stevens:Okay, so let's take one little baby step here. How do we make this work for two eyes?

15:57Hannah Fry: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 want it to fully work 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 ten and a half millimeters wide. You need to be about a centimeter.

16:28Michael Stevens:So what would it look like if it was only 6.5 millimeters wide? Would it be?

16:35Hannah Fry:It would just be a bit janky. It would work for sort of one point of your retina and not the rest of it. But your retina is going to be, you know, amalgamating the image. So it wouldn't really work.

16:46Michael Stevens:Okay, so that's really good to know. So we need, you're saying, 10 millimeters.

16:52Hannah Fry: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 started. So in total, for both eyes, you actually need more like 7.05 centimeters. Whoa, that's big. It is because the distance of your eyes is so big compared to the distance that you're sort of holding away.

17:23Michael Stevens:Right. So I've got a tape measure here. I'm going to get seven centimeters. I mean, 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. Unfortunately, your eyes are wide. but the sun isn't this wide when i if i were to look at it no but think about how much things

17:49Hannah Fry: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

17:58Michael Stevens:how much it jumps so is this is this calculation for making it so that when i switch my eyes the sun is still within this boundary? Exactly. Okay, I see. So, let me just... For those of you who are listening only... I'm looking at other 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 focus on the distant sun, then this is like... It looks like this. I've got this huge thing.

18:33Hannah Fry:No, look, look, Michael, I'm sorry, but it's not going to be perfect unless you can shrink your eye down to one point. It's... Yeah.

18:41Michael Stevens:Okay. 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 and i

19:09Hannah Fry: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 essentially at such a short distance you know it's about 10 of the distance it's about 10 percent 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.

19:31Michael Stevens:That's so cool. See, this is why we're a good team, because I just assumed one eye, that's what we're going to do.

19:37Hannah Fry:Everyone's going to be fine. And you're like,

19:38Michael Stevens: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. Exactly. So this gets better as we get bigger.

19:53Hannah Fry:Right. 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.

20:03Michael Stevens:Yeah. So how big do the glasses have to be? Like, obviously, just to put 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 it's too gaseous or plasmatic or whatever. But the point is, how big of a pair of glasses do we have to get such that two-eyed viewing is a pleasant, funny scene?

20:40Hannah Fry: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?

20:55Michael Stevens:Yeah, 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.

21:07Hannah Fry:Yeah, right. 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 uh 100 centimeters so it'd be about a meter i mean look you can basically divide it by 100 about a meter about a meter yeah 100 meters away that's very doable that's very doable we can make

21:38Michael Stevens: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? Is there like one little spot you're going to have to stand at where the illusion works, but someone next to you is not going to like it?

22:25Michael Stevens:Correct.

22:25Hannah Fry: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 hands together.

22:38Michael Stevens:Okay. But I am falling in love with this, Hannah. I didn't realize what these numbers would sound like. But this was so easy. 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 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.

23:14Michael Stevens: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.

23:25Hannah Fry:and I need to know how to build it.

23:27Michael Stevens:Okay, so now I know that I can please one person at a time with a meter wide pair of sunglasses that's a hundred meters away from them. Exactly. Okay, but let's go up. Let's get bigger.

23:36Hannah Fry:In this sculpture garden, can we, just to back ref a previous episode, can we also make sure that this is where your skeleton resides, picking its own nose?

23:45Michael Stevens:Oh, yeah. Yeah, and people can, I can be dressed up for different holidays. Yeah, the Halloween version is going to be easy. That would be easy, just naked.

23:55Hannah Fry: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.

24:00Michael Stevens: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?

24:12Hannah Fry:You want to go higher? Let's go to the ISS, which is not that high. I mean, it's outer space, but it's not crazy high. No, it's not. 400 kilometers? That's nothing. I feel like 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 fine.

24:30Michael Stevens:And every time we bring up the ISS height, we say the same thing, which is, is that all? Is that all? That's puny. That's nothing. We've got to up those numbers, ISS. Guys, pathetic. But it's true. I mean, 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.

24:51Hannah Fry:It's nothing. It's nothing. Okay, so 400 kilometers up, divided by 107, we'll tell you how wide it needs to be. So if it's just you, it's going to be 3.7 kilometers big. That's how big it needs to be.

25:02Michael Stevens:Yeah, okay, whoa, whoa, we've gone up by order of magnitude here. That's big.

25:05Hannah Fry:That's big, that's big, right? But it's further away, so it's going to take up, it needs to be much bigger in order to take up the same angle of your field of view. One benefit that you do get here, though, is that 3.7 kilometers, the six centimeters for your eyes doesn't make any difference anymore. Right.

25:21Michael Stevens:So two-eye viewing is fine. We don't need to even worry about that anymore. Absolutely fine. But that's only going to look good from one person's perspective?

25:30Hannah Fry: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 going to choose my hometown. 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.

25:57Michael Stevens: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.

26:08Hannah Fry:Exactly.

26:09Michael Stevens:44 kilometers is big, but it is doable.

26:13Hannah Fry:It's doable.

26:13Michael Stevens:We could build such a thing.

26:16Hannah Fry:I think if we really put the effort in, one tiny problem, it will also pass over in about six seconds because of how the ice is moving. So you're going to have to be quick. You're going to have to be quick.

26:27Michael Stevens:Oh, shoot. Yeah. So it's going to 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 going to just transit the sun. it's going to take six seconds to completely um 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 yeah yeah yeah this is

27:04Hannah Fry:the problem if you want if you want binocular vision you know you're still too close at 400 You're still way too close to get it to work really properly. You're gonna need to go much, much further out. I know. So that the distance between people in London, for instance, is like a tiny, tiny, tiny rounding area for how wide the sunglasses are.

27:25Michael Stevens:Yeah. Now, if instead of orbiting, we put these 44-kilometer-wide sunglasses at the top of a structure that was 400 kilometers tall, then it wouldn't be 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 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.

28:08Michael Stevens:But the sun is only half a degree wide in our field of view. So you're gonna have like, what, a minute where the illusion kinda 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.

28:37Hannah Fry:Is that right? 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 going to look too big.

28:50Michael Stevens:Yeah. So I'm just thinking, is this going to 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 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.

29:11Hannah Fry:So you've got like a solid... I just want to add for anyone who's not watching these visuals, it's the most delightful thing because of 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 you've got i'd say maybe

29:32Michael Stevens:a minute where it's gonna look good and that's it but that's a minute every day that's a minute every day and everyone's gonna have to well you have to worry about north and south as well there don't you 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 you're gonna have to

29:54Hannah Fry:move it north and south. This is going to have to be a moving flag.

29:56Michael Stevens:Oh, I see. Yes. We don't have to make the tower taller. We just have to move it north and south every day.

30:02Hannah Fry:Look, I think the best solution here is to go further out. I think 400 kilometers is nowhere near enough, you know?

30:08Michael Stevens:Okay, let's go further out.

30:09Hannah Fry:I think if you push it out to geostationary orbit, for example, then we're talking, which, as I learned approximately one week ago, is really, really far away.

30:19Michael Stevens:It's really far away. 35 ,786 kilometers. Far enough away that, one, we solve the problem of motion, because everyone on one side of the Earth, 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?

30:54Hannah Fry:I mean, definitely one. What do you mean one? 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. How wide?

31:10Michael Stevens:334 kilometers. Oh, so we went from 44 kilometers to over 300. That's not a huge jump. No, it's fine. It's easy just to make it a slightly bigger flag.

31:19Hannah Fry:But only one person would enjoy it? I mean, look, that's the number for one person. But you could have people spaced, you know, five kilometers apart, and they'd still be able to see largely the illusion. I mean, what kind of tolerance do you want here? Within 1 %? Then you can only be within a space of three kilometers.

31:37Michael Stevens:Oh, I see what you're saying. Yeah. 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.

31:54Hannah Fry: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.

32:04Michael Stevens: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.

32:19Hannah Fry:Exactly. That it would just ruin the illusion. Right. Because at that distance, even at 35 ,000 kilometers away, the radius of the Earth is not a rounding error. It's so big.

32:29Michael Stevens: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. I think for no one. For no one.

32:47Hannah Fry:They would be too big.

32:49Michael Stevens:So is there some magic point where this solves itself and suddenly, obviously when they're on the sun itself, then everyone gets to enjoy it.

32:58Hannah Fry:then it's fine. But this is it. The closer that you get to the sun, the smaller the earth gets in the distance. And the smaller the distance between people on the earth becomes as part of the overall picture. 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 stage where the triangle, as it were, between the sunglasses and the observer, where the distance between observers is basically, it doesn't, it makes no difference.

33:34Hannah Fry:So we are talking way, way, way, way, way closer to the sun than geostationary orbit. For it to actually probably work for more people than just you and your mate who's standing right next to you.

33:45Michael Stevens: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

34:08Hannah Fry:so i 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 yeah do you know about the Lagrange point?

34:22Michael Stevens:I do. Do you want to talk about it?

34:24Hannah Fry:No, you go ahead about what the Lagrange point is.

34:26Michael Stevens: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 much further from the Earth. But you reach a point where you're also falling towards the sun. It's not stable, though, 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 pole becomes stronger, it wins out and you fall to earth. You get closer to the sun, you fall to the sun. But 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.

35:04Michael Stevens:And you would always be right in between the earth and the sun. Earth's rotation wouldn't get into the way. so you'd always be right there in front of anyone's view of the sun yes exactly now the thing about

35:18Hannah Fry:this point is that it is uh that the l1 sits at one and a half million kilometers from earth okay all right so if it's at the lagrange point it needs to be 14 000 kilometers wide 14 000 kilometers wide again i think we could do it you think we could do it and this would allow everyone in

35:40Michael Stevens:London to see sunglasses on the sun, what's the benefit of doing the LeBronge point? You'd get more than London for that.

35:46Hannah Fry:I reckon if you're willing to accept sort of 10 % tolerance.

35:51Michael Stevens:One way or the other.

35:52Hannah Fry:Then you, you know, you can have a thousand kilometers easy.

35:56Michael Stevens: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, um, then Lagrange Point 1, there are five, by the way. We can talk more in depth about them, because 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.

36:27Hannah Fry:I think that's, honestly, I think that's our best bet so far.

36:29Michael Stevens:Can we do better?

36:33Michael Stevens:How big do you want to go? Well, what I want is for everyone who can see the sun to see sunglasses on the sun all day.

36:44Hannah Fry:I think you only need 140 million kilometers.

36:47Michael Stevens:140 million kilometers wide. For everyone on Earth, yeah. So how big do the sunglasses need to be?

36:54Hannah Fry:Sunglasses need to be 1.3 million.

36:56Michael Stevens:Okay, 1.3 million kilometers wide. That's how big we need to build this pair of sunglasses. And then we put it... 140. million kilometers 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 yeah they will

37:20Hannah Fry:they will and they'll only be one percent too big and right right okay so the the sunglasses are one

37:26Michael Stevens:percent bigger than they need to be yeah that'll still look fine it'll still look cool still look great. Still look great.

37:34Hannah Fry:And everybody will have this experience. I mean, depending on how you're doing the propulsion system, but everybody can have this experience of the sun kind of moving through the sunglasses.

37:45Michael Stevens: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.

38:07Hannah Fry:And what better monument to your life would there be than that?

38:12Michael Stevens: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. 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.

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40:22Michael Stevens:Okay, welcome back. So, 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 going to need to build a disk. I'm just going to give our final answer here for those of you that joined us halfway through, which is nobody. But I love this. Not how YouTube works. Go on. You're going to need a disk, a disk of safe solar viewing film that reduces the sun's brightness by 100 ,000 times. So we can all look at it and look up in the sky and see a safe to look at ball.

40:58Michael Stevens:OK, 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. No big deal. 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. Yeah.

41:27Hannah Fry:Yeah. And I think life on Earth quite likes the sun's light.

41:30Michael Stevens:Yes. It would be as dark on Earth as it is during a total solar eclipse. Okay. It would really mess up photosynthesis. It would mess up. Birds wouldn't know what hit them. Birds would not know. They would be very unhappy. And I don't think birds would go, yeah, but it was worth it, dude.

41:48Hannah Fry:Look, the sun's got sunglasses on. That doesn't make any sense. You'd have owls hooting in the middle of the day.

41:54Michael Stevens:It's just like the sun on the hot tamales box. However, there are some realistic proposals that are less silly that have said, yeah, but you know, if you dim the sun only one or two percent, 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, one to two percent, we could stop at least one consequence of climate change.

42:32Hannah Fry:Eh? I mean, guys, it feels like there's easier ways to do this, but sure.

42:37Michael Stevens:Go on. 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 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. It would have to constantly do this because 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.

43:14Michael Stevens:We would not, of course, stop toxic chemicals in the air, the acidification of the oceans, 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 sunshade in space and then give ourselves more license to just keep polluting or pollute more.

43:38Hannah Fry:Yeah. You know that story about the woman who swallowed a fly?

43:42Michael Stevens:Yes, exactly. Sort of feels a little bit like that.

43:45Hannah Fry: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 that may end up, like, falling as rocks on Earth, but no big deal. Guys, guys, guys, don't swallow the fly.

44:01Michael Stevens: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. 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.

44:30Hannah Fry:So the energy at nighttime.

44:32Michael Stevens:This idea is actually more real than even I knew. 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 going to be like two, three times as bright as the full moon if we do this right.

45:11Hannah Fry:Right.

45:12Michael Stevens:So we'll increase productivity in the cities. We'll increase productivity of farms. Because photosynthesis can go on for longer.

45:19Hannah Fry:I mean, it's sort of unlimited energy. If you get it right, it's sort of got unlimited energy. There's plenty of sun going around.

45:26Michael Stevens:Exactly. And so believe it or not, 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 couldn't control this mirror to 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. Apparently, the observers felt like they just noticed a quick flash of light and that was it.

46:12Michael Stevens:So it wasn't like, whoa, they just lit up my town at night. Where is this mirror now? Has it just floated off? It burned up in the atmosphere right after it was used once. And then they built a second one, and it got caught on one of Mira'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. Sorry, what? So imagine that someone goes missing in the woods or a boat is missing. It's too dark. You have to 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.

46:58Michael Stevens:Boom. You've got bright light.

47:00Hannah Fry:Hang on. There's actual funding for this.

47:02Michael Stevens:Yeah.

47:03Hannah Fry:Right. Where?

47:06Michael Stevens:They're planning an array of 50 ,000 mirror-bearing satellites to orbit the Earth.

47:11Hannah Fry: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. I think this is a really bad idea.

47:26Michael Stevens:But Hannah, it's for search and rescue.

47:29Hannah Fry:Use a torch, okay?

47:31Michael Stevens:Well, okay. I'll tell you, here's the details. The startup is called Reflect Orbital. They're out of Hawthorne, California. And the United States government just approved a mission to launch this giant mirror to test this out. 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. Right.

47:58Hannah Fry: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. It's 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.

48:37Hannah Fry:And we can't really do it because actually energy is expensive. But actually, if you had free unlimited energy, you 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 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 going to do some startup and then ruin the planet for everybody else.

49:20Hannah Fry:I may have got slightly angry. I don't like it. I don't like it at all. Make it go away. I feel like there's a lot of stuff. 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.

49:41Michael Stevens: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 65 millimeter version, I could do that one. Also, I was joking. But Reflect Orbital is not.

50:02Hannah Fry:Oh, this was never aimed at you. I'm fully supportive of the sunglasses idea. Is this search and rescue one that's bothering me? Go on.

50:12Michael Stevens:Reflect Orbital is not joking. They got approval on the 9th of July of 2026 and they've got a satellite. It's called Erendel 1. That sounds like a Lord of the Rings name, by the way. Arendelle. Look at that. They're even stealing humanity's characters. It's going to launch 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. I mean, is there a world where this can be okay if they only focus the light down on solar panels?

50:51Hannah Fry:Yes. Of course.

50:52Michael Stevens:Of course.

50:53Hannah Fry:I mean, I sort of think that there's different things you could do, but sure. Yes, I agree. You know what 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 then there's this amazing like robotics that blah, blah, blah, blah, blah, blah, blah, blah. And then I went to go and talk to some disability advocates and they were like, Like, you know what we really need? Ramps. It's boring, but that's what we actually need.

51:26Hannah Fry:And like, you know what the world needs? Like renewable energy and good batteries. You know what it doesn't need? Frickin' mirrors in space.

51:33Michael Stevens:Yeah, the sexiness factor matters so much when it comes to getting funding. And at the moment, and especially even a few years ago, space was such a sexy thing to invest in, but there just weren't enough options. And so 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. Here's someone who's like, I'm going to put mirrors in space. Think about how cool that would be. We could make the night bright.

52:08Michael Stevens:Cool, here's a bunch of money. Here's a satellite, do it.

52:10Hannah Fry: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'm... hate the world.

52:18Michael Stevens: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 sunshade 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 a big surface area and the sun is shooting out a lot of light, and light can push things.

52:52Hannah Fry:Solar sails. Exactly, solar sails.

52:55Michael Stevens: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 darn thing literally gets pushed by the light. So whatever giant structure we put out there, whether it be to light up the night or put sunglasses on the sun, is going to have to deal with the fact that it's going to need to deal with and maybe 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? It's got momentum, though, doesn't it?

53:34Michael Stevens:It does. It has momentum. But that doesn't really answer the question, because how can you have momentum without mass? You know, physics 101s 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 based on its mass.

54:17Michael Stevens:However, the answer must be different for an object at rest versus that same object moving. A moving object has more energy. So – but E 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 in our day-to-day lives about how much momentum like a bowling ball has.

55:03Michael Stevens:We don't need to—

55:04Hannah Fry:I mean, it's nothing in comparison to the energy that's involved in the mass.

55:07Michael Stevens:That's right. That's right. And when it comes to a photon which has no mass, that doesn't mean that it has 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.

55:23Hannah Fry:Basically, E equals MC squared is not the equation that Einstein wrote down. It's just the cute version that fits on t-shirts.

55:31Michael Stevens: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...

55:46Hannah Fry:How are we 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?

55:50Michael Stevens:Yeah, you can make it a mesh. You're never going to get rid of the push from light altogether. But you can make a mesh. You can try to redirect so it reflects in different directions. All of this has been thought about a lot by people who are trying to design sunshades. Which people have, for real. Which people have, for real. Yeah. I don't think it'll ever happen because I think there are much better ideas to do what a sunshade attempts to do.

56:25Hannah Fry:Okay, I've got one final thing to say about 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 disk, an opaque disk. The only tiny problem about this is that light that's sort of bending around a smooth edged opaque disk, there'll be like a bright spot right at the dead centre of its shadow. It's called the arago spot.

57:00Hannah Fry:I don't know if you thought about this.

57:01Michael Stevens:I did not think about this.

57:03Hannah Fry:This is going to be difficult. But there's a way around it. We can easily get around this, which is, people have been talking about this since the 1800s, by the way. The way around this is that we can 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 worms.

57:26Michael Stevens:Wait, yeah. What is the deal with the petals? Because NASA is building a star shade, which we hadn't talked about because 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.

57:55Hannah Fry:This is why.

57:56Michael Stevens:Because I was only worried about our sun, I didn't look deeper into why it had petals. Why does it have petals?

58:02Hannah Fry:So it's to prevent, because essentially if you imagine that you've got a disk and then you shine a really bright light on it.

58:08Michael Stevens:Yeah.

58:08Hannah Fry:then what happens is that you've got the kind of light going all around, right? And the light is essentially, because you're talking about such large distances, it's going to be bending around this smooth edged opaque disk. And then you're going to get this central point in the middle.

58:22Michael Stevens:Oh, wow. Okay. So I'm loving this idea even more now. We're going to put sunglasses on the sun with a flower. Exactly. Okay. So what have we learned?

58:37Hannah Fry:that some people have too much money. That's my main takeaway.

58:40Michael Stevens: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.

58:53Hannah Fry: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.

59:01Michael Stevens:Yeah, exactly. And 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 therestisscience at goalhanger.com. Send us an email.

59:18Hannah Fry:Absolutely. Or leave us a comment wherever you are watching or listening to this podcast, or hop on over to our Reddit, r slash therestisscience. And we will see you next time.

59:28Michael Stevens:Yep. See you next time. Bye-bye.

59:40I see you.

59:41Michael Stevens:Avatar Fire and Ash is now streaming on Disney+. It's the film critics are calling the best Avatar yet. Go, go, go, go! A true epic and completely jaw-dropping.

59:52Hannah Fry:This is the only pure thing in this world.

59:55Michael Stevens:Return to Pandora on Disney+. It will be an adventure for the whole family. and watch the Oscar-winning phenomenon at home.

1:00:02Hannah Fry:This is sick!

1:00:05Michael Stevens:Avatar Fire and Ash, now streaming on Disney +, rated PG-13. I'm Glenn Washington, host of Snap Judgment from KQED. Every week, Snap drops you inside someone's biggest decision. The kind of decision you can only make once. With everything on the line, what do you believe? What do you want? and what would you risk to get it? Find out. Tap to listen now to Snap Judgment from KQED on Spotify.