Daniel and Kelly’s Extraordinary Universe - Listener Questions #18

Episode Date: September 30, 2025

Daniel and Kelly answer questions about gravtational slingshots, megafauna and microscopic black holes!See omnystudio.com/listener for privacy information.

This is an I-Heart podcast. Hi there, this is Josh Clark from the Stuff You Should Know podcast. If you've been thinking, man alive, I could go for some good true crime podcast episodes, then have we got good news for you. Stuff You Should Know just released a playlist of 12 of our best true crime episodes of all time. There's a shootout in broad daylight, people using axes in really terrible ways, disappearances, legendary heists, the whole nine yards. So check out the Stuff You Should Know true crime playlist.

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My name is Nicole Garcia, and on Burn Sage, Burn Bridges, we aim to explore that culture. Somewhere along the way, it turned into this full-fledged award-winning comic shop. That's Dr. Lee Francis IV, who opened the first Native comic bookshop. Explore his story along with many other Native stories on the show, Burn Sage Burn Bridges. Listen to Burn Sage Burn Bridges on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. It's important that we just reassure people that they're not alone, and there is help out there. The Good Stuff podcast, season two, takes a deep look into One Tribe Foundation, a non-profit fighting suicide in the veteran community. September is National Suicide Prevention Month, so join host Jacob and Ashley Schick as they bring you to the front lines of One Tribe's mission.

One Tribe, save my life twice. Welcome to Season 2 of The Good Stuff. Good Stuff Podcast on the iHeartRadio app, Apple Podcasts, or wherever you get your podcast. I'd like us to quickly explore deep into space. Can gravity slingshots help us win that race? If ancient humans killed off the giant kangaroo, why didn't we kill off the African elephant, too? How much do tiny black holes like to eat? If I adopt one, will it nap at my feet? Whatever questions keep you up at night, Daniel and Kelly's answers will make it right.

Welcome to Daniel and Kelly's extraordinarily curious universe. Hello, I'm Kelly Wainer-Smith. I study parasites and space, and I am excited to talk about charismatic megafauna today. Hi, I'm Daniel. I'm a particle physicist, and I never wanted a woolly mammoth as a pet. Oh, yeah, no, I didn't either. I don't think I want anything bigger than the goats that we have right now. What about a goat-sized black hole? Or do goats basically operate as black holes because they eat everything? You know, our goats are surprisingly picky about what they eat, which I'm finding a little frustrating because I was like, oh, we're going to get walking garbage disposals. But ours are like, no, I don't want that. I don't want that either. So your goats are not the goat?

I wouldn't say that. They are definitely the greatest of all time. And one of them is named greatest of all time, aka Kevin for short. That's the full name that's on its certificate because we let children name our goats. So one of them is the goat. Literally. Literally. All right.

But I had a particularly negative experience with wildlife this morning. I just walking up the hill accidentally smacked a wasp of some sort and my thumb is expanding. Uh-oh. And we'll see how big it is. the end of the episode, but I wanted to focus on more positive interactions, since we're going to be talking about, like, Africa's charismatic wildlife, I wanted to know what is the most, like, awe-inspiring experience you've had with an animal in nature? Because you and Katrina hike. You're, like, outdoor people. I think one of the most incredible experiences I've had was

seeing a mountain lion close up. I grew up in the mountains of northern New Mexico, and they were mountain lines because they were mountains. And, you know, sometimes you'd get up in the morning and you'd see their pop prints in the snow. But one time I actually did spot one. And it's incredible how much they are like house cats, you know, in their smoothness and their feline charisma, but also how terrifyingly huge they are. Because I've often wondered, like, what would my house cat do if it weighed a thousand pounds? And now I know the answer is it would eat me. I mean, most of the time, Mountain Lions don't go after people.

I want to clear the Mountain Lions name. And if anybody wants to read more about that, they should check out Mark Elbrock's book on Mountain Lions. But yeah, so that is beautiful. So, like, did you see it at a distance? Where, like, how far away was it? It was, like, maybe 80 feet or something. My parents' backyard was right up against National Forest.

And so there was just, like, a lot of trees and canyons back there. And I saw one walking. to the trees and I saw it and it saw me and we like share it a little moment and I was like I'm gonna tiptoe back inside you can have the whole outdoors right this is all yours now buddy and I assume that it did it did not approach you or anything like that no no it did not and I'm very happy that it went on its way but it reminds me of this incredible story I heard about European lions you know lions used to roam Europe also but they were killed off many many years ago. And the story I heard is that for a long time there was a question about European

lions. Do they have mains like African lions or not like North American lions? And we answered this question through cave drawings. People found drawings of lions from tens of thousands of years ago from Paleolithic artists. And it's basically like a lab book and they don't have mains. And you can see like hunting pairs even. And so that's incredible to me that like humans tens of thousands of years ago, answered our current science question. Okay, so that is amazing. But how does the absence of seeing mains, like, so, you know, in Africa, when you get groups that are hunting, it's usually groups of females that are hunting.

The females do most of the hunting. So how do you know that, you know, the worthless males just weren't drawn on the insides of the caves? Yeah, great question. I shouldn't have said hunting pair. I think they had some reason to believe this is like a family unit. Okay. But it sort of connects us over space and time to, you know,

early humans who were also awed by the majesty of megafauna. Yes, absolutely. Very cool. And when I asked you that question, I assumed you we're going to pick megafauna because almost everyone picks, you know, some large mammal. The largest mammal they've ever seen is usually the answer to that question. Oh, I see. You've asked other people this question and nobody has said the time I saw a rat in my kitchen.

Well, of course not. No, no one has said that. But I think my moment was I was in the woods and I was helping a friend with the snake survey. and a bunch of wood frogs had recently, like, metamorphosed from tadpoles into, like, adults, but they were, you know, into the land stage, but they were really tiny, and there were literally hundreds of them. And I noticed them because I, like, moved my foot forward, and all of these tiny things hopped out of the way. Oh, my gosh. And I realized, like, oh, my gosh, I'm surrounded by, like, hundreds of little froglets, these little tiny frogs. And I just sat down, and, like, the sun was coming through the trees in just the right way that it kind of had, like, a magic.

feeling. And every time I'd sort of move my hand, all of these little froglets would like hop around. And it was just like, I don't know, it was magic. It was like being in some sort of a fairy book or something. And I think for me, that was my moment. Well, I have a moment I recall from being in your neck of the woods. It was more of a horror story than a magical moment. Katrina and I were hiking and backpacking. I think it was in the Blue Ridge Mountains. And we set up our camp and we had a nice campfire. And at the end of the evening, we were ready to go back to our tent. And it was only then that we noticed that the ground just outside the extent of the fire was covered in daddy long legs. They were like millions and millions of them. And they were

crawling all over our tent and everywhere. It was incredible. We were so grateful that we had zipped our tent closed. So we like sprinted back to the tent, open it, jumped in, closed it, and was just like terrified of the carpet of daddy long legs that were outside. I was like, wow, Virginia's crazy. But they can't hurt you. They can't hurt you, but I'm still not going to, like, lie down and have daddy long legs crawl all over me. All right. Well, I don't, I don't, I feel like I'm not, uh, not really feeling the fear that you. All right, Zach, you are free to release daddy long legs all over Kelly while she's asleep.

Sure. Sure. Wow. Amazing. You don't have the same reaction to spiders as other people do, huh? Well, daddy long legs aren't really spiders. Oh, my gosh. Wow.

And that scientific knowledge puts you at ease? It does. Let me make sure that's right. Our daddy long leg spider. I think they're arachnids. Oh, maybe they do still count as spiders. Yeah, all right. So sorry, I guess they are still.

Daddy long legs are still. See, I told you they were creepy. Like, that makes a difference. All right. Sorry, you were right. Daddy long legs looks like they are spiders. But I do feel like knowledge makes a huge difference. So we've got these, they're called rabid wolf spiders.

Bad name. Right? And they're like big and they do look kind of scary. And when I first moved here, I freaked out because my son likes to roll around in the grass. And these are like grass spiders that are in the grass. And so he'd roll around and I'd always see like a big spider running away. And it really freaked me out. But I read about them and now I'm not scared of them. Like they're not going to go after my son. It's fine. I do feel like knowledge makes a big difference in my fear outside in general. You're totally right. And of course, knowledge is our business. We're here to help everybody understand. universe better and be less afraid of it or maybe appropriately afraid of it. So let's get into it because today we're not here just to talk about my experiences with spiders and Kelly's magical moment with the frogs. We're here to talk about your questions about the universe. Things you wonder about places where you wish you had more knowledge. So we regularly ask our listeners to send in their questions and we'd love to hear from you. Please write to us to questions at

Daniel and Kelly.org. We always write back to everybody and sometimes we pick questions to answer here on the pod because we think a lot of people might be interested in the answer or because I think it'd be fun to joke about it with Kelly. Or because when I get the email from you, I don't know the answer and I need to stall for time. That's the other reason. So speaking of stalling for time, here's our first question, which is about how to quickly explore the outer solar system using gravity's help. Hi, Daniel and Kelly. I was thinking about how we use gravitational slingshots for missions such as Voyager, Cassini, and New Horizons. As we venture further into the solar system,

would we use this method for human crew missions? And what forces would the crew feel if they were slingshotted to their destination? Thanks. All right, and that question was from Rob Pixley. Rob, thank you very much for writing in. Kelly, you're an expert on space and exploration. What do you think about this?

Well, I believe that gravitational slingshots are when you use massive objects to gain some speed, but I'm going to wait for you to give some more information. But when I read this, it reminded me of Jules, Verne's, I can never say his name right. Jules Verne. Thank you. Because he's French. Okay, all right.

That guy, that French guy, he wrote a book from the earth to the moon, and it was about a Baltimore gun club who decided they were going to build a giant gun and shoot people to the moon. And it works, but the problem is actually, if you were to calculate how many Gs the crew would have felt, they would have been probably like liquefied in Chapter 20 on the way to the moon. They wouldn't have made it. So, so let's hear about gravitational slingshots. Would this kill humans or not? Gravitational slingshots are super awesome. They're a way to boost your speed and change your direction without using any fuel. You know, one of the big issues for getting

around the solar system or getting around the universe is that fuel is heavy. And if you use fuel to propel yourself, you need fuel to help you push that fuel. And then you need more fuel to help you push that fuel. And pretty soon you have a gas tank the size of Jupiter just to get anywhere. So it's very nice if you can navigate the universe without using fuel because then you don't have the additional mass and the need to propel that, all that stuff. So for decades, NASA and other folks have been using this technique called a gravitational slingshot, which essentially steals a little bit of speed from a planet or a moon. The way it works is you can approach a big planet and its gravity will change your direction. So for example, say you're

just coasting, you have no rockets on and you're approaching Jupiter and you swing around the back of Jupiter and come out the other side. Now you're going in a new direction, right? That's not something you can otherwise accomplish usually without burning some fuel without thrusters. Because a change in direction, even if your overall magnitude is the same, is still an acceleration. To change your direction in space, you've got to have some acceleration. Somebody's got to pull on you. So basically, you use the gravity of the planet to pull on you. And you can even come out the other side, But not just with a change in direction, but with a net increase in speed. And that's why it's called a gravitational slingshot.

Wow. I don't think we've ever used this on a vehicle carrying humans because the only option would be the trip to the moon. But have we used this for our rovers or anything? Or probes? We've used it for lots of probes, absolutely, because a lot of times we don't have the fuel to get them all the way out to the outer solar system, and we want to keep them light. And it saves fuel. and then also you get to add like another planet on your trip you're like hey i want to go to

saturn but i'd like to swing by mars on the way or i'd like to go to neptune can we stop by jupiter and then you also get to take pictures of jupiter because you know none of these things are so well explored that like one more trip is boring yeah right so that's like a bonus for these things and i think the physics of it is really fascinating because it's a little bit counterintuitive like it feels like free energy like where is this speed coming from and the answer to that question is that you're really taking that speed from the planet. Like if you swing by Jupiter, it changes your direction, it accelerates you, and it effectively slows down.

It slows down in its orbit around the sun. So, for example, if you take a normal space probe and you swing it around Jupiter and it gets sped up in the process, Jupiter slows down by just by a tiny little bit because the mass of Jupiter is so huge compared to the mass of the probe that it loses, I did this calculation, one times 10 to the negative 25 kilometers per second of its velocity. So basically negligible. However, if you scaled this up and you did like 10 to the 25 space probes because you want to explore the whole galaxy,

you might start having an impact on Jupiter. But basically think about Jupiter as a huge battery of momentum and you're tapping into that a little bit and adding it to your space probes. So if an aggressive alien civilization wanted to screw up our solar system, could they just send probes by Jupiter enough times to get it to sort of move around and wreck havoc on the rest of us? I guess they could, but, you know, if they were capable of doing that, they should just like nudge an asteroid towards Earth. I mean, I'm not giving advice to malevolent aliens.

Okay, sounds like I am. So, you know, please, if you are malevolent aliens, don't do this or anybody shouldn't do this. But, yeah, the most dangerous thing you could do in the solar system is nudge a comet, for example, because comets, the time they get to the inner solar system are going really, really fast because they fall from so far away. And they're really hard to see in advance because their periods are so long. So yeah, nudging a comet would be the most dangerous thing. I guess the most subtle thing would be tweaking Jupiter. That would be a cool basis for a science fiction novel. Get on it, Daniel. And for those of you still trying to like grok how this works,

you know, another analogy is like, think about a moving train and you have a tennis ball. If you throw your tennis ball against the front of the train, then the train's velocity and he gets added to the tennis ball's velocity when it bounces off, right? So now it comes back. It's going not just the same velocity as it was when it hit the train. Like if you bounce a tennis ball against a wall, it comes back with the same speed. If you bounce a tennis ball against a train rushing at you, it comes back much, much faster. It slows down the train a tiny little bit, nobody's ever going to notice, but it speeds up the tennis ball.

And so it's this huge mass ratio that makes this possible. All right. So how fast does this speed up happen? And is it going to be fast enough that you're going to liquefy the huge ball? Right. So two really interesting questions, and that's really what Rob was asking about. So number one, this isn't a great idea for human missions, but not because of the G forces, but because usually it involves going pretty far out of your way. Like you're sending a probe to Pluto, you know it's going to take forever to get there. If you can make it lighter by stopping by Saturn on the

way, you don't really care if it's going to slow you down by five years, because it can slow you down by five years. So it's a more economical way to get this speed, but often it requires going really far out of your way. And if you're doing space missions, the goal is to get there fast, to spend less time in space, less time exposed to low gravity, less time exposed to radiation, all this kind of stuff. Now, sometimes it can work if the solar system is just right. You know, if you want to get to Saturn and Jupiter happens to be in just the right place, then maybe a flyby if Jupiter can help you get to Saturn faster. But usually you just want to go directly there. And so you want chemical rockets or ion thrusts.

or fusion power or something. So this is good for long missions where you don't really care how long it takes, which is not what we're going for here. Okay, so it would take longer, but, I mean, if you're doing like a generation ship on an interstellar journey, you know you're not going to be alive

when you get to the final destination anyway. So why not add an extra five years to your trip to speed it up for the next few generations? So let's assume we decide we're going to do this anyway. Or are you going to get killed by the speed? Well, before we answer that, there's another wrinkle, which is you're right. If you're like aiming for Alpha Centauri and you want to be redirected and you don't want to spend all that fuel, this is not a terrible idea. But it's going to be more effective the closer you get to the planet, right, because then the more powerful the gravity.

However, the closer you get to Jupiter, the more you have to worry about the radiation of Jupiter, right? So there's really a tradeoff there. But here's the thing about the G forces. There aren't any. You don't feel any of these G forces. That's right. because gravity is not a force. You feel G forces when you're being accelerated.

So, for example, if you're on a roller coaster and you reach the bottom of the roller coaster and you start going up, you feel those G forces. Or if you're in a car and you're turning really fast, you feel those G forces. Or if you're in a rocket and somebody burns the rocket really fast to get you going off the planet, you feel those G forces. When you are falling just under gravity, you feel no G forces. So, for example, you jump out of an air. airplane, you don't feel any G forces, right? You're in free fall. You only feel G forces when

there's acceleration, and you only feel acceleration when you're fighting gravity. If you just chill with gravity, man, and go with the flow of the universe, you feel no G forces. Okay, well, so what if, what if you're on this interstellar ship, you want to get the Jupiter speed up, and the folks who made your interstellar ship planned ahead for the problem that low gravity is bad for human bodies and now you are spinning. So you have artificial gravity in your habitat to help with things like reproduction. Now are you feeling the G forces with the artificial gravity? You feel the artificial gravity, absolutely, because that's not gravity, right? That's the force from the structure of the ship holding itself together. The same with like on the surface of the

earth, you feel what we call gravity, but it's really the earth pushing you up against the natural inclination you have to fall towards the center of the curvature. So yes, you feel the G-forces from the spinning. That's real because it's not gravity. But falling into Jupiter's gravitational field doesn't change that at all. Yeah. Okay. So sorry. So I knew that you would feel the artificial gravity. But what I meant is when you're feeling artificial gravity and then you accelerate. Yeah. Do you feel that acceleration more strongly when you're under artificial gravity relative to no? No. Okay. No. No, you don't. Cool. Yeah. So that's sort of also. because, you know, you don't feel gravity at all,

even though it's having an impact on you, even though you are accelerating, you're changing your vector from the point of view of somebody distant, right? This is the thing about GR is that it's very confusing. It depends on who's watching. From your point of view, you never feel acceleration due to gravity. Somebody else looking from far away who doesn't see the curvature, they're like, well, according to my coordinates, your velocity is changing,

and therefore you're having acceleration. And that's cool. GR lets everybody have their own calculation. But what do you feel is what's important, right? You're not going to be torn apart. Your stomach's not going to be shredded. Your organs are not going to be squished. Feel free to dive close to Jupiter in your arc ship on your way to Alpha Centauri. You have to worry about the radiation, but you don't have to worry about the G forces. And because I have a POD in physics, I know that GR stands for General Relativity. Yes. Absolutely. Jail relativity, one of the most fascinating, beautiful, and I think widely misunderstood theories,

mostly because of that dang rubber sheet bowling ball analogy out there, which confuses everybody. Yeah, but we're here to clear up misconceptions. All right, so let's find out if Rob wants to use a gravitational slingshot for his journey to interstellar space that I've decided he's going on. Hi, Daniel and Kelly. Your explanations not only make sense, but offered a great way to look at the scenario for multiple interesting different perspectives.

Thanks again. Hi there, this is Josh Clark from the Stuff You Should Know podcast. If you've been thinking, man alive, I could go for some good true crime podcast episodes, then have we got good news for you. Stuff You Should Know just released a playlist of 12 of our best true crime episodes of all time. There's a shootout in broad daylight, people using axes in really terrible ways, disappearances, legendary heists, the whole nine years. yards. So check out the stuff you should know

true crime playlist on the iHeart radio app, Apple Podcasts, or wherever you get your podcasts. There's a vile sickness in Abbas town. You must excise it. Dig into the deep earth and cut it out. The village is ravaged.

Entire families have been consumed. You know how waking up from a dream. A familiar place can look completely alien. Get back, everyone. He's going to next. And if you see the devil walking around inside of another man, you must cut out the very heart of him. Burn his body and scatter the ashes in the furthest corner of this town as a warning. From IHeart Podcasts and Grimm and Mild from Aaron Manky,

this is Havoc Town, a new fiction podcast set in the Bridgewater Audio Universe, starring Jewel State and Ray Wise. Listen to Havoc Town on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. The devil walks in Aberstown. It may look different, but native culture is very alive. My name is Nicole Garcia, and on Burn Sage, Burn Bridges, we aim to explore that culture. It was a huge honor to become a television writer because it does feel oddly, like, very traditional. It feels like Bob Dylan going electric, that this is something we've been doing for a hundred of a year.

you carry with you a sense of purpose and confidence. That's Sierra Teller Ornelis, who with Rutherford Falls became the first native showrunner in television history. On the podcast, Burn Sage, Burn Bridges, we explore her story, along with other Native stories, such as the creation of the first Native Comic-Con or the importance of reservation basketball. Every day, native people are striving to keep traditions alive

while navigating the modern world, Influencing and bringing our culture into the mainstream. Listen to Burn Sage Burn Bridges on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. Culture eats strategy for breakfast. I would love for you to share your breakdown on pivoting. We feel sometimes like we're leaving a part of us behind when we enter a new space, but we're just building. On a recent episode of Culture Raises Us, I was joined by Volisha Butterfield, Media Fan. founder, political strategist, and tech powerhouse for a powerful conversation on storytelling,

impact, and the intersections of culture and leadership. I am a free black woman who worked really hard to be able to say that. I'd love for you to break down. Why was so important for you to do C? You can't win as something you didn't create. From the Obama White House to Google to the Grammys, Malicia's journey is a masterclass in shifting culture and using your voice to spark change. A very fake, capital-driven environment and society. We'll have a lot of people tell half-truths.

I'm telling you, I'm on the energy committee. Like, if the energy is not right, we're not doing it, whatever that it is. Listen to Culture raises us on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. Okay, we're back, and we're answering questions from listeners today. question is about major animals. Major animals. I've never heard them referred to as major before. Majorly awesome. So I have never been to Spain. How would you pronounce this name, Daniel? Do you know? Ademar? I've never heard this name before, but it looks to me like Ademar. Yeah. All right. Anyway, fantastic name. And so Adamar from Spain. Let's hear your

question. Hello, Daniel and Kelly. My name is Ademar and I'm asking this question from Spain. I recently learned that elephants seem to identify different human languages and react differently depending on what they hear, thus being able to know if a group of humans poses a threat to them based on the language they speak. Differently, rhinos seem to adopt a more aggressive behavior whenever they spot a skinny ape like us. Some suggesting that this could be a consequence of biological adaptation as they evolved in contact with humans for many, many times, in fact, since the beginning of the hamlets.

What is most intriguing is that this could be the reason why great animals don't exist in any continent other than Africa. As our arrival, there was much more sudden, and they didn't have time to evolve and adapt to us before we hunted them down to extinction. May Kelly give us a deeper insight in this matter? Thank you for having me, and congratulations for providing us with such an awesome podcast.

All right, so my opinion on the matter is pretty much that Adamar was right, right on the money. And so we're just going to go ahead and talk about the science here because it's a really fascinating story, and Adomar seems to have a good handle on it. But I'm just going to go ahead and reiterate how cool all this stuff is. Yeah, Kelly, tell us the history. How much more awesome was the Earth 10 or 50,000 years ago? Before humans came along. All right. So there were a lot more large mammals on the planet. And by large, we mean greater than 44 kilograms.

Why 44? Not 45, not 40. Who decided? Was there some meeting the 44 kilogram threshold? I don't know who decided on the 44 kilogram threshold. No doubt it was arbitrary. Somebody must have looked at an animal and been like, you are too small. How much do you weigh? You are now our threshold. Oh, I hope they didn't hurt that poor animal's feelings anyway. I mean, the good news, if you're below the threshold, it sounds like you were maybe less susceptible to extinction caused by humans.

So maybe you'd be happy if you were under this arbitrary threshold. So tell us about these megafauna that used to roam the earth. Okay, so this large megafauna includes things like you mentioned at the beginning of the show, like lions that used to be found much farther north. There were also giant ground sloths. There were kangaroos that were much larger, and there were mammoths. There were just lots of much larger mammals that were roaming the earth. And somewhere between 10 to 50,000 years ago, about 200 of these large mammal species went extinct.

Wow. And this is called the Quartinary megafauna Extinction, QME. And is there a bigger trend here that, like, animals get bigger and bigger? Because, like, dinosaurs got really huge. and then the biggest ones died out and then mammals come along and then they eventually got big is this some sort of evolutionary trend

eventually somebody finds the niche of being too big to eat I mean I'm sure a lot of things go into optimal size for animals being big does make you too big to eat but you know let's say T-Rex was a big predator and you know blue whales are also big predators but they're going after krill

so they don't have to be big so that they can be vicious and take down large animals they're eating teeny tiny little creatures called krill, little crustaceans in the ocean. And so, yeah, I'm sure a lot of things go into being big, but it is a niche that sometimes makes sense to fill. Yeah, so about 10 to 50,000 years ago, we lost a bunch of these species. And the question is why? And, you know, probably because it's biology. The answer is it depends. There's probably a lot of different factors. It could have been that climate was changing at the time as well. But there's pretty good evidence to suggest

that one of the major causes of this decline in large mammals was the fact that humans were starting to move into new areas. So lunch, basically. Lunch, yes, that's right. So, for example, about 14 of the 16 large mammal species that were present in Australia went extinct between about 30,000 and 50,000 years ago and humans showed up about 40 to 60,000 years ago.

So the numbers don't match up exactly, but some evidence that humans show up and those species go extinct. In North America, humans arrived around like 12 to 20,000 years ago, and 83% of the large mammals in North America, which was 34 out of 47 species, went extinct. Somewhere between 11 to 15,000 years ago. So we show up, 83% of the large mammals disappear. And I guess it's easy to draw the dotted line to say, like, well, humans probably killed them or humans maybe ate them or whatever.

But it could also be more complex, right? Humans arriving could change the ecosystem and these megafauna could be sensitive to, like, the web underneath them that are changing. Maybe humans are eating something else the megafauna were eating or something. It could just be more complicated, right? Absolutely. Or maybe we brought some disease that jumped from us to them or we brought some other domesticated animals along that killed them in some way. So, yeah, there's a lot of complicated. It's not just necessarily that we, like, ran after them with spears and we ate them.

But we probably did a little of that, too, didn't we? I'm sure there's some fossil evidence that we did some of that, too. Yeah. We needed to eat. But anyway, so there is this like correlational evidence that when humans show up on a new continent, not that long after a large percent of their large mammals go extinct. And I guess another thing we should consider is there could be like confounding factors also, like something which caused humans to move there and could also cause the extinction,

but which wouldn't put the blame on humanity really at all. Yeah, that's right. So if there were, for example, large climactic changes that made some areas bad for humans and they were migrating in search of new food sources or something, it could be that humans came at the same time as the climate changed. There have been people who have looked into this. And I think that they conclude that humans still probably played a role even after you control for some other stuff that we know was happening at that time. But it probably wasn't just humans. But it looks like mammals, large mammals in particular, were susceptible to death around this time, not just because we were hunting. them or whatever it was about humans that caused them to die. But in particular, large mammals

tend to have, they live a long time, but it takes a long time for them to start making babies. And when they do make babies, they don't make a lot of them. You may remember our conversation about K-selected species that we had with Katie Golden. So large mammals tend to not have a lot of babies to begin with. So it's a little bit easier to kill them off because they can't sort of bounce back as quickly. Fascinating. And so the question really is, like, why didn't the same thing happened in Africa. It seems like Africa has more megafauna than the other continents. Why is that? Yeah, that is a great question. So during this same period, only 10 of the 48 large mammal species in Africa went extinct. And in Eurasia, they lost

nine of 26 species. So that's a much lower percent. The current hypothesis is that humans evolved in these areas. So as our species was picking up the skills for hunting and stuff like that we were evolving alongside of these species. So as we got better, we didn't get like great at this overnight, but as we got better, selection was sort of favoring traits in, for example, elephants to help them essentially stay away from humans and keep themselves alive in the face of this sort of growing super predator, which we would end up becoming. And so the idea here is that Africa and Eurasia, those animals evolved with us. And so they were just better able to escape us. But then when we went to somewhere new like Australia, North America, and these animals had never

seen anything like us, all of a sudden we show up with our spears, and they are just no match for the super predators who just landed on their continent. Wow, fascinating. So it's actually good luck to evolve together with humans, right, rather than just have them show up at your doorstep. You want humans to be bugging you for thousands of years, not just showing up unannounced. When we show up unannounced, we are particularly problematic guests. And what about this other idea that Atomar raises about how elephants can, like, understand human language and guess our intentions? Is there anything to that? I don't think Atomar in particular was trying to say that elephants can understand language. They sent a paper from 2014 by McComb at all in

proceedings of the National Academy of Sciences. And essentially what they did hear was they tried to figure out if elephants just from hearing an audio clip would respond in a way that suggested that they knew that different kinds of people differed in the risk that they posed to the elephants. Wow. Yeah, for example, the Maasai people are pastoralists, so their grazing land and waterholes are often areas that are also used by elephants

and are used by the livestock that the Maasai are sort of walking around the area. And so they often come in conflict, and sometimes the elephants will kill Maasai and the Messiah need to essentially defend themselves. On the other hand, the Kamba people are more agricultural, and they rarely have run-ins with elephants. And when they do have run-ins, it's usually like a male elephant

who has sort of invaded their field. And so the Kamba people are often leaving alone females and the groups of females with their babies. And additionally, human females, whether the Maasai or Kamba, are unlikely to kill elephants. And young boys are also unlikely to kill elephants. The people who are most likely to kill elephants are adult men. So they did this playback.

experiment where they had everybody read the same sentence. And the sentence was, look over there, a group of elephants is coming. And they said it in their own language. And then they looked to see what the elephants did. And when you played a Maasai man saying that sentence, the elephants would bunch together in a defensive like huddle. And they would respond in a way that suggested that they were experiencing fear because a predator was around. Wow. Yeah. And they did less of that when they heard the combo people. And remember, these are people that often don't end up in positions where they need to defend themselves and their livelihoods against elephants. The elephants also responded more strongly to the sound of men than women and more strongly to

the sound of men, Maasai, relative to boy Maasai. So the elephants do seem to have a pretty good sense that, like, humans are risky, but some kinds of humans are more risky than others. And so they seem to have this, like, nuanced ability to tell human risk and respond accordingly. I told you the elephants are listening. They got those big ears, and they can tell when you're talking about them. That's right. That's right. And they never forget. And so you got to be extra careful. But it's worth noting that many animals respond to humans as though we are risky. And that's even in North America, in Europe, they've done playback experiments with badgers that freak out. If you play sounds at waterholes in Africa, just about every species that hears the sound of a human, they'll respond more strongly to the sound of a human than the same. sound of a lion. So in general, just about any continent you go to, the animals know humans often mean trouble. We should leave town or get defensive. We're going to cause some trouble. So this is sort of an unrelated topic than the extinction of animals. This is more like what are animals doing

in this day and age to respond to humans. And maybe elephants were doing this back in the past and that's what saved them from getting killed by us. They just kind of gave us space. But in general, humans can be devastating to wildlife. Well, let's say some nice things about humans to balance it out. You know, I think it's incredible that we can unravel these stories. Recently, Hazel made this comment. She was like, all science has basically stuff happened and we figured out why. And I was like, you know, that's a pretty good point, Hazel.

And it touches on this incredible thing that we do in science, which is like, gather these clues that are just like randomly accidentally left imprinted on the world to figure out what happened, right? to unravel this incredible story you're telling over tens of thousands of years about these huge animals that no longer exist and why and a complicated interplay between species. It's incredible to me that we can unravel these stories, that we can pull them out, sometimes literally out of the ground, to learn the deep history of our universe. Yeah, and you are always a more optimistic and uplifting person than I am, and so that's

beautiful. And I'll attempt to follow in your footsteps by noting that by understanding the damage we've done in the past, you know, we can try to ameliorate the damage that we might be doing now or the damage we might be doing in the future. And so, you know, the conservation movement has grown in, you know, the last couple decades and we're getting better at caring about this stuff and trying to at least slow the decline of some of these species. And in other cases, turn around the decline of species. So, you know, if we can recognize our past mistakes, we can hopefully use that information to do better in the future. Well, could I say something controversial?

Do you think that an individual species on its own has like inherent value? Like should we be conserving every species or should we take a bigger, broader view and say, look, diversity is important. And that means that new species should be evolving and sometimes species disappear. It's just part of the process. Should we be trying to hold on to every individual species or should we be maintaining diversity sort of in a larger sense? Wow.

All right. So that question deserves like a whole episode. But the short answer is, so there's a background extinction rate, so you shouldn't necessarily lose sleep if a species goes extinct because that was going to happen whether humans were here or not. But I believe the extinction rate is something like, it's at least 10 times higher than we would expect it to be based on background levels. So we are really speeding things up. For any particular species, I mean, there are some species that play less important roles in ecosystems, losing them would have less of an impact. But that's still like a unique product of millions of years of evolution that is beautiful and well adapted to its environment and is being wiped out, you know, because of us. And that to me does feel catastrophic and really sad.

And, you know, I feel like you have to be able to accept nuance in these conversations and way pros and cons and stuff like that. But, you know, I do feel like any time we lose the species, it is sad, even if it's just a beetle that was present in only one place or something. Well, it is an incredible output of evolution. You know, it's like effectively billions of years of biological computation, you know, to design this creator that can do something amazing or has incredible chemicals in it. Anyway, stay tuned, everybody, for the upcoming episode where Daniel argues that extinction is good, actually. What? Like all the extinction or just like, I mean, this might be the last episode of the DKEU's extraordinary universe. All right. Well, we'll dig into that for another episode. In the meantime, let's hear from Atamar about whether Kelly answered their question.

Hello again, guys, and thank you for your response. It makes me glad to think that possibly we are not. The only reason that those animals went extinct and Kelly's context really makes me think in a wider spectrum. Like, for example, what if it was a disease? What if it was a glaciation? The same we were running away from. Or maybe it was another super predator. Anyway, who knows? Thank you. And keep up the world's work. Hi there. This is Josh Clark from the Stuff You Should Know podcast. If you've been thinking, man alive, I could go for some good true crime podcast episodes.

Then have we got good news for you. Stuff You Should Know just released a playlist of 12 of our best true crime episodes of all times. There's a shootout in broad daylight, people using axes in really terrible ways, disappearances, legendary heists, the whole nine yards. So check out the stuff you should know true crime playlist. On the iHeart Radio app, Apple Podcasts, or wherever you get your podcasts. There's a vile sickness in Abbas Town. You must excise it.

Dig into the deep earth and cut it out. The village is ravaged. Entire families have been used. consumed. You know how waking up from a dream? A familiar place can look completely alien? Get back everyone! He's going to be next!

And if you see the devil walking around inside of another man, you must cut out the very heart of him. Burn his body and scatter the ashes in the furthest corner of this town as a warning. From IHeart Podcasts and Grimm and Mild

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My name is Nicole Garcia, and on Burn Sage, Burn Bridges, we aim to explore that culture. It was a huge honor to become a television writer because it does feel oddly, like, very traditional. It feels like Bob Dylan going electric, that this is something we've been doing for a hundred of years. You carry with you a sense of purpose and confidence. That's Sierra Taylor Ornelis, who with Rutherford Falls

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Culture eats strategy for breakfast. I would love for you to share your breakdown on pivoting. We feel sometimes like we're leaving a part of us behind when we enter a new space. but we're just building. On a recent episode of Culture Raises Us, I was joined by Volusia Butterfield, media founder, political strategist, and tech powerhouse

for a powerful conversation on storytelling, impact, and the intersections of culture and leadership. I am a free black woman who worked really hard to be able to say that. I'd love for you to break down. Why was so important for you to do C? You can't win as something you didn't create.

From the Obama White House to Google to the Grammys, Belisha's journey is a masterclass in shifting culture and using your voice to spark change. A very fake, capital-driven environment and society will have a lot of people tell half-truths. I'm telling you, I'm on the energy committee. Like, if the energy is not right, we're not doing it, whatever that it is. Listen to Culture raises us on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. All right. I'm still trying to decide if I can forgive Daniel for hinting that extinction caused by humans isn't always bad. But you know what? I'm going to distract myself with this insightful question from a listener. Hey, Daniel and Kelly. Really enjoying the podcast. Thank you. I have a question for you. And it's about black holes, of course. Black holes are usually described as unslakeable consumers of all matter and energy. But I know in some cases, the rate of consumption is limited by the energy pressure of that consumption,

process, pushing back on the gravitational pull and starving it of additional fuel. I would love to hear more about what this might be like on a more human scale. Would a tiny primordial black hole around the earth generate a tiny accretion disk of matter spinning around the light speed, generating the same kind of pressure and shooting out x-ray lasers from the top and bottom? How big or small would a black hole be to have these sorts of effects? When we think of a black hole interactive with normal human scale matter, what would that look like? How long could the Earth survive with a little black hole orbiting around inside of it. Really looking forward to hearing more about this. Thank you.

Thank you, David, very much for that question. And I hope that our answer is not going to help you build a miniature black hole to embed into the Earth. So we're just going to take it in good faith and answer your question anyway. That also sounds like a good topic for a sci-fi novel. Let's talk about black holes and start with the basics because David's question involves a lot of sophisticated black hole science. Remember, black holes are regions of space time.

where there's so much mass, that there's so much curvature that anything that falls beyond the event horizon eventually reaches the center. It can never escape. And something I think a lot of people don't understand about black holes is that the event horizon is not a physical surface. It's just a designation we make, a dotted line we draw. And actually, you never know where the event horizon is at any moment. You can only tell in the infinite future. Essentially, think about going to the infinite future, and then asking the question, in what regions of space did particles never escape? We'll now draw that to be the event horizon. And so you can't ever really actually know you can use GR to predict at any moment, but

the event horizon is just like a categorization. We say beyond this point, nothing has escaped or can escape, and outside that point, things can escape. But this is just gravity, right? Black holes don't, like, suck with infinite power or anything. If you replace the sun with the black hole of the same mass, we would feel that. it's gravity and the Earth would continue to orbit, right? So it really is just a gravitational object. Okay, so it black hole, the same size as our sun, wouldn't pull on us any more than

our sun because they're similarly dense? If it has the same mass and it's at the same distance, it would have the same gravity. Okay. All right. I'm with you. It wouldn't be as bright and the Earth would be chilly and it would be bad, but not gravitationally. All right. I don't want that to happen. Does the event horizon always stay at the exact same spot or does it shift over time? if the black hole like absorbs big things that make it bigger. Absolutely. The event horizon depends on the mass of the black hole and not just on the stuff inside the event horizon, right?

This is why you shouldn't think about the event horizon as some physical barrier. As you approach a black hole, your gravitational energy contributes to the mass of the black hole. So the black hole's event horizon actually grows out to meet you as you fall into the black hole. So you approach the event horizon, the event horizon approaches you. It's like a little gravitational hug. Oh, yeah, we talked about what would happen if Zach was in a black hole. And if we threw him sandwiches, would they ever get to him? Exactly.

And the reason we need to understand that black holes are not infinitely powerful is to think about the stuff that's orbiting them. Because that's what David's question was about, this accretion disk. When you think about the images of black holes out there in space, they look like a glowing donut with a black core. Or, you know, the visualization in interstellar or whatever. They have these luminous disks around them. What is that and why aren't they getting sucked in? Well, they're not getting sucked into the black hole. For the same reason, the Earth is not getting sucked into the sun, right?

There's a lot of gravity from the sun. Why aren't we just getting sucked into the sun? And the answer is we have velocity. There are stable orbits around the sun. In the same way, there are stable orbits around a black hole because it's just a gravitational object. So if you approach a black hole, as long as you were outside the event horizon, you could orbit around it forever without falling in.

if you are stable and, crucially, nothing bumped into you to knock you out of your orbit. That doesn't sound fun. I'll pass. And that's the same story for the Earth, right? Now, the accretion disk is very different because it's not just like one chunk of matter here, one chunk of matter there. It's like a huge hot disk of gas and dust that has a lot of internal friction. That's why it's glowing. The tidal forces from the black hole are massaging it and heating it up and it's rubbing against itself. And so it's very hot and it's glowing. So that's why we can see these. black holes. These telescope pictures of black holes, really, they're pictures of the hot accretion

disc around the black hole. So the accretion disk is stuff that's orbiting the black hole, hasn't fallen in yet, is going too fast, probably will fall in eventually, because there's a lot of friction there, so you can't just like have a stable orbit. But it's sort of like on deck to go in the black hole, but not there yet. And what is that made out of? Is that just like the basic dust and junk that you find in space? It's made out of whatever you've been feeding your black hole. But yeah, the universe is mostly hydrogen. So it's always a safe bet to say it's mostly hydrogen because that's what the universe is. And so, yeah, there's gas and this dust and whatever, but mostly it's hydrogen there.

And here's where the black hole starts to work against itself because the accretion disk is so hot that it glows. That's what we can see these things. So it emits radiation pressure. So it pushes stuff away. We talked in a recent episode about how stars are a balance between gravity and radiation pressure, The fusion at their hot core is producing a lot of energy, and that pushes out on the star. And if the star is too big, it can actually blow the star apart. That's why you have like a maximum size of stars.

Well, a similar thing is happening here. The heat from the accretion disk creates radiation, which pushes other stuff away. So the hotter the black hole, the bigger the black hole, the bigger the accretion disk, the more it slows down its ability to eat stuff. So black holes cannot grow infinitely quickly. There's no theoretical upper limit to the size of a black hole, but because of this process, this radiation pressure, there is an upper limit to how rapidly they can grow. You put a black hole in a blob of stuff, it can't just like slurp everything out really quickly

because it's going to get hot and glow and push its own food away from itself. So what determines the size of the accretion disk? It's just how much stuff was there around it. Like if you put a black hole in deep space and nothing is there, it'd have no accretion disc. It would just be black. We could not see it except for its gravitational effects. You drop in the middle of a really big blob of stuff,

it's going to get a big accretion disk. So it just depends on what's around it. And this is a really interesting puzzle in physics right now because this limit on how quickly black holes can grow is one of the reasons why we don't understand supermassive black holes. We see super massive black holes in the early history of the universe, like a billion years after the Big Bang, already we have black holes with like billions of times the mass.

of the sun. But if you do the calculations, the limit on the speed of which black holes can grow tell you that's impossible. So how did they get so big? That's the big question of super massive black holes. Although we have some interesting hints that like if the accretion disc is asymmetric, then you can have what they call super Eddington accretion rates. It's fascinating. But David's question is about tiny black holes. And he's asking, can they also have accretion disks? And you told me that it doesn't matter how big a black hole is. is what matters is the stuff that started. So could a tiny black hole have a huge accretion disc?

So a tiny black hole can have accretion disk and this Eddington limit of the rate of which black holes can grow scales with mass. So the limit is smaller with smaller mass. So there's like less radiation pressure from a smaller mass black hole because you can't heat up its accretion disk as much. But at a very small black hole,

something else happens, right? Black holes don't just glow because of their accretion disc. They are themselves not totally black. We think they glow themselves with a little bit of hawking radiation. This is this bizarre radiation that happens because you have an event horizon in quantum fields. And there's this hand-wavy story out there about particles and antiparticles or one falls into the event horizon. That's just cartoons. It's not really the physics of what's happening.

But we do think that hawking radiation might be real. And the crucial thing about it is that it's bigger for smaller black holes. Really big black holes, almost no hawking radiation. Smaller black holes, dramatic hawking radiation. So the smaller the black hole gets, the bigger the hawking radiation. So this is actually going to be more radiation pressure than the accretion disc. For a small enough black hole, the hawking radiation will be brighter than the glow from the accretion disc. Okay, so you, I'm picturing it now.

There's a black hole in the middle. There's an accretion disc around it, and then hawking radiation extends even beyond that. And if you were looking, do we have a kind of telescope that could distinguish those three objects? We have a telescope that can see black holes, and we've only been able to see supermassive black holes. And so those have essentially no hawking radiation or unmeasurable amounts. We've never seen hawking radiation. But if you had a small enough black hole and it was nearby, it would be very bright in hawking radiation. So it did the calculation, and if you had a black hole with like 40 billion kilograms, which is about the mass of the Hoover Dam,

then the glow from the accretion disc would be about as bright as the glow from hawking radiation. So, like, pretty bright. But this is tiny, you know, this is like a tiny fraction of the mass of the earth even. So this is really a very small black hole. Like the gravity from the Hoover Dam is not very powerful. Remember we had that episode about like measuring gravity and like the Scottish guys climbing around that mountain? That was a much bigger effect because that's a much bigger mountain and still very hard to measure. Gravity is super weak.

So a black hole with the mass of the Hoover Dam would not. not be very powerful, but it would be as bright as its accretion disc. So yes, David, small black holes can have accretion disc. It depends on what they've been eating. So if you have a tiny black hole out in the middle of nowhere, no accretion disc. You take a tiny black hole and you plop it in a huge bed of plasma and hydrogen and whatever, it will get an accretion disc as well, but it will also glow with its own hawking radiation. Wow. Okay, so then let's get to the last part of David's question. If you put that black hole in the center of the earth, How long could Earth survive?

Yeah, this is a fascinating question as well. And I thought it was a little unrealistic because we can't make a black hole the size of the Hoover Dam. We're unlikely to see one. So I thought, well, let's take it another step. Let's say we did make a black hole because, for example, at the Large Hadron Collider, we're trying to make black holes all the time. What? We smash protons together.

Yeah, we smash protons together in the hopes that occasionally they will create a tiny miniature black hole. And remember, smaller black holes radiate. faster than bigger black holes. So if we make a black hole to a large adjunct collider, it's going to have like 10 to the minus 24 kilograms of mass. It'll almost instantly radiate itself away. But it would be awesome because then we'd learn something about quantum gravity. It would be incredible. But if you did take one of these things and somehow was able to feed it before it irradiated away and you put it in the center of the earth and then you do a calculation of like how long would it take to slurp up everything and consume the entire Earth, that would take about 10,000.

years. All right. So I wouldn't be around anymore, so fine. That's right. That's all right. And you would have time to jump on that arc ship to Alpha Centauri. And we'd even have time to design and build it, you know, because 10,000 years is a good amount of time, even for space projects. But this ecologist feels like you probably shouldn't do anything to destroy the Earth, because there are a lot of species that would get left behind. And every species is important, Daniel. Extinction's not okay. Do you want to extinctify the mosquitoes? How about that wasp that bit you earlier? How's that thumb doing, by the way? It's about 30% bigger than the other thumb right now. But I still think that the wasps, they're important pollinators in some cases.

They control, like, other insect populations. I think it deserves to stay. I might get rid of mosquitoes. I'm glad we got rid of smallpox. I accept that there's some nuance, but your answer didn't sound like it was going to be very nuanced, Daniel. I was just trying to be controversial. Mm, success. Thank you, David, for that question. Let us know if we answered it, and follow-up questions are always welcome. Hi, Daniel and Kelly.

Thank you so much for that answer. You did indeed answer the question, my only follow-up. If I were to wave my magic wand and create a Hoover Dam-sized black hole and situated somewhere that I could watch it feed from 10 or 20 meters away, would I be able to enjoy the spectacle for the three or 400 microseconds it would take for the stream of gamma rays and exotic particles to shred my spacesuit? or would it be a tainty little firefly I could enjoy until my oxygen ran out? Either way, I promise not to destroy the earth anytime soon. Thanks again.

Thanks, everyone out there for joining us on this episode. We love hearing from you. Please send us your questions. It inspires us. It motivates us. It gives us something to do. Write to us to questions at daniel and Kelly.org. We look forward to hearing from you. Produced by IHeart Radio. We would love to hear from you. We really would. We want to know what questions you have about this extraordinary universe.

We want to know your thoughts on recent shows, suggestions for future shows. If you contact us, we will get back to you. We really mean it. We answer every message. Email us at Questions at Danielandkelly.org. Or you can find us on social media. We have accounts on X, Instagram, Blue Sky, and on all of those platforms, you can find us at D and K Universe.

Don't be shy. Write to us. Hi there, this is Josh Clark from the Stuff You Should Know podcast. If you've been thinking, man alive, I could go for some good true crime podcast episodes, then have we got good news for you. Stuff You Should Know just released a playlist

of 12 of our best true crime episodes of all time. There's a shootout in broad daylight, people using axes in really terrible ways, disappearances, legendary heists, the whole nine yards. So check out the Stuff You Should Know true crime playlist on the iHeart radio app apple podcasts or wherever you get your podcasts there's a vile sickness in abyss town you must excise it dig into the deep earth

and cut it out from iHeart podcasts and grim and mild from aaron mankey this is havoc town a new fiction podcast sets in the bridgewater audio universe starring jewel state and ray Listen to Havoc Town on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. It may look different, but Native Culture is alive. My name is Nicole Garcia, and on Burn Sage, Burn Bridges, we aim to explore that culture. Somewhere along the way, it turned into this full-fledged award-winning comic shop. That's Dr. Lee Francis IV, who opened the first Native comic bookshop. Explore his story along with many other Native stories on the show, Burn Sage, Burn Bridges.

Listen to Burn Sage Burn Bridges on the iHeart Radio app, Apple Podcasts, or wherever you get your podcasts. I'm Dr. Scott Barry Kaufman, host of the Psychology Podcast. Here's a clip from an upcoming conversation about how to be a better you. When you think about emotion regulation, you're not going to choose an adaptive strategy which is more effortful to use unless you think there's a good outcome. Avoidance is easier. Ignoring is easier. Denials easier.

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