Distant time and the hint of a multiverse
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0:00 - 0:02The universe
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0:02 - 0:04is really big.
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0:04 - 0:07We live in a galaxy, the Milky Way Galaxy.
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0:07 - 0:10There are about a hundred billion stars in the Milky Way Galaxy.
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0:10 - 0:12And if you take a camera
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0:12 - 0:14and you point it at a random part of the sky,
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0:14 - 0:16and you just keep the shutter open,
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0:16 - 0:19as long as your camera is attached to the Hubble Space Telescope,
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0:19 - 0:21it will see something like this.
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0:21 - 0:24Every one of these little blobs
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0:24 - 0:26is a galaxy roughly the size of our Milky Way --
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0:26 - 0:29a hundred billion stars in each of those blobs.
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0:29 - 0:32There are approximately a hundred billion galaxies
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0:32 - 0:34in the observable universe.
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0:34 - 0:36100 billion is the only number you need to know.
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0:36 - 0:39The age of the universe, between now and the Big Bang,
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0:39 - 0:41is a hundred billion in dog years.
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0:41 - 0:43(Laughter)
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0:43 - 0:46Which tells you something about our place in the universe.
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0:46 - 0:48One thing you can do with a picture like this is simply admire it.
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0:48 - 0:50It's extremely beautiful.
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0:50 - 0:53I've often wondered, what is the evolutionary pressure
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0:53 - 0:56that made our ancestors in the Veldt adapt and evolve
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0:56 - 0:58to really enjoy pictures of galaxies
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0:58 - 1:00when they didn't have any.
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1:00 - 1:02But we would also like to understand it.
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1:02 - 1:06As a cosmologist, I want to ask, why is the universe like this?
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1:06 - 1:09One big clue we have is that the universe is changing with time.
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1:09 - 1:12If you looked at one of these galaxies and measured its velocity,
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1:12 - 1:14it would be moving away from you.
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1:14 - 1:16And if you look at a galaxy even farther away,
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1:16 - 1:18it would be moving away faster.
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1:18 - 1:20So we say the universe is expanding.
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1:20 - 1:22What that means, of course, is that, in the past,
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1:22 - 1:24things were closer together.
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1:24 - 1:26In the past, the universe was more dense,
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1:26 - 1:28and it was also hotter.
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1:28 - 1:30If you squeeze things together, the temperature goes up.
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1:30 - 1:32That kind of makes sense to us.
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1:32 - 1:34The thing that doesn't make sense to us as much
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1:34 - 1:37is that the universe, at early times, near the Big Bang,
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1:37 - 1:39was also very, very smooth.
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1:39 - 1:41You might think that that's not a surprise.
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1:41 - 1:43The air in this room is very smooth.
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1:43 - 1:46You might say, "Well, maybe things just smoothed themselves out."
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1:46 - 1:49But the conditions near the Big Bang are very, very different
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1:49 - 1:51than the conditions of the air in this room.
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1:51 - 1:53In particular, things were a lot denser.
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1:53 - 1:55The gravitational pull of things
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1:55 - 1:57was a lot stronger near the Big Bang.
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1:57 - 1:59What you have to think about
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1:59 - 2:01is we have a universe with a hundred billion galaxies,
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2:01 - 2:03a hundred billion stars each.
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2:03 - 2:06At early times, those hundred billion galaxies
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2:06 - 2:09were squeezed into a region about this big --
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2:09 - 2:11literally -- at early times.
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2:11 - 2:13And you have to imagine doing that squeezing
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2:13 - 2:15without any imperfections,
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2:15 - 2:17without any little spots
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2:17 - 2:19where there were a few more atoms than somewhere else.
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2:19 - 2:22Because if there had been, they would have collapsed under the gravitational pull
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2:22 - 2:24into a huge black hole.
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2:24 - 2:27Keeping the universe very, very smooth at early times
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2:27 - 2:29is not easy; it's a delicate arrangement.
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2:29 - 2:31It's a clue
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2:31 - 2:33that the early universe is not chosen randomly.
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2:33 - 2:35There is something that made it that way.
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2:35 - 2:37We would like to know what.
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2:37 - 2:40So part of our understanding of this was given to us by Ludwig Boltzmann,
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2:40 - 2:43an Austrian physicist in the 19th century.
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2:43 - 2:46And Boltzmann's contribution was that he helped us understand entropy.
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2:46 - 2:48You've heard of entropy.
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2:48 - 2:51It's the randomness, the disorder, the chaoticness of some systems.
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2:51 - 2:53Boltzmann gave us a formula --
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2:53 - 2:55engraved on his tombstone now --
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2:55 - 2:57that really quantifies what entropy is.
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2:57 - 2:59And it's basically just saying
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2:59 - 3:01that entropy is the number of ways
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3:01 - 3:04we can rearrange the constituents of a system so that you don't notice,
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3:04 - 3:06so that macroscopically it looks the same.
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3:06 - 3:08If you have the air in this room,
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3:08 - 3:11you don't notice each individual atom.
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3:11 - 3:13A low entropy configuration
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3:13 - 3:15is one in which there's only a few arrangements that look that way.
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3:15 - 3:17A high entropy arrangement
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3:17 - 3:19is one that there are many arrangements that look that way.
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3:19 - 3:21This is a crucially important insight
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3:21 - 3:23because it helps us explain
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3:23 - 3:25the second law of thermodynamics --
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3:25 - 3:28the law that says that entropy increases in the universe,
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3:28 - 3:30or in some isolated bit of the universe.
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3:30 - 3:32The reason why entropy increases
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3:32 - 3:35is simply because there are many more ways
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3:35 - 3:37to be high entropy than to be low entropy.
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3:37 - 3:39That's a wonderful insight,
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3:39 - 3:41but it leaves something out.
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3:41 - 3:43This insight that entropy increases, by the way,
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3:43 - 3:46is what's behind what we call the arrow of time,
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3:46 - 3:48the difference between the past and the future.
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3:48 - 3:50Every difference that there is
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3:50 - 3:52between the past and the future
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3:52 - 3:54is because entropy is increasing --
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3:54 - 3:57the fact that you can remember the past, but not the future.
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3:57 - 4:00The fact that you are born, and then you live, and then you die,
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4:00 - 4:02always in that order,
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4:02 - 4:04that's because entropy is increasing.
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4:04 - 4:06Boltzmann explained that if you start with low entropy,
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4:06 - 4:08it's very natural for it to increase
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4:08 - 4:11because there's more ways to be high entropy.
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4:11 - 4:13What he didn't explain
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4:13 - 4:16was why the entropy was ever low in the first place.
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4:16 - 4:18The fact that the entropy of the universe was low
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4:18 - 4:20was a reflection of the fact
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4:20 - 4:22that the early universe was very, very smooth.
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4:22 - 4:24We'd like to understand that.
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4:24 - 4:26That's our job as cosmologists.
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4:26 - 4:28Unfortunately, it's actually not a problem
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4:28 - 4:30that we've been giving enough attention to.
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4:30 - 4:32It's not one of the first things people would say,
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4:32 - 4:34if you asked a modern cosmologist,
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4:34 - 4:36"What are the problems we're trying to address?"
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4:36 - 4:38One of the people who did understand that this was a problem
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4:38 - 4:40was Richard Feynman.
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4:40 - 4:4250 years ago, he gave a series of a bunch of different lectures.
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4:42 - 4:44He gave the popular lectures
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4:44 - 4:46that became "The Character of Physical Law."
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4:46 - 4:48He gave lectures to Caltech undergrads
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4:48 - 4:50that became "The Feynman Lectures on Physics."
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4:50 - 4:52He gave lectures to Caltech graduate students
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4:52 - 4:54that became "The Feynman Lectures on Gravitation."
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4:54 - 4:57In every one of these books, every one of these sets of lectures,
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4:57 - 4:59he emphasized this puzzle:
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4:59 - 5:02Why did the early universe have such a small entropy?
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5:02 - 5:04So he says -- I'm not going to do the accent --
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5:04 - 5:07he says, "For some reason, the universe, at one time,
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5:07 - 5:10had a very low entropy for its energy content,
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5:10 - 5:12and since then the entropy has increased.
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5:12 - 5:15The arrow of time cannot be completely understood
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5:15 - 5:18until the mystery of the beginnings of the history of the universe
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5:18 - 5:20are reduced still further
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5:20 - 5:22from speculation to understanding."
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5:22 - 5:24So that's our job.
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5:24 - 5:26We want to know -- this is 50 years ago, "Surely," you're thinking,
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5:26 - 5:28"we've figured it out by now."
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5:28 - 5:30It's not true that we've figured it out by now.
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5:30 - 5:32The reason the problem has gotten worse,
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5:32 - 5:34rather than better,
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5:34 - 5:36is because in 1998
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5:36 - 5:39we learned something crucial about the universe that we didn't know before.
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5:39 - 5:41We learned that it's accelerating.
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5:41 - 5:43The universe is not only expanding.
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5:43 - 5:45If you look at the galaxy, it's moving away.
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5:45 - 5:47If you come back a billion years later and look at it again,
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5:47 - 5:50it will be moving away faster.
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5:50 - 5:53Individual galaxies are speeding away from us faster and faster
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5:53 - 5:55so we say the universe is accelerating.
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5:55 - 5:57Unlike the low entropy of the early universe,
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5:57 - 5:59even though we don't know the answer for this,
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5:59 - 6:01we at least have a good theory that can explain it,
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6:01 - 6:03if that theory is right,
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6:03 - 6:05and that's the theory of dark energy.
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6:05 - 6:08It's just the idea that empty space itself has energy.
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6:08 - 6:11In every little cubic centimeter of space,
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6:11 - 6:13whether or not there's stuff,
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6:13 - 6:15whether or not there's particles, matter, radiation or whatever,
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6:15 - 6:18there's still energy, even in the space itself.
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6:18 - 6:20And this energy, according to Einstein,
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6:20 - 6:23exerts a push on the universe.
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6:23 - 6:25It is a perpetual impulse
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6:25 - 6:27that pushes galaxies apart from each other.
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6:27 - 6:30Because dark energy, unlike matter or radiation,
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6:30 - 6:33does not dilute away as the universe expands.
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6:33 - 6:35The amount of energy in each cubic centimeter
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6:35 - 6:37remains the same,
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6:37 - 6:39even as the universe gets bigger and bigger.
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6:39 - 6:42This has crucial implications
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6:42 - 6:45for what the universe is going to do in the future.
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6:45 - 6:47For one thing, the universe will expand forever.
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6:47 - 6:49Back when I was your age,
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6:49 - 6:51we didn't know what the universe was going to do.
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6:51 - 6:54Some people thought that the universe would recollapse in the future.
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6:54 - 6:56Einstein was fond of this idea.
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6:56 - 6:59But if there's dark energy, and the dark energy does not go away,
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6:59 - 7:02the universe is just going to keep expanding forever and ever and ever.
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7:02 - 7:0414 billion years in the past,
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7:04 - 7:06100 billion dog years,
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7:06 - 7:09but an infinite number of years into the future.
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7:09 - 7:12Meanwhile, for all intents and purposes,
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7:12 - 7:14space looks finite to us.
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7:14 - 7:16Space may be finite or infinite,
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7:16 - 7:18but because the universe is accelerating,
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7:18 - 7:20there are parts of it we cannot see
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7:20 - 7:22and never will see.
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7:22 - 7:24There's a finite region of space that we have access to,
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7:24 - 7:26surrounded by a horizon.
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7:26 - 7:28So even though time goes on forever,
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7:28 - 7:30space is limited to us.
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7:30 - 7:33Finally, empty space has a temperature.
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7:33 - 7:35In the 1970s, Stephen Hawking told us
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7:35 - 7:37that a black hole, even though you think it's black,
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7:37 - 7:39it actually emits radiation
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7:39 - 7:41when you take into account quantum mechanics.
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7:41 - 7:44The curvature of space-time around the black hole
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7:44 - 7:47brings to life the quantum mechanical fluctuation,
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7:47 - 7:49and the black hole radiates.
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7:49 - 7:52A precisely similar calculation by Hawking and Gary Gibbons
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7:52 - 7:55showed that if you have dark energy in empty space,
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7:55 - 7:58then the whole universe radiates.
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7:58 - 8:00The energy of empty space
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8:00 - 8:02brings to life quantum fluctuations.
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8:02 - 8:04And so even though the universe will last forever,
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8:04 - 8:07and ordinary matter and radiation will dilute away,
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8:07 - 8:09there will always be some radiation,
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8:09 - 8:11some thermal fluctuations,
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8:11 - 8:13even in empty space.
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8:13 - 8:15So what this means
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8:15 - 8:17is that the universe is like a box of gas
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8:17 - 8:19that lasts forever.
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8:19 - 8:21Well what is the implication of that?
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8:21 - 8:24That implication was studied by Boltzmann back in the 19th century.
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8:24 - 8:27He said, well, entropy increases
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8:27 - 8:29because there are many, many more ways
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8:29 - 8:32for the universe to be high entropy, rather than low entropy.
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8:32 - 8:35But that's a probabilistic statement.
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8:35 - 8:37It will probably increase,
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8:37 - 8:39and the probability is enormously huge.
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8:39 - 8:41It's not something you have to worry about --
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8:41 - 8:45the air in this room all gathering over one part of the room and suffocating us.
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8:45 - 8:47It's very, very unlikely.
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8:47 - 8:49Except if they locked the doors
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8:49 - 8:51and kept us here literally forever,
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8:51 - 8:53that would happen.
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8:53 - 8:55Everything that is allowed,
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8:55 - 8:58every configuration that is allowed to be obtained by the molecules in this room,
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8:58 - 9:00would eventually be obtained.
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9:00 - 9:03So Boltzmann says, look, you could start with a universe
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9:03 - 9:05that was in thermal equilibrium.
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9:05 - 9:08He didn't know about the Big Bang. He didn't know about the expansion of the universe.
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9:08 - 9:11He thought that space and time were explained by Isaac Newton --
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9:11 - 9:13they were absolute; they just stuck there forever.
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9:13 - 9:15So his idea of a natural universe
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9:15 - 9:18was one in which the air molecules were just spread out evenly everywhere --
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9:18 - 9:20the everything molecules.
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9:20 - 9:23But if you're Boltzmann, you know that if you wait long enough,
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9:23 - 9:26the random fluctuations of those molecules
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9:26 - 9:28will occasionally bring them
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9:28 - 9:30into lower entropy configurations.
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9:30 - 9:32And then, of course, in the natural course of things,
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9:32 - 9:34they will expand back.
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9:34 - 9:36So it's not that entropy must always increase --
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9:36 - 9:39you can get fluctuations into lower entropy,
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9:39 - 9:41more organized situations.
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9:41 - 9:43Well if that's true,
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9:43 - 9:45Boltzmann then goes onto invent
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9:45 - 9:47two very modern-sounding ideas --
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9:47 - 9:50the multiverse and the anthropic principle.
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9:50 - 9:52He says, the problem with thermal equilibrium
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9:52 - 9:54is that we can't live there.
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9:54 - 9:57Remember, life itself depends on the arrow of time.
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9:57 - 9:59We would not be able to process information,
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9:59 - 10:01metabolize, walk and talk,
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10:01 - 10:03if we lived in thermal equilibrium.
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10:03 - 10:05So if you imagine a very, very big universe,
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10:05 - 10:07an infinitely big universe,
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10:07 - 10:09with randomly bumping into each other particles,
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10:09 - 10:12there will occasionally be small fluctuations in the lower entropy states,
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10:12 - 10:14and then they relax back.
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10:14 - 10:16But there will also be large fluctuations.
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10:16 - 10:18Occasionally, you will make a planet
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10:18 - 10:20or a star or a galaxy
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10:20 - 10:22or a hundred billion galaxies.
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10:22 - 10:24So Boltzmann says,
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10:24 - 10:27we will only live in the part of the multiverse,
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10:27 - 10:30in the part of this infinitely big set of fluctuating particles,
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10:30 - 10:32where life is possible.
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10:32 - 10:34That's the region where entropy is low.
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10:34 - 10:37Maybe our universe is just one of those things
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10:37 - 10:39that happens from time to time.
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10:39 - 10:41Now your homework assignment
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10:41 - 10:43is to really think about this, to contemplate what it means.
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10:43 - 10:45Carl Sagan once famously said
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10:45 - 10:47that "in order to make an apple pie,
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10:47 - 10:50you must first invent the universe."
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10:50 - 10:52But he was not right.
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10:52 - 10:55In Boltzmann's scenario, if you want to make an apple pie,
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10:55 - 10:58you just wait for the random motion of atoms
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10:58 - 11:00to make you an apple pie.
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11:00 - 11:02That will happen much more frequently
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11:02 - 11:04than the random motions of atoms
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11:04 - 11:06making you an apple orchard
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11:06 - 11:08and some sugar and an oven,
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11:08 - 11:10and then making you an apple pie.
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11:10 - 11:13So this scenario makes predictions.
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11:13 - 11:15And the predictions are
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11:15 - 11:18that the fluctuations that make us are minimal.
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11:18 - 11:21Even if you imagine that this room we are in now
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11:21 - 11:23exists and is real and here we are,
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11:23 - 11:25and we have, not only our memories,
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11:25 - 11:27but our impression that outside there's something
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11:27 - 11:31called Caltech and the United States and the Milky Way Galaxy,
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11:31 - 11:34it's much easier for all those impressions to randomly fluctuate into your brain
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11:34 - 11:36than for them actually to randomly fluctuate
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11:36 - 11:39into Caltech, the United States and the galaxy.
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11:39 - 11:41The good news is that,
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11:41 - 11:44therefore, this scenario does not work; it is not right.
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11:44 - 11:47This scenario predicts that we should be a minimal fluctuation.
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11:47 - 11:49Even if you left our galaxy out,
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11:49 - 11:51you would not get a hundred billion other galaxies.
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11:51 - 11:53And Feynman also understood this.
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11:53 - 11:57Feynman says, "From the hypothesis that the world is a fluctuation,
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11:57 - 11:59all the predictions are that
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11:59 - 12:01if we look at a part of the world we've never seen before,
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12:01 - 12:03we will find it mixed up, and not like the piece we've just looked at --
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12:03 - 12:05high entropy.
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12:05 - 12:07If our order were due to a fluctuation,
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12:07 - 12:09we would not expect order anywhere but where we have just noticed it.
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12:09 - 12:13We therefore conclude the universe is not a fluctuation."
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12:13 - 12:16So that's good. The question is then what is the right answer?
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12:16 - 12:18If the universe is not a fluctuation,
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12:18 - 12:21why did the early universe have a low entropy?
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12:21 - 12:24And I would love to tell you the answer, but I'm running out of time.
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12:24 - 12:26(Laughter)
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12:26 - 12:28Here is the universe that we tell you about,
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12:28 - 12:30versus the universe that really exists.
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12:30 - 12:32I just showed you this picture.
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12:32 - 12:34The universe is expanding for the last 10 billion years or so.
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12:34 - 12:36It's cooling off.
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12:36 - 12:38But we now know enough about the future of the universe
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12:38 - 12:40to say a lot more.
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12:40 - 12:42If the dark energy remains around,
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12:42 - 12:45the stars around us will use up their nuclear fuel, they will stop burning.
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12:45 - 12:47They will fall into black holes.
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12:47 - 12:49We will live in a universe
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12:49 - 12:51with nothing in it but black holes.
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12:51 - 12:55That universe will last 10 to the 100 years --
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12:55 - 12:57a lot longer than our little universe has lived.
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12:57 - 12:59The future is much longer than the past.
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12:59 - 13:01But even black holes don't last forever.
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13:01 - 13:03They will evaporate,
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13:03 - 13:05and we will be left with nothing but empty space.
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13:05 - 13:09That empty space lasts essentially forever.
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13:09 - 13:12However, you notice, since empty space gives off radiation,
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13:12 - 13:14there's actually thermal fluctuations,
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13:14 - 13:16and it cycles around
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13:16 - 13:18all the different possible combinations
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13:18 - 13:21of the degrees of freedom that exist in empty space.
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13:21 - 13:23So even though the universe lasts forever,
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13:23 - 13:25there's only a finite number of things
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13:25 - 13:27that can possibly happen in the universe.
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13:27 - 13:29They all happen over a period of time
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13:29 - 13:32equal to 10 to the 10 to the 120 years.
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13:32 - 13:34So here's two questions for you.
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13:34 - 13:37Number one: If the universe lasts for 10 to the 10 to the 120 years,
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13:37 - 13:39why are we born
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13:39 - 13:42in the first 14 billion years of it,
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13:42 - 13:45in the warm, comfortable afterglow of the Big Bang?
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13:45 - 13:47Why aren't we in empty space?
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13:47 - 13:49You might say, "Well there's nothing there to be living,"
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13:49 - 13:51but that's not right.
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13:51 - 13:53You could be a random fluctuation out of the nothingness.
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13:53 - 13:55Why aren't you?
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13:55 - 13:58More homework assignment for you.
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13:58 - 14:00So like I said, I don't actually know the answer.
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14:00 - 14:02I'm going to give you my favorite scenario.
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14:02 - 14:05Either it's just like that. There is no explanation.
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14:05 - 14:07This is a brute fact about the universe
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14:07 - 14:10that you should learn to accept and stop asking questions.
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14:11 - 14:13Or maybe the Big Bang
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14:13 - 14:15is not the beginning of the universe.
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14:15 - 14:18An egg, an unbroken egg, is a low entropy configuration,
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14:18 - 14:20and yet, when we open our refrigerator,
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14:20 - 14:22we do not go, "Hah, how surprising to find
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14:22 - 14:24this low entropy configuration in our refrigerator."
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14:24 - 14:27That's because an egg is not a closed system;
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14:27 - 14:29it comes out of a chicken.
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14:29 - 14:33Maybe the universe comes out of a universal chicken.
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14:33 - 14:35Maybe there is something that naturally,
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14:35 - 14:38through the growth of the laws of physics,
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14:38 - 14:40gives rise to universe like ours
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14:40 - 14:42in low entropy configurations.
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14:42 - 14:44If that's true, it would happen more than once;
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14:44 - 14:47we would be part of a much bigger multiverse.
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14:47 - 14:49That's my favorite scenario.
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14:49 - 14:52So the organizers asked me to end with a bold speculation.
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14:52 - 14:54My bold speculation
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14:54 - 14:57is that I will be absolutely vindicated by history.
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14:57 - 14:59And 50 years from now,
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14:59 - 15:02all of my current wild ideas will be accepted as truths
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15:02 - 15:05by the scientific and external communities.
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15:05 - 15:07We will all believe that our little universe
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15:07 - 15:10is just a small part of a much larger multiverse.
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15:10 - 15:13And even better, we will understand what happened at the Big Bang
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15:13 - 15:15in terms of a theory
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15:15 - 15:17that we will be able to compare to observations.
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15:17 - 15:19This is a prediction. I might be wrong.
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15:19 - 15:21But we've been thinking as a human race
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15:21 - 15:23about what the universe was like,
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15:23 - 15:26why it came to be in the way it did for many, many years.
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15:26 - 15:29It's exciting to think we may finally know the answer someday.
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15:29 - 15:31Thank you.
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15:31 - 15:33(Applause)
- Title:
- Distant time and the hint of a multiverse
- Speaker:
- Sean Carroll
- Description:
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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.
- Video Language:
- English
- Team:
- closed TED
- Project:
- TEDTalks
- Duration:
- 15:34
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