Speaker 1: It's not easy to hold an entire universe inside your head.
Speaker 1: I mean, that's a whole lot of planets and stars,
Speaker 1: and I hope aliens to squeeze into a few pounds
Speaker 1: of brain matter. Instead, we try to whole date model
Speaker 1: of the universe in our minds. But you know, even
Speaker 1: that is hard because it turns out that the rules
Speaker 1: of the universe are pretty different from the rules of
Speaker 1: rocks and trees and water and all the things we're
Speaker 1: familiar with. You've got relativity and the expansion of space
Speaker 1: and things moving and light speed and all that jazz.
Speaker 1: So it can be quite a challenge. And when we
Speaker 1: try to bring all these crazy ideas into our minds,
Speaker 1: sometimes things don't quite fit together. But trust me, it
Speaker 1: actually does make sense, and it can make sense to you. Hi.
Speaker 1: I'm Daniel. I'm a particle physicist, and I've spent a
Speaker 1: lifetime trying to import a model of the universe into
Speaker 1: my brain. I spent a lot of time thinking about
Speaker 1: how the universe works and making sure that I understand
Speaker 1: what's going on looking at it, from the tiniest little
Speaker 1: particles to the huge cosmic scales of the universe. I
Speaker 1: want one single idea which explains everything. I want to
Speaker 1: have a coherent understanding of everything that's going on out there,
Speaker 1: and I believe, without evidence, without any reason to believe this,
Speaker 1: I do believe that the universe does make sense. I
Speaker 1: believe that we are capable of understanding some fraction of it,
Speaker 1: at least, that human mathematics and philosophy are able to
Speaker 1: build constructs in our mind which can map onto that
Speaker 1: external universe and explain it to us in a way
Speaker 1: that we can manipulate it and understand it and examine it.
Speaker 1: I don't know if we're capable of understanding the entire
Speaker 1: universe at all of its incredible glory, if we are
Speaker 1: smart enough, if we will ever develop the mathematics, indeed,
Speaker 1: if our mathematics are even capable of describing the entire universe.
Speaker 1: But I do know it's worthwhile, So welcome to the
Speaker 1: podcast Daniel and Jorge Explain the Universe, in which we
Speaker 1: attempt to take our current knowledge of the state of
Speaker 1: the universe and download it all into your brain. We
Speaker 1: talk about everything that we do understand, and we love
Speaker 1: to talk about the things that we do not yet understand,
Speaker 1: because even the smartest of human physicists do not have
Speaker 1: a complete model of how the universe works in her brains.
Speaker 1: We are still working on that. There are still pieces
Speaker 1: that do not fit together. We have ideas from lots
Speaker 1: of different physicists, and sometimes we can mail them together
Speaker 1: into one coherent theory, and sometimes they just do not
Speaker 1: play nicely. And we're still hard at work trying to
Speaker 1: get an understanding of the universe. But it's those scientists
Speaker 1: asking those questions, trying to make one theory of the universe,
Speaker 1: trying to understand how everything out there can make sense
Speaker 1: to our tiny little brains. That is what is driving
Speaker 1: science forward. If you are not an active scientist, you
Speaker 1: might have the impression that science just sort of rolls
Speaker 1: forward on its own, results being produced year after year
Speaker 1: as we push back the darkness and illuminate the universe
Speaker 1: with our ideas. But in fact, science is not a
Speaker 1: monolithic institution. It's very personal. It's all about individual people
Speaker 1: asking questions. It's because that little girl grew up and
Speaker 1: really needed to know how stars are formed. It's because
Speaker 1: that little boy really wanted to understand how the universe
Speaker 1: was expanding. It's because this person over here really wanted
Speaker 1: to know the smallest things about the universe. It's into
Speaker 1: a dual people asking their personal questions about the universe
Speaker 1: and spending their life trying to find those answers. That's
Speaker 1: how science moves forward. That is how we develop new
Speaker 1: understandings of the universe, and that is also how you
Speaker 1: developed new understandings of the universe by asking questions. And
Speaker 1: one thing we love on this podcast are your questions
Speaker 1: because we celebrate that kind of curiosity. The same curiosity
Speaker 1: to drive signs forward also drives you to understand the universe.
Speaker 1: And you'd be surprised how often the questions you have
Speaker 1: in your mind are the very same questions that scientists
Speaker 1: are asking. And so we want to bring you all
Speaker 1: the way up to the forefront of scientific knowledge and
Speaker 1: make sure you understand what scientists are puzzled about and
Speaker 1: what they have figured out. So Jorge is not here today,
Speaker 1: so I'll take the opportunity to be answering some of
Speaker 1: your questions about the universe. And today we have a
Speaker 1: doozy of an episode for your folks. It's all about
Speaker 1: time and space in the expansion of the universe and
Speaker 1: how that can all make sense, and it's going to
Speaker 1: require you to bend your brain a little bit to
Speaker 1: accept that the universe operates in a way that is
Speaker 1: very unexpected and very unusual and has very weird consequences. Remember,
Speaker 1: we live in a universe that's very different from the
Speaker 1: one that we thought we lived in just five ten
Speaker 1: thousand years ago, and only in the last hundred years
Speaker 1: have we revealed these deep and fascinating truths about the universe,
Speaker 1: which frankly upend every idea we had about how things
Speaker 1: move and what things mean, and what distance and time
Speaker 1: and velocity even are man And so we expect that
Speaker 1: in the future there will be more revelations, but until then,
Speaker 1: a lot of us are still catching up with some
Speaker 1: of the crazy, bonkers ideas that science has so far revealed.
Speaker 1: So today on the podcast, we'll be doing listener questions
Speaker 1: about space and time and the whole universe Earth, And
Speaker 1: so don't forget that you two can get your questions answered.
Speaker 1: If you write to me two questions at Daniel and
Speaker 1: jorgead dot com, you'll get an answer, and you might
Speaker 1: even get your question on the podcast. So, without further ado,
Speaker 1: let's get to the first listener question from Chris Hey.
Speaker 1: Daniel and Jorge. I've heard that as light travels from
Speaker 1: the edges of the universe over time, that more of
Speaker 1: the universe is visible today than yesterday because the light
Speaker 1: from farther away has had time to reach us. But
Speaker 1: also that the universe expands away from us at faster
Speaker 1: than the speed of light depending on how far away
Speaker 1: it is. In the far future, we wouldn't be able
Speaker 1: to see even the light from our closest neighboring galaxy.
Speaker 1: How can both of these facts be true? Thank you?
Speaker 1: I love the podcast. Thank you Chris for this wonderful question.
Speaker 1: At first, I'd like to point out that I love
Speaker 1: hearing the way Chris is doing physics. He's heard one thing,
Speaker 1: he's heard something else, and he is trying to fit
Speaker 1: those two ideas in his mind together. He wants to
Speaker 1: unify understanding of the nature of the universe, not just
Speaker 1: two cools owning ideas. He wants it all to gel
Speaker 1: in his head. And that's what everybody should be doing.
Speaker 1: When you hear an idea about science or read some
Speaker 1: news article, you should ask yourself, Huh does this make
Speaker 1: sense to me based on other things I've heard? How
Speaker 1: does it fit together with what I understand, and it's
Speaker 1: by doing that that you build an ever larger constellation
Speaker 1: of ideas in your mind that are all fitting together
Speaker 1: and giving you a unified understanding of the universe out there.
Speaker 1: So let's talk about Chris's question. He wants to understand
Speaker 1: two different ideas. One that our observable universe, the part
Speaker 1: of the universe that we can see, seems to be
Speaker 1: growing because as time goes on, light has had enough
Speaker 1: time to get to us, so we can see deeper
Speaker 1: and deeper into the universe. That is true. And he
Speaker 1: also wants to understand how the fact that the universe
Speaker 1: is expanding means that things near us will in the
Speaker 1: future no longer be visit bowl, which is also true.
Speaker 1: And so let's unpack these two different ideas. First, let's
Speaker 1: make sure we understand exactly what is meant by the
Speaker 1: observable universe, this first part of his question. So at first,
Speaker 1: let's just simplify things and assume that we were dealing
Speaker 1: with the universe where there is no expansion of space,
Speaker 1: so we don't have to worry about that for now,
Speaker 1: and just think about this question of the observable universe.
Speaker 1: What does that mean? Well, in the end, It comes
Speaker 1: down to the fact that light takes time to get somewhere.
Speaker 1: Something happens on Andromeda, which is millions of light years away,
Speaker 1: you don't see it immediately. In fact, Andromeda could get
Speaker 1: attacked by aliens and totally blown up. We would not
Speaker 1: see that happen for millions of years. The fact that
Speaker 1: light does not move instantaneously means we can't see infinitely
Speaker 1: far out there, because there could be something out there.
Speaker 1: There almost certainly is something out there galaxy so far
Speaker 1: away that light has not yet had time to get
Speaker 1: to us through the whole history of the universe. Imagine
Speaker 1: that a photon created in the very very beginning of
Speaker 1: the universe sent towards us, traveling for billions of years
Speaker 1: and still still not having arrived, still needing to fly
Speaker 1: through space to get to us. And so this portion
Speaker 1: of the universe where light has had time to get
Speaker 1: to us, that's what we call the observable universe. It's
Speaker 1: not really a physical thing. The edge of the observable
Speaker 1: universe is just defined by where we are and the
Speaker 1: age of the universe. It's centered literally around us, like
Speaker 1: our heads. If we lived on Jupiter, the observable universe
Speaker 1: would be centered at Jupiter. If we lived at Proximus Centauri,
Speaker 1: the observable universe would be centered at Proximus Centauri. And
Speaker 1: aliens living in another galaxy have a very different observable
Speaker 1: universe because light needs time to get to them, right,
Speaker 1: So you could have different, partially overlapping observable universes if
Speaker 1: you're observing it from different places. And the size of
Speaker 1: the observable universe grows every year because every year you've
Speaker 1: a loud more time for light to get to you
Speaker 1: from those distant galaxies. And so in a universe that's
Speaker 1: not expanding, the observable universe grows by one light year
Speaker 1: in radius every year, and so if you wait, the
Speaker 1: observable universe gets bigger and bigger, and you've had time
Speaker 1: for that light to get to you from further and
Speaker 1: further places. And so what's at the edge of the
Speaker 1: observable universe. The edge of the observable universe is the
Speaker 1: oldest light that we can see. And in the universe
Speaker 1: that's not expanding, that's not actually light from the very
Speaker 1: very early universe. Because in the very early universe, everything
Speaker 1: was hot and nasty and interacting with each other it
Speaker 1: was a big, crazy plasma sort of like the inside
Speaker 1: of our sun, and so any light that was created
Speaker 1: then was also we absorbed almost immediately, and so it
Speaker 1: doesn't exist any longer. The oldest light that we can
Speaker 1: see comes from the cosmic microwave background, this moment when
Speaker 1: the plasma cooled so that it became transparent, and then
Speaker 1: light created in that moment. Could they fly free through
Speaker 1: the universe. So the oldest light we can see comes
Speaker 1: from the first moments that his cosmic microwave background radiation
Speaker 1: was created. So we can't actually see light from the
Speaker 1: very very earliest universe, all right. So that's the concept
Speaker 1: of the observable universe, and it seems to make sense
Speaker 1: that as time goes on we can see further and
Speaker 1: further out. But there's this other thing going on at
Speaker 1: the same time the universe is expanding. You might have
Speaker 1: in your mind this idea of the Big Bang is
Speaker 1: like an explosion. A lot of people think of the
Speaker 1: Big Bang is like a tiny dot of stuff the
Speaker 1: whole universe swished down to an atom, which then explodes
Speaker 1: and moves through space. But the modern idea of the
Speaker 1: Big Bang is actually quite different. It's that the universe
Speaker 1: was infinite and always has been infinite, and that the
Speaker 1: singularity wasn't a single point in space, but it was
Speaker 1: a singularity in density. That is, the universe was very
Speaker 1: hot and dense everywhere, and the Big Bang was an
Speaker 1: expansion of that. Density is stretching of space itself, so
Speaker 1: not stuff moving out from a little dot through space,
Speaker 1: but an expansion of space itself. That means that new
Speaker 1: space is being created, and that expansion is still happening.
Speaker 1: In fact, that expansion is still accelerating. That's what we
Speaker 1: call dark energy. So the universe is expanding, by which
Speaker 1: we mean new space is being created between stuff all
Speaker 1: the time. And this new space is being created between
Speaker 1: our galaxy and other galaxies for example, So the distance
Speaker 1: between us and other galaxies can be increasing even if
Speaker 1: we have no relative velocity, right, just like create new
Speaker 1: space between them. So this is called the recession of velocities,
Speaker 1: basically the rate at which the distance is increasing even
Speaker 1: if you don't have relative velocity. Now, a lot of
Speaker 1: people wonder a lot of second, is this new space
Speaker 1: that's being created only out there between galaxies or is
Speaker 1: it also here in our solar system? Why isn't the
Speaker 1: Sun getting expanded away from the Earth, Why is it
Speaker 1: Mars getting blown out of the Solar System. Well, this
Speaker 1: process dark energy that's accelerating the expansion of space is
Speaker 1: actually a very small effect per unit space, So like
Speaker 1: in our Solar system, it's pretty weak, and gravity is
Speaker 1: actually much much stronger, so it's strong enough to hold
Speaker 1: the Solar system together. But dark energy adds up as
Speaker 1: you get more space. Over distances, it adds up, so
Speaker 1: you get more and more dark energy, for example, over
Speaker 1: the scale of a galaxy. But galaxies are still strong
Speaker 1: enough to hold themselves together. Even galaxy clusters are strong
Speaker 1: enough to hold themselves together, we think. But between clusters
Speaker 1: of galaxies and these much much larger distances, that's where
Speaker 1: dark energy wins. That's where the expansion of space is
Speaker 1: stronger than the local gravity and it's tearing things apart.
Speaker 1: What does that mean for how far we can see
Speaker 1: for this question of the observable universe, Well, what it
Speaker 1: means is that we can actually see much further than
Speaker 1: thirteen point eight billion light years away, which is the
Speaker 1: age of the universe times the speed of light. The
Speaker 1: reason is that we can see photons from stuff that's
Speaker 1: now much further away because space has been expanded. Right,
Speaker 1: something sent us a photon a long time ago. It's
Speaker 1: flying towards us, and since then, that thing, that galaxy
Speaker 1: or whatever, the bit of the CMB has been expanded
Speaker 1: away from us, so it's now much much further away.
Speaker 1: So we can see things now that are forty six
Speaker 1: and a half billion light years away, and that's because
Speaker 1: of the expansion. So on one hand, the universe is
Speaker 1: tearing things away from us. On the other hand, it
Speaker 1: means that we can see things which are now much
Speaker 1: much further away than you would expect if the universe
Speaker 1: is not expanding all right, So that means that we
Speaker 1: have two things going on at the same time. One
Speaker 1: is that we have more time for photons to get here,
Speaker 1: so the observable universe is growing. On the other hand,
Speaker 1: the universe is expanding faster and faster. So let's dig
Speaker 1: into how those two things play off each other. What's
Speaker 1: happening between us and a distance galaxy is that the
Speaker 1: space between us is being expanded, So the distance between
Speaker 1: us and that galaxy is getting bigger, and the further
Speaker 1: away something is, the more bits of space are being
Speaker 1: expanded and so the faster something is moving away from us.
Speaker 1: So the further something is away from Earth, the faster
Speaker 1: it's moving away from us. And this was Hubble's discovery. Actually,
Speaker 1: this is just Hubble's law that this recession velocity grows
Speaker 1: with the distance. The further you are away from us,
Speaker 1: the faster you're moving away from us. And this can
Speaker 1: actually add up to a speed moving away from us
Speaker 1: faster than the speed of light. And remember this is
Speaker 1: not motion through space. This is the expansion of space.
Speaker 1: And while special relativity says nothing can move through space
Speaker 1: faster than the speed of light, there's no limit how
Speaker 1: fast space itself can expand. And their objects out there
Speaker 1: whose distance is growing faster than the speed of light,
Speaker 1: and what does that mean? Can we see those things?
Speaker 1: This area of the universe is called the Hubble volume.
Speaker 1: Things that are moving away from us less than the
Speaker 1: speed of light are inside this Hubble volume, this Hubble sphere,
Speaker 1: and things that are moving away from us faster than
Speaker 1: the speed of light are outside the Hubble sphere. And
Speaker 1: so you might imagine, I guess things that are outside
Speaker 1: the Hubble sphere we will never see because they're moving
Speaker 1: away from us faster than the speed of light. And
Speaker 1: you might be wondering, what about these things outside the
Speaker 1: hubble sphere, these things with a distance between us and
Speaker 1: them is growing faster than the speed of light. Can
Speaker 1: we see those things? Well, what happens if a photon
Speaker 1: is emitted from those things while that photon starts moving
Speaker 1: through space, but space is expanding, And if that space
Speaker 1: is expanding faster than the speed of light, then it's
Speaker 1: like Usain Bolt is running towards you, but somebody's putting
Speaker 1: down new track faster than he's running. So the distance
Speaker 1: actually grows between us and that photon. It's sort of
Speaker 1: like the photon is moving towards us, but it's a
Speaker 1: distance is growing, right, It's not moving away from us,
Speaker 1: but the distance between us and that photon sent by
Speaker 1: something whose distance to us is growing fast than the
Speaker 1: speed of light. Our distance that photon is also growing,
Speaker 1: all right. So you might think, well, we're just never
Speaker 1: going to see those things. And this is a common
Speaker 1: misconception because this hubble sphere, this point at which the
Speaker 1: universe is moving away from us faster than the speed
Speaker 1: of light, this actually is expanding. Right. As the universe expands,
Speaker 1: so does the hubble sphere. So eventually this photon can
Speaker 1: make some progress and the hubble spear catches up to it,
Speaker 1: and then it enters a part of the universe that's
Speaker 1: moving away from us less than the speed of light,
Speaker 1: and so we can actually see those things. So because
Speaker 1: that hubble spear itself is expanding, it means we can
Speaker 1: see light from things that we're moving away from us
Speaker 1: faster than the speed of light. But that's because the
Speaker 1: hubble spear was expanding. The hubble sphere was expanding. But
Speaker 1: around seven and a half billion years into the age
Speaker 1: of the Universe, something happened. Dark energy took over and
Speaker 1: started to accelerate the expansion of the universe to make
Speaker 1: this expansion happen faster and faster, and that has the
Speaker 1: effect of actually shrinking the hubble sphere. It means that
Speaker 1: things are moving away from us faster and faster, closer
Speaker 1: than they were before. Right, it's speeding up this expansion,
Speaker 1: so you don't have to be as far away from
Speaker 1: us anymore to be moving away from us at faster
Speaker 1: than the speed of light. So this hubble sphere is
Speaker 1: shrinking and shrinking and shrinking, and now there are things
Speaker 1: that are so far away from us and moving away
Speaker 1: from us so quickly that we will never see them.
Speaker 1: And in fact, the hubble sphere itself is shrinking and
Speaker 1: shrinking and shrinking, and if dark energy continues, then it
Speaker 1: will shrink basically to zero. Almost all of the universe
Speaker 1: would be moving away from us at faster than the
Speaker 1: speed of light, making it invisible because we can only
Speaker 1: see photons and things moving away from us faster than
Speaker 1: the speed of light if the hubble sphere is expanding
Speaker 1: and catches up with those things. But the hubble sphere
Speaker 1: is now shrinking due to dark energy and the accelerated
Speaker 1: spansion of the universe. So what's the furthest thing that
Speaker 1: we will ever see. Well, currently we can see things
Speaker 1: that are about forty six and a half billion light
Speaker 1: years away. Photons from things that are now sixty two
Speaker 1: billion light years away will eventually make it here. They'll
Speaker 1: make it here just as space expands away to make
Speaker 1: it impossible. Things that are further away will never catch
Speaker 1: up to the hubble sphere. The hubble sphere is now shrinking,
Speaker 1: so things that are sufficiently far away can never have
Speaker 1: their light get to us because the hubble sphere is
Speaker 1: shrinking and they will never catch it, meaning their photons
Speaker 1: will travel through space towards us, but will never actually
Speaker 1: get closer to us. So this is a great question.
Speaker 1: Thank you very much Chris for asking this. The observable
Speaker 1: universe is growing, but space is expanding even faster, and
Speaker 1: so it's pushing things out past the edge of the
Speaker 1: observable universe. And for most things, that means we will
Speaker 1: never see them, and those things will disappear year and
Speaker 1: in the far far future, most of the universe will
Speaker 1: be dark. What remains to be seen is how much
Speaker 1: of the universe can stay local to us. Can gravity
Speaker 1: hold our galaxy together or will it get torn apart
Speaker 1: by dark energy? Cannot hold our solar system together or
Speaker 1: will dark energy tear that apart as well. The problem
Speaker 1: is we just do not understand dark energy, and we
Speaker 1: don't know how to predict its future, and so it's
Speaker 1: very difficult to know exactly what the future holds. So
Speaker 1: thanks for that really wonderful question. I want to answer
Speaker 1: a couple of more questions, but first it's time for
Speaker 1: a break. All right. We're back and we are trying
Speaker 1: to import the entire universe into our minds. And understand
Speaker 1: its expansion and the speed of light and what happens
Speaker 1: when you move through the universe nearly at the speed
Speaker 1: of light. And so there is from Norway has a
Speaker 1: question about just that. Hello Daniel and her Head. Thanks
Speaker 1: very much for a great show. I have three interesting
Speaker 1: questions for you. The first is the speed of light
Speaker 1: and the distances. As we know, the universe is so
Speaker 1: huge that even for the light, it takes ages to
Speaker 1: reach such a vast distanceance. But my question is actually
Speaker 1: we know that the faster you go, the time for
Speaker 1: you for some who who is observing you go slower.
Speaker 1: So my question is what's the time of photons or
Speaker 1: of the light? Does light have time? The second question
Speaker 1: is if we can also reach such a huge distances
Speaker 1: by having a very strong gravitation field very close to us,
Speaker 1: for instance, to build a space shuttle that has numerous
Speaker 1: black holes around, which should decelerate time for those observing us.
Speaker 1: So this is the second question, and the third question
Speaker 1: is concerning the gravitational waves. Does gravitation have the same
Speaker 1: speed as the speed of light. So we know that
Speaker 1: if we remove the Sun from the middle of our
Speaker 1: solar system, then it will take like eight minutes to
Speaker 1: realize there is no sun. But shall we continue for
Speaker 1: eight minutes to spin around or it will be immediate disappearance.
Speaker 1: Thank you all right, Thank you very much Andrews for
Speaker 1: your enthusiasm about the universe and these fine questions. I'll
Speaker 1: answer his other questions of email, but I want to
Speaker 1: talk about this first question here on the podcast, this
Speaker 1: question about what's the time for a photon? Do photons
Speaker 1: experience time or are they frozen? This is a very
Speaker 1: common question. I think there's some misunderstandings about it, and
Speaker 1: there's some satisfactory and also sort of unsatisfactory answers that
Speaker 1: I can give you. First, let's review what we know
Speaker 1: about time and clocks. The basic thing to understand is
Speaker 1: that moving clocks run slowly. What that means is that
Speaker 1: if you're looking at a clock and it has velocity
Speaker 1: relative to you, it will be moving more slowly than
Speaker 1: a clock you are holding in your hands that has
Speaker 1: no velocity relative to you. So moving clocks run slowly.
Speaker 1: And there's a really key concept built into that statement,
Speaker 1: which is that somebody has to be measuring that motion,
Speaker 1: so motion is always relative. You can't say I'm on
Speaker 1: a spaceship. I'm going near the speed of light. My
Speaker 1: time must be running slowly because you have to talk
Speaker 1: about your velocity as relative to somebody else. Somebody who's
Speaker 1: measuring your speed and your time will only be running
Speaker 1: slowly for that person, the person who is measuring you
Speaker 1: to be moving quickly. Because if you're on the spaceship
Speaker 1: and you're holding a clock and you're looking down, you're thinking,
Speaker 1: I'm going super fast. Where's the relativistic effects. I want
Speaker 1: to feel time go slow? Right. You never feel time
Speaker 1: going slowly because your clock is always motionless relative to you. Right.
Speaker 1: This is the clock that tells you how life is
Speaker 1: being experienced, and it's not moving relative to you, so
Speaker 1: it does not slow down. Somebody else on Earth who
Speaker 1: is measuring your velocity to be very very high, they
Speaker 1: do see your clock moving slowly. So there's that inherent disagreement.
Speaker 1: You look at your clock, you say it looks normal.
Speaker 1: They look at your clock and they say, well, you're
Speaker 1: moving quickly. I see it running slowly, right, And so
Speaker 1: there's this contradiction between what two different observers report, and
Speaker 1: that's at the heart of relativity. It tells you that
Speaker 1: observations are relative, right, that time itself is relative. So
Speaker 1: then Anders wants to take this to the extreme, and
Speaker 1: he says, well, say I'm in my spaceship and I'm
Speaker 1: going at the speed of light relative to Earth. When
Speaker 1: somebody looks at my spaceship, where they see my time
Speaker 1: slowing down so much that it's almost stopped. And the
Speaker 1: answer that, of course is yes. As you approach the
Speaker 1: speed of light, your time goes slower and slower and slower,
Speaker 1: And the closer you get to the speed of light,
Speaker 1: the slower your clock goes. Again, according to the person
Speaker 1: for whom you have that high speed, you don't have
Speaker 1: a high speed relative to yourself, so you don't observe
Speaker 1: that effect. So what happens if you do go the
Speaker 1: speed of light? Does time actually stop? Right? Is something
Speaker 1: moving at the speed of light like frozen in time?
Speaker 1: So unfortunately, this is not a question we can really
Speaker 1: answer in the satisfactory way because no spaceship with a
Speaker 1: person and a clock and it can go at the
Speaker 1: speed of light. Those things all have mass, and nothing
Speaker 1: that has mass can go at the speed of light.
Speaker 1: That's a fundamental rule. Light always moves at the speed
Speaker 1: of light, no matter who is measuring it. But thing
Speaker 1: that have mass, particles, people, lava, hamsters, nothing that has
Speaker 1: mass to it can move at the speed of light.
Speaker 1: So that means that you can't do that experiment and
Speaker 1: see time stop for the ship moving at the speed
Speaker 1: of light because the ship cannot move at the speed
Speaker 1: of light. That's fine because and it's actually asked a
Speaker 1: slightly different question. He said, what is time for a photon?
Speaker 1: Because photons do move at the speed of light. Can
Speaker 1: we think about what it's like to be a photon
Speaker 1: or can we think about what time is like for
Speaker 1: a photon? Unfortunately, this isn't a question that really has
Speaker 1: a satisfactory answer because it's not something we really even
Speaker 1: know how to measure. I mean, think about the measurement
Speaker 1: we did where we had somebody on the spaceship. We
Speaker 1: had their clock and we had our clock, and we
Speaker 1: were comparing our clock and their clock. If we want
Speaker 1: to think about a photons version of time, where's the
Speaker 1: photons clock? Right? How do we compare our clock to
Speaker 1: their clock? Basically we need in both frames to have
Speaker 1: a clock that's stationary with respect to the object. So
Speaker 1: when you were on the spaceship, you had a clock
Speaker 1: that was not moving relative to you, And I had
Speaker 1: a clock that was not moving relative to me, and
Speaker 1: I was comparing your clock to my clock, and you
Speaker 1: were moving quickly relative to me, So I saw your
Speaker 1: clock running slowly. But a photon doesn't have a clock, right,
Speaker 1: You can't put a clock in the photons frame. Another
Speaker 1: way to think about this is that we could measure
Speaker 1: time passing in the photons frame by having it passed
Speaker 1: two stationary clock like put a clock on one side
Speaker 1: of a track and on the other side of the track,
Speaker 1: and fire a laser beam. And the way we measure
Speaker 1: time on something that's going from one side of the
Speaker 1: track to the other. As we look at the clock
Speaker 1: that thing is holding, we asked how much time is
Speaker 1: elapsed from the beginning to the end, And typically we
Speaker 1: put two synchronized clocks, one of the beginning of the
Speaker 1: race and one of the end of the race, so
Speaker 1: we can compare it to the one that the thing
Speaker 1: that's moving fast is carrying. But again, a photon cannot
Speaker 1: carry a clock, right, I cannot hold anything. And in fact,
Speaker 1: special relativity only deals with inertial frames, frames of reference
Speaker 1: where you can measure things some way. You can have
Speaker 1: things at rest or things in motion, but a photon
Speaker 1: cannot have a frame. There is no frame of reference
Speaker 1: for the photon. That's because a photon can't be at
Speaker 1: rest with respect to anything. Anybody who measures a speed
Speaker 1: of light will always find it to be moving at
Speaker 1: the speed of light. There's no way you can like
Speaker 1: catch up to a photon. I remember, a photon is
Speaker 1: nothing but motion. It has no mass to it, so
Speaker 1: if you caught up to a photon, it wouldn't be
Speaker 1: anything anymore. So there is no frame of reference. You
Speaker 1: can't do these special relativity calculations to transform yourself from
Speaker 1: our frame to the photons frame and calculate how much
Speaker 1: time has passed. It's a bit of an unsatisfactory answer
Speaker 1: because while the limit special relativity as you approach the
Speaker 1: speed of light seems to suggest that time stops at
Speaker 1: the speed of light, actually doing these calculations at the
Speaker 1: speed of light gives you nonsense answers, right, And what
Speaker 1: that means is that essentially photons can't be observers. They
Speaker 1: can't like make measurements about the universe. You can't calculate
Speaker 1: what it's like to be a photon using special relativity,
Speaker 1: because special relativity requires these frames of reference to have
Speaker 1: an observer who's capable of making these measurements, and photons
Speaker 1: just are not. And I totally understand that it's an
Speaker 1: unsatisfactory answer. We'd like to say something awesome and cool,
Speaker 1: like photons don't experience any time, and when they're born
Speaker 1: in the Drameda, they're here the next moment, But really
Speaker 1: that's not something that we can strictly say. We can
Speaker 1: write science fiction novel about it, we can think it's
Speaker 1: really cool, but we can't actually know what it's like
Speaker 1: to be a photon. And if you're a science fiction
Speaker 1: enthusiastic might be imagining, like, well, what if we had
Speaker 1: a being who has made only out of photo potons?
Speaker 1: Could you have a conscious observer made out of photons? Well,
Speaker 1: these photons needs to be moving at the speed of light,
Speaker 1: and photons don't interact with each other, so it seems
Speaker 1: pretty incomprehensible to like build an intelligence or consciousness or
Speaker 1: an observer out of these photons. And even if you
Speaker 1: could remember, it would still have no frame of reference.
Speaker 1: It couldn't measure anything in the universe. All right, Andrews,
Speaker 1: thanks very much for that question. Unfortunately, we do not
Speaker 1: know what it's like to be a photon. We cannot
Speaker 1: strictly answer that question. We know that as you approach
Speaker 1: a speed of light, your time seems to slow down
Speaker 1: from the point of view of other people, but we
Speaker 1: don't actually know what it's like to go at the
Speaker 1: speed of light, and we don't even know if it's
Speaker 1: like anything. Well, with that in mind, let's take another
Speaker 1: break and I'm gonna dig into one more big question
Speaker 1: about the universe. Alright, we're back and we're tackling the
Speaker 1: biggest of questions today about the expansion of the universe
Speaker 1: and the speed of light. And here's one about the
Speaker 1: very age of the universe. Hi, then, Jorge, I may
Speaker 1: do from Argentina, I have a question for you. So,
Speaker 1: general relativity tells us that there is no absolute frame
Speaker 1: of reference by which you can measure time. It always
Speaker 1: depends on the observer. So if that's the case, how
Speaker 1: can we mark the passage of time in such a
Speaker 1: way that allows us to define the edge of the
Speaker 1: universe as being thirteen point eight billion years old? What
Speaker 1: is the frame of reference used to arrive to that
Speaker 1: number and how can it change? If you choose another frame.
Speaker 1: Thanks a lot, guys. All right, Eduardo, excellent question again.
Speaker 1: Eduardo is trying to understand how two things can't fit
Speaker 1: together in his mind. On one hand, we say all
Speaker 1: the time, and I just said a few minutes ago,
Speaker 1: that the passage of time depends on who is observing
Speaker 1: the clock and what velocity is being measured, and who
Speaker 1: is measuring that velocity. There is no absolute frame of reference.
Speaker 1: On the other hand, we also say the universe is
Speaker 1: thirteen four and eight billion years old, and there's been
Speaker 1: a certain amount of time for a light to get
Speaker 1: to us. For example, how can we talk about the
Speaker 1: universe as having an age like one single number if
Speaker 1: everybody's clocks are different, if there is no universal sense
Speaker 1: of time. And really interesting consequence of relativity is that
Speaker 1: you can't really talk about simultaneity of events if they
Speaker 1: are separated in space. That the order of events of
Speaker 1: things that are far apart can be different according to
Speaker 1: your perspective. If you're moving fast or if you're moving slow,
Speaker 1: if you're over here, if you're over there, you might
Speaker 1: see A happen before B, and somebody else might see
Speaker 1: B happen before A, and you can both be correct,
Speaker 1: because in this universe, the order of things is not
Speaker 1: necessarily nailed down if you're far enough apart. We dug
Speaker 1: into this in a whole episode about a foot race
Speaker 1: running really really fast near the speed of light, and
Speaker 1: who wins the race depends on who is watching and
Speaker 1: their velocity relative to the race, And so all of
Speaker 1: this up ends the whole concept of having a single
Speaker 1: age of the universe, which suggests a simultaneous event happening
Speaker 1: all across the universe, all at once. And so Eduardo
Speaker 1: is asking, is that possible? Does that make any sense?
Speaker 1: And if it doesn't, then how do you define the
Speaker 1: age of the universe. So the short answer is, Eduardo,
Speaker 1: you're absolutely correct. The age of the universe is not
Speaker 1: a fixed number. It does indeed depend on your frame
Speaker 1: of reference or who is asking the question to think about, like,
Speaker 1: the length of time that our universe has existed is
Speaker 1: not even a number, right that it depends on who
Speaker 1: is asking the question. And that's because the passage of
Speaker 1: time does seem to depend on your beat. Right, Your
Speaker 1: observation of how clocks are taking depends on how fast
Speaker 1: those clocks are taking relative to You can't go all
Speaker 1: the way to the speed of light, like we talked
Speaker 1: about a minute ago. But you can get pretty dramatic
Speaker 1: effects by going or of the speed of light. So
Speaker 1: what does that mean. It means that like, something moving
Speaker 1: really really fast relative to most of the rest of
Speaker 1: the stuff in the universe will have its clock running
Speaker 1: more slowly than our clocks, and if it ever slows
Speaker 1: down and comes back into our frame of reference and
Speaker 1: stops moving relative to us, it will seem younger than
Speaker 1: everything else in the universe. And this is not something weird.
Speaker 1: We observed this all the time. Take for example, a muan.
Speaker 1: A muon is a sub atomic particle and it only
Speaker 1: lasts for a few micro seconds before it decays into
Speaker 1: electrons and other stuff. But we have muans which are
Speaker 1: created in the upper atmosphere and make it all the
Speaker 1: way down here to the ground. How is that possible
Speaker 1: if mu wants only live for a few microseconds. Well,
Speaker 1: the answer is that they're moving relativistically, and so their
Speaker 1: clocks aren't running slower, and so when they get here
Speaker 1: there are only a few microseconds old in their frame
Speaker 1: of reference, right, and so they're like younger than they
Speaker 1: otherwise would have been. They haven't aged as much as
Speaker 1: they otherwise would have because relativity has kept them young.
Speaker 1: And when they come down here to the service of
Speaker 1: the Earth and they slow down and they stop, they
Speaker 1: see the rest of the universe is sort of older
Speaker 1: than they expect. So if you've been flying around, moving
Speaker 1: really really fast relative to the rest of the stuff
Speaker 1: in the universe for most of the life of the universe,
Speaker 1: and then you slow down and you sort of hang
Speaker 1: out with the rest of the stuff in the universe
Speaker 1: and stop your motion, you will see the rest of
Speaker 1: the universe is sort of surprisingly old. You will have
Speaker 1: only lived for a billion years, for example, and you'll
Speaker 1: be surprised to find the rest of the universe seeming
Speaker 1: to be like fourteen billion years old. So there's an
Speaker 1: idea and here that's been implicit in my answer that
Speaker 1: I want to make explicit, which is the stuff in
Speaker 1: the universe. Eduardo asks, if time is actually relative, then
Speaker 1: how do we measure the age of the universe. Because
Speaker 1: malogists have this idea of a proper time, they say, well,
Speaker 1: everybody measures time differently, so let's just agree on a
Speaker 1: definition of time as being according to a clock that's
Speaker 1: stationary with respect to an object. So I can talk
Speaker 1: about my proper time. It's the time measured by a
Speaker 1: clock that I'm I'm holding. Somebody else moving fast past
Speaker 1: me might see my clock moving slowly, but they can
Speaker 1: agree that I measure my time and that's my proper time.
Speaker 1: So this is a long way of saying that we
Speaker 1: define one specific frame of reference when we talk about
Speaker 1: the age of the universe, and that's the frame of
Speaker 1: reference that's at rest with respect to most of the
Speaker 1: stuff in the universe. Because when we're talking about the
Speaker 1: age of the universe, we want to know, like how
Speaker 1: old is the stuff around us? How long has this
Speaker 1: stuff been here? I mean, I know there's also a
Speaker 1: deeper level on which we want to ask the question
Speaker 1: of like the whole existence of the universe, separate from
Speaker 1: the question of like the stuff in it. But we
Speaker 1: have to pick one frame of reference when we talk
Speaker 1: about this. So we pick the stuff in the universe,
Speaker 1: and most specifically, we picked the frame of reference in
Speaker 1: which the cosmic microwave background radiation is at rest. That
Speaker 1: early universe plasma that was created three hundred eighty thousand
Speaker 1: years after the birth of the universe, that had a
Speaker 1: frame of reference. There's a frame in which that is,
Speaker 1: on average, not in motion, and that's not an absolute
Speaker 1: reference frame, right, Like, the universe doesn't prefer that reference
Speaker 1: frame for any reason. It just happens to be the
Speaker 1: one in which that stuff is at rest. The laws
Speaker 1: of physics still work in other reference frames. That's what
Speaker 1: we mean when we say there's no absolute reference frame.
Speaker 1: But there is a reference frame at which most of
Speaker 1: the stuff in the universe is at rest, and that's
Speaker 1: the one where the CMB is at rest. And so
Speaker 1: that's how we define the age of the universe. The
Speaker 1: universe is thirteen point eight billion years old according to
Speaker 1: clocks that are not in motion relative to the cosmic
Speaker 1: microwave background radiation. Now is the Earth in motion relative
Speaker 1: to the cosmic microwave background radiation? Actually, yes, we are
Speaker 1: moving through the CMB because you know, our motion is
Speaker 1: actually quite complicated when moving around the Sun. The Sun
Speaker 1: is moving around the center of the galaxy. Our galaxy
Speaker 1: is moving around the center of mass of the local
Speaker 1: galactic cluster, So our motion is quite complicated. And we're
Speaker 1: actually moving through the CMB. It's several hundred kilometers per second.
Speaker 1: So what that means is that the age of the
Speaker 1: universe according to Earth clocks is a tiny little bit
Speaker 1: different than it would be if our clocks were not
Speaker 1: at motion relative to the CMB, But even several hundred
Speaker 1: kilometers per second is pretty small compared to the speed
Speaker 1: of light, and so it's a very small effect. In fact,
Speaker 1: it's much much smaller than our uncertainty on the age
Speaker 1: of the universe, so it's not a dominant effect at all,
Speaker 1: and velocity is actually not the only effect. There are
Speaker 1: other things that can slow down your clocks. If you
Speaker 1: are near an object with mass, like a black hole
Speaker 1: or a son, that also slows down your clock. That's
Speaker 1: called gravitational time dilation, and we'll dig into that in
Speaker 1: another episode the reasons for that. But for example, if
Speaker 1: you get near a black hole, then your clock slows down.
Speaker 1: So if you've spent a good fraction of your life
Speaker 1: near a black hole, then you will not have aged
Speaker 1: as much and you would think the universe is surprisingly old.
Speaker 1: So the short answer to Edwardo's question is that we
Speaker 1: do pick a frame of reference. It's the CMB, and
Speaker 1: the age of the universe is quoted as relative to
Speaker 1: the CMB. For observers that have not been near very
Speaker 1: massive objects, So thanks very much for that question, Edwardo.
Speaker 1: Thanks for thinking hard about whether these ideas really fit together.
Speaker 1: I hope I help those click together in your mind.
Speaker 1: And thanks to everybody who has sent in questions, who
Speaker 1: continues to support the podcast, who shares with us your curiosity,
Speaker 1: and it's been along for this super fun ride exploring
Speaker 1: our bonkers universe. If you have questions you like answers to,
Speaker 1: please don't be shy send them to me to questions
Speaker 1: at Daniel and Jorge dot com. I know you're out there.
Speaker 1: I know you have a question. I know you haven't
Speaker 1: sent it to me. I just don't know why. Thanks
Speaker 1: for listening, and remember that Daniel and Jorge explained the universe.
Speaker 1: It's a production of I Heart Radio. For more podcast
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