The Rest Is Science - This Glass Was Made By Lightning

Episode Date: January 29, 2026

Could a bolt of lightning become a permanent geological relic? How small would you have to squash a hamster to turn it into a black hole? Professor Hannah Fry and Michael Stevens dismantle our perceptions of scale and texture, moving from the glassy "fulgurites" forged in sandy soil to the mathematical threshold of the Schwarzschild radius. They explore the counter-intuitive geometry of the Earth, calculate the extreme density required to collapse domestic life into a gravitational singularity and examining the crystalline remains of atmospheric discharge. This is an investigation into the smooth, the sharp, and the impossibly dense, proving that the world we touch is rarely as it seems. ------------------- For more information about Cancer Research UK, their research, breakthroughs and how you can support them, visit ⁠⁠https://cancerresearchuk.org/restisscience⁠⁠ 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 OakleyVideo & Social: Bex TyrrellAssistant Producer: Imee MarriottProducer: Becki HillsSenior Producer: Lauren Armstrong-CarterHead Of Digital: Samuel OakleyExec Producer: Neil Fearn Learn more about your ad choices. Visit podcastchoices.com/adchoices

This episode is brought to you by Cancer Research UK. So when most people think of naked mole rats, their unusual relationship to cancer probably isn't the first thing that comes to mind. But maybe it should be because it is incredibly rare for them to develop cancer, which could be partly down to their unique immune system, or it might be the way that their cells respond to damage. So scientists are studying their biology for its cancer-fighting secrets. It's a reminder that discoveries can sometimes come from places, you don't expect. Cancer Research UK is the world's largest charitable funder of cancer research. Thousands of scientists of doctors and nurses work across more than 20 countries to help turn discoveries in the lab into new tests, new treatments and new innovations. And the impact is clear. Over the past 50 years, the charity's pioneering work has helped

double cancer survival in the UK, meaning more people living longer, better lives free from the fear of cancer. For more information about Cancer Research UK, their research, their breakthroughs and how you can support them, visit Cancer Research UK.org forward slash rest is science. Welcome to the rest of science. This is field notes. This is a kind of podcast expedition diary where Michael and I are going to trade the curious objects or thoughts or sometimes feelings that are occupying our minds. And we'll answer the strange questions that are troubling yours. Because every week one of us is going to bring sort of strange, spectacular object or story onto the show. And together we're going to see what kind of uncharted territory it takes us to.

But we want to hear your questions, your theories and your thought experiments too. So send them in and stay tuned to see where we end up. Yes. Now, later today, I'm going to be showing off a very rare physical monument of something we only ever experience as a split second flash. That's my hook and tease for you, Michael. Ooh, that's a good hook. I know.

Any guesses so far? Is it like how photons travel? I mean, there's photons involved, but you've got to stay tuned for the second half if you want to find out more. But for this first off, what we thought we'd do is we would dive into our mailbag. As ever, you can send us your questions, you can send us your own objects, your own thoughts and sometimes feelings. But our first question to say, Madav has got a question. This I think is one for you, Michael. Why do mirrors flip us horizontally, but not vertically? Yeah. That's a great question. I've done a whole bunch of videos and TikToks about mirrors.

And it is weird. How come when I approach a mirror my right hand is on the left side and vice versa, but my head isn't where my feet should be? Why is it just doing this horizontally? And of course, the answer is it's not flipping you horizontally. it's flipping you inside out. Everything that you present to a mirror gets reflected right back. When you look at like a letter R, you're like, yeah, that looks normal. But then you turn it to the mirror. You were the one who flipped it horizontally. You turned it.

And it's just getting sent right back to you. But also if you lie down, it still knows that your left hand and your right hand and where your feet are. Like, if you make yourself horizontal, right, then it suddenly flips vertical, suddenly makes your left hand to your right hand, but your head is not switched through your feet. Yeah. So it's like, how does it know? How does it know that to only ever flip what's horizontal to you? That's right. Yeah. If I turn the mirror, it continues this horizontal reversing. If I put myself upside down, it continues the horizontal reversing. Words are reversed right to left. They're not reversed vertically. They're not reversed vertically. They're not.

flipped over. And the answer is that the mirror isn't flipping anything. You are. You see, all mirrors do is give back exactly what hits them. And when I say have some text on my notepad and it looks normal and then I turn it to a mirror, I'm the one who just turned it. Now, it's as though I'm looking through the paper because that letter is hitting the mirror and it's coming back to me without being changed. Because if you'd written on a piece of tracing paper and we're holding it up and looking through the tracing paper, you would see exactly what is reflected back at you in the mirror. It would be unchanged, it would be unchanged, the sort of the backside of the tracing paper and what you're seeing in the mirror. That's right.

And so another way this has been explained is that mirrors actually flip things inside out. All right? You can think of it this way. When you look in a mirror, the closest thing to the mirror becomes the closest thing to you in the mirror image. So if my nose is closest to the because I'm facing it. Then that means that in my mirror reflection, the nose will be closer to me, and the back of my head is behind my nose. So I've been pushed, literally,

my back and my front have been pushed through each other, seemingly, apparently, and now I'm looking at myself squished inside out. You know, like those little suckers that you get, that's sort of a stable, and then you can pop them inside out? Yeah. It's a bit like that, right? It's like the mirror is doing that to you as a human. It's sort of like grabbing you by the nose and like,

popping you inside out. That's right. So top and bottom stay on the same axis. Right and left stay on the same axis. It's just that now the front and the back are in different orientations relative to left and right. And so we think, well, if I froze my mirror image and walked around to join it to face in the same direction, it would be reversed. But no, no, no. You have reversed yourself by turning around to join it and face the other way.

I love that question. I mean, in general, I just really love, like, thinking very hard and long about things that feel like they should be obvious and then getting really confused. Yeah. Great question. Absolutely great question. Let's move on to a question from Brandon who asks, how dense would a hamster have to be to become a black hole? Okay, I cannot tell you how much fun I had this afternoon doing the calculations for this. Because the answer is actually quite surprising, I think. Okay, so I looked up the average weight of a hamster. I've gone for, if you're interested, a chubby Syrian hamster, 150 grams, that's their general weight.

If you wanted to turn one of those into a black hole, the problem is that you have to shrink it down to be so small that it's not just about squishing it, it's about obliterating the concept of space within it. Excellent. Here's the sort of breakdown, right? The sparse child radius, this is a calculation, it's an equation that tells you how wide something needs to be before essentially it becomes a black hole. Before it's, the density becomes so great that it becomes a black hole.

So when you run the calculation for a hamster at 0.15 kilograms, you work out that the sparse child radius is 2.2 times 10 to the minus 28 meters. Okay. which is, I'm going to say it, small. To put that into perspective, a proton is 10 to the minus 15 meters. Oh, no. So you've got to squish all the hamster's mass into a volume smaller than a proton. Oh, yeah. I mean, like size of atoms, forget it.

That's like gigantically vast in comparison to the size of this hamster's got to get down. It's got to be 10 trillion times smaller than a single proton, which if you want to put that in perspective, it's the hamster is to a proton at the moment what a grain of sand is to the entire earth basically, it's going to be so small. So, okay, then the consequence of what happens when you do that is phenomenal.

Does it hurt the hamster? I don't think we've got a sort of Honey I Shrunk the Kids type smallizer machine. Okay, I love it. I love it. Hamster miniaturizer machine. I think the hamster's fine. Imagine that in the hamster.

is fine the whole time. It just like starts to realize, hey, my my gravitational force is getting stronger. I've become a black hole. Dang it. Damn, damn it. Okay, I'm going to tell you the hamster is fine. Spoiler alert, not everyone else is. Just bear with me for a second. Because here's the thing, right? The density, we know with the hamster weighs, it's 150 grams of like fur and cheeks, but it's now squished into this subatomic spec, which means the density that's required. is 3.3 times 10 to the 81 kilograms per meter cubes, okay? People can check my calculations on this if you like. But just to visualize that, that sort of crunch, water, 1,000 kilograms a meter cube. Steel, 8,000. The core of the sun, 150,000. A neutron star, which is the most

densest object in the entire universe, is 10 to the 17 kilograms for meter cubed. Our hamster, remember, 10 to the 81. Okay, so basically it needs to be, I mean, many, many, many, many, many gazillions denser than a neutron star for this to work. Well, sure. I mean, we're trying to make a black hole. Like, it's got to be denser than any regular matter. Sure.

But this is like even denser than that, even denser than that. And the problem is that, okay, according to hawking radiation, little black holes will evaporate over time. but this tiny little hamster black hole is going to be so unstable that it will probably only last for about 10 to the minus 26 seconds, right? So really, I mean, it barely exists. But what that means is that once it's down to this tiny size, it instantly converts its entire mass of 150 grams back into pure energy, right? And it equals MC squared. So what this means is the moment that you finish miniaturizing your hamster, it would detonate and the energy release would be about 3.2 megatons of TNT, which is about 200 times more powerful

than the atomic bomb dropped in Hiroshima. Yeah. So, I mean, you can if you want to, Brandon, but I would say don't. Wow. If you turned your hamster into a black hole, you would create a nuclear bomb. Probably be at least. the end of the country you're in, if not wider. There'd be a nuclear winter that would wipe out much of the planet, I imagine. Now, this hamster, during its tiny fraction of a second

that it's a black hole, it will at least be free. It'll be able to leave its cage. Yeah. Look, I think when people try and say that one small creature cannot make a difference, I think this is evident to the contrary. It depends how you define can. Look, I can imagine. I can imagine some, like, oppressed hamster saying, one of these days, I will compress my mass into a size smaller than a proton. And then you'll all be sorry. You'll all be sorry. Hey, you know what?

I think we've just found a new plot for a new Pixar film. Yeah. The hamster who became a black hole. Yeah. Copyright. The rest is science, 2026. Too right. Okay.

Speaking of shrinking objects, Dan, I've got another question for you, Michael. This one's from Edward. He asks, I've heard that if the earth was shrunk down to the size of poor, ball, it would be smoother than any other man-made object. Is this true? I mean, first of all, wouldn't be small enough to be a black hole? No, it wouldn't. The Earth's Schwarzschild radius is funny enough. I actually literally have it right here. Yeah, we'd had this in gravity, right? It'd be about, like, I think, 0.8 centimeters. Yeah. So if all of Earth's mass existed in this volume, you could be so close to all that mass that even light couldn't escape.

But if we're just shrinking it down to the size of a pool ball, I mean, we're still talking about something dangerous, denser than a neutron star, but not quite able to capture light. I think light could be, you know, very bent by it. But here's to answer your question, Edward, it would not be particularly smooth. In fact, the earth squeezed down to the size of a pool ball would be about as rough as 320 grit sandpaper. So the next time you're at a hardware store,

find the 320 grit and feel that. That's what a giant would feel if they grasped Earth. Now, the myth that the earth is smoother than a pool ball comes from a misreading of the International Pool Association's rules. I don't know if that's the actual governing body, but they say a pool ball must be built with a diameter of 2.25 inches plus or minus 0.005 inches. All right?

So that's five thousandths of an inch. And people have taken that to mean that a pool ball can have craters and bumps that are five thousandths of an inch. And at the scale of Earth, that would mean 28 kilometer high mountains and trenches. So obviously, the Earth is smoother than a pool ball. But that's not what the regulation means. the regulation isn't telling us about the texture. It's telling us about the spherical nature of the ball, how off, how oblate it can be.

And so the, if you actually look at real pool balls, they have like sub-micron scratches on them for real. Like a really well-used, quite scuffed up pool ball is going to have these little tiny scratches that you can see under a microscope. And they correspond. to bumps and crannies that are actually much smaller than the Marianas Trench on Earth or Mount Everest would be at that scale. So, sorry to say, the Earth is not smoother than a pool ball. It is as smooth as 320 grit sandpaper, which I'm trying to think of things in real life that would feel that way.

I think maybe like... Well, where is 320 on the spectrum? from like if you start off with, you know, the coarsest of all where you're like just trying to get the surface down, what numbers that? If you're trying to like remove material, you're using an extra coarse sandpaper that could be like a 24, a 30, a 36. These things are like hilarious. It's almost like a saw, a piece of paper that's a saw. These are all macro grit sandpaper. You look at it and it looks like someone glued a bunch of rocks to some paper.

But when you get into the, they call them microgrit sandpapers, very fine ones are about 240 grit. But for Earth's texture, we need extra fine. Okay. Between 320 and 360s. So those are going to be, you know, used for wood polishing to initiate polishing. The idea that a pool ball can have these 5,000th sized pits and craters, that's describing 120 grit, which is, one of those, like, that'd be fine. That can't even remove varnish or paint on wood, it's so fine.

But that's not Earth. All right, okay, so this is sort of somewhere in the middle. So, I mean, if you sort of run your finger along it, along this sandpaper, you know, it's not like your finger is sort of getting stuck as you're going. It's like you can run your finger across it. It's just you can also feel that it's not perfectly smooth. Yes. You would say, wow, this is not smooth, for sure.

Right. Because the other one I've heard is that is about the fingerprint. Have you heard this one? What's this one? That if you shrunk the earth down to the size of a pool ball and a giant held it in their hand, then the craters and peaks of their fingerprint would be greater. I mean, I was making some quite strong assumptions about the biological surfaces of this giant and the fingertips of this giant.

but that the craters in your fingerprint are greater than you see on Earth. Yes, this is the thing. These are the numbers that I think you might have wanted, that human fingers can feel objects as small as 13 nanometers. Really? We are incredibly sensitive to the vibrations caused by touching an object like that, which means that if your finger was the size of the earth, you could touch the earth and feel the difference between a house and a car.

No. Our sense of touch is a miracle. It blows your mind. Wow. Yeah. That's incredible. I'm just, sorry, I'm just feeling these scratches on my table just to see, like, the, you're right, you know. They're like really tiny little scratches.

You can actually feel them. Yeah. And we might not be able to count the scratches, but we can tell between, two different surfaces how they feel and that one's different than the other because of sub-microscopic texture differences. So a house and a car on the earth to a giant whose finger was as big as the planet, it would feel different. They'd be like, oh, that's a parking lot.

Oh, there's no buildings here. They would be able to tell. Oh, Buckingham Palace. This whole Earth is smoother than a pool ball nonsense. Come on. Let's grow up. Earth is bumpy. It's bumpiness deserves some credit.

Stop doing that internet. Okay, well, we've got some bumpiness for you in the second half of this, because boy, have I got an object for you. We'll be back right after this break. This episode is brought to you by Cancer Research UK. We often think of beating cancer as treatment, but imagine stopping it before it begins. After years of work, Cancer Research UK scientists

are launching a clinical trial of lung vac. the first vaccine designed to prevent lung cancer. It builds on Tracer X, the world's largest cancer evolution study, which tracked lung cancer cells over many years to uncover the disease's earliest warning signs. Lung Vax is designed to train the immune system to spot these signs early on, destroying faulty cells before cancer develops. So it's not treatment, but preventative, with the potential to stop lung cancer before it starts. The first stage of the trial starts this year, focusing on people at higher risk.

It shows what long-term research makes possible. For more information about Cancer Research UK, their research breakthroughs and how you can support them, visit cancerresearchukuk.org forward slash the rest is science. And we're back. Now, Michael, can you describe what I am holding in my hand? It looks like a short twig. Yes, it does. A very bumpy stick-colored stick.

I've got another one here. Oh, and now you've just pulled up a shorter one. It looks like petrified wood, like a petrified twig, because it's sort of grayish-brown and rough. It looks very organic. Again, it's only about as long as a pinky finger, slightly bumpy and gray. and brown. It might be hollow. You are right. It is. So I deliberately showed you this side because my God, my nails are so bad. We could not put out on. I was going to say, Hannah, I cannot believe those nails. You know, I know you're being sarcastic, but you would not believe the amount of

criticism that will come my way. I'm saying it so that they don't. I'm stealing their thunder. Nails more like snails. I don't know. Does that make sense? So right, you are absolutely right that it's like, it's about the length of a pinky finger. It's very nobly. It's brown. It doesn't look very interesting at all.

You would walk past this lying on the ground and not notice it at all. But if I turn it round, this might give you a slightly better clue because on the other side, you can see that it's hollow and inside it has this glassy texture. Oh, like a geode almost. Almost, almost. And it feels, it feels like glass. Oh, yeah. I can hear you banging it on your table

and it sounds like a piece of fine porcelain. It does indeed. Okay, here is my big reveal. What I am holding in my hand is fossilized lightning. Oh, wow. Isn't that cool? So what happens is that lightning, often, you know, millions of years ago,

but you can, I mean, all basically throughout the entire history of the air, lightning hits sand. Can you see that one? This one's a bit clearer, I think. And it turns it to glass. And it turns it to glass, yes, because what happens is, I mean, if you imagine having a bucket of sand and, like, shining a mega powerful laser in there, then any sand that's touching the laser will just, like, instantly melt into goo.

The goo then sort of, like, cools down into this hollow tube of glass. But the very middle of it doesn't just melt. It actually vaporizes, where the, where the lightning hits. the center of this, it will actually vaporize. So you have this this hollow tube, this hollow glass tube. And then what happens is that the sort of gas from the vaporized sand expands outwards creating this, this, this basically this glass straw, right? Isn't that cool? Yeah, okay, right. So it's so hot in the middle, it turns not into a liquid, into a goo, but into a gas. Yeah. Glass gas, exactly. And what is amazing about,

these things. Okay, so Darwin in particular, he was obsessed by these. They're called Folgerites, by the way. They're really cheap. I got these on eBay. They're like nine, 10 quid each. I mean, there's like, there's loads of them. They're not like, you know, you're not going to find this on a walk in Epping forest, you know? Like, you've got to go to the right part of the world to see a lot of these. But like across the world, there's a lot of them because there is a lot of lightning that's happening at any moment in time. Yeah. In fact, actually, there is a really brilliant website, which is called blitz-or-tung.org, where you can see it's a live map of where lightning is striking across the world. It comes with sounds as well. So it plays a sound for every

lightning strike that occurs on Earth, in real time? In real time, exactly. And there's way more lightning going on than you would imagine. I mean, right now, there's a little pocket going on in southern Europe, loads across Australia, a big band. Essentially by the equator is where you get lots and lots of it, almost very rarely get lightning at the poles. Yeah. And one of the theories about that, by the way, is that it's cosmic rays that give the sort of potential to,

in order to sort of trigger off a lightning strike where you get some potential difference in clouds. Oh, no kidding. Cosmic rays once again. Once again, those guys. Particles of mini-hats. Yeah. So they like seed the process required. for lightning to happen.

So I think it's the other way around. So there's this, the sort of main theory is about how within clouds you have ice and you have sort of like sloppy hail, right, like soft hail that interact with each other, that crash into each other because of the turbulence in clouds. And then they end up separating. They have different charges, but they end up separating because one's lighter than the other. So that's how you get the sort of the potential difference between different layers of the cloud. But there is this idea, this theory that cosmic rays then act as like the seed for that

lightning to start, which I really like. Wow. All right. So why did you pick these up? Just as decoration, did you use them to teach? Hey, it's a glass straw, Michael. What's what's not to like? No, I read about how much Darwin liked them. And I just wanted to see how easy it was to get hold of. because they are really amazing that you would have something because essentially they're so rough and noblier on the outside because that's where the sand is, right? The sand sucked to it. But on the inside, I mean, it's a shame that I can't give this to you in person,

but they are so smooth and glassy on the inside. It's like it's really, it sort of feels otherworldly. It sort of feels like this is a freak moment that has created this. Yeah, it was a freak moment. It's natural glass. It's accidental glass. glass. Have you ever used one as a straw? No, I was joking. I should do that. I mean, it's not going to hurt you. But you should, right? If they're only nine quid each. Yeah,

exactly. I mean, how brittle are they? Could you carry it around as a reusable straw? Should I try and snap it? Yes. I reckon you good, shall I? For the purposes. For the purposes of this video, it's worth it. Ready? Okay, I'm going to try to stuff it. Whoa, that was easy. Yeah. That was brittle. That was. That was. That was. That was. That was now, now I've just either half the price or doubled it. Yeah, you may have doubled it. The thing that's nice about these is that because they've been being created across the whole history of the earth, what they do is that as that glass sort of melts and then hardens, it traps these little air bubbles inside it. So there's all the way down here, there'll be all these

little air bubbles. And that's a sort of like a taste, as it, as it were, of the, of the the air at that moment in time whenever the fulgarite happened. So what scientists do is they take these, they work out how odd they are, and they allow them to sort of see what the atmosphere was like in the Sahara Desert, for example, you know, 15,000 years ago, see what kind of plants were there, see what kind of carbon isotopes were in the air. You really sort of get this way to look backwards in time using these little things. It's cool, isn't it?

So by breaking that one, you just probably, you may have released a few molecules of prehistoric air. I think this one's quite a new one, to be honest. Smells, smells modern. Smells Victorian, maybe. Victorian, exactly. You know, there is this, Folgorite has this quite terrifying cousin, which is called Trinotite. Yes, I was going to say, that is not naturally formed glass. It's glass that humans made.

by blowing up and testing nuclear weapons. Exactly. So when the first atomic bomb was detonated in New Mexico, the Trinity test, the heat from that melted the desert sand. And it turned it green as well. It's this radioactive glass. That one I did not buy on eBay. That's harder to get because it doesn't just happen every time there's a lightning storm.

Or not every time. But you have to test a nuclear weapon around sand. And then you've got some transatlantic. which of course is named after the Trinity site where the first atomic full-scale testing occurred. But I think it's probably called Trinotite no matter where it forms now. I think it probably is. Yeah, I think it probably is. You've got history of buying and buying radioactive objects just for your own interest. I assume you remember you're telling us about some radioactive lead at one point.

Yeah, yeah. Well, you can get radioactive lead isotopes just mail order. you could probably get them off of Amazon today. My mom got me some radioactive lead for my bubble chamber when I was a kid. And it was at the tip of a needle inside a little test tube, and she made me keep it in the garage. But we also just bought some otinite, which is a uranium-bearing mineral. My colleague, who lives in a different city, acquired a large amount of this mineral. And it came in a lead-lined box with a big warning on.

it that says stay five meters away. So we found a suitable location and there's like warning labels and stuff, but it's still not really concentrated enough. We want to remove as much of the elements that aren't uranium as possible. Now, we don't quite know how to enrich the uranium. Yeah. But I might make some calls. I would say that's closely guarded state secrets, isn't it? Uranian and enriched. Yeah, I mean, we've looked into how to enrich uranium. And the old centrifuge process. Just, I don't know how to make a good enough centrifuge. And you've got a, they don't really tell you exactly how to do it.

Today, I think they use a lot of lasers. And in the future, I think they'll do nothing. I think that we're developing ways to power nuclear power stations just with uranium ore that doesn't need to be processed and enriched, which is great, which is really great. Well, I think that that's bringing us towards the end of this episode. But you can tune in next week to hear more about Michael's adventures into deeply troubling uranium enrichment. Or find out whether he's been smuggled away by the FBI. I know.

I feel like maybe I shouldn't have said that because I've ruined the surprise, but not for everybody. All right. Well, we'll be back on Tuesday with hopefully another episode as long as Michael remains a free man. So, yeah, catch us then. I will be in a week. I'll see you guys then.