Why Your Body Needs Gravity

Does space travel make astronauts younger? Professor Hannah Fry and Michael Stevens explore Einstein’s twin paradox, time dilation and NASA’s famous Twins Study with astronauts Scott and Mark Kelly. After Scott Kelly spent 340 days aboard the International Space Station, he returned milliseconds younger than his identical twin. But microgravity had also affected his eyesight, bones, blood, gut microbiome, body temperature, genes and telomeres. So what exactly happens to the human body in space, and which changes remain after astronauts return to Earth? In this episode, Hannah and Michael discover how deeply the human body depends on gravity. Along the way, they uncover why your head ages faster than your feet, what a 63 day cave experiment reveals about our sense of time, and whether future humans could ever truly adapt to life beyond Earth. ------------------- For more information about Cancer Research UK, their research, breakthroughs and how you can support them, visit ⁠https://www.cancerresearchuk.org/our-research/rest-is-science 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: Teo Ayodeji-Ansell Animator: Sam Benson Video & Social: Bex Tyrrell Assistant Producer: Lucy Lipscombe Producer: Simona Rata Senior Producer: Lauren Armstrong-Carter Chief Digital Officer: Samuel Oakley Exec Producer: Neil Fearn Learn more about your ad choices. Visit podcastchoices.com/adchoices

Speaker names marked ? are inferred from speaking time and the show’s known hosts — they are not voice-verified.

• 0:01Hannah Fry?

  Welcome to The Rest Is Science. I'm Hannah Frank.

• 0:02Michael Stevens?

  And her hair is ginger.

• 0:05Hannah Fry?

  Yes, it is. I haven't mentioned it. It doesn't really come up.

• 0:07Michael Stevens?

  I hadn't noticed, honestly. I'm very colorblind, you know, in that way.

• 0:12Hannah Fry?

  Imagine if you were colorblind and you just...

• 0:14Michael Stevens?

  I mean, colorblind, like, I just don't even know. You're just a human to me.

• 0:19Hannah Fry?

  Just another human.

• 0:20Michael Stevens?

  Anyway, my name's Michael Stevens, and this is The Rest Is Science.

• 0:25Hannah Fry?

  Okay, Michael, for today's episode, I want to tell you a story about two men. They had the same face. They were twins. That's not that much of a mystery. One of them gets on to a rocket ship in Kazakhstan. The other one drives home to Arizona. Now, I know what you're thinking. Einstein talked about this. He said, "Take two twins, put one in a rocket ship, send them away from the Earth. They'll come back. They'll be different ages." I want to talk to you, Michael, today about what actually happened when people actually tried it for real.

• 1:07Michael Stevens?

  This episode is brought to you by Cancer Research UK.

• 1:10Hannah Fry?

  As if they weren't cool enough already, some astronauts have now added cancer scientists to their CEB. They are making use of microgravity conditions in space to carry out experiments.

• 1:20Michael Stevens?

  That's because the same thing that lets astronauts experience weightlessness also affects how cancer cells grow, behave, and respond to drugs.

• 1:29Hannah Fry?

  And back down on Earth, Cancer Research UK is working with astronomers to explore tumors with technologies that are typically used to map the Milky Way, which I think gives you an idea of just how complex this disease is.

• 1:42Michael Stevens?

  That's right. There are over 200 different types of cancer. But by pushing boundaries and embracing the latest technologies, Cancer Research UK has helped double cancer survival in the UK over the past 50 years.

• 1:54Hannah Fry?

  And today, it is continuing to save and improve lives around the world. For more information about Cancer Research UK, their research and breakthroughs, and how you can support them, visit cancerresearchuk.org.

• 2:06Speaker 02

  Observe the North American dad. 50s, great polo collection. He enters his vehicle information on Carvana. An offer appears in minutes. Remarkable. He squints, reads it again, smiles, and says, "Dang, that's a good offer." The highest form of approval. No haggling, no hassle. No one called him "boss." He's still on the couch. Astonishing. Sell your car today on carvana.com. Pickup fees may apply.

• 2:37Speaker 03

  This episode is brought to you by Michelob Ultra. Good times mean competitions between you and your friends, like seeing who can throw some pigskin the farthest or rush the field the fastest. Winner gets an ultra. Because it's not just a superior-like beer, it's also giving fans superior access to their favorite sporting events. Superior is worth playing for. Visit michelobultra.com/superioraccess to enter. Enjoy responsibly. Copyright 2026 Anheuser-Busch InBev Michelob Ultra Beer, St. Louis, Missouri.

• 3:06Hannah Fry?

  The Rest Is Science.

• 3:15Michael Stevens?

  This is a real story.

• 3:16Hannah Fry?

  Oh, I am made up.

• 3:17Michael Stevens?

  You're talking about the bald twins.

• 3:18Hannah Fry?

  Huh? Are they bald? Do you know what? I actually don't know what they're like.

• 3:22Michael Stevens?

  I'm pretty sure they are. I know every bald man.

• 3:26Hannah Fry?

  They are bald. Aren't they lovely men?

• 3:29Michael Stevens?

  I can't remember their names. Kelly?

• 3:31Hannah Fry?

  Yeah, Scott Mark Kelly, yeah.

• 3:33Michael Stevens?

  One of them is like a senator.

• 3:35Hannah Fry?

  Oh my God, that's Mark Kelly?

• 3:37Michael Stevens?

  Yeah, yeah.

• 3:39Hannah Fry?

  I cannot believe I've been researching this for like two days and I didn't put two and two together.

• 3:45Michael Stevens?

  Wait, were they both astronauts or just one of them?

• 3:48Hannah Fry?

  They were both astronauts. Mark was a retired astronaut.

• 3:50Michael Stevens?

  Because Mark has done a wonderful job of literally finding a way to be known for something besides the fact that you're an astronaut.

• 3:59Hannah Fry?

  Right.

• 3:59Michael Stevens?

  He comes up in the news all the time, and I'm like, "He's an astronaut." They don't even mention that at this point. Can you imagine having a life like that? Where you're like, "Oh yeah, I've been to space." But that's not even what they say about me most of the time.

• 4:11Hannah Fry?

  It's not even the most interesting thing about me.

• 4:13Michael Stevens?

  I know.

• 4:14Hannah Fry?

  Wild. I cannot believe I didn't put two and two together because I follow American politics all the time. Like, all the time. No, all about Mark Kelly. Genuinely was so interested in what happened in this experiment. It never even occurred to me to think that that name might ring a bell. Have you met any astronauts, by the way?

• 4:31Michael Stevens?

  Yeah, I have.

• 4:32Hannah Fry?

  Which ones have you met?

• 4:33Michael Stevens?

  Don Pettit.

• 4:34Hannah Fry?

  Right.

• 4:34Michael Stevens?

  And I met him like twice, so technically...

• 4:38Hannah Fry?

  Two.

• 4:38Michael Stevens?

  Two, because he's made of different atoms the second time I met him.

• 4:41Hannah Fry?

  I completely agree. You know, no man standing in a river is the same man. It's a different river, different man, right? Yeah, different astronaut, different you as well.

• 4:48Michael Stevens?

  It was a different me. Have I even met anyone? This collection of atoms has not.

• 4:54Hannah Fry?

  Here's the thing. I think astronauts, they're as close to a superhuman species as we have. You know, I think if you filtered off just the astronauts, did an astronaut breeding program, you know? I think quite quickly they would diverge from the rest of us. They are so special, so smart. They're like the smartest people, incredibly fit, unbelievably brave, and also just really, really kind and decent.

• 5:24Michael Stevens?

  And there's a demeanor to them that I don't know if they always have that and they self-select for astronaut-hood, or if they become that way as they become closer to astronaut-hood.

• 5:38Hannah Fry?

  I also wonder whether having to face your own mortality, which I think you have to do if you are going to do something that is really as, you know, I mean, you're taking your life and you're strapping yourself to what is essentially a bomb and blasting yourself out of the atmosphere of our planet.

• 5:57Michael Stevens?

  But not for the thrill of it. The guys who do that with Red Bull are very different. An astronaut has this sort of humility, this kind of service-oriented personality. "I'm here to serve this agency, the American people, or the people of my country." It's very, it's very cool.

• 6:18Hannah Fry?

  It's really, really, really cool. And I think that this experiment of Scott and Mark Kelly demonstrates just how willing they are to give themselves over in the pursuit of knowledge and science for all of humanity, not for just themselves.

• 6:35Michael Stevens?

  And astronauts are not perfect, by the way.

• 6:38Hannah Fry?

  Sure.

• 6:39Michael Stevens?

  I just want to say that because you can read some of their biographies and you'll be like, "Oh." But in terms of public persona, they are like a kind of person I've only met a few of in my life.

• 6:50Hannah Fry?

  Right. I find them so much more impressive than actors, than musicians.

• 6:56Michael Stevens?

  Than podcasters. Can you imagine looking up to us?

• 7:02Hannah Fry?

  I find them so much more impressive than basically anything else. Athletes can be really impressive, the sort of single-minded pursuit of a goal, of an ambition, of like pushing the boundaries of what's humanly possible.

• 7:14Michael Stevens?

  And the natural talent to do what they do. And it's like, I will never do that. I will never win an Olympic gold medal.

• 7:21Hannah Fry?

  No, agree.

• 7:23Michael Stevens?

  I will also never go to the moon.

• 7:24Hannah Fry?

  No.

• 7:25Michael Stevens?

  I mean, I might be able to, but I will choose not to so that what I just said will stay true.

• 7:32Hannah Fry?

  I mean, frankly, absolutely the same. Okay, shall I tell you about this experiment? Because it's genuinely fascinating. All right, so let's kick off with what Einstein said about twins.

• 7:40Michael Stevens?

  Okay.

• 7:41Hannah Fry?

  The twin paradox, is it not?

• 7:42Michael Stevens?

  Classic.

• 7:43Hannah Fry?

  So he proposed this as a thought experiment in 1905. And what he said is, "Okay, so you've got two twins born on the same day on Earth. You pop one on a spaceship, send them off on a journey 10 light years away from Earth. But they're traveling at, this wasn't Einstein's numbers, but let's say 86.6% of the speed of light."

• 8:01Michael Stevens?

  That was a specific percent.

• 8:02Hannah Fry?

  It was specific, wasn't it?

• 8:03Michael Stevens?

  Yeah.

• 8:04Hannah Fry?

  Now, the reason why I chose that number is because at that speed...

• 8:08Michael Stevens?

  88.6% the speed of light.

• 8:10Hannah Fry?

  86.6.

• 8:11Michael Stevens?

  86.6, got it, okay.

• 8:13Hannah Fry?

  Yeah, okay. I can see you're going to do the calculations in your head, Michael.

• 8:16Michael Stevens?

  No, I'm not. I just want to remember because there's a cool fact about it you're about to tell me.

• 8:20Hannah Fry?

  There is.

• 8:20Michael Stevens?

  And I don't want to forget.

• 8:21Hannah Fry?

  Okay, you ready?

• 8:21Michael Stevens?

  86.6% the speed of light is special because?

• 8:24Hannah Fry?

  It's special because it means that time on Earth will travel faster than the speed on the spaceship by a factor of two.

• 8:37Michael Stevens?

  Double, okay, perfect. All right, so at that speed, I'm traveling at that speed. My stationary twin on Earth is going to be aging twice as fast.

• 8:46Hannah Fry?

  Correct.

• 8:47Michael Stevens?

  Right.

• 8:47Hannah Fry?

  So if you go off your pop, right, accelerate away, 10 light years away, turn around, come back, by the time you come back, it's going to have taken you 11 and a half years.

• 8:57Michael Stevens?

  They're twice as old as me now, even though we were born on the same day. And it took me 11 and a half years to go 10 light years?

• 9:06Hannah Fry?

  Correct.

• 9:07Michael Stevens?

  But then I have to come back.

• 9:08Hannah Fry?

  Yeah.

• 9:09Michael Stevens?

  Why did it only take me 11 and a half years? Shouldn't it take me 10 years to go at light speed?

• 9:14Hannah Fry?

  So in total, the distance that you've traveled is 20 light years.

• 9:18Michael Stevens?

  Okay, yeah, because you went 10 light years away and back.

• 9:20Hannah Fry?

  Correct. However, you're going at 86.6% the speed of light, so that's where you get the 23 from.

• 9:26Michael Stevens?

  That's how long it took, according to your twin on Earth.

• 9:30Hannah Fry?

  Yeah, your twin on Earth is like, right, off you go. You, distance and time, it's very simple. Speed is distance over time. I know your speed, 86.6% the speed of light. I know the distance, 10 light years.

• 9:41Michael Stevens?

  I'll see you in 23 years.

• 9:42Hannah Fry?

  See you in 23 years.

• 9:43Michael Stevens?

  But for you, who's traveling so quickly, how long has elapsed according to your watch?

• 9:48Hannah Fry?

  11 and a half years.

• 9:50Michael Stevens?

  Just half as long.

• 9:51Hannah Fry?

  Half the time.

• 9:53Michael Stevens?

  Wow.

• 9:53Hannah Fry?

  And you come back and you're like, looking in your twin's face and you're seeing your future, essentially. You are seeing what lies ahead of you in your body.

• 10:02Michael Stevens?

  11 and a half years from now.

• 10:04Hannah Fry?

  Exactly.

• 10:04Michael Stevens?

  You say, "I will look like that."

• 10:06Hannah Fry?

  Right.

• 10:07Michael Stevens?

  Bummer.

• 10:07Hannah Fry?

  Bummer.

• 10:08Michael Stevens?

  And your old twin is like, "Oh, maybe great." Or maybe great. Probably great.

• 10:13Hannah Fry?

  Probably great.

• 10:13Michael Stevens?

  With the way twins are, you know?

• 10:15Hannah Fry?

  Yeah.

• 10:15Michael Stevens?

  And your earthbound twin is looking at you going, "Oh, 11 and a half years ago, I had that appearance."

• 10:23Hannah Fry?

  So fresh-faced.

• 10:24Michael Stevens?

  More or less. I mean, things can happen. Maybe you, you know, decided to grow a beard and your twin didn't. I don't know, there could be some differences.

• 10:31Hannah Fry?

  There could be.

• 10:32Michael Stevens?

  But the point is...

• 10:33Hannah Fry?

  Maybe your twin on Earth really took up smoking and drinking heavily.

• 10:35Michael Stevens?

  Can I ask you another question?

• 10:36Hannah Fry?

  Go ahead.

• 10:37Michael Stevens?

  So, okay, you're on the ship.

• 19:12Michael Stevens?

  Yeah.

• 19:13Hannah Fry?

  Yeah, maybe don't do that. But that's the theory anyway. That's like Einstein's theory of how one would age compared to the other. But NASA aren't stupid, right? They sort of knew that they wouldn't be able to

• 19:24Michael Stevens?

  Go fast enough, yeah.

• 19:25Hannah Fry?

  Actually measure the difference of five milliseconds of life. But what they did do, and the reason why they did this twin study, was to look at the impact of physically being in space.

• 19:36Michael Stevens?

  Ah.

• 19:37Hannah Fry?

  Not just because of general relativity.

• 19:39Michael Stevens?

  But all the other things.

• 19:40Hannah Fry?

  All of the other things.

• 19:41Michael Stevens?

  That have much bigger effects, like no G force on your body for a year.

• 19:45Hannah Fry?

  Right, between the two brothers.

• 19:48Michael Stevens?

  Probably more radiation.

• 19:49Hannah Fry?

  Right. And let me tell you, Michael, the effects of being in space are mental.

• 19:55Michael Stevens?

  Right.

• 19:56Hannah Fry?

  Completely insane.

• 19:57Michael Stevens?

  All right, tell me, what happened?

• 19:59Hannah Fry?

  Okay, so one of the big things, um, one of the astronauts I got to meet, he is not allowed to be in space anymore. Because being in space has changed the physical shape of his eye.

• 20:10Michael Stevens?

  Hmm.

• 20:11Hannah Fry?

  Because of the way that the pressure changes. So on the ISS, they have to have loads of spare pairs of glasses.

• 20:20Michael Stevens?

  Oh.

• 20:20Hannah Fry?

  Up there, floating around.

• 20:22Michael Stevens?

  Your prescription changes.

• 20:23Hannah Fry?

  Your prescription.

• 20:24Michael Stevens?

  Why do your eyeballs change? Because surely the air pressure inside can be made similar to Earth's.

• 20:32Hannah Fry?

  Right. It's not just about pressure, though. I mean, so your optic nerve swells, you get all these folds that appear. You get, uh, it's sort of like wet wallpaper, basically.

• 20:42Michael Stevens?

  Yes, because there wouldn't be gravity keeping the blood away from your eyes. The blood just is like, I'm gonna be here.

• 20:48Hannah Fry?

  So, so, I mean, the whole shape of it changes.

• 20:51Michael Stevens?

  Yeah.

• 20:51Hannah Fry?

  And then, and permanently so in some cases. There are some astronauts, there's one guy called, uh, John Phillips, who went up, had 20/20 vision when he went up. And then when he came back, it was 20 out of 100. I don't know loads about how those numbers work, but I think that's not good.

• 21:05Michael Stevens?

  That means that what, what, uh, a, a, a person with normal vision can see from 100 feet away.

• 21:10Hannah Fry?

  Right.

• 21:11Michael Stevens?

  He'd have to be 20 feet away to see.

• 21:12Hannah Fry?

  Okay.

• 21:13Michael Stevens?

  Yeah.

• 21:13Hannah Fry?

  Which is, I would say, bad.

• 21:15Michael Stevens?

  Which is worse than 20/20.

• 21:17Hannah Fry?

  So there were all these theories about why, what is it, what's going on? I mean, something's definitely increasing the pressure on astronauts' eyes. But, uh, what is it? Is it to do with lymphatic fluid that's moving around in, in microgravity? Uh, and NASA calculated that the equivalent of two large plastic bottles of Coke.

• 21:37Michael Stevens?

  Mm-hmm.

• 21:37Hannah Fry?

  Um, shifts from your legs when you're in space towards your head.

• 21:43Michael Stevens?

  Wow.

• 21:44Hannah Fry?

  You got a lot of fluid. I mean, that's sort of why they look a little bit like they're being choked.

• 21:48Michael Stevens?

  That's one of the main reasons I wouldn't wanna go up. I think that if my face got all puffy and was all, like, congested, I would just feel so grumpy. They wouldn't invite me back anyway.

• 21:59Hannah Fry?

  No, I mean, you wouldn't do 340 days up there, would you? Um, the thing is, is that they did a study into this in 2015. And they were looking specifically at people on the Varmic comet to, to work out how much pressure was in their heads in that moment. And they were actually surprised to find out that intercranial pressure, right, intercranial, so inside, actually drops during the periods of lower gravity.

• 22:23Michael Stevens?

  Okay.

• 22:24Hannah Fry?

  So the, the current theory about why is all of this stuff going on in your eye? So even though your face is getting puffy and your lymph is up in your face, it's inside your head, you're actually okay.

• 22:34Michael Stevens?

  Okay.

• 22:34Hannah Fry?

  Right? So the current theory is that it's your, uh, cerebrospinal fluid.

• 22:39Michael Stevens?

  Oh.

• 22:40Hannah Fry?

  That is floating around. This is the stuff that helps to cushion your brain, by the way. And when you are not in a sort of upright position, when you don't have gravity assisting you. That pressure is like all over the place and your eye ends up changing as a direct result of it.

• 22:58Michael Stevens?

  Jeez.

• 22:59Hannah Fry?

  Mm-hmm. Not good, is it?

• 23:00Michael Stevens?

  Why does your face change?

• 23:03Hannah Fry?

  Because of the lymph, the lymphatic fluid.

• 23:04Michael Stevens?

  Because of the lymph, lymphatic fluid.

• 23:06Hannah Fry?

  Exactly.

• 23:06Michael Stevens?

  Your eyeball is harmed by this cerebrospinal fluid stuff, because the optic nerve goes right into the brain.

• 23:12Hannah Fry?

  Exactly.

• 23:13Michael Stevens?

  So you're like, wow.

• 23:14Hannah Fry?

  Yeah, not good.

• 23:16Michael Stevens?

  There's more you're going to tell me.

• 23:17Hannah Fry?

  Oh, there's, oh, there's more. Not everybody is affected in the same way with their eyes. It affects some people worse than others. But we're talking, we're talking the whole of the body is essentially like really not happy when it goes into space.

• 23:28Michael Stevens?

  I can see that this would be a big challenge for long space flights.

• 23:32Hannah Fry?

  Mm-hmm, very definitely.

• 23:34Michael Stevens?

  We've gotta come up with some artificial gravity.

• 23:36Hannah Fry?

  Yes.

• 23:37Michael Stevens?

  Maybe some more stuff. You tell me, what else goes wrong?

• 23:39Hannah Fry?

  Because this is the thing, you just don't expect that gravity would make that much of a difference. But it turns out that, that the Earth is the reason why our bodies function in the way that we do. We are, we are of this planet, right? We really are of this planet.

• 23:54Michael Stevens?

  We really are. This is our womb and our crib and our world and our grave. And we think, oh, it would be so cool to have no gravity around, uh, and then our bodies tell us, actually, this is what I was built for. What are you doing?

• 24:11Hannah Fry?

  Okay, so let me give you some examples about the ways in which our body is built for this. Body temperature.

• 24:16Michael Stevens?

  Hmm.

• 24:17Hannah Fry?

  Is so much harder to control when you're in space.

• 24:20Michael Stevens?

  Is it really?

• 24:21Hannah Fry?

  Than it is when you're on Earth. Because if you're on Earth, right, you lose most of your, your body heat through, through convection. You've got air that's moving around you, you've got cooler air. You've got that sort of movement, turning on a fan on a hot day, all of that kind of thing. But on the ISS, there's, there's no convection.

• 24:37Michael Stevens?

  Right, because there's no buoyancy. More air doesn't rise. More air stays kind of around us as an envelope around your body, yeah.

• 24:46Hannah Fry?

  So sweat sticks to your skin as well. And astronauts' temperatures when they're up in space rises by about one degree C. You're constantly hot and puffy.

• 24:56Michael Stevens?

  Yeah.

• 24:57Hannah Fry?

  And you can't see.

• 24:58Michael Stevens?

  Hot and puffy. A fan would help.

• 25:01Hannah Fry?

  A fan would help, agreed, because then you're mechanically moving the air. Humans have sort of outsourced a lot of our, our thermoregulation to the planet. Which I think is a kind of crazy idea. There's also our circadian rhythm, our, our sense of time, our sort of concept of days and nights. Just goes completely all over the place when you're up in space. Because, because that, again, is tied to the orbit of our planet around the Sun, right? And, and the rotation of our planet, um, on its own axis. So there've been lots of studies into this.

• 25:35Michael Stevens?

  Okay, in the ISS, they can control their lights, but they've got, what, 17 sunrises and sunsets a day? So you have to just rely on the artificial lighting in the ISS for night and day.

• 25:48Hannah Fry?

  You do.

• 25:48Michael Stevens?

  I wanna write this down, or a couple of things down, because I, I'm obsessed with the ways in which we offload our mind to calendars and computers and, and notifications and, and alarms. But you're right, even before technology, our bodies were offloading functions to physics in general.

• 26:10Hannah Fry?

  Right.

• 26:10Michael Stevens?

  I need to dump heat. Luckily, convection exists.

• 26:13Hannah Fry?

  Thank you very much.

• 26:14Michael Stevens?

  I need to, uh, not have my eyeballs get all fricked up. Luckily, gravity will keep those fluids at bay.

• 26:23Hannah Fry?

  That's the closest I've ever heard, heard you get to swearing.

• 26:26Michael Stevens?

  I can't believe that. If my mom hears that I said fricked, I said it again. I'm gonna have to go to space.

• 26:35Hannah Fry?

  Just to get away. But also, this idea of like when to rest, you know, our bodies automatically know when to rest. And that is something else that we have completely outsourced to the planet.

• 26:45Michael Stevens?

  We have.

• 26:46Hannah Fry?

  In the form of our circadian rhythms and daylight being the, the, the primary driver of it. There have been much longer, older studies into this about how messed up we get about the passage of time when we don't have those, those cues. One that I really liked was in the 60s, a 23-year-old geologist got put into a cave. Um, and then just sort of left there. This is in the Ligurian Alps, uh, about 130 meters down. I would say not a nice place to spend some time. So it's like, really cold.

• 27:20Michael Stevens?

  It depends on how introverted you are. Not really, no. It depends on how much you treasure your sanity, because, yeah.

• 27:27Hannah Fry?

  It sends you completely wild. So there's a one-way phone line he could call up. And the team would not call down, but they were instructed to never mention the time or the date whenever he called them. Um, and every time he woke, or he ate, or he, he prepared to sleep, he would, uh, ring them so that they could log it. So they could have little clockens that he wasn't allowed to see. And he just had the freedom to eat when he was hungry and sleep when he was tired and so on. Anyway, when he eventually came out, he had been down for 63 days. Would you like to guess how long he thought he'd been down for?

• 28:04Michael Stevens?

  Yeah, hold on. My guess is that he thought he'd been down for, ah, gosh, I don't even know, longer or shorter. I, I'm gonna say he thought he'd been down for 90 days.

• 28:17Hannah Fry?

  Right, which would have been my guess as well. I would have, I would have assumed that it was torture. And that it just felt like time had stretched in the, in the sense of psychologically. Actually, he thought he'd only been down for 35 days.

• 28:29Michael Stevens?

  Whoa.

• 28:30Hannah Fry?

  Right.

• 28:31Michael Stevens?

  As long.

• 28:31Hannah Fry?

  Yes, I mean, it's the factor of two again. It's the same as if you send your twin in a spaceship, but except you haven't actually aged for real. Which I think is wild, but basically what happened is his circadian rhythm stretched a little bit day by day by day by day. Until he was living not twenty four hour days, but sort of thirty forty hour days and just not even realizing that that was what was happening.

• 28:54Michael Stevens?

  Wow.

• 28:55Hannah Fry?

  But the same thing, if you're not careful, um, without the light cues, without the sort of the sense of time and light cues, you just, you just cannot keep track of time. I mean, I think clocks sort of help this as well. As you say, you've sort of outsourced this to a clock.

• 29:07Michael Stevens?

  Yeah, yeah. So if we lived in like this room with no natural light, but we had a clock, we could still say, oh, I think it's bedtime now. But I, I spent three days in an isolation room with no clocks.

• 29:18Hannah Fry?

  And the lights were on, weren't they?

• 29:19Michael Stevens?

  And the lights would never go off.

• 29:21Hannah Fry?

  Oh.

• 29:22Michael Stevens?

  And no meals were delivered because I didn't want to ever get any clue. So all the food was there with me. And I didn't ring anyone up because I was afraid that even if I rang up to be like, oh, hey, I've decided to go to sleep. That I could tell what time it was based on how groggy the person sounded.

• 29:39Hannah Fry?

  Right.

• 29:39Michael Stevens?

  If it was like the main producer answering, it's the middle of the day. If it was some younger, uh, production assistant, I'd be like, it's the middle of the night. Okay. And I had them surround the whole chamber with loud fans because I didn't want to be able to hear. That sounds like someone who's doing some maintenance work that wouldn't be happening at night. It's not night. So. Uh, but I got immediately confused. By the next day, I thought it had been two days or something. Uh, but, and that was only a three day isolation.

• 30:09Hannah Fry?

  So you thought it was longer.

• 30:11Michael Stevens?

  Yes, I thought, okay, it's definitely Sunday by now. And then down on the screen, they're like, it's Friday at eleven P M. Like I was just way off. I would take little naps and wake up and think, oh, man, it's been a whole night. But I, uh, I, I couldn't turn the lights out.

• 30:27Hannah Fry?

  Did you have an eye mask?

• 30:29Michael Stevens?

  No, no, but I used like the pillows in the bedding and I had a little, there were cameras in there the whole time too. So I had a like a kind of a private ish toilet, but um, I just meditated. I didn't have, I started playing with my sock and I said, no, this is about nothing coming into my brain.

• 30:48Hannah Fry?

  Right.

• 30:49Michael Stevens?

  And it felt like it whizzed by looking back.

• 30:55Hannah Fry?

  At the time then?

• 30:55Michael Stevens?

  At the time, it felt like, is this ever going to end? Are they going to leave me in here? But I have no memories from it because nothing happened.

• 31:03Hannah Fry?

  Yeah. Oh my gosh. I can't, I can't, I, I would worry that I would permanently psychologically damage myself in that situation. You are much braver.

• 31:11Michael Stevens?

  I hoped I would, the opposite of worry, but I'm fine as it turns out.

• 31:17Hannah Fry?

  As it turns out, we think, still too early to tell.

• 31:21Michael Stevens?

  Is it? Is it? All right. So circadian rhythms.

• 31:25Hannah Fry?

  Yeah, your sense of time anchored to the planet. Your bones also, by the way, another thing extremely anchored to, to gravity. And what's happening is astronauts, they, they're losing bone at this unbelievable rate.

• 31:40Michael Stevens?

  They just don't need it.

• 31:40Hannah Fry?

  They just don't need it because you shouldn't think of your bones as though there's this storage unit. You should instead think of it, it's almost like a, a bank, right? Sort of like making deposits and withdrawals all the time. And when you stop requiring these withdrawals, it just sort of decays away, right? Just stops like remaking itself. The thing is, is that the way that your skeleton exits your body, do you know this?

• 32:04Michael Stevens?

  No.

• 32:05Hannah Fry?

  You essentially urinate it out.

• 32:07Michael Stevens?

  Oh, really? So it's put into the liquid waste.

• 32:11Hannah Fry?

  Yes, you're pissing out your own bones.

• 32:15Michael Stevens?

  This sounds kind of fun, actually, not gonna lie.

• 32:19Hannah Fry?

  But kidney stones are a really big problem in space because that's essentially one of the, one of the things, right? As you are urinating, um.

• 32:28Michael Stevens?

  Your bones away.

• 32:29Hannah Fry?

  Right.

• 32:29Michael Stevens?

  They collect in the.

• 32:31Hannah Fry?

  Exactly. They're crystallizing in your kidneys. And the thing is, once you get back to earth, you're still in trouble from all of this. So there was one study in twenty twenty two, which looked at astronauts a year after they'd been in space. And if they had been for six month missions, they still hadn't recovered a year after. Essentially their shin bones looked a decade older.

• 32:53Michael Stevens?

  Wow.

• 32:54Hannah Fry?

  Than they would have been had they stayed on earth.

• 32:57Michael Stevens?

  We need that gravity.

• 32:58Hannah Fry?

  We need that gravity. Uh, blood also, by the way, you are breathing out your dead blood cells as you're in space. You can tell because every time a red blood cell dies, it releases one molecule of carbon monoxide.

• 33:11Michael Stevens?

  Oh.

• 33:11Hannah Fry?

  And so you can tell how much blood is decaying in somebody by giving them a breathalyzer, essentially by testing.

• 33:19Michael Stevens?

  More CO two in their breath.

• 33:21Hannah Fry?

  Right.

• 33:21Michael Stevens?

  More blood cells die.

• 33:22Hannah Fry?

  And space anemia is a really well documented thing that people are losing blood cells in space. At a rate of one and a half times what you would be getting on earth.

• 33:32Michael Stevens?

  Wow, one and a half times.

• 33:34Hannah Fry?

  It's a lot, isn't it? So you're urinating out your skeleton, you're breathing out your.

• 33:38Michael Stevens?

  Blood.

• 33:39Hannah Fry?

  Breathing out your blood. You can't see anything unless you've got the glasses that they've got on the ISS.

• 33:44Michael Stevens?

  Can't really sleep.

• 33:45Hannah Fry?

  Can't, you can't really sleep.

• 33:46Michael Stevens?

  You're hot.

• 33:47Hannah Fry?

  It's hot and you're sweaty, but your sweat's not going anywhere and your face is all puffy. I mean, this is like, frankly, it doesn't sound that great, does it?

• 33:55Michael Stevens?

  No.

• 33:55Hannah Fry?

  One good thing is your DNA is all right. You're okay. Your DNA is okay. Uh, and we know this because there was an experiment where they took some mouse sperm, freeze dried it, popped it up to the ISS. Left it there, you know, just sitting in storage for a little while, brought it back down to earth.

• 34:13Michael Stevens?

  Thawed it.

• 34:13Hannah Fry?

  Thawed it, used it for IVF, put it in a mouse, absolutely totally fine, happy mouse babies.

• 34:20Michael Stevens?

  Congratulations.

• 34:21Hannah Fry?

  Grandchildren, mouse babies, absolutely fine. Your DNA, fine. If it's in a sperm cell, no big deal. Happy.

• 34:27Michael Stevens?

  Okay.

• 34:28Hannah Fry?

  In your body, however, slightly different, slightly different story. So while your DNA is, it's like a recipe of how to make the whole of you as a human. The way that those genes are expressed, changes in your whole body, changes all of the time, right? When you sort of, I don't know, do a lot of exercise or, or even going back to the bone thing, right? When you're, when you're under gravity, there'll be certain expressions that are dominant over others. When they did the testing during the, the twin study, when they were testing Scott's genes. In space and then Mark's genes on earth. So they described it as like a set of fireworks going off in Scott's body. Scott was the one in the ISS.

• 35:09Michael Stevens?

  Yeah.

• 35:10Hannah Fry?

  You would definitely expect to see some of these changes if you take a human and you put them in a really stressful situation, right? There's like lots of changes that the body is adapting to. But what was really interesting was just the, the sort of the scale of the magnitude and the speed of this. But also that when he was brought back down to earth, about ninety percent of these changes reversed. But ten percent of them lingered.

• 35:35Michael Stevens?

  They didn't go back.

• 35:36Hannah Fry?

  They didn't go back. The suggestion for why this might be the case is that he's exposed to so much more radiation than his brother was.

• 35:43Michael Stevens?

  Oh.

• 35:44Hannah Fry?

  And this matters because these systems are things that are involved with immune function, with DNA repair, with like. You know, inflammation, with oxidative stress, with like bone formation, all of this stuff.

• 35:57Michael Stevens?

  And this was primarily, they think, because of radiation damage?

• 36:01Hannah Fry?

  Potentially.

• 36:01Michael Stevens?

  Right.

• 36:02Hannah Fry?

  Cause the thing is, is that Mark Kelly, who was on earth, his gene expression has changed too, but because he'd lived a year.

• 36:08Michael Stevens?

  Sure. But he also had this giant atmosphere, uh, protecting him.

• 36:12Hannah Fry?

  Right.

• 36:12Michael Stevens?

  From radiation.

• 36:14Hannah Fry?

  Right.

• 36:14Michael Stevens?

  Which his brother up in space, more exposed.

• 36:16Hannah Fry?

  Much more exposed. And that brings us then to the biggest change of all.

• 36:21Michael Stevens?

  Oh.

• 36:21Hannah Fry?

  Everything you had so far is like, okay, yeah, sure. Your, your face is puffy, whatever, your bones are, you're pissing out your bones. Um.

• 36:29Michael Stevens?

  Big deal.

• 36:29Hannah Fry?

  Big deal. Who cares?

• 36:31Michael Stevens?

  I call that Tuesday.

• 36:33Hannah Fry?

  But this one is like really dramatic. Do you know what a telomere is?

• 36:38Michael Stevens?

  Uh, yeah, it's like the end of the DNA.

• 36:41Hannah Fry?

  Exactly. It's sort of, it's best described as the, the cap on the end of a shoelace.

• 36:46Michael Stevens?

  Okay.

• 36:46Hannah Fry?

  That protects it. When DNA is replicating itself, you know, when cells are dividing and replicating themselves, this thing gets shorter and shorter and shorter over time. And it's actually a really good way to assess how old somebody is.

• 36:58Michael Stevens?

  Uh huh.

• 36:58Hannah Fry?

  So their epigenetic age, as it were, as opposed to their chronological age.

• 37:02Michael Stevens?

  Right.

• 37:03Hannah Fry?

  You want long telomeres.

• 37:04Michael Stevens?

  Got it. Yeah.

• 37:05Hannah Fry?

  You want nice and long, means you're nice and young. You've got plenty more replications to go before you lose more and more of that shoelace. When Scott Kelly was in orbit, his telomeres got longer.

• 37:17Michael Stevens?

  Oh.

• 37:17Hannah Fry?

  Right?

• 37:18Michael Stevens?

  I was just going to ask, what can I do to lengthen my telomeres?

• 37:22Hannah Fry?

  Right.

• 37:22Michael Stevens?

  Are there any like snake oil salesmen selling, you know, telomere lengthening cream?

• 37:27Hannah Fry?

  Yeah, NASA.

• 37:29Michael Stevens?

  Go to space and it's NASA. Whoa.

• 37:31Hannah Fry?

  NASA are selling.

• 37:32Michael Stevens?

  So, okay, do we have any ideas why?

• 37:35Hannah Fry?

  Why that's going on? Well, okay. I should also add, it sounds really good. While you're in space, you've got nice long telomeres. Isn't that lovely? Kind of makes up for all of the other horrible stuff. When he came back down to earth, the length of his telomeres collapsed. And were then shorter than they would have been before.

• 37:53Michael Stevens?

  What a rug pull.

• 37:54Hannah Fry?

  What a rug pull.

• 37:55Michael Stevens?

  You had me ready to buy my ticket to the moon.

• 37:58Hannah Fry?

  Cause this is it. Then it's like Einstein was right. You live longer when you go into space. As long as you don't come home.

• 38:04Michael Stevens?

  You go, oh, I like that.

• 38:08Hannah Fry?

  I should say, it wasn't like all of them were suddenly way short, but he had an increased number of critically short telomeres.

• 38:16Michael Stevens?

  It sounds painful.

• 38:18Hannah Fry?

  It sounds critical, frankly.

• 38:19Michael Stevens?

  It's, it does sound critical. It sounds, it sounds bad.

• 38:22Hannah Fry?

  It sounds bad. Okay. So the question as to why this might be the case, there's another twin study where they took two pairs of twins and they took them to Everest. Two of the twins, right, from different families, uh, went up the mountain. Their brothers stayed down the bottom of the mountain and they did the same thing. They were like doing all of this testing on them. They had to like draw blood from them while they're at the top of the mountain, which I think was apparently quite dramatic.

• 38:44Michael Stevens?

  I bet, yeah.

• 38:45Hannah Fry?

  Um, anyway, they saw the same thing, that the telomeres elongated when they were at the top of the mountain. The same thing as you get if you're on board the ISS, which is like a really strange thing. But once again, once they came down, it didn't end well.

• 39:01Michael Stevens?

  Right.

• 39:02Hannah Fry?

  So the suggestion as to why this might be the case, there's only two ways that the human body can lengthen their telomeres. One, when you're born, there's a thing called telomerase. It's an enzyme that you find in embryos. You also find it in stem cells. I mean, it does exist, but just not in the normal run of things in your, in an, in the adult body. The other way that you can lengthen your telomeres is something called ALT, which is alternative lengthening of telomeres.

• 39:27Michael Stevens?

  Hmm.

• 39:27Hannah Fry?

  And this isn't rebuilding. It's more that it's using a sort of DNA trick. It's like copying the sequence from another chromosome. It's extremely rare and you really only see it in certain types of cancer cells.

• 39:40Michael Stevens?

  How?

• 39:41Hannah Fry?

  Mm hmm.

• 39:42Michael Stevens?

  So why would that be happening to people in space and then at the top of Everest?

• 39:47Hannah Fry?

  Like who, honestly, who knows? It sort of seems as though what is happening is this ability for your cells to lengthen their own telomeres is switched off, but kind of remained dormant unless you push that human into a really extreme environment, at which point your body sort of starts acting like it's cancer cells.

• 40:11Michael Stevens?

  Or it starts acting like it's being born again.

• 40:13Hannah Fry?

  Like it's being, well, although it's a different, it's a different mechanism.

• 40:16Michael Stevens?

  Different mechanism. It's ALT, not the telomerase.

• 40:18Hannah Fry?

  That's a really important point. In both the Everest and the twin study, when they were in, it was Scott and Mark Kelly, it was the ALT mechanism.

• 40:28Michael Stevens?

  That's it. Okay.

• 40:29Hannah Fry?

  Not the young, cute little baby one that you get in embryos.

• 40:32Michael Stevens?

  Bummer for them.

• 40:33Hannah Fry?

  Bummer for them. This raises this really interesting question because it means that we know that humans can lengthen their own telomeres. And this is like a very important thing. If you're talking about lengthening people's lives, it's been demonstrated that you can force people into a situation where their telomeres lengthen. You also then, everybody ended up worse off, right? As, as soon as they had this happen to them, they ended up worse off. But going back to Einstein's idea of who ends up being older, the one on the spaceship or the one on earth, this telomere thing, while you're in space, you maybe have a longer lifespan, but get back, the one who went to space is worse off. It's the opposite way around to what Einstein said.

• 41:16Michael Stevens?

  So to them, less time has passed.

• 41:19Hannah Fry?

  Mm hmm.

• 41:20Michael Stevens?

  They may have aged visibly on the surface less.

• 41:23Hannah Fry?

  Mm hmm.

• 41:23Michael Stevens?

  But deep in their DNA, they might actually be older.

• 41:27Hannah Fry?

  Might actually be older. Right. If you actually put them on a, the rocket ship that went away for 10 nineties.

• 41:33Michael Stevens?

  So who would you rather be? The twin on earth or the twin in space?

• 41:37Hannah Fry?

  On earth. Thank you very much. Much rather stay on it. You mentioned earlier, how are we going to get people long distance space travel if all of this stuff is going on?

• 41:46Michael Stevens?

  Yeah.

• 41:47Hannah Fry?

  Maybe they just need to be born in space.

• 41:50Michael Stevens?

  Yep.

• 41:50Hannah Fry?

  Maybe they just need to not have the history of, of gravity in their midst.

• 41:57Michael Stevens?

  Okay. All right. So, right. If we start delivering babies in space, they will be prepared for it.

• 42:05Hannah Fry?

  Yeah. I mean, there has been a bit of delivering babies in space, just not human.

• 42:08Michael Stevens?

  Not human. Yeah. They've done like rat babies, I think.

• 42:10Hannah Fry?

  Fish as well.

• 42:11Michael Stevens?

  Oh, fish. Or in the case of the telomeres, we maybe, maybe the change we kind of like, but coming back, we don't.

• 42:20Hannah Fry?

  Mm hmm. I mean, yeah, I think that the only solution to this is that we, we do some pregnancy.

• 42:25Michael Stevens?

  We've got to do a lot more reproduction in space.

• 42:28Hannah Fry?

  We need, we need a maternity ward in space.

• 42:30Michael Stevens?

  Yeah. Get on it, guys.

• 42:31Hannah Fry?

  Um, and we started this episode by saying that astronauts are willing to, uh, to give themselves over for the study of, of science. I think a newborn astronaut, it's not beyond the realms of what humans can achieve.

• 42:42Michael Stevens?

  The newborn can't consent to being an astronaut.

• 42:45Hannah Fry?

  It's all right. They're doing it for humanity.

• 42:48Michael Stevens?

  We can thank them later.

• 42:49Hannah Fry?

  We can thank them later.

• 42:51Michael Stevens?

  Well, find a way to lengthen their telomeres back.

• 42:53Hannah Fry?

  Yeah.

• 42:54Michael Stevens?

  It's okay. So those of us on earth have a lot to be very, very grateful for. And we have a lot to be grateful for to those who go out into space for us because they're living through time differently and they're changing their DNA.

• 43:11Hannah Fry?

  For us. Thank you astronauts and thank you listeners.

• 43:17Michael Stevens?

  Yeah. Whereas we do this show for you.

• 43:20Hannah Fry?

  Huh?

• 43:22Michael Stevens?

  And we love doing it. So thanks for being with us today. Be sure to subscribe to us on YouTube or follow us on whatever platform you're using to, to watch or listen. And, uh,

• 43:33Hannah Fry?

  Field Notes later in the week. We'll see you then. Bye bye.

• 43:36Michael Stevens?

  Bye.

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