Listener Questions #24

23:43And then if you're talking about organisms other than humans, you can get some other slightly different sorts of signals that are approximately 24 hours. So then what parts of the human body are affected by it? Obviously, you sleep in this sort of 24-hour cycle. Is there other stuff going on also? Yeah, lots of things. So like sleep is important. Metabolism is also important. Like your body temperature is impacted by circadian rhythms. Metabolism. So is that why I shouldn't eat chips at 10 p.m.? That could partly be it or be involved. I think cortisol is to some extent involved in helping with digestion and stuff.

24:20And so your cortisol kind of peaks at times of day when you're like expected to be hungry and expected to be eating. What's cortisol? Cortisol is a hormone that folks usually associate with recovery from stress. So when you get stressed out by something, your body releases cortisol, and then cortisol helps your body sort of return to homeostasis or like a normal state after you've been stressed out. Cortisol sounds great. I should get some more of that. Well, if you're like, you know, running away from a lion, cortisol is great. Like it releases a bunch of energy all at once that you can outrun the lion, and then when you're in a safe spot, it helps you sort of return to normal.

24:56But if your cortisol level is elevated for a very long time, you can start having diseases because you're constantly in a stressed out state. So cortisol is more supposed to like get you out of an intense acute situation. But if it's elevated chronically, it can be bad. And there's a really interesting book on this called Why Zebras Don't Get Ulcers by Robert Sapolsky. Cool. All right. Yeah. Well, I wonder if zebras go to my physics faculty meetings if they feel stressed out and need cortisol afterwards. Oh, yeah. Faculty meetings. I think what I'm stressed out about in faculty meetings is that I could be spending my time way better doing anything else.

25:31But, you know, I haven't had to go to faculty meetings for a while. So that's great. But our question today is not about silly arguments in physics faculty meetings. It's about the rhythms of the body. So you're telling us that these things happen. It impacts sleep and metabolism and temperature. But what's the mechanism for it? Like, what is driving it? Yeah. OK, so most of the work that we've done to figure out the mechanism has been done in organisms like mice and fruit flies. But here's how we think that it works, how we're guessing that it works in humans based on what we've seen in lab animals.

25:58So there's a little part of your brain called the hypothalamus. And inside of the hypothalamus, there's a little region called the super chiasmatic nucleus or the SCN because I'm not going to try saying that a bunch of times. It sounds a lot like supercalifragilisticexpialidocious, but all right. Yeah. I hear that every time you say that now. Okay. No one's going to be able to pay attention to the rest of the episode. they're going to be singing just like Mary Poppins, but try to focus people. All right. So you've got the SCN and the SCN is connected to the optic nerves. And so optic nerves, these are the nerves that go to your eyes.

26:32And so we've talked in the past about how your eyes have specialized cells called rods and cones, and those help you detect like patterns and colors and to see your world. But you also have cells that just detect the intensity of light. And those cells send information through your optic nerve back to the SCN, and your brain uses that information to tell the rest of your body how the rhythm should be working. And so it does this by either sending messages through the nerves or by directing the production of hormones that will then go to the rest of your body and talk to your cells and basically say, okay, hey, guys, it's morning.

27:11And when it's morning, your body increases its heart rate, increases its blood pressure, increases temperature. Your body is not making melatonin at this point. Melatonin is associated with sleep. So this is generally how your body collects the information about what should be setting the timing for the rhythm. But each one of your cells also has its own circadian clock. So we have these special cells in our eyes. instead of just using the information which already exists in the rods and cones, we like evolved a separate pathway just for this. Wow, this is pretty weird engineering. Well, but it's detecting something different.

27:49Your rods and your cones are detecting like colors and, you know, patterns of like dark against light and stuff like that. These cells are just detecting the intensity of the light that's coming in. So you're like, oh, it's dawn now or oh, it's noon. And so it's detecting that. I feel like I could write a computer program to extract the same information from the data produced by the cones and the rods, but that's fine. Obviously, the brain is not engineered by physicists. And that's not how evolution works. Evolution doesn't say, okay, yeah, I'm going to start from scratch and come up with the best system.

28:20It's like, well, what do I have and how can I work with it? I just think it's a fascinating clue that something happened there, that this is what we ended up with. You know, it's just another example of how, like, obviously this is not well organized, just what kind of worked. And so do we know this because we've, like, done studies in mice where we've, like, tweaked those cells or got rid of those cells or, like, shined light on those cells or something, and it's changed the way the mice behave? Yeah. So if you mess up a mouse or a fly's SCN, then they'll start free cycling. So essentially they won't show these 24-hour cycles.

28:52They're not so expiadocious anymore? That's right. That's right. It's truly quite atrocious. All right. So that's one. Oh, wait, hold on. You got it. Okay, that was pretty good. That was pretty good. Yeah, you're begrudgingly patting me on the back there. I was so excited about my next question. I didn't even register your joke, but that was excellent. 10 points for Kelly for everybody who's keeping score. But so you're telling us that not only these light signals tell us when to be fragilistic, but also the rest of our body responds in some way? Yeah. So first, I just want to mention real quick that organisms from bacteria to humans show these clocks.

29:31And so this is like not every organism uses their SCN. And so this sort of like feeds back on our conversation about how you just use what you have. This has like been going on for a really long time. Anyway, so not every single cell has its own circadian clock. I misspoke earlier. But many cells, even if you take them out of the body and you put them in like a Petri dish, they will show like a 24-hour schedule for the activities that they do. Like they'll repair DNA at a certain time, stuff like that. And so here's how we think that works. So your cell is making a protein called clock protein.

30:09The clock protein at dawn moves into your nucleus where the genetic information is stored, and it binds to literally thousands of different sites on your DNA. And when it binds, it's telling your DNA to start making certain things. And it could be making certain hormones, like it can say, hey, start making that cortisol. It could start doing, you know, whatever is needed to increase body temperature or heart rate, etc. So thousands of things are turned on when your clock proteins go in there and bind to lots of different spots. Another thing that's being made, though, during the course of the day are proteins that will shut this down.

30:45And these proteins are called period proteins. And so the period proteins will build up over the course of the day because the clock protein said, hey, start making these. And at some point they've built up to high enough levels that now they go into the nucleus and they pull the clock proteins off of the DNA. and that stops all of the stuff that the clock proteins had been turning on and getting made. As the night goes on, the period proteins break down and go away. And then the next morning, the clock proteins go back in and the cycle starts again. So like many clocks, you have some sort of process which has a natural timescale built into it, right?

31:22And so there's this chemistry where things are sloshing back and forth and naturally at the same rate as our 24-hour cycle. Is that what's happening? Yeah, yeah. That's a good summary. And I saw you looking something up. Was I wrong about something or was it unrelated? I was wondering if CLOCK was an acronym for something in a really tortured way. Oh, okay. All right. I was hoping it stood for something ridiculous. Okay. Because if it was a physics acronym, it definitely would have a ridiculous name. Got it. Anyway. So light is important for determining these cycles. Right. But it also is fine-tuned by things like the temperature you find yourself at, when during the day you're eating.

32:02And these cues are important for other animals as well. So it's not just light, but light does seem to be a super important factor. So it's interesting you have several different kind of things going on. Can they get out of sync or is there something that tries to keep them in harmony? Well, so once you develop a cycle, your body is pretty good at keeping that cycle going. So like if you stayed in a dark room for two days, your body would still have some of its normal cycle. It's not like it breaks down immediately, But over time, if you deprive your body of that light cue or you mess your cues up, then you can start having problems.

32:36So, for example, if you work the night shift, you know, any of us who have had jet lag have had like a temporary period where circadian rhythm was like something's not right and that it's had to get back to normal. But being off of your normal circadian rhythm too often, for example, working the night shift increases your risk of diabetes, obesity, depression, dementia and some kinds of cancers. So these circadian rhythms seem to be important for a lot of reasons, at least for humans. Wow. And how widespread are circadian rhythms? Are they everything on Earth has a circadian rhythm? Well, so like bacteria, humans, plants, lots of stuff has circadian rhythms.

33:14But our listener had a really great question, which is, how on Earth do you have a circadian rhythm if you live in a place with no light? Yeah. And so the three different situations that I looked into where there's no light are if you are, for example, at the North Pole during the time of year where you don't see the sun. There's like, I think, a couple months where that's the case. If you live in a cave or if you live in the deep sea. And then there's also organisms like naked mole rats that live underground. But I think naked mole rats can peek their heads out every once in a while and see the sun to help entrain their clock.

33:47So they get some light cues still. So let's start with if you live far north. So one thing that's important to note, which we touched on just a second ago, is that clocks don't go away immediately just because you're in total darkness. So at the start of the long night, they're probably fine because their body hasn't forgotten the rhythm yet. And during the time of year when it's all light, they're probably also fine because the intensity of the light still changes over the course of the day. and your eyes are focusing on light intensity, not just whether it's there or not. So they can still keep their rhythms at that time of year.

34:24But there is a point in the winter where their clocks have not had appropriate input for long enough because it's been dark for so long. And they do seem to stop showing signs of 24-hour cycles if you look at the reindeer. So it looks like at some point they do start free cycling, essentially, and their circadian rhythms start to break down. They don't seem to have physiological problems associated with that. I'm not quite sure why, but it looks like they do break down at some point. So it's bad for them to be in the dark. It would be better if they got light occasionally to sort of correct their cycles or get them back on track.

35:00Yeah, so I was trying to figure out the answer to that. And I think that the answer is to some extent that it's complicated. So part of why we have cycles is that it helps us figure out when we should be eating and stuff like that. And part of that, if you're a wild animal, is about when your food is even available. But if you're in the dark and all the other animals are in the dark all the time too, you might not need to have a circadian rhythm. Because it might, you know, if you're, say you're a fox living in the Arctic Circle, you want to make sure you're awake at the same time as the bunnies.

35:35But if it's complete darkness and everybody's free cycling, you don't really need to get up at a certain time because the bunnies could be out there at any time. And so it's not a helpful cue anymore to like have your activities sync to a certain time of day. I was wondering more about the internal stuff, like you mentioned earlier, that if your rhythm gets messed up, you're at risk for diabetes and cancer and stuff. Is the same thing true for reindeer when they're free cycling? So I don't know the answer for reindeer, but I did try to find the answer to that question. I was able to find some information about blind cave fish.

36:06So there are fish that live in caves that have closely related ancestors that live outside of caves. Oh, okay. And so you can compare, you know, essentially these like sister species or sibling species and see how they differ. And the species that live in caves don't have shorter lives. And in a lot of cases, they have longer lives than their surface living counterparts. Okay, and then let me tell you about their circadian rhythms. That's the important piece here. So people were trying to figure out if cave fish have circadian rhythms, and they were trying to figure out, like, okay, in the absence of light, how do you do that?

36:46And so one idea they had was that maybe bats that go in and out of the cave are what they're cuing into because when the bats leave or they come back, they poop in the water, and that poop provides food for a lot of cave fish. So maybe that's what they're sinking to, but there was no evidence that that was actually happening. So it looks like they're not sinking to that. But there could be a lot of similar effects, right? Where things are happening outside the cave and like even the microbes in the air or something like that could be affecting. You could be sensing indirectly the fact that there's day and night outside the cave from inside the cave.

37:20Yeah, that's right. But so people have also brought the cave fish into the lab. And when they bring them into the lab and they expose them to normal light cycles, they can develop circadian rhythms. So they still, even though they don't have eyes, they still must have like the cells needed to detect light and they can develop rhythms. But it looks like various parts of their circadian clock are messed up. So if you look at things from the genetic level, it's kind of messed up. And it looks like they also, for the most part, are losing their circadian rhythms in the cave, but that doesn't seem to be shortening their lives.

37:55Wow. Fascinating. Yeah. Which is not what I expected. Like I spent a long time being like, well, no, what is the cue? They have to have circadian rhythms. And then I found a review paper that pretty much was like their rhythms kind of seem to disappear in caves. They're just kind of like moving around whenever and sleeping whenever. And that's OK. And that's OK. We're not judging. Teenagers everywhere are like, see, mom, it's fine for me to stay up until 2 a.m. Right. And their rooms are kind of like messy caves. And I see lots of lots of points of comparison here. All right. But what about at the bottom of the ocean where light doesn't filter down?

38:31Okay, so this is interesting. So after I had finished the research on caves, my expectation was that animals that live in the deep sea are also going to not really show circadian rhythms. Because they're, you know, they're down in an area where the light isn't getting to. But I thought, well, you know, maybe there's still some cues like a lot of the food that comes to the deep sea comes from things just sort of like dying and raining down. and I thought maybe there's like a daily pattern to how the food rains down. But this is actually really hard to study. To when the carcass snacks happen. Yeah, well, you know, I pay attention to snacks.

39:08Kids, dead bodies are falling. Come on outside. What is it like to be a parent to the bottom of the ocean? I mean, when a whale falls down, it is like buffet for months. The videos are messed up. But so the way that folks tend to study this is, you know, they either take the deep sea animals into the lab. But if you bring them into the lab, you're often like turning on lights to study them. And so now they're in like a not natural environment. And, you know, labs in general are not like the bottom of the sea. But another way that people study it is they will put like essentially mobile labs. They'll lower them down to the bottom of the sea, and then they'll take pictures or videos and try to see if they can detect cycles and what's happening down there.

39:50But another problem there is that they turn on lights often when they do that. And so this stuff is hard to study, but I found a study that did use cameras with lights, and they found that a lot of species didn't show detectable patterns, but it was also hard to get large sample sizes. but they did find that there is a kind of worm that lives in a tube and there's a pattern to when it sticks its head out of the tube to like try to get food they were looking at different things that were changing in the environment and it looks like this behavior is correlated with the tides so you can still feel the tides at the bottom of the sea which is amazing i didn't realize that yeah thank you moon see it's all connected yes it turns out there is a through line for the whole episode.

40:34That's right. I guess bagettification is kind of interesting. But I guess, so it's not just the tides, but the tides have different temperatures. So it could be temperature that's queuing this. The tides also can bring food. So maybe it's the food that's queuing the clocks. And this information is largely observational. So there's still a lot that we have left to learn, but some indication that tides can be what's impacting timing in the deep sea. And that's literally everything I know about circadian clocks, because this is complicated. All right. So bottom line for us, what do we know about how circadian rhythms work in animals that never see the sun?

41:08Sometimes you're pwned and you can't create a circadian rhythm because you just don't have the cues. Other times you can find something that correlates with the light. Or sometimes instead of having a circadian rhythm, you have a circatidal rhythm or something. And you're queuing in on some other environmental thing that can help you maintain a rhythm in your life. Well, I thought that answer was supercalifragilisticexpialidocious, but let's hear what our listener says and see if there are follow-up questions. Wow, thanks for answering my question. Never would have thought that deep-sea creatures could sense the tides all the way down there.

41:44That's pretty cool.

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44:35We are back to Jupiter, a fascinating planet that could kill us all. Arthur, what do you want to know about Jupiter? Hi, Dan and Kelly. Nice to talk to you guys. I have a rocket thrust issue. I know that we can't escape the gravity of a black hole, no matter how powerful our rocket is. But I don't know the largest mass of a planet we can escape from with our current technology. I mean, can an average space rocket lift off from Jupiter? From the Sun? From a neutron star? I hope you have some fun answering this. Thanks. See, Jupiter is just so attractive gravitationally. People can't stop thinking about it.

45:19It's beautiful also. All right. So Arthur is wondering about taking off from planets because the more massive the planet, the stronger the gravity, the harder it is to lift off of. And he wants to know if an average rocket could actually get you off of Jupiter or even more exotic and denser locations. So the first thing I want to know is I can tell from the answer it's going to require math. Did you do these calculations or is there a website that has this information? There are a lot of websites that have this information, but I never trust them because you can find mistakes on those websites, which is the source of a lot of mistakes in like chat GPT because it just like strips them from the websites and gives you the answer.

46:03Sometimes in the wrong context or whatever. So I always double check these myself. All right. Wow. So what matters for lifting off the surface? You're probably going to think escape velocity. And first I want to say it's not about escape velocity, but then it's going to turn out to be about escape velocity. Physicists. I know. So escape velocity famously is the speed you have to be going so you can escape the gravitational pull of an object. Right? So, for example, if I'm standing on the surface of the Earth, how fast do I have to throw a baseball straight up so that it just keeps going forever?

46:34that its kinetic energy overcomes the potential energy well that has to climb out of, right? So as you move up away from the earth, you're gaining potential energy that has to come from somewhere, comes from your kinetic energy. If you have enough kinetic energy, then you can go forever, essentially. If your kinetic energy overcomes the potential well you have to climb. So you have to go fast to escape the earth. But the reason that's not what this is about is that that's not how rockets work, right? Rockets, you don't slingshot them from the earth in one push. I mean, people are working on that and it's hilarious, but traditional rockets - Wait, why is it hilarious and not inspirational?

47:13It's just like the grownup version of a nine-year-old boy's idea for how to get to space. You know, like, let's pull back a really big rubber band, you know? I mean, I knew they're working on centrifuges and it's pretty cool, but it seems like impractical to me also because the G-forces are insane. So I think you could probably launch like stuff into space, but not people. Anyway. That's my sense too. Yeah. You're launching stuff, hardened payloads, not humans. Anyway, that's not how traditional rockets work. If you notice when a rocket takes off, it's not going super duper fast. It's very slowly climbing, right?

47:46And that's because rockets don't have a single hard push at the beginning where they gain a lot of speed and then gradually lose it as they rise. They have a continual force. They have an engine on them. So rockets only have to go non-zero velocity in order to move up, right? As long as the force from the rocket is greater than the force of gravity from the Earth, it's going to be moving up. Okay. Right. So it can move up super duper slowly. It could take like a year to take off from the planet. It doesn't matter. It's not about escape velocity. It's about putting enough energy in to overcome the potential energy of the Earth, but it doesn't have to all be upfront.

48:21So that's why it's sort of not about escape velocity. Okay, but so say you were lifting off at like a foot per second, that would be much more energetically expensive, wouldn't it? Because you're needing to like maintain the mass you're trying to send up as you slowly go up. And if you do it all faster, that's probably more efficient. It's more efficient. Yeah. And if you do it all at once, just by giving it one big push, then you don't need to bring any propellant with you and you can just accelerate the payload, right? Whereas if you're climbing up at a foot per second, yeah, you've got to bring the rest of the ladder with you essentially, and you've got to lift that fuel.

48:56So we're definitely going to get there. So what we do need to do is think about how much fuel we have to bring and how we can overcome this energy. And so you have to calculate how much kinetic energy do you need to overcome the potential energy of the earth. It's not important that it all be at once and rockets do it gradually, but the way to calculate that is to calculate what they call the delta V, the change in velocity that a rocket can provide. And in the end, this turns out to be very similar to the escape velocity. And it makes sense that it's similar because they're both connected to essentially how much energy you need to climb out of this gravitational well.

49:33And so on Earth, the escape velocity is about eight kilometers per second. That's how fast you would have to throw a baseball or launch a payload from the surface. But it's also very closely connected, we'll use the rocket equation in a minute to how much fuel you have to bring with you. And that's going to turn out to be the limiting factor of whether you can lift off the planet is, can you practically bring enough fuel to get this much delta V? So on earth, you need like eight kilometers per second. And then you go to the rocket equation. The rocket equation says, how much mass do you need to bring so that you can do this?

50:05So you can climb out of this gravitational well, because remember a rocket, what is it doing? It's throwing stuff out the back, right? The way it works is it's conserving momentum. Imagine you're like in a rowboat and you have a pile of bricks. You throw the bricks out the back of the rowboat. The bricks go backwards. You go forwards. That's how a rocket works. It's throwing stuff out the back. So you have to have that stuff in the rocket to throw out the back in order to propel it. It's helpful if that stuff also has the energy you can use to push the propellant. It doesn't have to be. You can have those things be uncoupled.

50:38But in a chemical rocket, you have fuel, which is both propellant and the source of energy. And the rocket equation tells you what your mass ratio is. So on Earth, for example, you need a delta V of about 10 kilometers per second. That tells you your mass ratio is nine, which means you need a nine to one fuel to payload mass ratio. That's not great. It's not great. Yeah. If you have like a hundred kilogram person and a thousand kilogram spaceship around them, you need nine times as much mass and fuel to get that thing into orbit. Wow. Yeah. How much worse is it for Jupiter? And does it scale linearly?

51:13It scales exponentially, which is the bad news, right? And so say you're on a super Earth, which has the mass of like five to 10 times the Earth. Then the escape velocity is like 25 to 30 kilometers per second. Okay, that's not so bad. It's three to four times as much. But the mass ratio is 93. It's 10 times as bad as it is here on Earth. 93 to one. 93 to one. So instead of having a nine to one fuel to payload ratio, you have a 93 to one fuel to payload ratio. So now like your rocket is basically just fuel, right? And that's just for a super earth. Now go to Jupiter. Jupiter is so massive that it's escape velocity is like 40 kilometers per second.

51:55And this gives a mass ratio of more than a thousand, right? So you'd need a fuel tank that's a thousand times bigger than your payload. This is probably even an underestimate, but essentially this is impossible for chemistry, right? Like chemical rockets cannot achieve this. And if you went to the sun, right, then the escape velocity from the sun is 400 kilometers per second. And so now the mass ratios are just astronomical. From a neutron star, the escape velocity is four tenths the speed of light. And so I couldn't even get my calculator to give me a number on this one. It's just so big. And so the bottom line is that chemical rockets, where you have this fuel and you're slowly climbing out of the gravitational well, they work pretty well if the escape velocity is low because the mass ratio is pretty small.

52:42You can afford nine to one, which sounded bad already to you, right? But on a bigger planet like a super Earth, it's pretty hard to use. And on Jupiter or the sun or a neutron star, it's definitely not practical. So what about like a Project Orion style propulsion system? So if you were exploding nuclear bombs out the back of your rocket to send you up. Could you get off of Jupiter? Yeah, you could. This is limited to chemical rockets, right? And you can have other strategies. You could build a space elevator, right? You could have nuclear propulsion, absolutely. And especially if you're launching the nuclear weapons behind the rocket somehow so that they don't have to come along with the rocket, then you can escape this trap of having to bring all of your propellant with you.

53:26Yeah. Or if you have like a light sail with a laser behind it, You can use that to lift off of a planet. Or you could just build your thing in space anyway. Why are you building it on the surface of Jupiter? That doesn't really make a lot of sense, unless that's where your supervillain hideout is. In which case, I have to wonder, Jupiter doesn't get a whole lot of light. I wonder how your circadian rhythms are going in your supervillain lair. Well, you better hope that Brad isn't sending the Earth careening into your supervillain lair on Jupiter. Oh, maybe he's saving us, right? Maybe he's using the Earth.

53:57That doesn't really work, does it? No, no. Using the Earth to crush somebody else's supervillain there? Yeah. Not a great plan, Brad. I'm not paying you to write that plot, Daniel.

54:08So yeah, chemical rockets work essentially only on smaller planets. On bigger planets, you need other technologies. But those technologies are not impossible. So I think if we did evolve on Jupiter or on the surface of a neutron star, technically it's still possible to get off of those, but not with rockets. All right. Let's see if Arthur has any follow-up questions. and maybe he'll tell us what his supervillain plan is. Thanks, guys, for the kind answer to my question. I knew that lifting off from a bigger planet would be hard, but I didn't know it would be almost impossible, at least for chemical rockets, as you explained.

54:45I think that this either puts a big asterisk on plans for space exploration, since the range of celestial bodies we could visit would be very limited, or either forces research for more efficient technologies, as you mentioned. All I have to say about supervillains is that they love rockets these days, so I do not doubt they have plans for this massive madness. Thanks a lot. All right. Thank you, everybody, for sending in your questions. Remember, you can write to us to questions at danielandkelly.org and send us your thoughts about the universe, your musings, your wonderings, your philosophical meanderings.

55:22Please, we'd love to hear from you. Can't wait to hear from you. Thanks, everybody. Stay curious.

55:33Daniel and Kelly's Extraordinary Universe is produced by iHeartRadio. 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 &K Universe. Don't be shy. Write to us. Introducing the all-new Mazda CX-5.

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