Riddles of the Multiverse
While the idea remains hotly debated, some of the brightest minds in contemporary physics take very seriously the possibility that our universe is just one among many. How did this notion of multiple universes—a multiverse—arise? Could we ever cross over to other universes, and if we could, might we find another world identical to our own? In this interview, Clifford Johnson, Professor of Physics and Astronomy at the University of Southern California, offers his perspective on truly otherworldly ideas that science is just beginning to tackle.
BIRTH OF THE MULTIVERSE
NOVA: The idea of the multiverse grew, in part, from another big idea in modern cosmology, so let's start there. What's the theory of inflation?
Clifford Johnson: Inflation's one of those wonderful ideas that helped solve some very important problems in modern cosmology. There were these huge problems concerned with how very widely separated parts of the universe seemed to be related to each other in ways that didn't make sense. They have certain properties, things like temperature, which are in equilibrium with each other. Inflation explained this. It said that parts of the universe that seemed now to be connected in ways that were not possible really began connected and then expanded very, very rapidly apart.
What caused the universe to expand so rapidly? What did Alan Guth [the theoretical physicist, now at MIT, who originated of the inflation model] propose?
At the core of the inflation idea, there is a form of energy that's essentially driving the universe apart. So spacetime itself expands at this incredible rate. It's a very, very rapid expansion driven by a field we call "the inflaton." We know how to describe it mathematically, but we don't know its origin. So the universe doubled its size many, many times in fractions of a second.
Another universe might be essentially right next to ours.
Was the expansion somehow faster than the speed of light?
No. What actually happens is that space itself expands, so nothing within the space moves faster than the speed of light, but the actual spacetime itself expands in this incredibly fast way.
How can we think about the energy driving the universe apart? Are we talking about the energy of empty space?
It can be thought of as an intrinsic energy that space itself has that makes it want to expand, pull itself apart in all directions. Ultimately, we don't know why space has that property. What we do know is that it had that property for a certain amount of time, and that gave rise to certain phenomena that we now understand.
A lot of current research is looking at what the origin of inflation was. Was it some other mechanism that kicked the universe into this state of rapid expansion? Or are there intrinsic properties in space and time that we've yet to understand?
How did the theory of inflation lead some scientists to start taking seriously the idea that there might be multiple universes, a multiverse?
When you start looking at inflation deeply and you see how it works in the mathematics and the physics, you realize that it never really stops. It actually continues, creating more and more things that you can consider to be potential universes.
The conditions for inflation just allow more bubbles of expansion. Think of a glass of beer. You don't just have one bubble. If the conditions in the glass of beer can create one bubble, you'll have others.
So the mathematics of inflation leads to the multiverse idea?
It's one way of looking at the mathematics. Now, it's an issue of interpretation, but it really seems to be driving you to that conclusion. This happens a lot in physics: You write down an equation, and you look at the solutions of the equation, and some of the solutions turn out to be real things that we hadn't thought of before.
[Editorial note from Clifford Johnson: While ideas based on inflation are one route to the idea of possible multiple universes, other approaches also can suggest multiple universes. In string theory, for example, which may suggest that there are more dimensions of space than just the three we observe, several distinct universes can coexist by being separated along one or more of the extra dimensions. Such approaches are also under active research.]
PERCEIVING THE MULTIVERSE
How can we see these other universes or prove that they're there?
It's a wonderful subject of discussion. We don't yet have any theory that's worked out completely that tells us how the properties of other universes, if they exist, would somehow show up in our universe in ways that we could unambiguously say, "Ah! That's evidence of other universes."
It may be that even if those other universes exist, they might as well be figments of our imagination, because we'll never detect them. But there are signs in various approaches to physics, such as string theory, that suggest that maybe there are ways of understanding the presence of other universes. We'll have to see where the research takes us over the next several years.
What do you think these other universes might be like? Would they have the same laws that our universe has?
The kinds of universes that may exist could be very, very broad in their properties. So there could be some that are very much like ours. What we consider the constants of nature—how strong gravity is, how strong electromagnetism is, all these other forces that we're familiar with—maybe those are slightly different.
Maybe there are other forces that are much stronger in those other universes, or much weaker. Even if you change just slightly the properties of some of the forces—and you can do this on paper; as a theoretician you can change the value of a given parameter and see what happens—you get very different outcomes.
They could have very different forms of life, or no forms of life, or maybe all sorts of things that we haven't thought of.
How many universes are we talking about here? Hundreds? Billions?
That's another issue of debate. It may be simply infinite. It may be some countable but still very large number. Then there's the issue of how many of them live for very long. How many of them expanded and became large, relatively old universes like ours? No one's worked that out yet.
Could we ever visit one?
It's lovely to discuss in the context of fiction, but it's also something that scientists can explore. In working out the physics of how those different universes work, we'll know whether it makes sense to cross over from one to another.
There are really interesting scenarios that suggest that the stuff we're made of—the matter we're made of and the forces that glue it together—are sort of glued to our universe. They don't allow us to leave our four-dimensional universe—three space and one time—and travel to another one.
But there are other forces, such as gravity, that seem to pervade all of the universes. Gravity is in some ways more universal than all the other forces. So you can imagine scenarios, in your wild dreams, where somehow we communicate with other universes through gravitational effects. But again, this is very speculative, and we're nowhere near where we need to be to make sense of this.
One of the wonderful things about science is that it's about evidence; it's not about belief.
Where would these other universes be in relation to ours? Is there a way to envision it?
Well, we live in three spatial dimensions: We move back and forth, up and down, left to right. And then there's time, so that's our four-dimensional universe. Another universe might be essentially right next to ours by going in another direction that's not one of those four. We might call it "another kind of sideways."
You could think about a slice of bread. Say there's a bug moving around on it. If there's another slice of bread right next to it, with another bug moving around on that slice of bread, the bugs might not know about each other. They'd be in separate worlds.
How similar could two universes be? Could there be essentially duplicate universes?
Could there be another universe about the same age as ours, 13.7 billion years, with essentially identical copies of all the things we're familiar with? Perhaps I'm giving a version of this interview in another universe with very, very similar conditions?
My own feeling is that it's nice to speculate about, but at least as we understand things now, it's highly unlikely. A universe that had even slightly different parameters 13.7 billion years ago would probably evolve very differently over that time. Even if those universes were, as near as you can determine, identical at their inception, there are so many chaotic things that can happen through the evolution of all of the stars and the galaxies and the things that give rise to the conditions that ultimately give us planet Earth and then people.
So unless there's some very unexpected mechanism that really guides how these universes' physics stay on similar tracks, my guess is that duplicates of that kind would be extremely unlikely.
How would you feel if there were a duplicate universe to ours?
I'm not sure if that would make me feel less unique as a person, or maybe grateful because it means all those things that I've never found time to do are maybe being done by some copy of me somewhere else!
FUTURE OF THE MULTIVERSE
There are a lot of critics of the multiverse concept. Why do you think I should believe it?
My own feeling is that if you choose not to believe it, that's perfectly fine, because we scientists have not given you any evidence yet. One of the wonderful things about science is that it's about evidence; it's not about belief. Hopefully one day we will be able to shape our theories to a point where we can make a prediction based on this multiple universe idea, and that prediction will be verified or it won't.
Ultimately, I think right now it's very important to be aware of the kinds of things that we're thinking about, if you're interested in science and the progress of science. But until we've done an experiment, until we've come up with more than just the ideas—however compelling they are—I think it's okay not to believe.
Where do you think the idea of the multiverse will be a hundred years from now?
Oh, gosh. These ideas can take a long time to get right—not just because it's difficult technologically to build the right kinds of experiments to test them, but also because the ideas themselves are very difficult to work out. The mathematics is difficult, the possibilities are so vast that it takes a while to sort them out. So it may be that we figure this all out in a decade, or it may be a hundred years, or maybe a thousand years.
Think of the idea of the atom. The idea is actually over a thousand years old. So, just like the idea of the atom, it may be that the multiverse idea may take a very long time to work out. It may take a thousand years.
I hope it takes sooner, because I'd like to see what direction this goes in. It's still possible that it's completely wrong, and we come up with a better idea, and it may turn out to be right and we find a way of testing it. One of the interesting things that's also possible is that we simply never know. We abandon the idea because something better and more urgent came along to work on.
There are all sorts of possibilities. Another wonderful thing about science is that we don't know precisely where the research is going. But the sheer promise and excitement of the various directions is what makes us explore them. So we'll explore this one until we either figure it out or something better comes along that gets us going in other directions. Either way, everybody wins because we learn things about our universe.