Daniel and Jorge Explain the UniverseListener Questions: the expanding observable Universe, what it's like to be a photon and the age of the Universe
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It's not easy to hold an entire universe inside your head. I mean, that's a whole lot of planets and stars and iahope aliens to squeeze into a few pounds of brain matter. Instead, we try to hold a model of the universe in our minds. But you know, even that is hard because it turns out that the rules of the universe are pretty different from the rules of rocks and trees and water and all the things we're familiar with. You got relativity and the expansion of space and things moving at light speed and all that jazz. So it can be quite a challenge. And when we try to bring all these crazy ideas into our minds, sometimes things don't quite fit together. But trust me, it actually does make sense, and it can make sense to you. Hi. I'm Daniel. I'm a particle physicist, and I've spent a lifetime trying to import a model of the universe into my brain. I spend a lot of time thinking about how the universe works and making sure that I understand what's going on looking at it, from the tiniest little particles to the huge cosmic scales of the universe. I want one single idea which explains everything. I want to have a coherent understanding of everything. That's going on out there, and I believe, without evidence, without any reason to believe this, I do believe that the universe does make sense. I believe that we are capable of understanding some fraction of it, at least, that human mathematics and philosophy are able to build constructs in our mind which can map onto that external universe and explain it to us in a way that we can manipulate it and understand it and examine it. I don't know if we're capable of understanding the entire universe at all of its incredible glory, if we are smart enough, if we will ever develop the mathematics, indeed, if our mathematics are even capable of describing the entire universe. But I do know it's worthwhile, So welcome to the podcast Daniel and Jorge Explain the Universe, in which we attempt to take our current knowledge of the state of the universe and download it all into your brain. We talk about everything that we do understand, and we love to talk about the things that we do not yet understand, because even the smartest of human physicists do not have a complete model of how the universe works. In her brains. We are still working on that. There are still pieces that do not fit together. We have ideas from lots of different physicists, and sometimes we can meld them together into one coherent theory, and sometimes they just do not play nicely. And we are still hard at work trying to get an understanding of the universe. But it's those scientists asking those questions, trying to make one theory of the universe, trying to understand how everything out there can make sense to our tiny little brains. That is what is driving science forward. If you are not an active scientist, you might have the impression that science just sort of rolls forward on its own, results being produced year after year as we push back the darkness and illuminate the universe with our ideas. But in fact, science is not a monolithic institution. It's very personal. It's all about individual people asking questions. It's because that little girl grew up and really needed to know how stars are formed. It's because that little boy really wanted to understand how the universe was expanding. It's because this person over here really wanted to know the smallest things about the universe. It's individual people asking their personal questions about the universe and spending their life trying to find those answers. That's how science moves forward. That is how we develop new understandings of the universe, and that is also how you develop new understandings of the universe by asking questions. And one thing we love on this podcast are your questions because we celebrate that kind of curiosity. The same curiosity that drives science forward also drives you to understand the universe. And you'd be surprised how often the questions you have in your mind are the very same questions that scientists are asking. And so we want to bring you all the way up to the forefront of scientific knowledge and make sure you understand what scientists are puzzled about and what they have figured out. So Jorge is not here today, so I'll take the opportunity to be answering some of your questions about the universe. And today we have a doozy of an episode for you, folks. It's all about time and space and the expansion of the universe and how well that can all make sense. And it's going to require you to bend your brain a little bit to accept that the universe operates in a way that is very unexpected and very unusual and has very weird consequences. Remember, we live in a universe that's very different from the one that we thought we lived in just five ten thousands years ago, and only in the last one hundred years have we revealed these deep and fascinating truths about the universe, which frankly upend every idea we had about how things move and what things mean, and what distance and time and velocity even are man And so we expect that in the future there will be more revelations, but until then, a lot of us are still catching up with some of the crazy, bonkers ideas that science has so far revealed. So today on the podcast, we'll be doing listener questions about space and time and the whole universe. And so don't get that you too can get your questions answered. If you write to me two questions at Danielandhorhead dot com, you'll get an answer, and you might even get your question on the podcast. So, without further ado, let's get to the first listener question from Chris.
Hey, Daniel, and Jorge. I've heard that as light travels from the edges of the universe over time, that more of the universe is visible today than yesterday because the light from farther away has had time to reach us. But also that the universe expands away from us at faster than the speed of light, depending on how far away it is.
In the far.
Future, we wouldn't be able to see even the light from our closest neighboring galaxy. How can both of these facts be true? Thank you? I love the podcast.
Thank you Chris for this wonderful question. And first I'd like to point out that I love hearing the way Chris is doing physics. He's heard one thing, he's heard something else, and he is trying to fit those two ideas in his mind together. He wants a unified understanding of the nature universe, not just two cool sounding ideas. He wants it all to gel in his head. And that's what everybody should be doing. When you hear an idea about science or read some news article, you should ask yourself, Huh does this make sense to me based on other things I've heard? How does it fit together with what I understand? And it's by doing that that you build an ever larger constellation of ideas in your mind that are all fitting together and giving you a unified understanding of the universe out there. So let's talk about Chris's question. He wants to understand two different ideas. One that our observable universe, the part of the universe that we can see, seems to be growing because as time goes on, light has had enough time to get to us, so we can see deeper and deeper into the universe. That is true. And he also wants to understand how the fact that the universe is expanding means that things near us will in the future no longer be visible, which is also true. And so let's unpack these two different ideas. First, let's make sure we understand exactly what is meant by the observable universe this first part of his question. So at first, let's just simplify things and assume that we were dealing with the universe where there is no expansion of space, so we don't have to worry about that for now, and just think about this question of the observable universe. What does that mean? Well, in the end, it comes down to the fact that light takes time to get somewhere. Something happens on Andromeda, which is millions of light years away. You don't see it immediately. In fact, Andromeda could get attacked by aliens and totally blown up. We would not see that happen for millions of years. The fact that light does not move instantaneously means we can't see infinitely far out there, because there could be something out there. There almost certainly is something out there, a galaxy so far away that light has not yet had time to get to us through the whole history of the universe. Imagine that a photon created in the very very beginning of the universe sent towards us, traveling for billions of years, and still still not having arrived, still needing to fly through space to get to us. And so this portion of the universe where light has had time to get to us, that's what we call the observable universe. It's not really a physical thing. The edge of the observable universe is just defined by where we are and the age of the universe. It's centered literally around us, like our heads. If we lived on Jupiter, the observable universe would be centered at Jupiter. If we lived at Proximus c Entari, the observable universe would be centered at Proximus entari and aliens living in another galaxy have a very different observable universe because light needs time to get to them, right, So you could have different, partially overlapping observable universes if you're observing it from different places. And the size of the observable universe grows every year because every year you've allowed more time for light to get to you from those distant galaxies. And so in a universe that's not expanding, the observable universe grows by one light year in radius every year, and so if you wait, the observable universe gets bigger and bigger, and you've had time for that light to get to you from further and further places. And so what's at the edge of the observable universe, the edge of the observable universe is the oldest light that we can see. And in the universe that's not expanding, that's not actually light from the very very early universe. Because in the very early universe, everything was hot and nasty and interacting with each other. It was a big, crazy plasma, sort of like the inside of our sun, and so any light that was created then was also reabsorbed almost immediately, and so it doesn't exist any longer. The oldest light that we can see comes from the cosmic microwave background, this moment when the plasma cooled so that it became transparent, and then light created in that moment could then fly free through the universe. So the oldest light we can see comes from the first moments that its cosmic microwave background radiation was created. So we can't actually see light from the very very earliest universe, all right. So that's the concept of the observable universe, and it seems to make sense that as time goes on we can see further and further out. But there's this other thing going on at the same time, the universe is expanding. You might have in your mind this idea of the Big Bang is like an explosion. A lot of people think of the Big Bang as like a tiny dot of stuff the whole universe wished down to an atom, which then explodes and moves through space. But the modern idea of the Big Bang is actually quite different. It's that the universe was infinite and always has been infinite, and that the singularity wasn't a single point in space but it was a singularity in density. That is, the universe was very hot and dense everywhere, and the Big Bang was an expansion of that density is stretching of space itself, so not stuff moving out from a little dot through space, but an expansion of space itself. That means that new space is being created, and that expansion is still happening. In fact, that expansion is still accelerating. That's what we call dark energy. So the universe is expanding, by which we mean new space is being created between stuff all the time. And this new space is being created between our galaxy and other galaxies. For example, So the distance between us and other galaxies can be increasing even if we have no relative velocity, right, you just like create new space between them. So this is called the recession of velocities, basically the rate at which the distance is increasing even if you don't have relative velocity. Now, a lot of people wonder a lot of second, is this new space that's being created only out there between galaxies or is it also here in our Solar system? Why isn't the Sun getting expanded away from the Earth. Why isn't Mars getting blown out of the Solar System? Well, this process. Dark energy that's accelerating the expansion of space is actually a very small effect per unit space, So like in our Solar system, it's pretty weak, and gravity is actually much much stronger, so it's strong enough to hold the Solar system together. But dark energy adds up as you get more space. Over distances, it adds up, so you get more and more dark energy, for example, over the scale of a galaxy. But galaxies are still strong enough to hold themselves together. Even galaxy clusters are strong enough to hold themselves together, we think. But between clusters of galaxies and these much much larger distances, that's where dark energy wins. That's where the expansion of space is stronger than the local gravity and it's tearing things apart. What does that mean for how far we can see for this question of the observable universe, Well, what it means is that we can actually see much further than thirteen point eight billion light years away, which is the age of the universe times the speed of light. The reason is that we can see photons from stuff that's now much further away because space has been expanded. Right, something sent us a photon a long time ago. It's flying towards us, and since then that thing, that galaxy or whatever, the bit of the CMB has been expanded away from us, so it's now much much further away. So we can see things now that are forty six and a half billion light years away, and that's because of the expansion. So on one hand, the universe is tearing things away from us. On the other hand, it means that we can see things which are now much much further away than you would expect if the universe is not expanding all right, So that means that we have two things going on at the same time. One is that we have more time for photons to get here, so the observable universe is growing. On the other hand, the universe is expanding faster and faster. So let's dig into how those two things play off each other. What's happening between us and a distance galaxy is that the space between ween us is being expanded, So the distance between us and that galaxy is getting bigger, and the further away something is, the more bits of space are being expanded, and so the faster something is moving away from us. So the further something is away from Earth, the faster it's moving away from us, and this was Hubble's discovery. Actually, this is just Hubble's law that this recession velocity grows with the distance. The further you are away from us, the faster you're moving away from us. And this can actually add up to a speed moving away from us faster than the speed of light. And remember this is not motion through space. This is the expansion of space. And while special relativity says nothing can move through space faster than the speed of light, there's no limit to how fast space itself can expand. And there are objects out there whose distance is growing faster than the speed of light. And what does that mean? Can we see those things? This area of the universe is called the Hubble volume. Things that are moving away from us less than the speed of light are inside this Hubble volume, this Hubble sphere, and things that are moving away from us faster than the speed of light are outside the Hubble sphere. And so you might imagine, hmm, I guess things that are outside the Hubble sphere we will never see because they're moving away from us faster than the speed of light. And you might be wondering, what about these things outside the Hubble sphere, these things where the distance between us and them is growing faster than the speed of light. Can we see those things? Well, what happens if a photon is emitted from those things, Well, that photon starts moving through space, but space is expanding, and if that space is expanding faster than the speed of light, then it's like Ussein bolt is running towards you, but somebody's putting down new track faster than he's running. So the distance actually grows between us and that photon. It's sort of like the photon is moving towards us, but its distance is growing, right, it's not moving away from us, but the distance between us and that photon sent by something whose distance to us is growing faster than the speed of light. Our distance that photon is also growing, all right, So you might think, well, we're just never going to see those things. And this is a common misconception because this hubble sphere, this point at which the universe is moving away from us faster than the speed of light, this actually is expanding.
Right.
As the universe expands, so does the hubble sphere. So eventually this photon can make some progress and the hubble sphere catches up to it, and then it enters a part of the universe that's moving away from us less than the speed of light, and so we can actually see those things. So because that hubble sphere itself is expanding, it means we can see light from things that we're moving away from us faster than the speed of light. But that's because the hubble sphere was expanding. The hubble sphere was expanding. But around seven and a half billion years into the age of the Universe something happened. Dark energy took over and started to accelerate the expansion of the Uni to make this expansion happen faster and faster, and that has the effect of actually shrinking the hubble sphere. It means that things are moving away from us faster and faster, closer than they were before. Right, it's speeding up this expansion, so you don't have to be as far away from us anymore to be moving away from us at faster than the speed of light. So this hubble sphere is shrinking and shrinking and shrinking, and now there are things that are so far away from us and moving away from us so quickly that we will never see them. And in fact, the hubble sphere itself is shrinking and shrinking and shrinking, and if dark energy continues, then it will shrink basically to zero. Almost all of the universe would be moving away from us at faster than the speed of light, making it invisible because we can only see photons from things moving away from us faster than the speed of light if the hubble sphere is expanding and catches up with those things. But the hubble sphere is now shrinking due to dark energy in the accelerated expansion of the universe. So what's the furthest thing that we will ever see. Well, currently we can see things that are about forty six and a half billion light years away. Photons from things that are now sixty two billion light years away will eventually make it here. They'll make it here just as space expands away to make it impossible. Things that are further away will never catch up to the hubble sphere. The hubble sphere is now shrinking, so things that are sufficiently far away can never have their light get to us because the hubble sphere is shrinking and they will never catch it, meaning their photons will travel through space towards us, but will never actually get closer to us. So this is a great question. Thank you very much Chris for asking this. The observable universe is growing, but space is expanding even faster, and so it's pushing things out past the edge of the observable universe. And for most things, that means we will never see them, and those things will disappear, and in the far far future, most of the universe will be dark. What remains to be seen is how much of the universe can stay local to us. Can gravity hold our galaxy together or reilly get torn apart by dark energy? Can hold our solar system together? Or will dark energy tear that apart as well. The problem is we just do not understand dark energy, and we don't know how to predict its future, so it's very difficult to know exactly what the future holds. So thanks for that really wonderful question. I want to answer a couple of more questions, but first it's time for a break with big wireless providers. What you see is never what you get. Somewhere between the store and your first month's bill, the price you thoughts you were paying magically skyrockets. With Mint Mobile, You'll never have to worry about Gotcha's ever again. When mint Mobile says fifteen dollars a month for a three month plan, they really mean it. I've used mint Mobile and the call quality is always so crisp and so clear. I can recommend it to you. So say bye bye to your overpriced wireless plans, jaw dropping monthly bills and unexpected overages. You can use your own phone with any mint Mobile plan and bring your phone number along with your existing contacts. So dit your overpriced wireless with mint Mobiles deal and get three months a premium wireless service for fifteen bucks a month. To get this new customer offer and your new three month premium wireless plan for just fifteen bucks a month, go to mintmobile dot com slash universe. That's mintmobile dot com slash universe. Cut your wireless bill to fifteen bucks a month. At mintmobile dot com slash universe, forty five dollars upfront payment required equivalent to fifteen dollars per month new customers on first three month plan only. Speeds slower about forty gigabytes on unlimited plan. Additional taxi speeds and restrictions apply. See mint mobile for details.
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Hello, Danielle Joge, Thanks very much for a great show. I have three interesting questions for you. The first is the speed of light and the distances. As we know, the universe is so huge that even for the light, it takes ages to reach such of us disesss. But my question is actually we know that the faster you go, the time for you for someone who is observing you goes slower. So my question is what is the time of photons or of the light? Does light have time? The second question is if we can also reach such huge distances by having a very strong gravitation field very close to us, for instance, to build a space shuttle that has numerous black holes around, which would decelerate time for those observing us. So this is the second question, and the third question is concerning the gravitational waves. Does gravitation have the same speed as the speed of light? So we know that if we remove the Sun from the middle of our solar system, then it will take like eight mini to realize there is no sun. But shall we continue for eight minutes to spin around or it will be immediate disappearance?
Thank you all right, thank you very much Andrews for your enthusiasm about the universe and these fun questions. I'll answer is other questions over email. But I want to talk about this first question here on the podcast. This question about what's the time for a photon? Do photons experience time or are they frozen? This is a very common question. I think there's some misunderstandings about it, and there's some satisfactory and also sort of unsatisfactory answers that I can give you. First, let's review what we know about time and clocks. The basic thing to understand is that moving clocks run slowly. What that means is that if you are looking at a clock and it has velocity relative to you, it will be moving more slowly than a clock you are holding in your hands that has no velocity relative to you. So moving clocks run slowly. And there's a really key concept built into that statement, which is that somebody has to be measuring that motion, so motion is always relative. You can't say I'm on a spaceship, I'm going near the speed of light. My time must be running slowly because you have to talk about your velocity as relative to somebody else. Somebody who's measuring your speed and your time will only be running slowly for that person, the person who is measuring you to be moving quickly, because if you're on the spaceship and you're holding a clock and you're looking down, you're thinking, I'm going super fast. Where's the relativistic effects? I want to feel time go slow?
Right?
You never feel time going slowly because your clock is always motionless relative to you.
Right.
This is the clock that tells you how life is being experienced, and it's not moving relative to you, so it does not slow down. Somebody else on Earth who is measuring your velocity to be very, very high. They do you see your clock moving slowly? So there's that inherent disagreement. You look at your clock, you say it looks normal. They look at your clock and they say, well, you're moving quickly. I see it running slowly, right, And so there's this contradiction between what two different observers report, and that's at the heart of relativity. It tells you that observations are relative, right, the time itself is relative. So then Anders wants to take this to the extreme, and he says, well, say I'm in my spaceship and I'm going at ninety nine percent of the speed of light relative to Earth. When somebody looks at my spaceship, well, they see my time slowing down so much that it's almost stopped. The answer that, of course, is yes. As you approach the speed of light, your time goes slower and slower and slower, And the closer you get to the speed of light, the slower your clock goes. Again, according to the person for whom you have that high speed, you don't have a high speed relative to yourself, so you don't observe that effect so what happens if you do go the speed of light? Does time actually stop? Right? Is something moving at the speed of light like frozen in time? So unfortunately, this is not a question we can really answer in a satisfactory way because no spaceship with a person and a clock in it can go at the speed of light. Those things all have mass, and nothing that has mass can go at the speed of light. That's a fundamental rule. Light always moves at the speed of light, no matter who is measuring it. But things that have mass, particles, people, lava, hamsters, nothing that has mass to it can move at the speed of light. So that means that you can't do that experiment and see time stop for the ship moving at the speed of light because the ship cannot move at the speed of light. That's fine because Anders actually asked a slightly different question. He says, what is time for a photon? Because photons do move at the speed of light? Can we think about what it's like to be a photon or can we think about what time is like for a photon? Unfortunately, this isn't a question that really has a satisfactory answer because it's not something we really even know how to measure. I mean, think about the measurement we did where we had somebody on the spaceship. We had their clock and we had our clock, and we were comparing our clock and their clock. If we want to think about a photon's version of time, where's the photon's clock, right, how do we compare our clock to their clock. Basically we need in both frames to have a clock that's stationary with respect to the object. So when you were on the spaceship, you had a clock that was not moving relative to you, and I had a clock that was not moving relative to me, And I was comparing your clock to my clock, and you were moving quickly relative to me, So I saw your clock running slowly. But a photon doesn't have a clock, right, You can't put a clock in the photon's frame. Another way to think about this is that we could measure time passing in the photon's frame by having it past two stationary clock Like put a clock on one side of a track and on the other side of track and fire a laser beam. And the way we measure time on something that's going from one side of the track to the other is we look at the clock that thing is holding. We ask how much time has elapsed from the beginning to the end, and typically we put two synchronized clocks, one of the beginning of the erase and one of the end of the race, so we can compare it to the one that the thing that's moving fast is carrying. But again, a photon cannot carry a clock, right, It cannot hold anything. And in fact, special relativity only deals with inertial frames, frames of reference where you can measure things some way. You can have things at rest or things in motion. But a photon cannot have a frame. There is no frame of reference for the photon. That's because a photon can't be at rest with respect to anything. Anybody who measures the speed of light will always find it to be moving at the speed of light. There's no way you can catch up to a photon. I remember, a photon is nothing but motion. It has no mass to it, so if you're caught up to a photon, it wouldn't be anything anymore. So there is no frame of reference. You can't do these special relativity calculations to transform yourself from our frame to the photons frame and calculate how much time has passed. It's a bit of an unsatisfactory answer because while the limit of special relativity as you approach the speed of light seems to suggest that time stops at the speed of light, actually doing these calculations at the speed of light gives you nonsense answers, right, And what that means is that essentially photons can't be observers. They can't like make measurements about the universe. You can't calculate what it's like to be a photon using special relativity because special relativity requires these frames of reference to have an observer who's capable of making these measurements, and photons just are not. And I told understand that it's an unsatisfactory answer. We'd like to say something awesome and cool, like photons don't experience anytime, and when they're born in the dramata, they're here the next moment. But really that's not something that we can strictly say. We can write science fiction novel about it, we can think it's really cool, but we can't actually know what it's like to be a photon. And if you're a science fiction enthusiast, you might be imagining, like, well, what if we had a being who is made only out of photons? Could you have a conscious observer made out of photons? Well, these photons need to be moving at the speed of light, and photons don't interact with each other, so it seems pretty incomprehensible to like build an intelligence or consciousness or an observer out of these photons. And even if you could remember, it would still have no frame of reference. It couldn't measure anything in the universe. All right, Andrews, thanks very much for that question. Unfortunately, we do not know what it's like to be a photon. We cannot strictly answer that question. We know that as you approach us of light, your time seems to slow down from the point of view of other people, but we don't actually know what it's like to go at the speed of light, and we don't even know if it's like anything. Well, with that in mind, let's take another break and I'm going to dig into one more big question about the universe. 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. Take water, for example, most dairy farms reuse water up to four times the same water cools the milk, cleans equipment, washes the barn, and irrigates the crops. 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All right, we're back and we're attacked. Calling the biggest of questions today about the expansion of the universe and the speed of light, and here's one about the very age of the universe.
Hidan Jorge I Medo from Margentina, I have a question for you. So general relativity tells us that there is no absolute frame of reference by which you can measure time. It always depends on the observer. So if that's the case, how can we mark the passage of time in such a way that allows us to define the age of the universe as being thirteen point eight billion years old? What is the frame of references to arrive to the number? And how can it change if you choose another frame?
Thanks a lot, guys, All right, Edwaro, excellent question again. Edwardo is trying to understand how two things can fit together in his mind on one hand, we say all the time, and I just said a few minutes ago, that the passage of time depends on who is observing the clock, and velocity is being measured, and who is measuring that velocity. There is no absolute frame of reference. On the other hand, we also say the universe is thirteen point eight billion years old, and there's been a certain amount of time for a light to get to us. For example, how can we talk about the universe as having an age like one single number if everybody's clocks are different, if there is no universal sense of time. And really interesting consequence of relativity is that you can't really talk about simultaneity of events if they are separated in space. That the order of events of things that are far apart can be different according to your perspective. If you're moving fast or if you're moving slow. If you're over here or if you're over there, you might see A happen before B, and somebody else might see B happen before A, and you can both be correct, because in this universe the order of things is not necessarily nailed down if you're far enough apart. We dug into this in a whole episodisode about a foot race running really really fast near the speed of light, and who wins the race depends on who is watching and their velocity relative to the race. And so all of this upends the whole concept of having a single age of the universe, which suggests a simultaneous event happening all across the universe, all at once. And so Eduardo is asking, is that possible? Does that make any sense? And if it doesn't, then how do you define the age of the universe. So the short answer is, Eduardo, you're absolutely correct. The age of the universe is not a fixed number. It does indeed depend on your frame of reference or who is asking the question to think about, like, the length of time that our universe has existed is not even a number, right that it depends on who is asking the question. And that's because the passage of time does seem to depend on your speed, right. Your observation of how clocks are taking depends on how fast those clocks are ticking relative to You can't go all the way to the speed of light, like we talked about a minute ago, but you can get pretty dramatic effects by going ninety nine percent or ninety five percent of the speed of light. So what does that mean. It means that like, something moving really really fast relative to most of the rest of the stuff in the universe will have its clock running more slowly than our clocks, and if it ever slows down and comes back into our frame of reference and stops moving relative to us, it will seem younger than everything else in the universe. And this is not something weird. We observe this all the time. Take, for example, a muon. A muon is a subatomic particle and it only lasts for a few microseconds before it decays into electrons and other stuff. But we have muons which are created in the upper atmosphere and make it all the way down here to the ground. How is that possible if muons only live for a few microseconds. Well, the answer is that they're moving relative and so their clocks are running slower, and so when they get here there are only a few microseconds old in their frame of reference, right, And so they're like younger than they otherwise would have been. They haven't aged as much as they otherwise would have because relativity has kept them young. And when they come down here to the surface, of the Earth, and they slow down and they stop, they see the rest of the universe as sort of older than they expect. So if you've been flying around, moving really really fast relative to the rest of the stuff in the universe for most of the life of the universe, and then you slow down and you sort of hang out with the rest of the stuff in the universe and stop your motion, you will see the rest of the universe as sort of surprisingly old. You will have only lived for a billion years, for example, and you'll be surprised to find the rest of the universe seeming to be like fourteen billion years old. So there's an idea in here that's been implicit in my answer that I want to make explicit, which is the stuff in the universe. Vardo asks, if time is actually relative, than how do we measure the age of the universe. Well, cosmologists have this idea of a proper time. They say, well, everybody measures time differently, so let's just agree on a definition of time as being according to a clock that's stationary with respect to an object. So I can talk about my proper time. It's the time measured by a clock that I'm holding. Somebody else moving fast past me might see my clock moving slowly, but they can agree that I measure my time and that's my proper time. So this is a long way of saying that we define one specific frame of reference when we talk about the age of the universe, and that's the frame of reference that's at rest with respect to most of the stuff in the universe. Because when we're talking about the age of the universe, we want to know, like how old is the stuff around us? How long has this stuff been here? I mean, I know there's also a deeper level on which we want to ask the question of like the whole existence of the universe, separate from the question of the stuff in it. But we have to pick one frame of reference when we talk about this. So we pick the stuff in the universe, and most specifically, we pick the frame of reference in which the cosmic microwave background radiation is at rest. That early universe plasma that was created three hundred and eighty thousand years after the birth of the universe, that had a frame of reference, there's a frame in which that is, on average, not in motion, and that's not an absolute reference frame, right, like the universe doesn't prefer that reference frame for any reason. It just happens to be the one in which that stuff is at rest. The laws of physics still work in other reference frames. That's what we mean when we say there's no absolute reference frame, but there is a reference frame at which most of the stuff in the universe is at rest, and that's the one where the CMB is at rest. And so that's how we define the aide of the universe. The universe is thirteen point eight billion years old according to clocks that are not in motion relative to the cosmic microwave background radiation. Now is the Earth in motion relative to the cosmic microwave background radiation? Actually, yes, we are moving through the CMB, because you know, our motion is actually quite complicated. We're moving around the Sun. The Sun is moving around the center of the galaxy. Our galaxy is moving around the center of mass of the local galactic cluster. So our motion is quite complicated, and we're actually moving through the CMB. It's several hundred kilometers per second. So what that means is that the age of the universe according to Earth clocks is a tiny little bit different than it would be if our clocks were not at motion relative to the CMB, but even several hundred kilometers per second is pretty small compared to the speed of light, and so it's a very small effect. In fact, it's much much smaller than our uncertainty on the age of the universe, so it's not a dominant effect at all, and velocity is actually not the only effect. There are other things that can slow down your clocks. If you are near an object with mass, like a black hole or a sun, that also slows down your clock. That's called gravitational time dilation, and we'll dig into that in another episode the reasons for that. But for example, if you get near a black hole, then your clock slows down. So if you've spent a good fraction of your life near a black hole, then you will not have aged as much and you would think the universe is surprisingly old. So the short answer to Eduardo's question is that we do pick a frame of reference. It's the CMB, and the age of the universe is quoted as relative to the CMB for observers that have not been near very massive objects. So thanks very much for that question, Eduardo, Thanks for thinking hard about whether these ideas really fit together. I hope I help those click together in your mind. And thanks to everybody who is sent in questions, who continues to support the podcast, who shares with us your curiosity, and has been along for this super fun ride exploring our universe. If you have questions you'd like answers to, please don't be shy. Send them to me to questions at Danielanjorge dot com. I know you're out there, I know you have a question. I know you haven't sent it to me. I just don't know why. 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. House US dairy tackling greenhouse gases, many farms use anaerobic digestors to turn the methane from manure into renewable energy that can power farms, towns, and electric cars. Visit you as dairy dot COM's Last sustainability to learn more.
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