# Columbia’s Greene Says You Have an Evil Doppelganger: Interview

The implications of current mathematics lead to the radical notion that our universe is not all there is: It’s just one among many.

“These ideas are speculative,” says Columbia University physicist Brian Greene. “But if they’re right, this would be the greatest upheaval ever to our understanding of reality.”

In his latest book, “The Hidden Reality: Parallel Universes and the Deep Laws of the Cosmos,” Greene describes the recent discoveries in physics and cosmology that point to the existence of a multiverse.

We spoke in his spartan faculty office, decorated only with mathematical equations.

Lundborg: How does the multiverse idea arise from the math?

Greene: Mathematics can reveal things we haven’t seen. Einstein’s math showed an expanding universe with black holes, later verified.

The math we’re studying today, which emerges from Einstein’s work, suggests our universe may not be the only universe.

Lundborg: There are universes which in principle we’ll never be able to access, but how do we find out about the other ones?

The Multiverse

Greene: There are two good examples of multiverse proposals where the other universes might interact with us in some way.

One is the idea that the big bang may not have been a unique event. The math we’re studying to understand the origin of our universe better suggests the event, the big bang, may have occurred many times at various and far-flung places throughout a much bigger cosmos. Each big bang gives rise to its own expanding realm.

Lundborg: Presumably we can collide?

Greene: Yes, if we had a fender bender with another universe there might be ripples created in the cosmic microwave background radiation, the heat left over from the big bang. Astronomers are looking at that heat right now to see if they can find such ripples.

Lundborg: What’s the second path?

Greene: The other is a proposal that comes from string theory, which can be tested at the Large Hadron Collider in Geneva.

Protons are slammed against protons at extraordinarily high velocity, and it’s possible according to the mathematics that some of the debris created in those violent collisions could be ejected off of our universe and migrate into other dimensions that surround our universe.

Lundborg: We would see them disappear?

Greene: Exactly. The debris would take away energy -- we’d have a missing energy signature that physicists are going to look for.

The Doppelganger

Lundborg: Everyone can have an evil doppelganger -- how does that work?

Greene: The argument for doppelgangers is pretty straightforward. Assuming that space goes on infinitely far, in any finite chunk, matter can only arrange itself in a finite number of ways, like cards in a deck.

You and I are just a configuration of particles, so sooner or later we’re going to repeat. Matter can almost repeat its configuration but not repeat it identically.

Your physical body may repeat, but your mental configuration can be a little bit different, so there might be an evil version of you, and a version that loves skydiving.

Lundborg: So there’s a universe where Christ never was born and another one where Hitler won the war?

Greene: Without a doubt, if these ideas are correct.

Lundborg: Is there a universe where the rules of math are different?

Greene: Could be. That may be more of a philosophical issue.

Lundborg: What’s your view of dark matter?

Greene: The most convincing candidates are supersymmetric particles. These are particles like the ones that make us up -- electrons, quarks and so forth -- but heavier versions.

I’d say within a decade we’ll certainly know what dark matter is.

Lundborg: In your own scientific life, what was the most exciting moment?

Greene: A result where we showed that unlike what Einstein would have thought, space can tear apart and repair itself. The technical term is topology changing transitions, but using the math of string theory we went beyond Einstein and came to this pretty startling conclusion.

Lundborg: What do you wish you could still discover?

Greene: I’d like to really understand where time comes from. It’s so familiar and dictates so much of how we live and what we do. Science still doesn’t have an answer to the question: “What is time?”

Lundborg: The saddest thing in your book is that more and more things will lie beyond our cosmic horizon.

Greene: The way we know space is expanding is we look out through powerful telescopes and we see galaxies rushing away. If the expansion is speeding up, those distant galaxies will go so far we won’t be able to see them.

Looking out at deep space, we’ll see stillness, darkness, blackness. That means our descendents a hundred billion years from now won’t even know space is expanding -- their observations may fool them.

Lundborg: Maybe our observations are fooling us?

Greene: It does raise questions about aspects of reality that we hold dear.

Lundborg: Doesn’t your head hurt when you think about these things?

Greene: Yes. In a good way.

(Zinta Lundborg is an editor for Muse, the arts and leisure section of Bloomberg News. The opinions expressed are her own. This interview was adapted from a longer conversation.)

To contact the reporter on this story: Zinta Lundborg at zlundborg@bloomberg.net.

To contact the editor responsible for this story: Manuela Hoelterhoff at mhoelterhoff@bloomberg.net.