The Rest Is Science - The Most Dangerous Rock In Hannah's Collection

Episode Date: July 1, 2026

When you think of dangerous geological specimens, does your mind jump straight to radioactive uranium, toxic heavy metals, or even asbestos? What if one of the most dangerous items sitting on a shelf is actually a polished, perfectly ordinary-looking piece of black onyx? Professor Hannah Fry and Michael Stevens dive into the surprisingly treacherous geology and chemistry of onyx. Hannah brings an onyx from her personal rock collection to explain exactly why this specific piece of material holds the title of the most dangerous rock she owns, unpicking the fascinating physical properties of the stone and exploring how its conchoidal fracture pattern can easily create edges sharper than a modern surgical scalpel. ------------------- 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 Oakley + Jack MeekAnimator: Sam BensonVideo & Social: Bex TyrrellAssistant Producer: Lucy LipscombeProducer: Simona RataSenior Producer: Lauren Armstrong-CarterHead Of Digital: Samuel OakleyExec Producer: Neil Fearn Learn more about your ad choices. Visit podcastchoices.com/adchoices

Welcome to The Rest of Science. I'm Hannah Frye. And I'm Michael Stevens. And this is an episode of Field Notes, a little podcast expedition journey thing where we dip into some of your questions, a little mailbag. And one of us brings an object every single week. Yeah. So Hannah's brought the object today. And I don't know what it is. And I love anticipation. So we're going to actually start with you all. And then after we answer your questions, Hannah will show me what she's got to show all of us today. do more of what you love? More of the beautiful, meaningful human work? Like reaching bigger goals ahead of schedule, growing careers, and the bottom line. Workday is the AI platform for HR and

finance that actually knows your business, anticipating your toughest challenges and how to better solve them. Connecting more dots in your organization so your people have more time to connect to what they love, because we all know AI is changing work forever. Now let's change it for the better. It's a new workday. Hello, mid-sized business. We see your big ambitions, but how do you achieve the wins you want when your technology is holding you back? SAP Grow is built to grow with your business, no matter its size, with AI embedded at its

core, working across every system, all ready to go from day one so you can hit the ground running. Bring it with SAP Grow. AI Cloud ERP for any size business. This episode is brought to you by Cancer Research. UK. Scientists have found that cancer risks usually increase with age and size, but some species defy the odds. For example, deep sea Greenland sharks. They can grow over six meters long, weigh more than a small car, and yet live for up to 400 years. Now, understanding how Greenland

Sharks cellular repair and immune systems seem to have managed to keep them cancer-free for centuries, that could open up exciting research pathways. Essentially, over the Over millions of years, evolution has been running the world's most successful cancer prevention trial. And sometimes breakthroughs can be found in unusual places. So by exploring the unexpected, cancer research UK scientists are uncovering new ways to tackle over 200 types of cancer. Their work has helped to double survival in the UK over the last 50 years and continues to save and improve lives around the world. For more information about Cancer Research UK, their research and break-th century, and how you can support them, visit cancer researchuk.org slash the rest is science.

I'm going to jump right in with a question that came from Ben. Now, Ben asks, if you were out in space, say, right in the vicinity of a pair of black holes colliding, would you be able to feel the gravitational wave from the collision pass through you? Would it depend on distance and if so, how close would you need to be to feel it? or is it inherently impossible to detect since the space around you is getting deformed the same way you are? Scary question.

I don't like big things because even the solar eclipse really terrifies me. Not so much that I don't want to see it. I want to see every one that I can. And I have ever since I got addicted to them. But in the moments before, like the day of, I start just my heart won't slow down because the scale of it,

the size of the moon and the sun, doing a thing together. I mean, not that they really know. It's just too big. I can't stop it. Sorry, I'm going on a tangent on solar eclipses. But a black hole collision would be the same way. I am so in the mode of control because of our modern world where I can pause a streaming

show, I can resume it later, I can rewind, that I just feel like I get to control everything. I click on the links that I want. I scroll when I want. But what the solar eclipse does not work that way. It's going to happen when it happens. And you know when it's going to happen and there's nothing you can do about it. And that is so alien to our world today that it freaks me out. It's terrifying.

I remember this moment the first time I had that realization of how small and insignificant we are in comparison to the vastness of space, the sort of the fact that we are along for the ride, right, and we are completely insignificant. And I was 14 years old and I was in a science class at school. school and honestly, this existential dread lasted for, I would say, about 10 days. I was like not sleeping at night. I was really, it really got to me. I suddenly felt just so unbelievably insignificant. I'm not sure I've quite got over that, ever, actually. I think it's, I think I was it's sort of a before time and after time, you know, before that realization, happy and free.

And then after, and you're still living in the after. I say congratulations. I think that's an important realization. Now, did you try the antidote, which is to instead focus on how much bigger you are than the quantum world? Then you start to feel really big. Does it just feel like a giant going around crushing things? I think that that would have untethered me from both directions simultaneously. I think I would have been adrift. You're right. It doesn't actually work because then you start realizing that the neutrons don't care about you either. Almost nothing cares about us, Michael. Almost nothing. But you know what, Ben?

I care about you. And you're probably taller than me, just on average. So the smaller world does care about you, kind of. So here you go. Gravitational waves. They are literally a changing in space itself. And they're flowing through us all the time. In order to detect them, we need really big ones.

The first one ever detected was detected by LIGO in September of 2015. And then they announced it in February of 2016. So they took a long time to be like, whoa, are we actually seeing waves in space, gravity waves? And the first thing you would feel if gravity waves went through your body that were powerful enough to be sensed. The first thing that would happen is you would hear them because your ear, your ear drum is your most sensitive vibrational detector. I don't know what it would sound like. because sound, especially just manual manipulation of the eardrum, or is it really manual, spatial, space-time warping of your eardrum would be like pressure waves in air as far as the

mechanism is concerned. But the frequency of a gravity wave can be pretty low, like half a hertz a second. So it's probably going to sound more percussive, more like, boom, boom, boom. And I want to give full credit to a Reddit username Carbon Cubit. I'm not afraid to admit that I'm just researching these things. It's not like off the top of my head. I know everything. But Carbon Cubit did a really great breakdown of that first detected gravitational wave, which came from a black hole binary collision that happened 1.3 billion light years away. That gravity wave was immensely powerful. Basically two black holes, one that was 36 solar masses and another that was 29 solar masses collided to form a 62 solar mass black hole. If you're doing the math at home, three solar

masses are unaccounted for. That mass, according to E equals MC squared, turned into energy. Gravitational wave energy of space expanding and contracting and it's spread out. But gravity waves, just like electromagnetic radiation, gravity waves, they spread out. And the energy, per area goes down by an inverse square. So if you're twice as far away from the collision, the amount of energy you receive is four times less, a quarter of it. However, the amplitude of gravitational waves just goes down linearly. The amplitude is the height of the wave. So what that means is that Earth, 1.3 billion light years away, gets hit by these gravitational waves and Earth itself really did squish and squeeze by about a distance of a dozen protons

in a line because of those gravitational waves. First of all, that shows you how amazing LIGO is, that it can detect something like that. Let's bring that collision closer. Rather than 1.3 billion light years away, let's put it just one light year away. Then what happens? Well, then Earth only expands and contracts by about 20 microns. 20 millionths of a centimeter. Still not much. No. Can you bring it closer still? Bring it closer. Okay, let's bring it as close as the sun. Okay. Let's bring it 93 million miles away.

Just eight, like 8.3 light minutes away. Then Earth would expand and contract as space itself ripples through changing. Earth would expand and contract about one meter, about three feet. And this would happen quickly. Earth expands and contracts by about a meter just because of the moon's presence. That's what the tides are causing these changes. But imagine if instead of taking 12 hours to happen, the frequency was like once every two seconds. That could be bad. Our bodies, I think, would be fine, but the earth would not. We would see geysers erupting, volcanic eruptions, massive, massive tsunamis as the ocean absorbed that energy.

But I think we could, some of us could survive it. And wait, why do you think that the human body, just because we've got enough flex and the scale of us, actually, it's not that big of a deal? I think that over the scale of a human body, I don't think it would be as bad on us. What is a real shame? Just going back to your point about ears, it is a real shame that your ear canals are not perpendicular to one another because then you could be your own individual ligo, right? you could sense the pressure difference in a much more attuned way. Because they're sort of, I mean, I know the S-shaped, but they're sort of on the same axis, aren't they? They're sort of like pointing towards each other. Yeah, I think we are just not massive enough to be as affected by gravitational radiation

as we are by electromagnetic radiation. So the Earth itself might, you know, expand and contract by a meter, but the human body would not. Yeah, I think you're right about this. the ear thing. I think you would detect it by hearing it or feeling some sort of strange sense of balance. Oh, yeah. Your sense of balance would get messed up. That's also a very sensitive organ in your body. I forget what it is, but our ability to detect an incline is actually amazing. I think we can tell the difference between like the second hand, standing

on a floor that's a second hand pointing at nine, so horizontal, versus a second hand that's like one second past the nine. We can tell how. Yeah, we can go, oh, our inner ear is like, hey, I'm different now. That's interesting. You'd probably get dizzy. You'd probably feel nauseous, and you would hear like a, like a wumping. But that would be very small in comparison to at the scale of the earth and the oceans, the volcanic stuff. In terms of your body, though, I think the key thing here would be the frequency of the wave, right? Because if you're talking about a really low frequency, I don't know, black holes orbiting each other, where it's, you know, maybe 0.1 hertz, 0.0.01 hertz.

You know, the waves are actually quite slow. Then I think the body could resist quite a lot. But if you're talking about, I mean, Ben talks about colliding black holes, doesn't he? Oh, yeah, from a collision. The collisions are higher frequency, like a hundred times a second. That could be very disruptive to your blood vessels, to just your makeup. I think you'd have a better chance of hearing it too. I mean, if the gravity wave is just compressing space in a large area,

then your eardrum's just moving with your body, with your nerves, and you might not hear anything. We need tidal forces that are small enough that the ear. ear drums moving differently than the rest of your body. The bones in your ears are being moved differently than each other. And that becomes a very hard to ignore noise. So I guess it really depends. I guess I'd say that there's definitely a way, if you could create any kind of gravitational wave, you could certainly affect the human.

After all, we're made of matter. But even black holes at a distance of our sun, they'd have to collide, create a very high frequency wave. And then, yeah, your body could get pulled. I mean, not pulled apart, but stretched and squeezed to the extent that you weren't, like blood flow would be interrupted. Well, it would be accelerating and decelerating. And that's the thing that would be difficult with it, right? I mean, parts of your body did not enjoy accelerating away from other parts of your body. Yeah. The differences in acceleration would be very uncomfortable. Like, if I just got moved altogether, it'd be like riding in a car.

But if my head is moving in one direction and the rest of my body's being accelerated in another direction, I get uncomfortable. Could I get torn apart? Like, yes, I can imagine and I can describe gravitational waves that would tear you apart. But that would actually be more pleasant than the kind that just bother you for like an extended period of time, cutting off brain to various organs. But the point is, gravitational waves from extremely powerful black hole collisions a billion light years away, we don't even notice them. We have to build extremely sensitive equipment and then analyze the results for months before we go, aha.

Yes. So, 1.3 billion light years away, two black holes collided. And in 200 milliseconds, the same amount of energy was released from them as all the stars in the universe do in the same amount of time. But it was so far away, we barely noticed it. Basically, spaghettification is going to be a problem before the gravitational waves are. You can have to be so close that you're going to be stretched within the social radius. You know what? It's not so much spaghettification as it is needification. You're being needed like dough. I want to leave as an exercise to the listeners. What amount of gravitational wave needing would actually just feel good? What would be a pleasant massage? You know, when you go to an airport and you sit in one of those massage chairs,

what intergalactic objects would we need to collide to create that sensation? All day, every day. So the answer to your question, Ben, is that, yes, we will feel it, and we can feel it goodly or badly. Okay, next question. We have got one from Dan, who asked people have been talking about space junk or trash. Thank you for the American translation there, Dan, appreciate it, in the Earth atmosphere for a while now,

But if there is so much of it, why can't we see it in photos from the ISFs? Great question. And you're right that actually there has been a lot of chat about space junk. There's a couple of reasons for this, right? That, by the way, when we say there's a lot of space junk, there is a lot of space junk. So there is, there's some estimates that put it at 54,000 tracked objects that are bigger than 10 centimeters. 54,000 of these things. And by the way, if you are willing to drop it down to any fragments above a centimeter,

so not just kind of like, 10 centimeters is massive, right? If you imagine like, I don't know, like a mobile phone, you're sort of orbiting the earth and a mobile phone come and wax you in the face, right? So anything bigger than that, anything big than mobile phone is this 54,000 of them floating around the place. But if you go smaller, if you're like anything down to the size of a centimeter and above, It jumps to 1.2 million, which I think is astonishing. That is like, that is litter city up there. It's absolutely phenomenal.

If you're like, okay, we'll go even smaller, anything above a millimeter. So, you know, like a screw, for example, or like just a little bit of shrapnel, then there's around 130 million bits of space junk. This is all stuff that we have just scattered around the place, by the way. This is like, we are solely responsible for this. 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? I think trash was for your benefit, frankly. 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. 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 shrapner was moving at like 28,000 kilometers per hour. And like, that is so fast that a tiny screw could really damage the outer exterior of a vessel, especially that these things are not.

You know, they're not armor-plated. They're not sort of supposed to, they're not kind of designed to like to deal with all of this stuff coming in. Can you even armor-plate yourself against this kind of a collision? 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 is called Eridium-33, and it collided with a defunct Russian satellite. So we're not talking about time, a bit of space trap, and we're talking about something quite big here. But, yeah, was just taken out by that collision. So, I mean, the problem 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 the amount of actual space that you have is vast, right? If you think about the entire surface 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. And so you're sort of talking here about like, I don't know, 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. And typically, even in the sort of busiest lanes, the busiest orbits, the nearest piece of junk will probably be 100 plus kilometers 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 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. 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 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. And that is a collection of both satellites and space junk. I'm not surprised. 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. 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,

where it just happens to catch the sunlight and it shoots it right down at you. And it was this spark of light. 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 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. I mean, like literally from my garden in London on a clear night.

I can see, you know, particular conditions where you have a really clear night and the right time of the evening. 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. 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 clean-up 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. 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, you know, less than a grain of rice. But at these sorts of speeds, this is genuinely something you have to worry about. I'm sure it'll be automated. But in the future, the job of space trash man would be so cool. I like the idea of going out there with a net, you know?

Yeah, with a big net and you get to keep what you find. And it's a hard day's work. Oh, a bit of a pollen. 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 felt worth. No. In 1958, after just three months of orbit, it burned up while reentering Earth's atmosphere. There's probably bits of Sputnik junk up there. There could be Sputnik junk.

Can I tell you a tiny thing about Sputnik? I just think it's 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 like this sound that could be heard on actual radios, right?

Like actual radios could pick up on this. If you were in the right place, the right time, you could hear this ping of Sputnik. going over. And it's just so ominous, the sound. 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. Oh my goodness me. Yeah, it's here, it's there. It's above us in the sky. 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. 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 a month than 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 sort of, 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 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.

In fact, you almost certainly have. Yeah, it's like a breath from Julia 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. This is why listening to this program is so great. Right now, you know, when someone tells you take deep breaths, now you can go, Julia 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 for future people to experience of me. V-sauce. Exactly. Exactly. My grandkids will be like, man, I miss the guy.

It's like he's right here in my heart. Yeah. 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, And hello, I love the show. Ah, 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? 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 going to work my muscles out more, you know? As it turns out, so yeah, 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. 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 spend 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. So it's the same. But of course, biomechanically, our bodies are not little simple machines and faster changes in

muscle motions are less efficient 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... Sosa. 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.

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. Hey, look, that's the difference between probably four peanuts across the course of a week. It's funny that you mentioned peanuts. Is it four peanuts?

I've got peanuts right here. I'm a big peanut head. So 39 peanuts is 170 calories. Okay, I did the math. And that's an 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'm 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? Let's try it. 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. That counts. That's amazing. Does that count? What about? 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? Yeah, I've got mirrors. I've got mirrors in the bathroom, and I've got little glass mirrors for little optics experiments. Okay, you can put in the comments below, by the way, of objects you would like to see if Michael has in his Aladdin's cave. I don't have a plate. You don't have a plate?

Yeah, I eat off of paper towels, basically. I don't have a need for a plate, but I do need a bag of my own beard hair. Yeah, and a UFO that makes a whistling noise. Yeah, okay. And I do need, you know, 20 peanuts. Well, look, you need to give yourself a little treat for all those stairs you climbed. Makes a lot of sense. Yeah, so bottom line is 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 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. 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. That's right. And there's fewer steps because although you're watching, walking up some steps up at the top every, you know, every so often, they're disappearing. There you go. Enjoy, enjoy your free peanuts with that piece of information. Okay. 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? Okay. Ooh. In Toronto, every arrival is a lot of time. a statement, and nothing says it better than this. Cadillac Optic was the number one selling luxury EV in Canada for 2025. Find your rhythm across a seamless 33-inch display and an immersive 19-speaker AKG surround audio system. This city demands agility and optic delivers with precision to make every drive extraordinary. Let's take the Cadillac. Find out more at Cadillac canada.ca. Luxury sales claim based on S&P Global Mobility Canadian New Vehicle Total Registrations for calendar year 2025 for the Cadillac definition

of luxury. One day, you're negotiating with suppliers. The next, you're installing a shelf in the back room. Running a business means moving in many directions all the time. TD's new small business banking accounts are built for how your business moves. It's how we're making banking more human. This spring, denim gets a softer, lighter update. Introducing Old Navy's drapey denim wide leg, a new fit that moves with you. It's everything you want denim to feel like for summer. Easy, breathable, and effortlessly cool. With a fit that creates natural movement and a wide leg that feels modern, not overwhelming. Plus, that signature, wait, for this price, moment.

Old Navy's drapey denim wide leg. All right, Michael, what have you got for me? What answers have you got? Your mind. Hey, dun, tan, tan, dun, dun, da. Confluence will get you everywhere, but it's wrong. Next. 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. Okay, it is not a crystal. It is a rock. Go on. It's sort of a rockette, 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. Oh, I love that. So it's like the sharpest naturally occurring thing. Yeah, which also happens to be basically the sharpest thing.

Okay, tell me more. Okay, the answer is, is a rock. And it's another one of my rock collection. Why are you laughing? Are you like ashamed of your, how many rocks do you have? We're getting, we're getting towards the end of, my rock collection. But I'll be honest with you, I love a good rock. 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. Okay, 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. Okay? This is, this is like a very good rock. That'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. Yes, that's a good description of it. It's very heavy as well. It's like super, super heavy. 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. 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 a thing, right? So Obsidian, it's a naturally occurring glass.

Okay, so 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. Okay. 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.

Right. 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? Yes, I was thinking, where does pumice fit into this? Because pumice is the only other kind of lava-based rock I can name. 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 rhyelitic 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 if it comes out and it's really frothy, all of that water is like bubbling inside 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 end of a volcano eruption sort of wants lots of, 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. That's when you end up getting this stuff, because you just don't have any of the bubbles. 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 It floats even. 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 that pumice ends up having more of a mineral structure. Right. I think, I think it has a little bit more time. Look, actual geologists can correct me in this. I'm an amateur. I'm a sort of a 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 you get when you normally break rocks. And so instead, if you smash it, I mean, this has been like beautifully polished, but if you smash it, it ends up breaking concordially, 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 clam shells, 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. 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. 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 scalples that are made manufactured to perfection by sort of human. and 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, 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 microscopic bread knife. 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? 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, the brittleness is going to become a factor. Yeah. But there are, I mean, they're much finer. You get much cleaner incisions. Wounds here. feel faster as a result because you haven't got this sort of like jaggedy edge that's between all the

tissue. The thing is that actually because they're quite brittle and they can chip, there's also this whole thing about diamond scalpals 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 being this glass thing. 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 of with glass is it's this amorphous solid. So it's essentially got the molecular configuration of a liquid, but it's sort of 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.

And so obsidian is like permanently, imperceptibly, but it is permanently sliding down this scale into just becoming an ordinary stone. Whoa. It will only exist for a short period of time. How short of a period of time? Not that short. Like, we're still talking probably millions.

Yeah, but compared to a normal rock. Yeah. I mean, basically, there is no obsidian at all anywhere on the planet there's older than 20 million years old. So this is the thing, right? The dinosaurs definitely had obsidian. There was all over the place, but we don't have it. Whoa.

So there must be transitional obsidian where it's like not, not pure anymore. It's becoming a crystallized mineral. 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 it's, it's literally dying obsidian in your hand. Okay, so if you buy a piece of obsidian, you're really pre-ordering some quartz and feldspar. You are. It's got a long lead time. But put it on

your shelf. It's worth the weight. It's worth the weight. It's worth the weight. 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. So one thing that's worth saying about this is that every volcano has got this unique chemical fingerprint. So there are like trace elements that are trapped in the glass that happen at the moment of eruption. 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. 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? 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. And it actually came from a volcano that was 95 kilometers away. Wow. 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 routes. There's also 13,000 years ago. So I'm sort of skipping quite a forward a lot. time here, but there is some obsidian 13,000 years old, sorry, 13,000 BC, so what's that? 15,000 years ago. Yeah.

My bad. We can talk again about how the calendar is nonsense. Okay, 15,000 years ago, this is, there's a cave in Greece where some obsidian from a different island turns up in this cave, okay, 15,000 years old. And this is the earliest evidence that we have that humans were seafaring. So this island is like there's a hundred kilometers 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. them. 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. 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 the obsidian, in part because

they didn't have steel, right? They, you know, while all of Europe was, you know, making steel blades and armor plating and, and, you know, chain mail and all of that, the Aztecs, they, 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 Makuhutil. 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. 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.

So if you are, you know, under the age of 25 or of, feeble nature than 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, you know, ready to invade, ready to take down the locals, the Aztecs could decapitate a horse with just one swipe of this obsidian marked bat. Wow, because it's just so sharp. It doesn't take much force to just go all the way through the horse neck. 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. 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.

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. 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. How are you feeling, by the way? I'm feeling fine. I think if there are any consequences, I'll notice them, you know, decades from now. 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 Aztecs were really big on, and it sort of, it kind of explains why this one is, 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, 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 mirrors going back 8,000 years. 8,000 years. By the way, manufactured glass is probably only about 4,000 years old. So, you know, 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?

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 now in the British Museum. Mm-hmm. This Elizabethan occultist, John D, 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 ball.

Oh, snap, you've got a crystal ball. Look at that. So Michael has brought up his own crystal ball. his 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 it. 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, Michael. I'm going to get this right. You can do this. What a cool comparison. We've got Earth made, not the ball, the humans polished yours into a sphere, but your material is a natural material obsidian.

Mine is completely human-made and very heavy. Go on, keep going, keep going. Up, 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's 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, we're 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 Restor Science, what are you watching this episode on now? If not a black mirror of our own modern invention, you know? Whoa. Oh, look how you tied that back to the show. A little cheesy endic 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. I think mineraloid sounds a bit like an insult, don't you? Yeah, you mineraloid. 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. 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 2 o'clock in the morning when I can't sleep. That's my habits. Yeah. So talk to us.

Give us some late night reading. Leave a comment and we'll see you next time. Absolutely. Bye-bye. This is an urgent appeal for. from the Disasters Emergency Committee. Two devastating earthquakes have hit Venezuela, resulting in widespread destruction.

Thousands have been killed and injured, and survivors desperately need food, water, shelter and medical care. Go to DEC.org.uk to donate, or text DEC to 701.50 to give 10 pounds. Texts cost your donation plus standard network rate. See website for T's and C's. Thank you.