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Distant time and the hint of a multiverse

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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?
  • 13:47 - 13:49
    You might say, "Well there's nothing there to be living,"
  • 13:49 - 13:51
    but that's not right.
  • 13:51 - 13:53
    You could be a random fluctuation out of the nothingness.
  • 13:53 - 13:55
    Why aren't you?
  • 13:55 - 13:58
    More homework assignment for you.
  • 13:58 - 14:00
    So like I said, I don't actually know the answer.
  • 14:00 - 14:02
    I'm going to give you my favorite scenario.
  • 14:02 - 14:05
    Either it's just like that. There is no explanation.
  • 14:05 - 14:07
    This is a brute fact about the universe
  • 14:07 - 14:10
    that you should learn to accept and stop asking questions.
  • 14:11 - 14:13
    Or maybe the Big Bang
  • 14:13 - 14:15
    is not the beginning of the universe.
  • 14:15 - 14:18
    An egg, an unbroken egg, is a low entropy configuration,
  • 14:18 - 14:20
    and yet, when we open our refrigerator,
  • 14:20 - 14:22
    we do not go, "Hah, how surprising to find
  • 14:22 - 14:24
    this low entropy configuration in our refrigerator."
  • 14:24 - 14:27
    That's because an egg is not a closed system;
  • 14:27 - 14:29
    it comes out of a chicken.
  • 14:29 - 14:33
    Maybe the universe comes out of a universal chicken.
  • 14:33 - 14:35
    Maybe there is something that naturally,
  • 14:35 - 14:38
    through the growth of the laws of physics,
  • 14:38 - 14:40
    gives rise to universe like ours
  • 14:40 - 14:42
    in low entropy configurations.
  • 14:42 - 14:44
    If that's true, it would happen more than once;
  • 14:44 - 14:47
    we would be part of a much bigger multiverse.
  • 14:47 - 14:49
    That's my favorite scenario.
  • 14:49 - 14:52
    So the organizers asked me to end with a bold speculation.
  • 14:52 - 14:54
    My bold speculation
  • 14:54 - 14:57
    is that I will be absolutely vindicated by history.
  • 14:57 - 14:59
    And 50 years from now,
  • 14:59 - 15:02
    all of my current wild ideas will be accepted as truths
  • 15:02 - 15:05
    by the scientific and external communities.
  • 15:05 - 15:07
    We will all believe that our little universe
  • 15:07 - 15:10
    is just a small part of a much larger multiverse.
  • 15:10 - 15:13
    And even better, we will understand what happened at the Big Bang
  • 15:13 - 15:15
    in terms of a theory
  • 15:15 - 15:17
    that we will be able to compare to observations.
  • 15:17 - 15:19
    This is a prediction. I might be wrong.
  • 15:19 - 15:21
    But we've been thinking as a human race
  • 15:21 - 15:23
    about what the universe was like,
  • 15:23 - 15:26
    why it came to be in the way it did for many, many years.
  • 15:26 - 15:29
    It's exciting to think we may finally know the answer someday.
  • 15:29 - 15:31
    Thank you.
  • 15:31 - 15:33
    (Applause)
Title:
Distant time and the hint of a multiverse
Speaker:
Sean Carroll
Description:

At TEDxCaltech, 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.
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Video Language:
English
Team:
closed TED
Project:
TEDTalks
Duration:
15:34
TED edited English subtitles for Distant time and the hint of a multiverse
TED added a translation

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