The Fabric of the Cosmos: Universe or Multi-verse?
The Fabric of the Cosmos, Hour 4: Is our universe unique, or could it be just one in an endless “multiverse”? Aired August 1, 2012 on PBS
Originally aired 11.23.11
Program Description:
Hard as it is to swallow, cutting-edge theories are suggesting that our universe may not be the only universe. Instead, it may be just one of an infinite number of universes that make up the "multiverse." In this show, Brian Greene takes us on a tour of this brave new theory at the frontier of physics, showing what some of these alternate realities might be like. Some universes may be almost indistinguishable from our own; others may contain variations of all of us, where we exist but with different families, careers, and life stories. In still others, reality may be so radically different from ours as to be unrecognizable. Brian Greene reveals why this radical new picture of the cosmos is getting serious attention from scientists. It won't be easy to prove, but if it's right, our understanding of space, time, and our place in the universe will never be the same.
NOVA | The Fabric of the Cosmos: Universe or Multi-verse
TRANSCRIPT:
NARRATOR: Lying just beneath everyday reality is a breathtaking world, where much of what we perceive about the universe is wrong. Physicist and best-selling author Brian Greene takes you on a journey that bends the rules of human experience.
BRIAN GREENE (Columbia University): Why don't we ever see events unfold in reverse order? According to the laws of physics, this can happen.
NARRATOR: It's a world that comes to light as we probe the most extreme realms of the cosmos, from black holes to the Big Bang to the very heart of matter, itself.
BRIAN GREENE: I'm going to have what he's having.
NARRATOR: Here, empty space teems with ferocious activity, the three-dimensional world may be just an illusion, and there's no distinction between past, present and future.
BRIAN GREENE: But how could this be? How could we be so wrong about something so familiar?
DAVID GROSS (University of California, Santa Barbara): Does it bother us? Absolutely.
STEVEN WEINBERG (The University of Texas at Austin): There's no principle built into the laws of nature that says theoretical physicists have to be happy.
NARRATOR: It's a game-changing perspective that opens up a whole new world of possibilities. Coming up: what if new universes were born all the time…
ALEX VILENKIN (Tufts University): In this picture, the big bang is not a unique event.
NARRATOR: …and ours was one of numerous parallel realities?
BRIAN GREENE: Somewhere there's a duplicate of you and me and everyone else.
NARRATOR: Are we in a universe or a multiverse? The Fabric of the Cosmos, right now on NOVA.
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BRIAN GREENE: New York City: they say there's nowhere else like it, home to 8,000,000 people, countless structures, monuments and landmarks, every one of them unique. Or so we think.
Uniqueness is an idea so familiar, we never even question it. Experience tells us people and objects are one of a kind. Why else would we visit museums and collect great masterpieces?
Yet a new picture of the cosmos is coming to light, in which nothing is unique. Not that the world's great masterpieces are fakes, instead, I'm talking about something far more profound: a new picture of the cosmos that challenges the very notion of uniqueness, one in which duplicates are inevitable.
And that's just the beginning. There might be duplicates not just of objects, but of you and me and everyone else. But if this new picture is right, where are these duplicates? And why haven't we ever seen them?
The answer may lie outside our universe. There was a time when the word "universe" meant "all there is," everything. The notion of more than one universe, more than one "everything," seemed impossible. But perhaps, if we could go beyond our solar system, beyond the Milky Way, even beyond other distant galaxies, past the end of the observable universe, we'll find that there's more, a lot more, that our universe is not alone. There may be other universes. In fact, there might be new ones being born all the time. We may actually live in an expanding sea of multiplying universes, a "multiverse."
If we could visit these other universes, we'd find that some might have basic properties of nature so foreign that matter, as we know it, couldn't exist. Others might have galaxies, stars, even a planet that looks familiar but with some surprising differences.
And if there are an infinite number of universes in the multiverse, somewhere there's a place where almost everything is identical to ours, except for the slightest details. Like maybe there's another Brian Greene who ends up in a different line of work.
STEVEN WEINBERG: If the multiverse is indeed infinite, then one is going to have to confront a lot of possibilities that are very hard to imagine.
ALAN GUTH (Massachusetts Institute of Technology): There will be other places where there will be Alan Guths who look and think and act exactly like me, as well as many where there will be Alan Guths who look and think almost exactly like me, but with some small differences.
LEONARD SUSSKIND (Stanford University): Is it science? Is it a part of metaphysics? Is it just philosophy? Is it religion? Physicists tend not to ask those questions, they just say, "Let's follow the logic." And the logic seems to lead there.
BRIAN GREENE: However unfamiliar and strange the multiverse might seem, a growing number of scientists think it may be the final step in a long line of radical revisions to our picture of the cosmos.
That is, there was a time when we thought that the earth was at the center of the cosmos, and that everything else revolved around us. Then, along came scientists like Galileo and Copernicus. And they showed us that it's the sun, not the earth that's at the center of our solar system. And our solar system? It's just a little neighborhood in the outskirts of a gigantic galaxy. And our galaxy? It's one of hundreds of billions of galaxies that make up our universe. Now, all of these ideas sounded outrageous when they were first proposed, but today, we don't even question them.
The idea of a multiverse may be similar. It simply may require a drastic change in our cosmic perspective. On the other hand, some scientists think that the multiverse is nothing but a dead end for physics.
ANDREAS ALBRECHT (University of California, Davis): I'm very uncomfortable with the multiverse. To become solid science it's got a lot of growing up to do.
DAVID GROSS: You know, it exists in the same way that, you know, angels might exist.
STEVEN WEINBERG: We have to make our bets, and I think, right now, the multiverse is a pretty good bet.
ALAN GUTH: I think there's a good chance that the multiverse is real, and that a hundred years from now people might be convinced that it's real.
BRIAN GREENE: So where did this idea come from, and what's the evidence for it? Well, several surprising discoveries suggest we really may be part of a multiverse.
The first of these discoveries has to do with the generally accepted theory of the origin of our universe: the Big Bang.
According to this theory, our universe began some 14 billion years ago in an intensely violent explosion. Over billions of years, the universe cooled and coalesced, allowing the formation of stars, planets and galaxies. As a result of that explosion, the universe is still expanding today. But if you could run the history of our universe in reverse, all the way back to the beginning, you'd find that the Big Bang theory tells us nothing about what sent everything hurtling outward in the first place.
ALAN GUTH: It's called the Big Bang theory, but the one thing it really says nothing about is the bang itself. It says nothing about what banged, why it banged, or what happened before it banged.
BRIAN GREENE: So what fueled that violent explosion? What force could have driven everything apart?
The quest to figure that out would bring scientists face to face with the multiverse.
One physicist whose work unexpectedly helped lay the foundation for the multiverse idea is Alan Guth. Today, he's a professor at M.I.T., but back in 1979, Guth and a colleague, Henry Tye, were pursuing a new idea about how particles might have formed in the early universe.
ALAN GUTH: Henry suggested to me that we should maybe look at whether or not this new process we were thinking of would influence the expansion rate of the universe.
BRIAN GREENE: Guth and Tye hadn't set out to investigate the expansion rate of the universe in the first moments after the Big Bang, but Henry Tye's question caused Guth to review their calculations one more time.
ALAN GUTH: I stayed up quite late that night and went over the calculations very carefully, trying to make sure everything was correct.
BRIAN GREENE: As the night wore on, Guth discovered something extraordinary in the equations describing how new particles might have formed in the early universe.
ALAN GUTH: I came to the shocking conclusion that these new-fangled particle theories would have a tremendous effect on the expansion rate of the universe. The kind of process Henry and I were talking about would drive the universe into a period of incredibly rapid exponential expansion.
BRIAN GREENE: What Guth found in the math was evidence that in the extreme environment of the very early universe, gravity can act in reverse. Instead of pulling things together, this "repulsive" gravity would repel everything around it, causing a huge expansion.
ALAN GUTH: I immediately became very excited about it and scribbled out the calculation in my notebook. And then at the end I wrote: "spectacular realization" with a double box around it, because I realized that, if it was right, it could be very important.
BRIAN GREENE: By discovering this repulsive gravity, Alan Guth had unintentionally shed light on the very beginning of the Big Bang.
Described mathematically, this force was so powerful it could take a bit of space as tiny as a molecule and blow it up to the size of the Milky Way galaxy, in less than a billionth of a billionth of a billionth of a blink of an eye. After this incredibly short outward burst, space would continue to expand more slowly, and cool, allowing stars and galaxies to form just as they do in the Big Bang theory.
Guth called this short burst of expansion "inflation," and he believed it explained what set the universe expanding in the first place. The powerful, repulsive gravity of inflation was the bang in the Big Bang.
But despite having made a momentous breakthrough, Alan Guth had an even more pressing concern.
ALAN GUTH: I had no idea what my employment might be. I was really looking for a more permanent job. The inflationary universe scenario looks very exciting, so I went on, actually, a pretty long trip, giving talks about this.
STEVEN WEINBERG: Suddenly this idea caught on.
ANDREAS ALBRECHT: Talks about inflation were packed with people from all areas of physics.
STEVEN WEINBERG: Lots of astrophysical theorists, including me, got very enthusiastic.
ANDREAS ALBRECHT: It was a very, very exciting time.
STEVEN WEINBERG: If you have a really good idea that allows other people to move the field forward, people are going to pay attention.
ALAN GUTH: …an amazing feeling that, suddenly, I had crossed that gap from being an unknown post-doc to being one of the major players. And it was very hard to absorb, but it certainly felt good.
BRIAN GREENE: One reason inflation was so exciting was that it made predictions that could be tested through observation. Scientists realized that, if the theory were correct, evidence for it should be found in the night sky.
Imagine that we could shut off the sun and take away all the stars. If our eyes could detect the rest of the energy that's still there, we'd see a warm glow everywhere in the cosmos. This sea of radiation is called the cosmic microwave background. It's the last remnants of heat from the Big Bang itself.
Theory predicted that the violent expansion of space during inflation would leave an imprint on this radiation. These telltale "fingerprints" would form a precise pattern of temperature variations—slightly hotter spots and slightly colder spots—that would look something like this.
But it would be about 10 years before the technology was sensitive enough to test this prediction. Then, in 1989, NASA launched the Cosmic Background Explorer satellite, followed by a second satellite, W.M.A.P., in 2001 that would put inflation to the test.
The missions measured the radiation with tremendous precision, and the results were stunning. The temperature variations found in the cosmos were an almost identical match with the predictions of the theory of inflation.
It's just a theory, mathematics on the page, until it makes predictions that are confirmed. W.M.A.P. found what the math of inflation predicted. That is enormously convincing.
ANDREAS ALBRECHT: So inflation has had a number of chances, now, to fail. It made predictions, data came in, and inflation has come through with flying colors.
BRIAN GREENE: Guth's work on inflation, along with that of other physicists, was hailed as a milestone toward understanding the origin of the universe. But soon, two Russian physicists would discover that the equations of inflation held a shocking secret: our universe may not be alone.
One of these Russian physicists was Andrei Linde, who had already made pivotal contributions to inflationary theory. The other was Alex Vilenkin, who happened to attend one of the talks Alan Guth gave during his road trip.
ALEX VILENKIN: He gave a wonderful talk. I hadn't met him before, but what I heard was rather unexpected. In one shot, inflation explained very well, many features of the Big Bang, and was quite remarkable—why the universe is the way it is. So I went home greatly impressed.
BRIAN GREENE: Alex Vilenkin was so impressed that, for months afterward, he couldn't stop thinking about inflation.
ALEX VILENKIN: Usually, I have my thought of the day in the shower, which I tend to take long.
BRIAN GREENE: The more Vilenkin considered the process of inflation, the more he wondered about what would make it stop. How would a region of space transition out of inflation? What exactly would happen at the moment inflation ends?
ALEX VILENKIN: As I thought about it, it turns out that the end of inflation doesn't happen everywhere at once.
BRIAN GREENE: Vilenkin suddenly realized that if inflation doesn't end everywhere at once, then there's always some part of space where it's still happening.
ALEX VILENKIN: So, in this picture, the Big Bang is not a unique event that happened. There were multiple bangs that happened before ours, and there will be countless other bangs that will happen in the future.
BRIAN GREENE: It was a striking and unexpected new picture, in which inflation would stop in some regions but always continue somewhere else. New big bangs are always occurring, and new universes are always being born, yielding an eternally expanding multiverse.
Linde and Vilenkin in particular pushed the idea that inflation might never end, that this ballooning process could happen over and over again, giving one universe after another after another.
So was this a revolution in science or just a theory that's full of holes?
The idea became known as "eternal inflation." And you can picture it something like this. Imagine that this block of cheese is all of space, before the formation of stars and galaxies. Now, according to inflation, space is uniformly filled with a huge amount of energy. And that energy causes space to expand at an enormous speed. As it does, here and there the energy discharges, sort of like a spark of static electricity.
But this is like static electricity on a cosmic scale, and when it discharges, all that energy is rapidly transformed into matter, in the form of tiny particles. That process is the birth of a new universe, what we have traditionally called the Big Bang.
Inside these new universes, which are like holes in the cheese, space continues to expand, but much more slowly. And sometimes, galaxies, stars and planets form, much as we see in our universe, today.
Meanwhile, outside of these new universes, the rest of space is still full of undischarged energy and still expanding at enormous speed. And more expanding space means more places where the energy can discharge into more big bangs and create more new universes.
And that means this process could go on forever. In other words, when it comes to eternal inflation, that cheese is more like Swiss cheese, in which new universes endlessly form, creating a multiverse.
The multiverse: a profound implication of eternal inflation. But as Alex Vilenkin would soon learn, one that would not be easily accepted.
ALEX VILENKIN: I thought I had realized something important about the universe, and I wanted to share this with my fellow physicists. And one of the first, of course, had to be Alan Guth.
Now we know that quantum fluctuations are different in different regions of space…
I thought he would be excited about it, but this encounter didn't go as planned.
…inflation will last longer than in others.
As I was describing to him my new picture of the universe, inflating regions and so forth, ahem, expansion, I noticed that Alan is beginning to doze off a little bit. Actually, I was, of course, very unhappy about that, so I thought that I probably should go.
BRIAN GREENE: One problem with the concept of a multiverse was that there seemed to be no way to detect it. Not only is each universe expanding, but so is the space in between them. That means that nothing, not even light, can travel from any of the other universes to reach us.
ALEX VILENKIN: Physicists did not really respond very well to this idea of eternal inflation. Once I said that I'm going to tell them something about things beyond our horizon that cannot, in principle, be observed, most of them just lost interest right there.
BRIAN GREENE: Alex Vilenkin thought he was on to something big, but others were skeptical. So Vilenkin reluctantly tried to put his work on eternal inflation out of his mind.
ANDREAS ALBRECHT: Who wants to talk about a universe you're never going to see? The multiverse can't make predictions, it can't be tested. You could make the case that it's not really science.
STEVEN WEINBERG: How can you ever be confident of it when you can't see the other parts of the multiverse? We can only see our little patch, our little expanding cloud of galaxies. How are we ever going to know?
PAUL STEINHARDT (Princeton University): You can't prove the multiverse exists. It's not wrong. You can't prove that it doesn't exist. So why should we believe it?
BRIAN GREENE: Alex Vilenkin tried to stop thinking about the multiverse. With no hard evidence to support it, the idea seemed to have hit a dead end.
ALEX VILENKIN: Many people thought it's just not science to talk about things that you cannot observe. So I did not return to the subject for almost ten years.
BRIAN GREENE: Meanwhile, Vilenkin's Russian colleague, Andrei Linde, kept the flame alive. He had independently come up with his own version of eternal inflation, but unlike Vilenkin, he would not be deterred.
ANDREI LINDE (Stanford University): Maybe I am a little bit more arrogant. When I got the idea for this multiverse, I understood that this may be the most important thing which I ever do in my life. And then, if somebody doesn't want to hear it, that's their problem.
BRIAN GREENE: Linde published more than a dozen papers, but his work would meet an equally chilly reception. It seemed no one wanted to hear about the idea of a multiverse.
If the equations of eternal inflation were the only clues pointing to the multiverse, that's where the story might have ended, but the multiverse idea would gain some unexpected support from two completely unrelated areas of science.
One was an idea called string theory, designed to explain how the universe works at the tiniest scales. The other was an astounding discovery made by astronomers exploring the universe on the largest scale, a discovery that's utterly mysterious if there's only one universe. But if we're part of a multiverse, it's a whole new ballgame.
It has to do with the expansion of the universe, and it's easy to explain using a baseball. Now, if I toss this ball up in the air, we all know what will happen. As it rises, it slows down because of gravity.
Now, astronomers knew that the universe was expanding. And they assumed that the expansion would slow down because of the gravitational pull of stars and galaxies, just as the ball slows down because of the gravitational pull of the Earth.
But when they actually did the measurements, they found something astonishing, something that rocked the foundations of physics. They found that the expansion is not slowing down. It's speeding up.
It's as if I took this baseball, and when I throw it, instead of slowing down as it rushes away, it speeds up. Now, if you saw a ball do that, you'd assume there's some invisible force that's counteracting gravity, pushing on the ball, forcing it to speed away ever more quickly.
Astronomers came to the same conclusion about the universe: that some kind of energy in space must be pushing all the galaxies apart, causing the expansion to speed up. Because we don't see this energy, the astronomers called it "dark energy."
RAPHAEL BOUSSO (University of California, Berkeley): It's among the most important experimental discoveries ever, in the history of science.
ANDREAS ALBRECHT: It took most of us completely by surprise.
CLIFFORD JOHNSON (University of California, Berkeley): And so, we're still trying to come to grips with that.
BRIAN GREENE: Discovering that dark energy is pushing every galaxy in our universe away from every other, at an accelerating rate, was shocking enough. But even more surprising was the strength of that dark energy.
For over a decade, scientists have been unable to explain why such a peculiar amount of it exists in empty space. But that mystery seems easier to resolve if we're part of a much larger multiverse.
Now, the idea that space contains any energy at all sounds strange. But our theory of small things, like molecules and atoms, the theory called quantum mechanics, tells us that there's a lot of activity in the microscopic realm, activity that can contribute an energy to space.
And according to the math, the amount of energy generated by that microscopic activity is enormous.
The problem is, when astronomers measured the amount of energy that's actually out there, the amount of energy required to force the galaxies apart at the accelerating rate that's observed, they get a number like this: A decimal point followed by 122 zeroes, and then a one! An incredibly tiny amount, very close to zero, and nothing at all like what the theory predicted. In fact, it's trillions and trillions and trillions and trillions of times smaller, a colossal mismatch.
LEONARD SUSSKIND: We have tried everything to explain why the dark energy is as small as it is. We have tried everything, and everything fails.
STEVEN WEINBERG: Hopeless. I once called this the worst failure of an order of magnitude estimate in the history of science.
DAVID GROSS: Does it bother us? Absolutely!
BRIAN GREENE: Finding that the amount of energy in space is so much less than our theory predicts is not just an academic problem. The precise strength of that repulsive gravity, well, that has profound implications for all of us. For example, if I were to increase the strength of the dark energy just a little bit, by erasing four or five of these zeroes, I still have a tiny number, but the universe would be radically different. That's because a slightly stronger dark energy would push everything apart so fast that stars, planets and galaxies would never have formed. And that means we simply would not exist.
And yet, here we are.
So, why is the amount of dark energy so much less than our theory predicts and also just right to allow the formation of galaxies, stars, planets and life? We just don't know. The mismatch between the theoretical predictions of dark energy and what astronomers have observed is one of the great mysteries that science faces today.
But consider this: if we do live in a multiverse, then the mystery of dark energy might not be so mysterious after all. In fact, if we're part of a multiverse, the value of dark energy we've measured might actually make total sense.
Hi. Reservation for Greene.
To see how the multiverse might solve the dark energy puzzle, imagine you're checking into a hotel, and you get a room number like this: Ten-million-and-one.
Hmmm. Thanks.
DESK CLERK: Enjoy your stay.
BRIAN GREENE: Ten-million-and-one would seem like a pretty strange room number. And getting a room number like this would be surprising, much as the value of dark energy in our universe is a number that scientists have found surprising.
But here's the thing: if this hotel had a huge number of rooms, say, billions and billions, then getting room rumber ten-million-and-one wouldn't be so surprising. In a hotel this big, you expect to find a room with that number.
Similarly, if we're part of a multiverse with a huge number of universes, each with a different value of the dark energy, then you'd expect to find one with the value as small as what we've measured.
If you think of each of these rooms as a universe, and each universe has a different value for the dark energy, then most of these universes won't be hospitable to life as we know it.
The reason is the value of the dark energy wouldn't allow the formation of galaxies, stars and planets. Universes with much less dark energy than ours would just collapse in on themselves, and universes with much more dark energy than ours would expand so fast, that matter would never have the chance to coalesce into clumps and form stars and galaxies.
So, of course, we find ourselves in a universe where the value of the dark energy is hospitable to life. Otherwise, we wouldn't be here to talk about it.
So if we're part of a multiverse, the mystery of dark energy becomes not-so-mysterious. But there's a piece of the puzzle missing. How do we know if there's enough diversity within the multiverse so that every value for dark energy, including the strange value we observe in our universe, can be found somewhere?
The answer would emerge from an entirely different area of physics. I'm talking about a ground-breaking theory that comes from investigating the universe on the tiniest scale.
We know that inside atoms are even tinier bits of matter, protons and neutrons, which are made of still smaller particles called quarks. But physicists realized this might not be the end of the line. These sub-atomic bits might actually be made of something even smaller: tiny vibrating strands or loops of energy called strings. This set of ideas, called "string theory," says everything that exists is made of this one kind of ingredient.
And just as a single string on a cello can produce many different notes depending on how it vibrates, strings can take on different properties depending on how they vibrate, creating many kinds of particles.
From this theory came the promise of elegant simplicity: a single master equation that would explain everything we see in the world around us.
LEONARD SUSSKIND: String theory would be beautiful, it would be elegant; and calculation from that very simple theory would produce the world as we know it.
BRIAN GREENE: But for this beautiful theory to work, there was a catch: the math of string theory required something that defies common sense: a feature that would open the door to the multiverse: extra dimensions of space.
We're all familiar with three dimensions of space: height, width and depth. But the math of string theory says these aren't the only dimensions.
JOSEPH POLCHINSKI (University of California, Santa Barbara): The mathematics works only if the strings move and vibrate, not just in the three directions that we see, but in those and say, six more, nine space dimensions in all.
BRIAN GREENE: So if string theory is right, where are these extra dimensions? And why can't we see them?
Think about the cable supporting a traffic light. From a distance, it looks like a line: one-dimensional. But if you could shrink down to, say, the size of an ant, you'd find another dimension exists, a circular dimension that curls around the cable. And string theory says that if we could shrink down billions of times smaller than that ant, we'd find tiny extra dimensions like this are curled up everywhere in space.
LEONARD SUSSKIND: At every point of space, there's extra dimensions of space that are curled up into little tiny knots that you can't see because they're too small.
BRIAN GREENE: And the shape of those extra dimensions determines the fundamental features of our universe. Just the way the air streams that are going through an instrument, like a French horn, have vibrational patterns that are determined by the shape of the instrument, the shape of the extra dimensions determines how the little strings vibrate. Those vibrational patterns determine particle properties, so all the fundamental features of our universe may be determined by the shape of the extra dimensions.
LEONARD SUSSKIND: The way those extra dimensions of space are put together is, in many respects, like the D.N.A. of the universe. They determine the way the universe is going to behave, just exactly the same way as D.N.A. determines the way an animal is going to look.
BRIAN GREENE: The problem was the more string theorists looked, the more ways they found that extra dimensions could be curled up.
And the math provided no clues as to which shape was the right one corresponding to our universe.
SHAMIT KACHRU (Stanford University): I think the consensus, right now, is that that number seems to be astronomical. There are published papers suggesting upwards of 10-to-the-500—that's 10 followed by 500 zeroes—different possible shapes.
BRIAN GREENE: Ten-to-the-five-hundred different possible shapes for the extra dimensions, each appearing equally valid. It seemed preposterous, especially for a theory that was looking for one single master equation to describe our universe.
But then it occurred to some string theorists that, perhaps, there was a different way to look at the problem, and this different perspective would breathe new life into the idea of a multiverse.
LEONARD SUSSKIND: Ten-to-the-500 different string theories—this sounded like a complete disaster. What good is it to have a theory that has 10-to-the-five-hundred solutions? You can't find anything in there! Well, that left string theorists somewhat unhappy, somewhat depressed.
My own reaction to it at the time is, "This is great. This is fantastic. This is exactly what the cosmologists are looking for: enormous diversity of possibilities. Don't be unhappy about this. This says that string theory fits extremely well with cosmology and with all the interesting ideas about multiverses."
BRIAN GREENE: Turning what seemed like a vice into a virtue, some string theorists became convinced that the multiple solutions of string theory might each represent a real and very different universe. In other words, string theory was describing a multiverse and an extremely diverse one at that.
CLIFFORD JOHNSON: To everyone's surprise, string theory was actually quite readily describing huge numbers of different kinds of solutions which…each of which corresponds to a possible universe.
ANDREI LINDE: So we just got this multiverse for free.
DELIA SCHWARTZ-PERLOV (Tufts University): Both from string theory and from inflation, you have these universes that are produced. These different universes would all naturally have different amounts of dark energy.
BRIAN GREENE: In fact, according to the math, the amount of dark energy would span such a wide range of values from universe to universe that the strange amount we've measured would surely turn up.
RAPHEL BOUSSO: String theory, without even trying, solved that problem.
BRIAN GREENE: So, over a decade after Linde and Vilenkin had come up with their ideas about eternal inflation, the multiverse was revived.
Three lines of reasoning were now all pointing to the same conclusion: eternal inflation, dark energy and string theory. Just the way it takes three legs to support a stool, these three ideas, taken together, support the argument that we may live in a multiverse.
When different lines of research all converge on one idea, that doesn't mean it's right; but when all the spokes of the wheel are pointing at one idea, that idea becomes pretty convincing.
Today the multiverse is hotly debated. Many critics remain. David Gross is going to tell us, "No, no, no." But multiverse advocates, like Alex Vilenkin, Alan Guth and Andrei Linde are no longer alone.
ALEX VILENKIN: The tide appears to be turning. And now these ideas are accepted to a much larger degree.
ANDREI LINDE: The genie is out of the bottle. You cannot put it back.
BRIAN GREENE: So, what would it be like, if we could travel to some of these other universes? What would we see? Some would be vastly different from our own, with properties unlike anything we've ever seen. In fact, some universes in the multiverse might not have light or matter or anything recognizable at all. And there might be other universes with features not unlike the familiar ones we know, but where life takes a completely different form, perhaps communicating in ways we'd find utterly bizarre.
And the math shows that if we were able to visit enough of these universes, we might, eventually, find ones like ours, with a Milky Way galaxy, a solar system and an Earth, except with some slight differences.
In one, maybe the asteroid that killed off the dinosaurs 65 million years ago missed, and evolution charted a new course.
In another, there might be an Earth with people similar to us but better at multitasking.
But there's something even stranger. Somewhere out there, we should find exact copies of our universe, with duplicates of everything and everyone.
How could this be? How could there be exact duplicates of ourselves out there in the multiverse?
To see how, take this deck of cards. It's made up of 52 different cards. And, if I deal them, everyone will get a different hand. But, over the course of many, many rounds, eventually some of the combinations will start to repeat.
SECOND BRIAN GREENE: That's because, with 52 cards, there's a limited number of different hands you can deal.
BRIAN GREENE: So, if you deal the cards an infinite number of times, then repeating hands are inevitable.
THIRD BRIAN GREENE: And in the multiverse, a similar principle applies.
BRIAN GREENE: That's because, according to the laws of nature, the fundamental ingredients of matter, or particles, are kind of like a deck of cards: in any region of space, they can only be arranged in a finite number of different ways.
So if space is infinite, if there are an infinite number of universes, then those arrangements are bound to repeat. And since each one of us is just a particular arrangement of particles,…
SECOND BRIAN GREENE: ...somewhere there's a duplicate of you and me…
ALL THREE BRIAN GREENES: ...and everyone else.
ALAN GUTH: This can be shocking.
SHAMIT KACHRU: It could be that in another universe I was a rock star, and my life is much better—or much worse, depending on your opinion of rock stars.
CLIFFORD JOHNSON: That means all those things that I've never found time to do are maybe being done by some copy of me somewhere else.
ALEX VILENKIN: I was rather depressed actually. This picture robs us of our uniqueness.
LEONARD SUSSKIND: It is a consequence of the ideas, and the ideas seem very well motivated.
BRIAN GREENE: Yet, critics argue the multiverse is just too convenient an explanation for things we don't understand, like the tiny value of dark energy in our universe and the huge number of possible shapes for the extra dimensions in string theory.
PAUL STEINHARDT: The problem with that kind of reasoning is that it doesn't explain why the dark energy is the way it is. It just says it's random chance.
DAVID GROSS: I don't find that satisfactory. You can apply this kind of reasoning anytime you don't have a better explanation.
BRIAN GREENE: On the other hand, supporters of the multiverse point out that sometimes a better or deeper explanation for the way things are simply does not exist.
Take for example, the earth's orbit around the sun. We find ourselves at a distance of 93-million miles, perfect for life. If we were much closer to the sun, our planet would be too hot for life, as we know it, to exist. And if we were much farther from the sun, it would be too cold for life.
So why are we in this sweet spot? Well, starting in the late 1500s, the famous astronomer Johannes Kepler asked that very question. And he spent years trying to find a physical reason, some law of nature that requires the earth to be 93-million miles from the sun. But Kepler never found it, because it doesn't exist. There isn't any physical law requiring the earth to be 93-million miles from the sun.
It's simply one possibility of the many you'd expect to find in a universe we know is full of solar systems.
LEONARD SUSSKIND: You might think it was an extraordinary accident. It's not. It's just that there are a lot of planets out there.
BRIAN GREENE: Similarly, some suggest that the true explanation for many of the fundamental features of our world will elude us, if we don't consider the possibility that we live in a multiverse.
ALAN GUTH: Clearly, if we had a good physical reason, that would be great, and we would understand it. We'd be much happier.
STEVEN WEINBERG: We may have to live with that. There's no principle built into the laws of nature that say that theoretical physicists have to be happy.
LEONARD SUSSKIND: It's a hypothesis. It's the leading hypothesis, because nobody has another hypothesis which makes as much sense.
BRIAN GREENE: The multiverse: a tantalizing possibility. But with no experimental evidence, should you believe it?
We can't believe in anything until there's observational or experimental support. But what we have found over the last few centuries is that mathematics provides a sure-footed guide to the nature of things that we haven't yet been able to see, observe or experiment with.
Math predicted things like black holes and certain subatomic particles long before we ever observed them. And math is suggesting that there may be these other universes. That doesn't mean it's right, but often it's leading you to a deeper understanding of reality.
CLIFFORD JOHNSON: If you choose not to believe it, that's perfectly fine, because we have not given you any evidence yet. And one of the wonderful things about science is that it's about evidence; it's not about belief.
BRIAN GREENE: And some scientists now think we might just be able to find evidence for a multiverse. They propose that if our universe and another were born close together, the two might have collided. That collision could have left its own fingerprint: ripples in the cosmic background radiation, the heat left over from the Big Bang.
LEONARD SUSSKIND: My guess is yes, that in 100 years, we will know one way or another whether these ideas are right.
STEVEN WEINBERG: A hundred years from now it may be an amusing historical episode. We don't know. But if you only work on the things that are already well-established, you're not going to be part of the next big excitement.
BRIAN GREENE: If we do verify the multiverse, it would change our perspective, much as Copernicus did 500 years ago, when he showed that the earth is not the center of the cosmos. And some might say that if our universe is just one of many, our descent from the center would be complete.
DELIA SCHWARTZ-PERLOV: Regardless, I think it's more important is that we're so lucky that we can understand the universe.
ANDREAS ALBRECHT: I think it's a great ride, and I think it's really good for physics that we have this tension. I don't know where we're going to end up.
BRIAN GREENE: So what does this all mean? Are there infinite duplicates of you and me and everything existing right now in an infinite number of other universes?
Is the multiverse the next Copernican revolution? We don't know, at least not yet. But if the idea that we live in a multiverse proves true, we'd be witnessing one of the most exciting and dramatic upheavals to our understanding of the fabric of the cosmos.
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