Daniel and Jorge Explain the UniverseDaniel answers Listener Questions about closed time-like curves, force fields, and the lifetime of the Earth's magnetic field
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So much of physics is about a journey into the impossible. We spend a lot of time in physics understanding what we see in the universe. How does the Sun produce energy? Why is the universe expanding? How does light get from point A to point B. We do all this by distilling what we observe in describing it mathematically, But there's another side to that. We can also push on the limits of what is possible to try to break down barriers and create something new that's never been done before. We can take our mathematical understanding of the universe and find the corners, explore the nooks and crannies and see if we can do something that's never been done before. Truly, physics is about exploring the impossible. Hi, I'm Daniel. I'm a particle physicist and my personal scientific fantasy is to do something people once thought was impossible. And Welcome to the podcast Daniel and Jorge Explain the Universe, a production of iHeartRadio podcas Casting which we explore what is possible in the universe and what might be possible and what's downright impossible. We mix it all up and we talk about it. We try to figure it out. We apply our minds to understanding what's out there in the universe and try to bring your intuition up to speed so that all of it makes some sense to you. And in the end, everything that we do in physics, and everything that we do in science starts with a question, a question how does that work? Or could we even do that? Or why is it this way and not that other way? And questions are wonderful because they are at the heart of science. They are the engine of our curiosity. They are the reason that science moves forward. People often think of science as this big, monolithic institution that rolls forward at a steady pace year after year. But instead you should imagine it as a big swarm of people pushing with their individual little hands on some envelope, expanding the sphere of knowledge by individual effort, by curiosity, by lonely pursuits sometimes. And so it's those questions asked by individuals that have resulted in everything we know about the universe. And that's why you should keep asking questions, and you should wonder about the universe, and you should put value in those questions. You should really cherish your curiosity because it's those moments of curiosity that have led us to where we are today. And that's why on this show we really value answering your questions, not just the questions that I find exciting or that Hoorge is willing to talk about, but also the questions that real people are wondering about, people like you who think about the universe and wonder how does this idea fit with that idea or I've heard about this a lot of times, but I've never really understood it. So that's what we're here for, to answer your questions, to make sure you actually understand the universe. Hey, the name of the podcast is Explain the Universe after all, And as you might have figured out already, Jorges not here today, so I'll be taking the opportunity to catch up on our backlog of listener questions. If you have a question about the universe you'd like to hear us break down, please send it to us to questions at Danielanjorge dot com. We answer every email, we respond to every tweet. We will eventually answer your question. And some of these questions are fascinating and tricky enough that we promote them right onto the podcast and answer them directly because we think it might be a question lots of people are interested in hearing the answers to, so on today's program we are tackling listener questions about timelike curves, force fields, and the ends of the Earth. Before we dig into today's questions, I want to say thank you to everybody who came to my public office hours recently. If you have questions about the way the universe works, that you're not into writing emails and you don't tweet, and you might like to ask a follow up question, then come hang out with me at my public office hours. You can find the instructions on my website sites dot UCI dot edu slash Daniel, and you'll find directions for how to sign up for my public office hours, where I hang out with folks, talk physics and answer questions. This last one was super fun and I even got asked a question I had never heard before, which for me is amazing because it means I get to think about something new, I get to put two ideas together I had never had in my head before. So, without further ado, here's our first listener question of the episode.
Hi, Daniel and Jorge. I'm Autumn and my question for you is about closed timelike curves and what are they. I've heard a bit about them and how their solutions to the theory of general relativity and that supposedly they allow for time travel, but other than that not a lot. Thank you, guys, and take.
Care all right. Thank you very much, Autumn for writing about clothes timelike curves. This is indeed the kind of thing we like to dig into because it's some thing you might have heard of if you're interested in science fiction or science and time travel and all that kind of stuff. Because the possibility of actual time travel being allowed by the laws of physics is something that would blow our minds, and hey, open the possibility to fix all those mistakes you made in life. So let's dig into it. She asks, what is a closed timelike curve and is possible to use it for time travel. Let's break it down first, Let's understand what is meant by curve in this context before we talk about what a closed timelike curve is. Let's make sure we're using this word in a way that makes sense to everybody. When we say curve here, we don't mean the shape of the bowl that you're eating your cereal in or the smooth shape of the surface of the Earth. We're talking about something very specific. We're talking about how a particle moves through space and time. So if you imagine your head some chunk of space and there's a particle in it, and that particle moves through space as time goes forward, then the path of that particle sometimes is called a worldline of the particle, where sometimes it's called its curve, and this is just the path of the particle as it moves through space. So that's pretty simple. Close timelike curve is a special case of other kinds of curves, other kinds of worldlines paths that particles can go in. So that's what curves mean. But there's a little bit more to it. Because a particle has to follow rules as it moves through space. You can't just have any curve. You can't have a curve with discontinuities in it. For example, you can't be here and then one instant later be in Andromeda. There are rules physics tells us how the present can turn into the future, and very specifically, you have a limit to where you can be in the universe based on where you are, because there is a limit to the speed anything can move through the universe, which is why, of course you can appear in Andromeda in a moment, because it's too far away. So imagine now your particle flying through space. Where can it exist in the future. You can exist in the future nearby where it is right because it can move, and it can move quickly, but only up to a certain speed. So this defines what we call a cone, the cone of future possibilities for where this particle can be. If it's at a certain place in a moment. Then that cone is projected forward in time and tells you where it's possible to reach if you're traveling at the speed of light or less. The surface of the cone tells you where you could reach if you're traveling at the speed of light. So for example, if you move one second forward, you have a circular slice of that cone with radius of one light second. If you move one year forward, then the cone of course expands and your slice of that cone is now one light year in radius. So that's why it's a cone. It starts the tip is at you or the particle, and then it expands forward in time. That's the light cone. It describes all the places that you could be in the future given that you are where you are right now. And again, the surface of the cone is as far as you could get if you're moving at the speed of light. If you're moving less than the speed of light, then of course the number of possibilities shrinks. You can technically go anywhere in that cone if you could travel up to the speed of light, but the cone tells you sort of the maximum possible places that you can go. Now, this is important because we're talking about what's possible to do in the universe. How is it possible to move? Can you move through space and through time? Now in normal space, in flat space, the kind of space you imagine when you think about just space and darkness and space ship's floating out there. Light cones are pretty simple. They're just cones. You're in a spaceship, but a certain location in space, then where you can go is described by your light cone. But what happens when space is weird, what happens near a massive object, Well, then space curves, and these cones get a little bit more complicated because light fergs would change its path near a massive object, like would change its path near a black hole, it would curve, for example. So as you get near a massive object, your light cone is not a simple geometric cone that you would imagine. It actually bends a little bit. The possible places that you could go, even if you're traveling at the speed of light change. And this makes sense if you think about, for example, what happens near a black hole. As you get near a black hole, your cone tends to tilt towards the black hole, and eventually, once you cross the event horizon, your cone is entirely inside the black hole. That's what we mean when we say that space inside a black hole is bent so much that all of your futures, all of your possible paths, lead towards the center of the black hole. Your entire light cone has now tilted towards the center, so that every single place in that cone is now towards the singularity. So your singularity is in every possible future. There's no part of your light cone that exists outside the black hole. So it makes sense to us now to think about the trajectories of a particle. That's what we call its curve or it's worldline as it moves through space. And we can also imagine the possible curves for a particle, like how it might move through space, and these are dictated by the light cone, which again depends on the mass and the energy around you, because that bends space itself, which is what determines the shape of this light cone. Okay, so we understand the curve part of it. What does it mean to have a closed curve? Well, this is actually pretty simple. It just means that it turns back on itself. But not like you walk around in a circle and you're in the same location you were here. We mean something more specific. We mean that it turns back on itself, returning to the same point in space and in time. So a closed curve would be one that returns to where and when you started. How is that possible, Well, our closed timeline curves possible. It's possible if you can take these cones that we talked about, and if they can tilt sort of more than ninety degrees. A light cone in flat space, if you take your units to all be one, has a forty five degree line defined by the speed of light. Right now, that cone tilts as you get towards a black hole, so that one edge of it sort of turns more than forty five degrees, eventually towards ninety degrees. But if you built some weird thing in space, something which distorted the fabric of space so much that your light cone tilted past ninety degrees, that would open up the possibility to effectively move backwards in time. And if you had a series of these cones, you stack them sort of on top of each other, then you could curve your world line back through time and space back to where you started. So it's essentially a circle in space time, a world line which doesn't move forward through space and time, but instead curves through space and time back to itself. And this works because if you imagine the individual particle, it can go anywhere inside its light cone. Right, that's moving into its local future. Somebody else looking at it from the outside, remember, might have a different sense of time, and so they would see this particle moving into the past. The particle itself is always sort of experiencing its own time. Just like if you get on a spaceship and you travel really really fast, other people might see your clocks go slow, but you experience your clock moving forward normally. In the same way, this particle would be experiencing its own normal time, but from the outside we would see it moving backwards in time. So what is a closed timelike curve. It's a series of light cones tilted so that they loop back on themselves and construct a path through space time that returns to the original point in time. Now is this possible? Is this something which can actually happen? So far, we've just sort of been describing what the phrase means. We haven't worried too much about whether the laws of physics actually allow for this, so answering this this question is a bit of a theoretical exploration. What we need to do is ask, is there a way to construct a universe that gives us a space time that works this way? And that's how general relativity works. General relativity describes how space and time are bent by mass and energy, and then how objects move through that bent space. So you can do it in a couple of directions. You can say, here's my mass and energy. General relativity, tell me how is space bent? What happens to space if I have this mass and energy configuration. You can also try to go the opposite direction. You can say, hey, I'd like to have a universe that's bent in this certain way. Is that possible? Is there some way to construct a configuration of mass and energy that gives me this space? And that's tricky. These equations are always very very hard to solve, and they've only ever been solved for a very few very simple configurations, such as an empty universe or a universe uniformly filled with stuff, or a flat universe with a black hole in it. So it's in general very hard to solve these equations. But there was somebody who several decades ago came up with a solution that allows for closed timelike curves in general relativity in a very weird configuration of mass and energy in the universe. So if you have an infinite spinning cylinder of dust, so something which goes on forever and is spinning, and it's this sort of compactified collection of tiny objects dust basically, and it's spinning, then you could generate in space the kinds of distortions you would need to have light cones bend past ninety degrees and construct a closed timelike curve. So that's the solution in general relativity. Is it actually possible to assemble the mass in such a way that would give you those curves? We don't know. I don't personally think it's very fee able to build an infinite spinning cylinder of dust, but it opens up the possibility. It suggests that maybe there are ways to assemble mass and energy in space so that it bends the fabric of space and time to allow for this kind of motion. Does this actually allow for time travel? Could that really happen? Well, First of all, this isn't like arbitrary time travel. It's not like back to the future. We just dial in when you want to go. This would be a very specific path, and this object would move along this path. But it could only move along this path. It's not like you could go to any specific time or any arbitrary time. You can only move along this world line through time and space, and it would be a closed loop. So basically you could be on it forever. You can't go back and change the past because that would be off of this closed timelike curve. And in order for the curve to exist, it has to reinforce itself, has to be a complete solution, so it doesn't allow for arbitrary time travel. But even still, can that be weird to have a particle doing a loop in space and time, even if it's stable, even if it doesn't get to go back and kill its own grandparent particle? Is that actually something which could happen on our universe? Doesn't that feel like it violates causality and all sorts of things. Well, theorists are not sure. It seems to work according to general relativity, but people suspect that in practice it probably wouldn't happen, that there's something preventing that. And we know that general relativity is a great theory. We also know it's not a perfect theory. We know that it cannot describe the universe as it is because it breaks down its singularities, like the Big Bang in the center of black holes and all sorts of crazy stuff. And so it may be that this is just an artifact of that theory, a mathematical construction which works most of the time but sometimes gives nonsense answers. And some future theory of space time like quantum gravity, would prevent this from happening. So most theorists, if you ask them, think that there's something out there that would block this from really existing. But to date we do not know, and our best theory about how the universe works, general relativity, which determines how space and time bend in the presence of mass, does not prohibit closed timelike curves, so the jury is still out. So thanks for that awesome question, Autumn. I've been looking forward to digging into time travel and close timelike curves. Thanks very much for sending that in. I want to get to some more questions, but first let's take a quick break with big wireless providers. 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Hi, Daniel and joje I have a science fiction inspired question. Our force fields possible like the ones in Star Trek. Everything comes down to particles and an arrangement of them. And giving that, for example, brick walls just a certain arrangement of particles, could we mimic that in the air?
Okay? Love this question, and mostly because I love the role of science fiction in pushing forward science and inspiring our curiosity. You know, science fiction authors are like the theorists of theorists. They think about new crazy things we might be able to do, or ways the universe might work, and they're not limited by mathematics or practical or anything. And sometimes you read something that they write and you go, hmm, I wonder if that is possible or maybe we could actually do that. And so thank you science fiction authors for injecting crazy ideas into the minds of everybody out there and being on the vanguard of creative thinking. So this question is about force fields. Could we build force fields? First of all, we have to talk about what we mean by a force field. And if I was going to commission a force field for my spaceship, for example, because I was about to go into intergalactic war or whatever, here's what I would want. I would want it to be invisible, or at least mostly invisible, because I want to be able to see through it. If I have a spaceship, for example, and a force field around it, I don't want turning on the force field to mean that I can't see anything outside in the universe. Then I'd be a huge strategic disadvantage. So it should be invisible or at least translucent, and then it should block weapons, right it should be able to absorb radiation weapons like lasers, and it should be able to stop not weapons like kinetic energy weapons, you know, bullets or other kind of momentum driven weapons. So that's what I'd like, and you know, as a bonus and be cool if you could, for example, be used to hold prisoners. Right, you could like trap somebody who you've captured and put them in the hold of your ship and use a force field so you didn't have to build cells and easily configure it and all this kind of stuff. So that's my wish list for a force field. It should be invisible, you should be able to block radiation weapons, and you should be able to block matter weapons, and if possible, you should also be able to touch it without actually being damaged. And that's a long list of requirements. So let's talk about what is actually possible and what might be possible in terms of force fields. So I did a little bit of research in this, and there are people out there actually doing research on force fields. Some of the things don't really seem like the kind of force fields we're talking about. For example, there is a company out there building electric armor. The idea here is to turn the skin of your spaceship or your tank or whatever into something which responds to a bullet. So if somebody shoots a bullet at you, it's not just an inner wall of matter which absorbs that energy and maybe gets destroyed, but it responds to it. So the way they do this is by making two layers of armor and having a huge electrical gap between them, and when something impacts on that armor, it basically closes that gap. It results in a huge electric discharge which pushes back on the bullet. It's sort of reactive armor which responds when you're hit with a force backwards. So that's pretty cool. But it's not really a force field. I mean, it's not invisible, you'd need to build it, you couldn't like turn it on or off, and only works on matter weapons. It doesn't stop lasers, for example. But you know, it's something that people are actually doing, and so it's something you might actually see out there in the world soon. But let's talk about what might be possible. You know, when I think about a force field, first, I think about the Earth's force fields, because the Earth actually does have a force field. We have a huge magnetic field that protects the Earth from particles. The big swirling masses of melted rock and metal in the core of the Earth are providing an enormous magnetic engine, which creates a huge magnetic field with sort of vertical field lines that go from the north to the south or the south to the north. And what this does is when charged particles hit a magnetic field, they get curved, right. This is the Lorentz law in physics. And so an electron, for example, generated in the Sun and given a huge amount of energy and shot towards the Earth, which might otherwise penetrate through the atmosphere and give you cancer instead is bent around these magnetic field lines and funneled up to the north pole or down to the south pole. So that's cool because it's real and it exists. It doesn't really satisfy all of our requirements. I mean, it is invisible and it can deflect matter, but with a couple of big caveats, like, it only works on charged particles, right, It works on electrons, it works on protons. It does not work on neutral particles because neutral particles don't feel those magnetic fields. It depends on the charge of the particle. Remember that electricity and magnetism are very tightly woven together. They're actually just two sides of the same coin. So these magnetic fields only deflect charge particles. Plus they don't really deflect it, they just sort of focus it on the North Pole and the South pole. People think, wow, Aurora borealis is really cool. It's cool, but it's radiation. So if you live near the North Pole or the South Pole, you actually get more cosmic radiation than anywhere else on Earth because the magnetic field sort of funnels it up there and funnels it down to the South Pole. So it's protecting the rest of the Earth, but at the expense of the North and the Southern caps, so not even really deflecting those charged particles. And then of course it doesn't work on radiation. A magnetic field will not stop a laser beam. A laser beam is made of photons, and photons don't feel magnetic fields, which you know, is kind of weird because photons are partially magnetic fields. They are oscillating electromagnetic waves, right, the electrical component oscillating to the magnetic component and then back. But photons are neutral, they have no electric charge, and so again they are not deflected by magnetic fields. So a magnetic field kind of like a force field, but not really and also kind of impractical. If you want it to have a really powerful magnetic force field, you need to be able to generate that inside your spaceship. And you don't really want to have enormous massive currents of iron and nickel sloshing around in the inside of your spaceship to generate this magnetic field. All right, So what else could we do?
Well?
The third possibility is a plasma shield. Plasma is another state of matter. Basically, you just take gas and you make it even hotter, and then the electrons have so much energy that they whizz off and they leave their protons and now they're free. So it's ionized gas. You take all the atoms in the gas and you break apart the electrons and the nuclei, and that's what a plasma is. It's nothing special or fancy or science fiction e. It's just gas that's been heated up so much that the electrons now run free. But it is super duper hot and it's electrically charged, and that means it has the capability to basically vaporize anything. I mean, imagine basically a slice of the Sun. The Sun is plasma, it's super hot, and it's ionized. So imagine, for example, having a shield around your spaceship that was a slice of the Sun. You threw anything in it, boom, it would melt. Also, it's opaque. Two lasers, right, A laser can't penetrate the Sun because it's filled with charged particles which would interact with the light in the laser, unless, of course, you tune the laser to be a specific frequency that the plasma didn't absorb. But these excited particles, these free particles, are not limited to absorbing only very specific frequencies, so it'd be very difficult to get your laser beam through a slice of the Sun through a plasma shield. So that's pretty cool. And actually we have a little bit of a plasma shield already on Earth, and the part of the atmosphere we call the ionosphere is basically plasmas filled with ions, and it blocks a lot of radiation from the Sun, neutral radiation, photons, et cetera. And our ionosphere is not very dense but mostly blocks very long wavelength radiation. But if you made a denser plasma, more like a slice of the Sun, then you could block shorter wavelengths. So this is the kind of thing you could actually build. Now, how do you make a plasma shield? How's that possible? Can you actually do that? It'd be pretty tricky. I mean, you need to have basically a shell of gas, and then you need to have that gas get excited to get very very hot. You need to dump a huge amount of energy into it. So you can imagine, for example, puffing out a shell of gas and then zapping it with a bunch of lasers to turn it into a plasma shield. Like essentially deposit so much energy in this shield around you that anything else that comes into it would get absorbed or interacted with or diffused. This is not something that's very easy to do, not something we have the technology to do at all. Already. It's difficult for us to make and control plasmas. This essentially is the task of fusion. We are trying to replicate what's happening on the Sun in laboratories on Earth, not just so we can build force fields, but so that we can generate essentially limitless energy through nuclear fusion, the same process that happens in the center of the Sun, and it's tricky. People have been working on it for decades. They're trying to make a donut of plasma, and they're containing it with magnetic fields. Because the stuff is so volatile it would vaporize any container you put it in. It's so hot and nasty and interactive, so they use a magnetic bottle to contain this plasma. They have basically a donut of the sun, and it's hard to get it to go, it's hard to keep it straight. This is a big engineering challenge, and there's a project going on right now called Eater, which is the biggest and very promising application of this, but it costs billions of dollars and it's not simple to set up. So this would be a very difficult engineering challenge, and you'd have to somehow create this shield around your spaceship, get it zapped up to turn it into a plasma, and then keep it in line with very strong magnets. So possibly you could do that potentially sometime in the future. It might be possible, but it sort of violates one of the first principles we asked for for a force field, which is that it's invisible if you surrounded yourself with sort of a sheet of the sun and a sphere around your ship. Then you wouldn't be able to see anything outside that sphere, and you'd be stuck inside basically a little slice of the sun, which I guess would get pretty hot. So that's sort of the best idea that's out there that I'm aware of to build an actual force field. But it has a lot of engineering challenges ahead of it, and even if we could solve all of those, it doesn't actually satisfy all the requirements we have. So fun question. Maybe somebody in the future will come up with a new way to think about how to actually build a force field, and that shouldn't solve anybody from imagining and from wondering and from being creative. So keep using force fields in the science fiction that you write, and keep an eye out in science fiction for other cool ideas that we might actually turn one day into reality. All right, I want to get to one more question for first, Let's take another break. When you pop a piece of cheese into your mouth, or enjoy a rich spoonful of Greek yogurt, you're probably not thinking about the environmental impact of each and every bite, But the people in the dairy industry are. US Dairy has set themselves some ambitious sustainability goals, including being greenhouse gas neutral by twenty to fifty. That's why they're working hard every day to find new ways to reduce waste, conserve natural resources, and drive down greenhouse gas emissions. 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All right, we are back and we are having a lot of fun talking about the future and time travel and force fields and future technologies. And now I want to take a trip even deeper into the future, wondering how long humans can survive on Earth.
Hi, Daniel and Joge. My name is Gavin and I live in South Wales in the United Kingdom. In the last podcast What's Inside the Earth, you got me thinking. You told us that the big lump of moon size core inside the Earth is getting bigger by one millimeter every year, and that at some point in the future the core will stop turning and the Earth will lose its magnetic force, like we think happened to Mars. My question is will this happen before our son goes super and nova?
All right, thank you very much for that question, and also on behalf of all humanity, thank you so much for thinking ahead, for worrying because we don't have time to about the deep future, and starting to make plans today for what we might have to do to prepare for it. So he's wondering which of these two calamities will we have to deal with first, the Sun exploding and fizzling out or the earth magnetic field dying, And the two are related, of course, because the earth magnetic field is important for protecting us from the Sun's radiation, so if it disappears, then we will be fried. But hey, if it's going to last longer than the Sun, then we don't need to worry about it. So which one will kill us first is basically the name of the game in this question. So first let's remind ourselves what the timeline is for our Sun. How long is this thing going to keep burning and keeping us toasty and keeping us keeping on. So our Sun is currently about five billion years old. It's about the age of the Solar system, because it basically is the Solar system. Remember that it coalesced together from a huge cloud of gas and dust and leftovers from population three in population two stars that gathered together in some gravitational event, slowly slurping together, And that most of the stuff in the Solar system is in the Sun. About ninety nine percent of everything that's in the Solar system is the Sun. So we're just like a little detail on top of the Sun. Now, that happened about four and a half to five billion years ago, and since then, what's the Sun been doing? Well, it's been burning hydrogen. Gravity gathers together all this material, mostly hydrogen, but also some helium left over from burning from previous generations and a few other heavier elements, but still overwhelmingly hydrogen. And that's good because that's the fuel for the Sun. Gravity squeezes together this hydrogen, and when it gets it close enough, then it confuse. Remember that hydrogen is essentially just a proton with an electron around it, but when things get hot, it's basically just a proton. And to make fusion happen, you've got to squeeze too these protons together. But protons are both positively charged, and so they resist. And until you get them closed enough together, when the strong force can take over and do nuclear fusion and release a bunch of energy, they will resist. And that's why fusion is hard to make happen. You need really high temperatures and pressures. That's why it's difficult for them to engineer it here on Earth. But anyway, it happens in the Sun, and it's been happening now for about five billion years. So what's going to keep the Sun from burning on forever. Well, eventually it's going to run out of fuel. It's basically a huge hydrogen thermonuclear device. And what happens when it burns hydrogen is that it creates ash. That ash is helium, and it starts at the core because that's where the fusion starts. That's the hottest, densest part. So hydrogen first burns out at the core, and you get this helium core now surrounded by hydrogen, and the fusion is now happening in sort of outer layers surrounding the core. This pushes out on the Sun, making it bigger and bigger. It fluffs it out larger and larger, and actually even makes it brighter and brighter. So every year the Sun gets a little bit hotter. In four billion years, for example, the Sun will be about forty percent brighter than it is today, and just that is enough to like boil all of the oceans on Earth and turn them into vapor. So right there you can see that in about four billion years, our sun will not make Earth a very cozy place to live in. But as the Sun grows, it's gonna get bigger and bigger. And you might think, well, how much bigger can it get. It's already huge, right, it's already a million times the volume of the Earth. Well it's gonna get bigger while a lot. It's going to grow by about a factor of two hundred. And that's bad news because it means it's gonna get so big that it's radius it's going to match the size of Earth's orbit. Right, It's gonna envelop all the inner planets, and the Earth will be right there, right about on the edge of the Sun. What does that mean to be like inside the Sun or right just past the edge of the Sun. Well, it's gonna be really hot. We to be surrounded by these huge sheaves of burning hydrogen, which is not going to be a good place to live in. Now, inside the Sun, you now have a helium core which is getting bigger and bigger because as hydrogen burns, it creates more and more helium, and eventually you can even fuse this helium together to make something even heavier carbon. And so this is awesome because it burns hydrogen for millions and millions and billions of years, and then all of a sudden it starts to burn helium. It passes this critical point and you can get this helium flash. And the amount of energy generated by the Sun in this moment is actually brighter than all the stars in the galaxy put together, although you don't get to see it because it's absorbed by the inner layers of the Sun. But you know, maybe that's good because otherwise it would fry everything on Earth. So it starts helium fusion, creating carbon, and our sun is not heavy enough. It's not massive enough to then take the next step and fuse carbon into heavier elements. You could eventually make oxygen and neon and silicon and all sorts of crazy stuff if you had a bigger star. Our star is not big enough to do that. So what's going to happen is it's going to cumulate helium, which will burn into carbon. But then it's sort of stuck there. The carbon is inert and it's like having a lot of ash in your fire. It makes it harder for things to burn, and so the sun will just sort of decrease in brightness from there, basically fizzling out and turning into a white dwarf. A white dwarf is just a hot blob of carbon. It's not fusing anymore, it's not producing any more energy, but it's still really really hot, right it glows. White dwarfs are called white because they do give off light again, not because they're actually fusing they are creating new energy. They're just glowing the same way that anything that's hot glows it's a white hot lump of carbon in the universe, and so it glows white the way like a really hot piece of metal that you stick into a blacksmith's furnace will also glow white hot, and eventually it will cool. And scientists think that white dwarfs, if given enough time, will eventually radiate off all all of their energy and turn into something else, something weird, something called a cold black dwarf. But that will take trillions of years. So let's review the timeline. For about five billion years into the life cycle of the Sun. It will keep burning for about five billion more years, after which it will envelop the Earth, will have the helium flash, and it'll convert into a white dwarf, which won't give off enough heat to keep the Earth warm. But the Earth will have already been fried at that point, and then eventually the white dwarf, in trillions of years, will cool off. So timeline for the Sun to fry us is a few billion years. Now, let's turn to the other side of the question, how long will we have a magnetic field for? To answer this question, we need to think about what causes the magnetic field and why that might come to an end. So, as we talked about a few minutes ago, the magnetic field we think comes from internal flows of molten rock and metal. And again it's good idea to sort of turn back the clock and remember how the Earth was formed and why it is the way it is. We think the Earth came together basically a bunch of bits of rock which gathered together into larger bits of rock, into larger bits of rock. So the early Earth was just a collection of rock basically squeezed together, and it wasn't actually melted in the middle yet. Gravity then took over and squeezed it further and further. That plus radioactive decay from certain heavy metals that were embedded in the rock, helps melt the center of the Earth. This might have taken a few hundred million years to melt the center of the Earth to make it molten rather than just hot rock. And this was crucial because it let all the heavy elements, the iron, the nickel, sort of melt down.
Right.
Things get hot, they get molten, they get liquid. Now you have a fluid instead of rock, so it's easier for things to sort of like slide around and rearrange themselves. So the heavier elements sinc to the core, and now you get the structure that we have today, which is a solid inner core, very very dense, surrounded by that liquid outer core. And then on top of that is the mantle, which isn't exactly liquid, but it is rock that sort of flowing. And then on top of that is the crust, the thing that we actually live on. And we think that it's this molten motion inside the earth. Essentially this liquid outer core that's generating the magnetic field that it's spinning, that there's electric currents in there, and that motion of electric charges creates magnetic fields. Because remember there's a very tight connection between electricity and magnetism. That really are just one thing, electro magnetism, which is why moving particles can generate magnetic fields and why magnetic fields can bend the path of charged particles. So what happens is this stuff is swirling around. It's basically a current, and that creates a magnetic field, and that magnetic field enhances the current. Right, the magnetic field pushes these particles in a circle, and then moving in a circle makes a bigger magnetic field. This is called a dynamo effect. Basically builds on itself, but makes a stronger and stronger magnetic field. But it relies on the flowing of this liquid molten rock and metal in the center of the Earth outside the inner core. You have to have the ability for this thing to slash around. You need the motion in order to have a magnetic field. And as the question mentioned, this inner core is growing. The Earth is cooling. Basically, it's freezing very slowly, and this core is growing by one millimeter per year, and that's just because things cool, right, We are giving off heat into space the same way that a white dwarf will eventually radiate out all of its energy into space and become a cold black dwarf. Entropy tells us that he should spread out. So you have an isolated hot blob of something in cold space. Eventually it will radiate out its energy. So that's what's happening to Earth. And the inner core is growing, and eventually it will grow and it will cool down the Earth. So how long will this take? Will it be faster than the Sun enveloping the Earth and frying us, or will it be slower. It's difficult to predict these things because it requires projecting out pretty far in time, and our understanding of how these things work inside the Earth is still a little speculative, right. We've never drilled down to the Earth. A lot of this stuff is reconstructed from seismographs, from basically bouncing waves off the inside of the Earth and understanding how those waves are reflected as they hit various layers inside the Earth. So there's still a lot of guesswork involved. But the best estimate I found is that it will take about ninety billion years for the Earth to lose this magnetic field, essentially for the Earth to freeze internally and to stop having the flowing liquid rock and metal necessary to have a magnetic field. So that's ninety one billion years. That's a lot longer than the Sun's expected lifetime. The Sun we think will fizzle out in about five billion years, So the Earth, if it survives that, if it doesn't just get like melted and slurped into the Sun, we'd continue on for a long time for tens of billions of years with a magnetic field spinning and reflecting charged particles happily, even if we're now orbiting a white dwarf. So we think the Sun will expand and it'll fry the Earth in a few billion million years, after which you'll collapse to a white dwarf, and then the Earth with its magnetic field, will orbit this white dwarf for billions of years till eventually it cools and solidifies and loses its magnetic field in something like ninety billion years. So that's definitely something to think about, something to plan for, something to wonder about. I think personally, we'd be lucky if we made it a billion years on this Earth, so that we had to worry about things like the Sun getting bigger and frying us. We have more immediate problems that we need to tackle, not force fields, not close timeline curves, but just sort of taking care of the planet so we can last that long. Anyway, It's fun to think about these things, and fun to understand the physics that goes into making a sun or making a magnetic field so that you can understand how long it will last. All right, Thanks everybody for sending in these questions and for letting us ride with you on your curiosity journey, for wondering about the nature of space, for wondering if it's possible to go backwards in time to build force fields, and how how long this planet Earth will be around for us to live on. It's a joy to get your emails and a joy to think about the ideas that are in your head, so please don't be shy send us your questions. Tune in next time. Thanks for listening, and remember that Daniel and Jorge Explain the Universe is a production of iHeartRadio. For more podcasts from iHeartRadio, visit the iHeartRadio app, Apple Podcasts, or wherever you listen to your favorite shows. When you pop a piece of cheese into your mouth, you're probably not thinking about the environmental impact. But the people in the dairy industry are. That's why they're working hard every day to find new ways to reduce waste, conserve natural resources, and drive down greenhouse gas emissions. 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