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Physicist Brian Greene was propelled into the spotlight thanks to The Elegant Universe, his bestseller on string theory. Now he鈥檚 turned his attention to parallel worlds. He tells Amanda Gefter about his favourite multiverse, why we might be living in a computer simulation and the questions that keep him awake at night

Why did you decide to write about parallel universes in your new book, ?

I think it鈥檚 important for the general public not just to learn about science that鈥檚 all settled, confirmed and in textbooks, but also to capture a picture of vital science in the making. That鈥檚 the stage we鈥檙e at now when it comes to the idea that our universe may be one of many. If it鈥檚 right, it鈥檚 hard to imagine a greater upheaval to our understanding.

In the book you talk about many different types of multiverse鈥�

Yes, I chose to focus on nine versions of the multiverse. But as far-out as the notion of parallel worlds sounds, it鈥檚 not as though we physicists are saying 鈥榳hat crazy idea can we think of next?鈥�. Rather, what we find with many fundamental theories, if we pursue them to their logical, mathematical conclusion, is that they all seem to bump into one or another version of parallel universes. It鈥檚 almost harder to avoid the idea than it is to dream it up.

Could these nine different types of multiverse ultimately be the same?

It鈥檚 conceivable that all nine types of multiverse are separate ideas, and that one or a few, or all of them are real or maybe none of them are. But some interesting potential relationships between them have emerged. For example, in the 1950s, the physicist Hugh Everett came up with the 鈥渕any worlds鈥� interpretation of quantum mechanics, which says if quantum mechanics predicts that X, Y and Z are possible, with different probabilities, then X, Y and Z will actually happen. All possible outcomes occur, but in different universes. That bears potential similarity to another kind of multiverse 鈥� the kind that arises if space is infinite.

In any finite region of space, matter can only arrange itself in finitely many different configurations, much as a deck of cards can be arranged in only finitely many different orders. If you shuffle the deck infinitely, the card orderings must necessarily repeat. Likewise, if the universe stretches on forever, the arrangement of particles will repeat too. That would mean that you and I are having this conversation out there, perhaps over and over, an infinite number of times. In both of these multiverses, every possible outcome is happening somewhere in some universe.

You mention in the book that you鈥檝e never seen physicists quite so heated as they are over the multiverse. Why all the drama?

Physicists love to argue, but usually they鈥檙e arguing about some detail of science like how to interpret a piece of data. When it comes to the multiverse, the nature of the argument is different. It becomes a meta level of argument: are multiverse proposals scientific? This debate has caused some antagonism, but my feeling is, when people fight it out passionately, that鈥檚 always a good thing for science.

The tension hinges on this: can you measure or access other universes, and if not, how can they count as science? I had that reaction too, when I first encountered these ideas. But as I delved into them more deeply it became clear that just because you鈥檙e invoking other universes does not mean you stand outside of falsifiability or testability. I think we have to be very careful about writing off this kind of pursuit too quickly, because it could well be the right direction.

What kinds of experiments could show evidence of parallel worlds?

We have no concrete evidence today of parallel universes. But there are multiverse scenarios in which we might have access to other universes. There鈥檚 the inflationary multiverse, for instance. This is the idea that our big bang was not a single event 鈥� instead, there are many, perhaps infinite, big bangs, each giving rise to its own universe. You can think of all these different universes as bubbles in a bubble bath. Sometimes, the bubbles collide and if our universe has collided with another universe, we might see evidence of that in patterns in the microwave background radiation left over from the big bang. That would be a very direct way of establishing that there are other universes out there.

Would the existence of parallel worlds have any impact on our daily lives?

My view is that if you want to really understand yourself and life more generally, you have to know what reality is, where it came from, what properties it has that you may not be aware of. I desperately want to know what the true nature of reality is, because I feel that it helps me get a firmer grasp on why I鈥檓 here and how we relate to this grand and strange cosmos that we find ourselves within.

So when I鈥檓 in the grocery store buying orange juice, does it affect me? Not at every moment. But when I鈥檓 walking along and thinking about the nature of life and the work that I do, does it affect me? It does, because these ideas affect my sense of what鈥檚 real.

One kind of parallel universe that really challenges one鈥檚 sense of reality is the idea that we are living in computer simulation. Do you ever think to yourself, wow, this might all be a simulation? And if it is, does it matter?

I catch myself thinking about simulations when I鈥檓 looking at my kids. When I see them doing things that are so unexpected, it鈥檚 hard for me to imagine that some computer simulation came up with that behaviour. I know that doesn鈥檛 hold much water, because a good enough computer simulation might do so.

Would it matter? In a way it wouldn鈥檛. If I have been simulated for all the years I鈥檝e been on this Earth, it鈥檚 been a fun ride. If The Simulator is listening to me right now: keep it going! You know, it鈥檚 been good. And the fact that we can use our minds, even if they are simulated minds, to apparently gain insight into the world, even if it鈥檚 simulated insight, is exciting.

You can imagine people reading religious connotations into The Simulator

The thing to bear in mind is that The Simulator of these simulated worlds would still be subject to some rock-bottom laws of physics. They would not require supernatural understanding, the way we generally envision that some divine being would.

If you had to bet money on whether or not there鈥檚 a multiverse and which kind, what would your bet be?

I鈥檇 say three things. I鈥檓 particularly fond of the brane multiverse, which is the idea from string theory that we may be living on a membrane that is floating in a higher-dimensional space. I think there鈥檚 a chance that we may test it very shortly. The Large Hadron Collider has the capacity, at least in principle. If, in the course of a particle collision, some debris exits our brane, we could recognise that by seeing a loss of energy in our universe.

I鈥檓 also excited by the string landscape multiverse. When you put together the idea of eternal inflation 鈥� infinite big bangs 鈥� with the extra dimensions of string theory, you get the string multiverse, which says that all the different universes produced by inflation are characterised by different shapes for the extra dimensions. I have a bias toward that, because I鈥檝e been working on the extra dimensions of string theory since I was 22 years old. Finally, in terms of mind-bending, the idea of infinite space giving rise to endless duplications of us is so shocking because it鈥檚 so simple. We just have to wait to see if any of them are right.

If you could go into any other profession, what would it be?

Probably music. To me music has a capacity to directly tap into something that feels transcendent or eternal. And that鈥檚 why I do science, too. I like the notion that what we鈥檙e revealing may be eternal truths, fundamental threads in the tapestry of reality. But I think there are other ways of tapping into that kind of eternality and music is one of them.

Is there any question that keeps you up at night?

I wish it was just one. There are two that, if I allow myself to think about them, make my heart sink. Why is there something rather than nothing? It鈥檚 a simple question that鈥檚 been asked for so long and the idea of nothing seems to me logically sensible. But when I truly imagine nothingness, well, I find it almost scary. Why 颈蝉苍鈥檛 there nothing?

The other question is the nature of time. Time is with us, every moment. I can鈥檛 even say a sentence without invoking a temporal word 鈥� moment. But what is time? When we look at the mathematics of our current understanding of physics, time is there, but there鈥檚 no deep explanation of what it is or where it came from.