The Most Dangerous Rock In Hannah's Collection

15:52Hannah Fry:This is all stuff that we have just scattered around the place, by the way. This is like, we are solely responsible for this.

15:59Michael Stevens:Now, no one wants to get hit by a screw traveling at hundreds of meters a second. You do not. So you're talking about a lot of material. However, it's spread over a very large area. And like, I don't know a lot about space junk or what space trash, is that what the Brits call it?

16:15Hannah Fry:I think trash was for your benefit, frankly.

16:17Michael Stevens:Oh, no, we call it space junk. We love the word junk. Anyway, I don't hear about space junk hitting the ISS or even satellites very often for that matter.

16:28Hannah Fry:No. Okay, so a lot of it, the first thing is that the ISS is actually quite low. So a lot of this stuff is like, is kind of around floating above in a different orbit to the ISS. I mean, I do think this is something to worry about in the sense that this shrapnel was moving at like 28 ,000 kilometres per hour. That is so fast that a tiny screw could really damage the outer exterior of a vessel. Especially that these things are not armour plated, they're not designed to deal with all of this stuff coming in.

17:06Michael Stevens:Can you even armour plate yourself against this kind of a collision?

17:10Hannah Fry:Well, I don't know. I mean, there was an example in 2009 where satellites have been genuinely destroyed by this stuff. There was the active communication satellite, it's called Iridium-33, and it collided with a defunct Russian satellite. So we're not talking about a tiny bit of space wrapping, we're talking about something quite big here. But yeah, it was taken out by that collision. So, I mean, the problem is, is that even if you could armour yourself against all the little bits, there are giant honking great big things out there, too, that can potentially get in your way. now i agree with you that um that the amount of actual space that you have is vast right if you think about the entire surface of of the atmosphere of earth right it is it is gigantic even if you're talking about you know 54 000 objects bigger than a mobile phone um and so you're sort of talking here about like i don't know it is it is sort of a grain of sand but in the size of a cathedral You know, and there's many, many, many of those cathedrals next to each other, but they are very spread out over a long, you know, really far distances.

18:18Hannah Fry:And typically, even in the sort of busiest lanes, the busiest orbits, the nearest piece of junk will probably be 100 plus kilometres away. So there is a lot up there, but there is also a lot of space. However, what I will say, this idea that you can't see space junk in images from space is not quite true because there is a Redditor called Responsibility Number 2097 who has stitched together all of these images of the Earth from the Artemis mission and created this video essentially of uh of our planet and what is really intriguing about this is that in each individual image there's like a little bit of kind of scattering around of light um but when you watch them in a row you can see just at the edge of the atmosphere you can see these satellites moving around do you see it yeah just on the edges it's really hard to see when you're looking directly at the sort of center of the globe you you can't really see them But just on the edges where they're catching the light from the sun, you can see these little specks floating above the air.

19:31Hannah Fry:And that is a collection of both satellites and space junk. I'm not surprised.

19:36Michael Stevens:I mean, these things reflect sunlight really well. So even though they're tiny, they can send off a lot of light. In fact, I was once walking around on a little night hike with Jake Chudnow, the guy who makes all my music. And we were talking about satellites. And he said, yeah, you can even see them in the sky sometimes. times. We looked up, we all immediately spotted one. It wasn't the ISS. It was a, we even saw like one of those, what do they call them? Like an iridium flash or a, where it just happens to catch the sunlight and it shoots it right down at you. And it was this spark of light.

20:08Michael Stevens:And we were like, oh my goodness, the coincidence. We were just talking about this phenomenon. So we can, yeah, we can see the space junk. We certainly see satellites in astrophotography and the number of satellites up there now i've heard really affects the quality of some images because you do time lapses and you wind up with streaks of all these satellites i have actually spotted them

20:29Hannah Fry:i mean like literally from my garden in london on a clear night i can see uh you know particular particular conditions where you have a really clear night and the right time of the evening um i have seen a starlink train where you sort of get five or six satellites kind of moving all in the line that's very fun to spot um the thing is about space junk is that this is going to be more and more of a problem as time goes on because more satellites are being launched the more risk that you get of debris being thrown off the higher risk of collisions unless you have this really a sort of collaboration a global collaboration to have cleanup or some sort of traffic management thing this is genuinely going to be a problem because actually even a tiny paint flick because it's moving at that sort of speed can end up damaging a spacecraft.

21:19Hannah Fry:There was one example of a paint chip hit a space shuttle window and left a crater in the window, right? And this paint chip weighed less than a grain of rice. But these sorts of speeds, this is genuinely something you have to worry about.

21:34Michael Stevens:I'm sure it'll be automated, but in the future, the job of space trash man would be so cool.

21:41Hannah Fry:I like the idea of going out there with a net, you know?

21:44Michael Stevens:Yeah, Yeah, with a big net and you get to keep what you find. And it's a hard day's work.

21:50Hannah Fry:Oh, a bit of Apollo. Yeah. Oh, hey, look. I mean, that'd be worth something, right? Go and grab Sputnik. It's still up there. Is it? I don't know, actually. Maybe it fell to Earth.

22:01Michael Stevens:No. In 1958, after just three months of orbit, it burned up while reentering Earth's atmosphere.

22:09Hannah Fry:There's probably bits of Sputnik junk up there.

22:11Michael Stevens:There could be Sputnik junk.

22:13Hannah Fry:Can I tell you a tiny thing about Sputnik that I just think is absolutely amazing? So Sputnik really freaked everybody out, right? Especially the Americans, because it's this idea that you feel like you've got control over your own airspace. And then there is this foreign object that you know is in the sky above you. And what the Russians did is they deliberately had Sputnik send out this ping, this sound that could be heard on actual radios, right? Actual radios could pick up on this. If you were in the right place at the right time, you could hear this ping of Sputnik going over. And it's just so ominous, the sound.

22:50Michael Stevens:You couldn't hear it with your ears directly. You had to tune in to its frequency. And people all across America, all across the world, were able to go, oh my gosh.

22:59Hannah Fry:Oh my goodness me. Yeah, it's here, it's there, it's above us in the sky.

23:03Michael Stevens:No ground invasion necessary. I'm just here at my dinner table and we're listening to this thing. Is it watching us? No one knew how well it was detecting what was down below.

23:14Hannah Fry:But then what happened was a couple of students, they realized that the ping on the radio, the frequency of it would change depending on which direction or where above the Sputnik was, right? Essentially, they had the Doppler effect. So when a car goes past you and it goes, there's like frequency of that noise changes as it moves across. and it was the same thing was happening with Sputnik's pings across the sky that it would that it would sort of speed up and then slow down as it moved closer and away from you and so these students like literally these people tuning into a radio managed to work out the trajectory of Sputnik and predict exactly where it was going to be and exactly where it was going to be going purely based on like a bit of you know a bit of Pythagoras and a radio it's absolutely

24:06Michael Stevens:phenomenal i love that story no that's really cool what a fun little project fun but kind of scary i mean sputnik was like the first it truly was the first human made eye of sauron it was not an angel or a demon that was watching over you it was a physical thing built by people with names that was watching or at least there but could be watching could be watching there might still be pieces of it up there. But you know what? Because it burned up, there are pieces of it down on earth still, right? There's Sputnik dust. You've probably touched some of it. You may have even breathed some in. You've probably breathed some in.

24:47Michael Stevens:In fact, you almost certainly have. Yeah, it's like a breath from Julius Caesar. It's out there in the air. And so Sputnik used to be watching over us. And now it is in all of us.

24:58Hannah Fry:This is why listening to this program is so great right now. You know, when someone tells you to take deep breaths now you can go julius caesar sputnik dinosaur farts right you know every breath has a little bit of wonder that's why i fart so much i want to leave a lot

25:15Michael Stevens:for future people to experience of me these saucy exactly exactly my grandkids will be like man I miss the guy. It's like he's right here in my heart.

25:31Hannah Fry:Yeah.

25:31Michael Stevens:And in many ways, he actually is. I want to do one more quick question because I love this one. And I actually recently learned about it. So Lawrence wrote in and asked, hello, I love the show. Thanks, Lawrence. And I have a mechanics question that's been bothering me forever as an engineer, but I can't get my brain around it. Let's say I'm leaving the London Underground, but I'm tired and I'm in a hurry. It's a normal day. I'm standing on the escalator, watching people walk up the escalator and people use the stairs. And I wonder, is it less or equal or more effort to walk up an escalator than compared to walking up the stairs?

26:09Michael Stevens:It feels like it's more effort to push against the rising escalator, but surely the escalator is helping. Please help. I love this because I think about this stuff all the time too about, you know, what's, what's going to work my muscles out more, you know, um, as it turns out. So yeah, when you, when you step onto an escalator, your body is accelerated by the escalator, but it's accelerated up to the escalator's speed. And then you're there. Any further steps you take are no different than just walking up regular stairs. If you step on the escalator and fight it, then momentarily you are doing more work, but no, if someone's walking stairs or walking up an escalator, they're already on, same experience.

26:50Michael Stevens:However, let's talk about stairs because it is actually really complicated and still a bit unknown. What's better if you want to burn calories and use up energy, walking upstairs one at a time or two? Yes. So here are the variables in play. First of all, if our bodies were just simple machines, it wouldn't make a difference because work is just your mass times gravity times how high up you go. And if you go slowly one step at a time, well, then, you know, you spent a certain amount of energy. But if you do it twice as fast because you're skipping every other stair, well, you spent more, your rate of energy expenditure was higher, but you spent half the time spending it.

27:35Michael Stevens:So it's the same. But of course, biomechanically, our bodies are not little simple machines and faster changes in muscle motions are less sufficient and therefore burn more calories. So researchers have worked this out and we can put some of the papers below. They have measured respiration and heart rate as a proxy for calories burned. And they found that taking the steps two at a time does burn more calories per minute. However, it's not shorter. Yeah. As it turns out, walking up a flight of stairs one at a time actually burns more calories because of the way all these biomechanical efficiencies work out.

28:20Michael Stevens:If you are going to be walking upstairs for a certain amount of time, like for, I'm going to spend 10 minutes walking upstairs, then you should do them two at a time. But if you're just walking up to your office, you know, you're just walking up, I don't know, two, three flights, then walking, taking them one at a time rather, will burn more calories. And they actually calculated in the paper that if you climb a 15 meter stairwell five times a day, that equals 302 kilocalories per week. If you take them one step at a time, 302, but if you take them two steps at a time, only 266 kilocalories.

28:55Hannah Fry:Hey, look, that's the difference between probably four peanuts across the course of a week.

29:00Michael Stevens:It's funny that you mentioned peanuts. Is it four peanuts? I've got peanuts right here. I'm a big peanut head um so 39 peanuts is 170 calories okay I did the math and that's an

29:13Hannah Fry:additional 8.4 peanuts big time I have a question for you now Michael if I mention anything have you got it in your room is there anything I like sort of feel like I say you know peanuts and suddenly they appear I say bath bag and suddenly it appears I say beard hair and suddenly it appears Is there anything I could say that you don't have?

29:34Michael Stevens:Let's try it. Let's just, I get one shot. I want you to name an object, a thing, a type of thing, and I'm going to see if I've got it. A whistle. I've got one in my mouth. Does that count? I honestly don't think I have a whistle. Hold on. Okay. So I cannot immediately imagine a whistle. I think this gyroscope whistles, but I don't have the cord that spins it, but it's got little holes along it. so that it can whistle. It's obviously supposed to sound like a UFO.

30:06Hannah Fry:That counts. That's amazing.

30:07Michael Stevens:Does that count?

30:08Hannah Fry:What about, I was trying to think of ordinary objects that most people would have that you wouldn't have. Do you have a mirror?

30:17Michael Stevens:Yeah, I've got mirrors. I've got mirrors in the bathroom and I've got little glass mirrors for little optics experiments.

30:23Hannah Fry:Okay. You can put in the comments below, by the way, of objects you would like to see if uh if michael has in his uh in his in his aladdin's cave i don't have a plate you don't

30:35Michael Stevens:have a plate yeah i don't i i eat off of off of paper towels basically because i don't have a need for a plate but i do need a bag of my own beard hair yeah and and a ufo that makes a

30:51Hannah Fry:whistling noise. Yeah. Okay.

30:53Michael Stevens:And I do need, you know, 20 peanuts.

30:58Hannah Fry:Well, look, you need to give yourself a little treat for all those stairs you climbed. Makes a lot of sense.

31:04Michael Stevens:Yeah. So bottom line is, uh, walking up the stairs per flight is going to burn you more calories because even though it does take a bit longer, but also the quicker movements or the, there's more movements required overall because you've got twice as many steps. So the math works out such that you do burn more calories per flight of stairs by taking them one at a time.

31:27Hannah Fry:I guess going back to the question then, taking the escalator and walking up them also takes less calories because there's less time on the escalator. There's less steps effectively.

31:37Michael Stevens:That's right. And there's fewer steps because although you're walking up some steps up at the top every, you know, every so often they're disappearing.

31:45Hannah Fry:There you go. Enjoy your free peanuts with that piece of information. OK, we are going to go for a little break, I think. And when we come back, Michael, I want to ask you, what is the sharpest object? I'm going to say in the universe. What is the sharpest object in the universe? OK.

32:05Michael Stevens:Ooh. Ooh.

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33:21Hannah Fry:All right, Michael, what have you got for me? What answers have you got?

33:24Michael Stevens:Your mind.

33:25Hannah Fry:Hey, dun-dun-dun-dun-dun. Confluence will get you everywhere, but it's wrong. Next.

33:30Michael Stevens:That's not what you're looking for. I mean, I don't know. I've always wanted to know more about sharpness, so I'm glad we're going to talk about it today. I'm assuming it's going to have to be something that is made of a very strong crystal that comes to an edge that's like one atom wide.

33:48Hannah Fry:Okay, it is not a crystal. It is a rock.

33:51Michael Stevens:Go on.

33:52Hannah Fry:It's sort of a rocket, if you like. And you're right that it comes to a few atoms wide. So technically, you can get things artificially made that are one atom wide, but they're not very good for cutting. So I want something that's like, hardcore good at cutting. Is this thing that you have a natural thing? It is a natural thing.

34:13Michael Stevens:Oh, I love that. So it's like the sharpest naturally occurring thing.

34:18Hannah Fry:yeah which also happens to be basically the sharpest thing okay tell me more okay the answer is is it is a rock and um it's another one of my rock collection why are you laughing this

34:32Michael Stevens:are you like ashamed of your how how many rocks do you have we're getting we're getting towards

34:38Hannah Fry:the end of my rock collection but i'll be honest with you i i love a good rock okay i love a good rock. I'm going to show you the rock because it's polished. Look. Whoa. Look at this. Isn't it pretty? It's like a little orb. It's very pretty. What is it? What I'm holding is a very heavy black orb. It's been perfectly polished into a really glossy sphere. And this, my friend, is obsidian. Oh, OK. I'm going to tell you a little bit about obsidian. But before we're done, I'm going to tell you how obsidian, this little black rock, can decapitate horses, has been crossed an entire continent to be buried and has helped people talk to angels.

35:18Hannah Fry:OK, this is this is like a very good rock.

35:21Michael Stevens:It's a very good rock. By the way, you're holding you're holding a sphere of pure obsidian. That's about it's a little smaller than a magic eight ball.

35:30Hannah Fry:Yes, that's a good description of it. It's very heavy as well. It's like super, super heavy.

35:35Michael Stevens:I can see the lights from your ceiling reflected in it, but otherwise it very much is just like a black hole. It's kind of glossy, actually.

35:43Hannah Fry:Yeah, it has a little stand. So it sort of looks a little bit like it belongs in the fortune tellers. It kind of looks like it would belong on a fortune tellers table. It's very mysterious. Okay, so here's the thing, right? So obsidian, it's a naturally occurring glass. Okay, so way, way, way, way, way before glass was actually manufactured, this stuff was around and was on the planet. And it's made, it comes out of a volcano and it gets made when lava spurts out and it cools so fast that all of the atoms are really disordered. And so it kind of freezes sort of mid panic. Right. Right. So this is how you end up with glass, essentially, is that it's like sort of a liquid state, but then frozen into solid.

36:33Hannah Fry:OK, so it hasn't had the chance to crystallize. Right. Because what would normally happen if you get, you know, lava spurting out of a volcano, if it manages to cool nice and slowly, then what happens is all of the atoms sort of take time and find this much more crystal-like structure, sort of a mineral. That's what would make it a mineral. So technically, this is not a mineral. Geologists call it a mineraloid, right? So it's the sort of same category as pearl and amber. Because it's not, hasn't got the same structure. Anyway, so this, by the way, is the exact same stuff as pumice. You know, pumice stone?

37:11Michael Stevens:Yes, I was thinking, where does pumice fit into this? Because pumice is the only other kind of lava-based rock I can name.

37:18Hannah Fry:Okay, so pumice and obsidian, exactly the same stuff. The only difference is the water content. So as it comes out of the volcano, so normally pumice comes out first, right? They're both made from rhyolitic magma, so exactly the same really high silica content. and what happens right at the very beginning of a volcano eruption there's like lots of water that is you know in gas gaseous state and so um if it comes out and it's really frothy all of that water is like bubbling inside of the of the of the lava and then it cools you end up getting pumice and so all of those like air bubbles effectively give it that sort of porous shape but if the kind of at the end of a volcano eruption, sort of once lots of the water has disappeared, if you have a really low water content and it cools really quickly, so usually at the edge of the lava flow, that is when you get this volcanic glass.

38:12Hannah Fry:That's when you end up getting this stuff because you just don't have any of the bubbles.

38:15Michael Stevens:So pumice and obsidian have the same like atomic or even molecular makeup. However, pumice has had so much water content in it at its formation that it's like not dense at all it floats even um does it if the does the pumice cool really quickly to the point that it's also like made of obsidian that's just like spongy i don't think so i think

38:41Hannah Fry:that pumice ends up having more of a mineral structure right i think i think i think it has a little bit more time look actual geologists can correct me in this i'm an i'm an amateur i'm a i'm sort of bystanding and like an extremely excitable bystander in all of this geology stuff. But the thing about this glass, okay, the reason why it ends up making the sharpest object, I'm going for it, in the universe, is that when it breaks, because it hasn't got this crystal structure, it means that there doesn't have this sort of preferential kind of cleavage break that that you get when you normally break rocks.

39:20Hannah Fry:And so instead, if you smash it, I mean, this has been like beautifully polished, but if you smash it, it ends up breaking concoitally, right, in these really smooth curved shells, but like normal glass, right? If you sort of take, you know, human manufactured glass, you get these sort of, they literally look like clamshells, right? Like these sort of these shells that come out. And that is when you get these incredibly, incredibly thin edges that can be like a couple of molecules thick, right? Like nanometers, unbelievably tiny. And this is the thing about it, right? These edges that you can get with obsidian are so tiny that they are thinner than the wavelength of visible light, which means that this cutting edge is literally too small for light to be able to see.

40:14Hannah Fry:You can't even see it. It doesn't matter how big your microscope is, you cannot even see it. For contrast, by the way, like a steel scalpel, even the best steel scalpel that are made, manufactured to perfection by sort of human design, if you magnify those, what you see is this like serrated ridge of metal grains. I mean, they're like boulders in comparison to this stuff. It's like ultimately a surgeon's knife, once you really get down to it, it's like a bread knife. It's sort of getting hacked apart by a microscopic bread knife.

40:50Michael Stevens:That comparison is really helpful. That's very, yeah. So a surgical scalpel is just like a cratered sand dune compared to an obsidian blade. Why don't we use obsidian blades in surgeries.

41:05Hannah Fry:But we do. So this is a new thing that is happening. Actually, lots of surgeons, modern heart surgeons in particular, are really pro having obsidian blades. The only thing is, it's very brittle. So there is a risk of it breaking. I was going to say, yeah,

41:19Michael Stevens:the brittleness is going to become a factor.

41:22Hannah Fry:Yeah. But there are, I mean, they're much finer, you get much cleaner incisions, wounds heal faster as a result because you haven't got this sort of like jaggedy edge that's between all the tissue. The thing is, is that actually, because they're quite brittle and they can chip, there's also this whole thing about diamond scalpels, because diamonds, you can manufacture it to get it incredibly clean, incredibly smooth, to get it down to sort of a few molecules thick as well. But yeah, this idea of us going back to these naturally occurring rocks to get really, really incredibly sharp stuff. I just want to go back to this whole thing about it being a, about it not being a crystal, about it being this glass thing.

42:06Hannah Fry:Because the thing is, is that obsidian is also the youngest rock on earth. And the reason for that is that every bit of obsidian that exists is sort of rotting from the inside. The thing with glass is it's this amorphous solid. So it's got, it's essentially got the molecular configuration of a liquid but it's been frozen into a solid state and that is not stable so what happens over time is that all of these atoms that are trapped in this state that they don't want to be in they very very very slowly start to crystallize um and so obsidian is like permanently imperceptibly but it is permanently sliding down this scale into just becoming an ordinary stone Whoa.

42:54Hannah Fry:It will only exist for a short period of time.

42:57Michael Stevens:How short of a period of time?

43:00Hannah Fry:Not that short. We're still talking probably millions.

43:04Michael Stevens:Yeah. But compared to a normal rock.

43:08Hannah Fry:Yeah. I mean, basically, there is no obsidian at all anywhere on the planet that is older than 20 million years old. So this is the thing, right? The dinosaurs definitely had obsidian. It was all over the place, but we don't have it. Whoa.

43:22Michael Stevens:So there must be transitional obsidian where it's like not pure anymore. It's becoming a crystallized mineral.

43:30Hannah Fry:There is. Have you seen it? It's called snowflake obsidian. Wait, wait. I think I have seen this. It's got like snowflakes in it. Yes. And those are the crystals. Those are the crystals. That is the little white florets are these little crystals. But it's basically the obsidian kind of surrendering. It's the obsidian dying in slow motion. And this is it. We sell it as jewelry because it's beautiful, but it's literally dying obsidian in your hand.

43:59Michael Stevens:Okay. So if you buy a piece of obsidian, you're really pre-ordering some quartz and feldspar.

44:06Hannah Fry:You are. It's got a long lead time, but put it on your shelf. It's worth the wait. It's worth the wait. It's worth the wait. Anyway. Okay. Let me tell you a tiny bit more about obsidian right because this stuff is so i love this stuff because it is i think miraculous to us now you know surgeons are using it but it has been miraculous through time right all of our ancestors were also obsessed with this extremely strange volcanic glass um so one thing that's worth saying uh about this is that every volcano has got this unique chemical fingerprint so there like trace elements that are trapped in the glass that happen at the moment of eruption.

44:46Hannah Fry:So what you can do is you can take a piece of obsidian and since the 1960s, there's this method of a way to analyze the minerals and work out exactly what volcano it came from and exactly when. Okay. And what this means is that when you find chunks of obsidian on archaeological sites, you know where they came from. So we are going back to your Manuport stuff, my friend.

45:12Michael Stevens:I was going to say, yeah, it sounds like so then we can look at old human, you know, inhabited places and say, oh, where did they get this from? How long did they carry this?

45:23Hannah Fry:Uh-huh. How long do they carry this? How far away did they carry it from? And there are some wild obsidian stories. So, okay, obsidian, it turns out, not only is it the youngest rock and the sharpest thing, it's also the oldest trade that we know of. Because 320 ,000 years ago, right at the beginning, beginning, beginning of our species, in Kenya, there are finely worked obsidian tools that were found at a site. This is sort of between layers of sediment. That's how they managed to age, you know, at what point in history they were laid there. They were found at a site with no obvious obsidian source of its own.

46:07Hannah Fry:And it actually came from a volcano that was 95 kilometers away okay that is way more way further than a hunter-gatherer would travel in you know in a whole year it's not just like have it in your pocket and off you go it's like this is the oldest evidence that we have of exchange between groups right that's the only real explanation of how that could have happened how cool that it must have been trade roots there's also 13 000 years ago so i'm sort of skipping quite a lot in time here but um there is some obsidian 13 000 years old uh sorry 13 000 bc so what's that 15 000 years ago yeah my bad um 15 we can we can talk again about how the calendar is nonsense um okay 15 000 years ago this is uh There's a cave in Greece where some obsidian from a different island turns up in this cave, 15 ,000 years old.

47:10Hannah Fry:And this is the earliest evidence that we have that humans were seafaring. So this island is like, there's 100 kilometres of open sea distance between the two of them. It was almost certainly that they traversed it in these reed boats. This is before farming, by the way, this is before pottery, this is before cities, this is before, I mean, civilization in any real form as we know it. And humans were sailing open waters, carrying this stuff with them.

47:42Michael Stevens:Wow. So obsidian has been cool to us for a very long time, almost just as long as we've been a species, but it also tells that story of our prehistoric past.

47:56Hannah Fry:It really does. And I think the reason why this stuff was so valuable is in part because of its sharpness. There are some stories, the Aztecs, by the way, were particularly obsessed with obsidian, in part because they didn't have steel, right? While all of Europe was making steel blades and and armor plating and chain mail and all of that. The Aztecs, they were a society that was really focused on obsidian. And there are stories from the Spanish conquistadors, is that how you say it? Conquistadors? There you go, thank you. The Spanish conquerors who went over to try and capture you know territory in Mexico and and sort of around Central and South America and the Aztecs had built these Makuhutl I don't know if I'm saying that right look Aztec experts you can let me know in the comments how I'm getting this wrong um but essentially it was like a basically a baseball bat but with blades of obsidian stuck all around the edge oh no And this is a bit grim.

49:04Hannah Fry:So if you are under the age of 25 or of a feeble nature, then closure is for a moment. But what would happen, this terrified the Spanish because the Aztecs, as the Spanish rode across the hill with their horses, ready to invade, ready to take down the locals, the Aztecs could decapitate a horse with just one swipe. of this obsidian marked bat.

49:35Michael Stevens:Wow, because it's just so sharp. It doesn't take much force to just go all the way through the horse neck.

49:41Hannah Fry:Right. I mean, that is grim. And also, of course, these sort of Spanish people were probably quite frightened and there was probably some exaggeration in the reports of it. But yeah, the Aztecs knew how to make, their weapons were obsidian weapons.

49:54Michael Stevens:Can I just say, I love how sharp this stuff is and how well you've explained it. And yet your sample of obsidian that you brought for field notes is the least sharp a piece of obsidian can be. You're like, it gets really sharp. By the way, here's a perfectly polished sphere of it.

50:11Hannah Fry:Look, I could smash it for you, but I think I don't have the relevant health and safety equipment to deal with the sharpness.

50:18Michael Stevens:Yeah, I'm not going to ask you to smash it. Remember when you asked me to lick an undersea nodule and we didn't even know the health consequences? That's not me. That's you.

50:28Hannah Fry:How are you feeling, by the way?

50:30Michael Stevens:I'm feeling fine. I think if there are any consequences, I'll notice them decades from now.

50:36Hannah Fry:Okay, so I have one last thing to say about obsidian, which I think really demonstrates how it connects us to our ancestral past. So the other thing that the Anztecs were really big on, and it sort of, it kind of explains why this one is designed as an orbed in the sort of the style that you would see in a fortune teller's tent. Because actually, what obsidian was also used for a lot uh well it's the earliest form of mirror that we had because it could be polished so perfectly that you can see your own reflection in it we have we have mirrors going back 8 000 years 8 000 years by the way uh manufactured glass is probably only about 4 000 years old so you know way way way way way before that but what people would do the aztecs in particular is that they would use this to connect to the spirit world right so they would they would look into polished obsidian and use it to talk to angels use it to talk to people in the underworld there's a very famous aztec obsidian mirror that is uh that is now in the british museum uh that uh this elizabethan occultist john d um would use it to talk to angels let me show you something real fast if you've got some obsidian michael i've got a crystal a crystal ball look at that so michael has has brought up his own crystal ball his is is manufactured glass rather than natural glass and is perfectly see-through but what is quite fun about your one because you can see through it you can see how your image has reversed through the other side go and put your head up really close to i want to see your head upside down there you are there you are down slightly oh gosh hold on hold on let me just come on my god you can do this you can do what a what a what a cool comparison we've got

52:29Michael Stevens:earth made uh not the ball the humans polished yours into a sphere but your material is a natural material obsidian mine is uh completely human made and very heavy go on keep going keep going

52:44Hannah Fry:up, up, up, up, up. Yeah. Isn't that amazing? That's good. I enjoyed that. I enjoyed that deeply. So there you go. That was my object for this week. You've got one of my favorite rocks, sort of perishable, the oldest thing we ever carried on purpose that we know of, the sharpest edge, sort of a mirror that we've gazed into for 8 ,000 years. That's really cool, Hannah. And here's what I like about this. Okay. So we know that our ancestors were obsessed with this stuff. We know that our ancestors would stare into a black mirror to see images that were beyond the world they existed in. Just like to ask you, listeners of Restless Science, what are you watching this episode on now, if not a black mirror of our own modern invention, you know?

53:29Michael Stevens:Whoa, look how you tied that back to the show. A little cheesy ending for you though. Hannah, thank you for telling me about obsidian today. I didn't know any of this stuff. I just thought it was like a neat rock, probably a mineral. I was one of those mineral believers. And now I know it's a mineraloid.

53:46Hannah Fry:I think mineraloid sounds a bit like an insult, don't you? Yeah, you mineraloid.

53:51Michael Stevens:Yeah, you didn't quite make it to be a mineral. And yet, when it comes to rocks, that's a pretty cool property to have. That is a pretty cool property to have.

53:58Hannah Fry:All right. Well, that's it for this week. As ever, you can send us your questions. The rest is science at goalhanger.com. or leave a comment under this video on Spotify or YouTube or wherever you're getting this. We actually, frankly, read way too many of them, normally at two o 'clock in the morning when I can't sleep. That's my habits.

54:18Michael Stevens:Yeah. So talk to us, give us some late night reading, leave a comment, and we'll see you next time. Absolutely. Bye-bye.

55:02Hannah Fry:Transcription by CastingWords Cross your donation plus standard network rate. See website for Ts and Cs. Thank you.