TEDxCaltech
Sean Carroll: Distant time and the hint of a multiverse
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Cosmologist Sean Carroll attacks -- in an entertaining and thought-provoking tour through the nature of time and the universe -- a deceptively simple question: Why does time exist at all? The potential answers point to a surprising view of the nature of the universe, and our place in it.
Sean M. Carroll - Physicist, cosmologist
A physicist, cosmologist and gifted science communicator, Sean Carroll is asking himself -- and asking us to consider -- questions that get at the fundamental nature of the universe. Full bio
A physicist, cosmologist and gifted science communicator, Sean Carroll is asking himself -- and asking us to consider -- questions that get at the fundamental nature of the universe. Full bio
Double-click the English transcript below to play the video.
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The universe
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is really big.
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We live in a galaxy, the Milky Way Galaxy.
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There are about a hundred billion stars in the Milky Way Galaxy.
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And if you take a camera
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and you point it at a random part of the sky,
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and you just keep the shutter open,
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as long as your camera is attached to the Hubble Space Telescope,
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it will see something like this.
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Every one of these little blobs
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is a galaxy roughly the size of our Milky Way --
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a hundred billion stars in each of those blobs.
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There are approximately a hundred billion galaxies
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in the observable universe.
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100 billion is the only number you need to know.
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The age of the universe, between now and the Big Bang,
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is a hundred billion in dog years.
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(Laughter)
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Which tells you something about our place in the universe.
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One thing you can do with a picture like this is simply admire it.
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It's extremely beautiful.
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I've often wondered, what is the evolutionary pressure
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that made our ancestors in the Veldt adapt and evolve
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to really enjoy pictures of galaxies
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when they didn't have any.
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But we would also like to understand it.
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As a cosmologist, I want to ask, why is the universe like this?
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One big clue we have is that the universe is changing with time.
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If you looked at one of these galaxies and measured its velocity,
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it would be moving away from you.
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And if you look at a galaxy even farther away,
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it would be moving away faster.
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So we say the universe is expanding.
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What that means, of course, is that, in the past,
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things were closer together.
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In the past, the universe was more dense,
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and it was also hotter.
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If you squeeze things together, the temperature goes up.
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That kind of makes sense to us.
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The thing that doesn't make sense to us as much
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is that the universe, at early times, near the Big Bang,
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was also very, very smooth.
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You might think that that's not a surprise.
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The air in this room is very smooth.
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You might say, "Well, maybe things just smoothed themselves out."
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But the conditions near the Big Bang are very, very different
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than the conditions of the air in this room.
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In particular, things were a lot denser.
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The gravitational pull of things
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was a lot stronger near the Big Bang.
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What you have to think about
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is we have a universe with a hundred billion galaxies,
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a hundred billion stars each.
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At early times, those hundred billion galaxies
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were squeezed into a region about this big --
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literally -- at early times.
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And you have to imagine doing that squeezing
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without any imperfections,
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without any little spots
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where there were a few more atoms than somewhere else.
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Because if there had been, they would have collapsed under the gravitational pull
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into a huge black hole.
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Keeping the universe very, very smooth at early times
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is not easy; it's a delicate arrangement.
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It's a clue
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that the early universe is not chosen randomly.
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There is something that made it that way.
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We would like to know what.
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So part of our understanding of this was given to us by Ludwig Boltzmann,
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an Austrian physicist in the 19th century.
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And Boltzmann's contribution was that he helped us understand entropy.
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You've heard of entropy.
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It's the randomness, the disorder, the chaoticness of some systems.
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Boltzmann gave us a formula --
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engraved on his tombstone now --
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that really quantifies what entropy is.
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And it's basically just saying
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that entropy is the number of ways
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we can rearrange the constituents of a system so that you don't notice,
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so that macroscopically it looks the same.
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If you have the air in this room,
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you don't notice each individual atom.
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A low entropy configuration
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is one in which there's only a few arrangements that look that way.
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A high entropy arrangement
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is one that there are many arrangements that look that way.
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This is a crucially important insight
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because it helps us explain
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the second law of thermodynamics --
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the law that says that entropy increases in the universe,
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or in some isolated bit of the universe.
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The reason why entropy increases
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is simply because there are many more ways
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to be high entropy than to be low entropy.
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That's a wonderful insight,
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but it leaves something out.
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This insight that entropy increases, by the way,
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is what's behind what we call the arrow of time,
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the difference between the past and the future.
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Every difference that there is
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between the past and the future
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is because entropy is increasing --
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the fact that you can remember the past, but not the future.
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The fact that you are born, and then you live, and then you die,
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always in that order,
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that's because entropy is increasing.
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Boltzmann explained that if you start with low entropy,
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it's very natural for it to increase
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because there's more ways to be high entropy.
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What he didn't explain
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was why the entropy was ever low in the first place.
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The fact that the entropy of the universe was low
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was a reflection of the fact
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that the early universe was very, very smooth.
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We'd like to understand that.
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That's our job as cosmologists.
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Unfortunately, it's actually not a problem
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that we've been giving enough attention to.
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It's not one of the first things people would say,
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if you asked a modern cosmologist,
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"What are the problems we're trying to address?"
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One of the people who did understand that this was a problem
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was Richard Feynman.
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50 years ago, he gave a series of a bunch of different lectures.
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He gave the popular lectures
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that became "The Character of Physical Law."
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He gave lectures to Caltech undergrads
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that became "The Feynman Lectures on Physics."
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He gave lectures to Caltech graduate students
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that became "The Feynman Lectures on Gravitation."
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In every one of these books, every one of these sets of lectures,
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he emphasized this puzzle:
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Why did the early universe have such a small entropy?
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So he says -- I'm not going to do the accent --
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he says, "For some reason, the universe, at one time,
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had a very low entropy for its energy content,
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and since then the entropy has increased.
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The arrow of time cannot be completely understood
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until the mystery of the beginnings of the history of the universe
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are reduced still further
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from speculation to understanding."
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So that's our job.
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We want to know -- this is 50 years ago, "Surely," you're thinking,
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"we've figured it out by now."
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It's not true that we've figured it out by now.
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The reason the problem has gotten worse,
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rather than better,
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is because in 1998
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we learned something crucial about the universe that we didn't know before.
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We learned that it's accelerating.
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The universe is not only expanding.
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If you look at the galaxy, it's moving away.
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If you come back a billion years later and look at it again,
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it will be moving away faster.
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Individual galaxies are speeding away from us faster and faster
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so we say the universe is accelerating.
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Unlike the low entropy of the early universe,
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even though we don't know the answer for this,
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we at least have a good theory that can explain it,
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if that theory is right,
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and that's the theory of dark energy.
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It's just the idea that empty space itself has energy.
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In every little cubic centimeter of space,
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whether or not there's stuff,
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whether or not there's particles, matter, radiation or whatever,
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there's still energy, even in the space itself.
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And this energy, according to Einstein,
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exerts a push on the universe.
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It is a perpetual impulse
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that pushes galaxies apart from each other.
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Because dark energy, unlike matter or radiation,
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does not dilute away as the universe expands.
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The amount of energy in each cubic centimeter
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remains the same,
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even as the universe gets bigger and bigger.
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This has crucial implications
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for what the universe is going to do in the future.
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For one thing, the universe will expand forever.
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Back when I was your age,
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we didn't know what the universe was going to do.
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Some people thought that the universe would recollapse in the future.
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Einstein was fond of this idea.
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But if there's dark energy, and the dark energy does not go away,
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the universe is just going to keep expanding forever and ever and ever.
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14 billion years in the past,
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100 billion dog years,
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but an infinite number of years into the future.
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Meanwhile, for all intents and purposes,
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space looks finite to us.
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Space may be finite or infinite,
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but because the universe is accelerating,
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there are parts of it we cannot see
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and never will see.
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There's a finite region of space that we have access to,
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surrounded by a horizon.
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So even though time goes on forever,
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space is limited to us.
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Finally, empty space has a temperature.
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In the 1970s, Stephen Hawking told us
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that a black hole, even though you think it's black,
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it actually emits radiation
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when you take into account quantum mechanics.
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The curvature of space-time around the black hole
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brings to life the quantum mechanical fluctuation,
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and the black hole radiates.
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A precisely similar calculation by Hawking and Gary Gibbons
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showed that if you have dark energy in empty space,
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then the whole universe radiates.
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The energy of empty space
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brings to life quantum fluctuations.
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And so even though the universe will last forever,
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and ordinary matter and radiation will dilute away,
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there will always be some radiation,
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some thermal fluctuations,
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even in empty space.
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So what this means
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is that the universe is like a box of gas
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that lasts forever.
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Well what is the implication of that?
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That implication was studied by Boltzmann back in the 19th century.
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He said, well, entropy increases
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because there are many, many more ways
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for the universe to be high entropy, rather than low entropy.
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But that's a probabilistic statement.
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It will probably increase,
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and the probability is enormously huge.
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It's not something you have to worry about --
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the air in this room all gathering over one part of the room and suffocating us.
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It's very, very unlikely.
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Except if they locked the doors
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and kept us here literally forever,
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that would happen.
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Everything that is allowed,
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every configuration that is allowed to be obtained by the molecules in this room,
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would eventually be obtained.
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So Boltzmann says, look, you could start with a universe
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that was in thermal equilibrium.
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He didn't know about the Big Bang. He didn't know about the expansion of the universe.
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He thought that space and time were explained by Isaac Newton --
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they were absolute; they just stuck there forever.
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So his idea of a natural universe
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was one in which the air molecules were just spread out evenly everywhere --
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the everything molecules.
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But if you're Boltzmann, you know that if you wait long enough,
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the random fluctuations of those molecules
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will occasionally bring them
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into lower entropy configurations.
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And then, of course, in the natural course of things,
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they will expand back.
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So it's not that entropy must always increase --
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you can get fluctuations into lower entropy,
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more organized situations.
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Well if that's true,
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Boltzmann then goes onto invent
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two very modern-sounding ideas --
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the multiverse and the anthropic principle.
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He says, the problem with thermal equilibrium
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is that we can't live there.
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Remember, life itself depends on the arrow of time.
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We would not be able to process information,
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metabolize, walk and talk,
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if we lived in thermal equilibrium.
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So if you imagine a very, very big universe,
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an infinitely big universe,
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with randomly bumping into each other particles,
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there will occasionally be small fluctuations in the lower entropy states,
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and then they relax back.
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But there will also be large fluctuations.
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Occasionally, you will make a planet
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or a star or a galaxy
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or a hundred billion galaxies.
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So Boltzmann says,
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we will only live in the part of the multiverse,
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in the part of this infinitely big set of fluctuating particles,
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where life is possible.
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That's the region where entropy is low.
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Maybe our universe is just one of those things
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that happens from time to time.
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Now your homework assignment
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is to really think about this, to contemplate what it means.
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Carl Sagan once famously said
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that "in order to make an apple pie,
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you must first invent the universe."
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But he was not right.
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In Boltzmann's scenario, if you want to make an apple pie,
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you just wait for the random motion of atoms
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to make you an apple pie.
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That will happen much more frequently
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than the random motions of atoms
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making you an apple orchard
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and some sugar and an oven,
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and then making you an apple pie.
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So this scenario makes predictions.
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And the predictions are
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that the fluctuations that make us are minimal.
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Even if you imagine that this room we are in now
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exists and is real and here we are,
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and we have, not only our memories,
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but our impression that outside there's something
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called Caltech and the United States and the Milky Way Galaxy,
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it's much easier for all those impressions to randomly fluctuate into your brain
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than for them actually to randomly fluctuate
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into Caltech, the United States and the galaxy.
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The good news is that,
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therefore, this scenario does not work; it is not right.
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This scenario predicts that we should be a minimal fluctuation.
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Even if you left our galaxy out,
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you would not get a hundred billion other galaxies.
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And Feynman also understood this.
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Feynman says, "From the hypothesis that the world is a fluctuation,
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all the predictions are that
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if we look at a part of the world we've never seen before,
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we will find it mixed up, and not like the piece we've just looked at --
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high entropy.
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If our order were due to a fluctuation,
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we would not expect order anywhere but where we have just noticed it.
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We therefore conclude the universe is not a fluctuation."
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So that's good. The question is then what is the right answer?
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If the universe is not a fluctuation,
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why did the early universe have a low entropy?
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And I would love to tell you the answer, but I'm running out of time.
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(Laughter)
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Here is the universe that we tell you about,
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versus the universe that really exists.
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I just showed you this picture.
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The universe is expanding for the last 10 billion years or so.
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It's cooling off.
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But we now know enough about the future of the universe
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to say a lot more.
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If the dark energy remains around,
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the stars around us will use up their nuclear fuel, they will stop burning.
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They will fall into black holes.
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We will live in a universe
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with nothing in it but black holes.
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That universe will last 10 to the 100 years --
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a lot longer than our little universe has lived.
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The future is much longer than the past.
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But even black holes don't last forever.
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They will evaporate,
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and we will be left with nothing but empty space.
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That empty space lasts essentially forever.
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However, you notice, since empty space gives off radiation,
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there's actually thermal fluctuations,
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and it cycles around
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all the different possible combinations
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of the degrees of freedom that exist in empty space.
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So even though the universe lasts forever,
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there's only a finite number of things
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that can possibly happen in the universe.
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They all happen over a period of time
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equal to 10 to the 10 to the 120 years.
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So here's two questions for you.
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Number one: If the universe lasts for 10 to the 10 to the 120 years,
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why are we born
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in the first 14 billion years of it,
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in the warm, comfortable afterglow of the Big Bang?
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Why aren't we in empty space?
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You might say, "Well there's nothing there to be living,"
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but that's not right.
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You could be a random fluctuation out of the nothingness.
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Why aren't you?
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More homework assignment for you.
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So like I said, I don't actually know the answer.
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I'm going to give you my favorite scenario.
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Either it's just like that. There is no explanation.
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This is a brute fact about the universe
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that you should learn to accept and stop asking questions.
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Or maybe the Big Bang
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is not the beginning of the universe.
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An egg, an unbroken egg, is a low entropy configuration,
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and yet, when we open our refrigerator,
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we do not go, "Hah, how surprising to find
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this low entropy configuration in our refrigerator."
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That's because an egg is not a closed system;
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it comes out of a chicken.
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Maybe the universe comes out of a universal chicken.
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Maybe there is something that naturally,
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through the growth of the laws of physics,
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gives rise to universe like ours
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in low entropy configurations.
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If that's true, it would happen more than once;
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we would be part of a much bigger multiverse.
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That's my favorite scenario.
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So the organizers asked me to end with a bold speculation.
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My bold speculation
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is that I will be absolutely vindicated by history.
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And 50 years from now,
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all of my current wild ideas will be accepted as truths
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by the scientific and external communities.
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We will all believe that our little universe
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is just a small part of a much larger multiverse.
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And even better, we will understand what happened at the Big Bang
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in terms of a theory
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that we will be able to compare to observations.
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This is a prediction. I might be wrong.
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But we've been thinking as a human race
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about what the universe was like,
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why it came to be in the way it did for many, many years.
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It's exciting to think we may finally know the answer someday.
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Thank you.
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(Applause)
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ABOUT THE SPEAKER
Sean M. Carroll - Physicist, cosmologistA physicist, cosmologist and gifted science communicator, Sean Carroll is asking himself -- and asking us to consider -- questions that get at the fundamental nature of the universe.
Why you should listen
Sean Carroll is a theoretical physicist at Caltech in Pasadena, California, where he researches theoretical aspects of cosmology, field theory and gravitation -- exploring the nature of fundamental physics by studying the structure and evolution of the universe.
His book on cosmology and the arrow of time, From Eternity to Here: The Quest for the Ultimate Theory of Time, was published in 2010. He keeps a regular blog at Cosmic Variance.
Sean M. Carroll | Speaker | TED.com