# Beyond Our Universe

## Interview with

Ben - Parallel Universes sound like an idea straight out of science fiction - but they are genuinely based in fact. The best mathematical models we have to explain the universe often predict some form of multiverse may exist as well. Brian Greene, professor of physics and mathematics at Columbia University, examines 9 different multiverse proposals in his new book, The Hidden Reality. I caught up with him at the Cambridge Science Festival...

Brian - We're asking one of the grandest of all questions which is, "Is our universe the only universe?" And that at first sight is a strange question because we're used to thinking of "universe" to mean everything, the totality, but a lot of research from a variety of different directions over the last few decades have suggested that what we long thought to be everything may not be everything. It may be a piece of a larger whole and that larger whole may contain other universes and that's where we come to the idea of multiple universe.

Ben - There are actually quite a few different theories born largely from the maths. How do we try and tie them altogether? Are they at the moment just competing ideas or are they all actually pieces of one large jigsaw?

Brian - Well, in my book, I actually describe 9 different variations on the theme of multiverse and there are relationships between them, but we have not yet develop any kind of meta-multiverse framework in which all of these proposals would sit. They're not necessarily mutually exclusive. We don't know if any of these are right. We will only know that when there is some experimental or observational evidence, but we're taking the idea seriously because our mathematical investigations lead us to this idea. We don't impose this idea from the outside. We follow the math, see where it takes us, and time and again, it's suggesting that there may be other universes.

Ben - Following the maths is obviously a very good way to get at what may be objectively the truth, but trying to understand these in human terms is very difficult. It's very counter intuitive. How do you get around trying to explain to people what the results of these numbers might actually mean?

Brian - Well, a number of the multiverse proposals are not that hard to grasp. For instance, we all know of the Big Bang, right? We have been trying to understand the Big Bang with greater precision in recent decades than we did in the past. In essence we've been trying to fill in a missing piece of the Big Bang theory, which is what started the bang. What was the bang? What started space to undergo this outward swelling? We now have a proposal on the table. It's called inflationary cosmology. The name is not all that important, but I bring it up because when we study the math of this proposal, it suggests that the Big Bang may not have been a unique one-time event. There may be many Big Bangs, each giving rise to its own swelling realm of space. It's as if our universe is a growing bubble in a grand cosmic bubble bath with other universes. That is a strange idea, but it's not that hard to wrap your mind around this possibility and this is one of the proposals that I discuss in the book.

Ben - As well as looking at things on the grand scale of the entire universe, we have to consider things at the subatomic scale, smaller than we currently know about. How does that fit into the idea of looking at parallel or multiple universes?

Brian - Well, you wouldn't think that it would. When studying tiny things- molecules, atoms, subatomic particles, that would not suggest that you were enroute to a theory of Parallel Universes. But, surprisingly have found that it does lead to that possibility from a number of different perspectives. Let me just give you one. Quantum mechanics, the study of the smallest ingredients in the world, broke the older Newtonian model of the world by saying that you can't predict with absolute certainty the result of any experiment. You can only predict the probability of getting one outcome or another. The electrons say, there's a 50% chance being here, 50% chance of being over there. Now, that's weird enough - a world governed by probabilities, but a puzzle that still persists to this day is, when you do a measurement of the electron, you find it at one location or another, so what happened to the other possible outcome? One suggestion is that it happened too. You saw the electron over here in one universe, but the maths suggests that there was a copy of you who sees the electron over there in another universe, two universes coming from the probabilistic framework of the quantum world - multiple universes.

Ben - So would the implication of there being multiple universes be that there are actually lots and lots of me, pointing lots and lots of microphones at lots and lots of you, all throughout this multiverse and with very, very slight differences between them?

Brian - It's quite possible. In some of the multiverse scenarios, exactly that would happen. For instance, in perhaps the simplest of all multiverses, it comes from imagining that space goes on infinitely far. We don't know that it does. Maybe it curves back on itself like the surface of the Earth, but if it does go infinitely far, there is a breath-taking conclusion along the lines of what you just mentioned which is this: In any finite region of space, matter can only arrange itself in finitely many different ways. It's like if you take a deck of cards, this is my favourite analogy to describe this, if you shuffle the deck, the cards come out in different orders. But there are only finitely many different orders for the cards, so if you shuffle the deck enough times the order of the cards, must repeat. No way around it. Similarly, as space goes on infinitely far then the order of the particles region, by region, by region must repeat too. There just aren't enough different arrangements to go around. Now you and I, we're just an arrangement of particles. If the arrangement repeats out there then we're having this conversation out there. And like you say, it's even easier for the particle arrangement to almost, but not exactly repeat. That would mean that perhaps I'm interviewing you in one of those universes. So it's a startling idea, but it comes from simple assumption. Space goes on infinitely far and also, the hidden assumption as well that the laws of physics that we know about here are the laws of physics everywhere. So we can actually say something sensible about what happens out there. But under those mild assumptions, you come to this startling conclusion.

Ben - How can other areas of science actually pay a part? We've already mentioned cosmology, we've mentioned astronomers, we've mentioned particle physics. How are these different groups all feeding in to find the same answer?

Brian - Well I think the different groups play different but overlapping and complimentary roles. When we talk about multiple Big Bangs, this comes from inflationary cosmology, which makes some predictions that observational astronomers can look to the sky and try to test. Inflationary cosmology says some very definite things about the microwave background radiation. This is heat left over from our Big Bang. It speaks to tiny temperature differences in the sky that inflationary cosmology implies should be there. The observational astronomers turn telescopes skyward and they have found those tiny temperature differences in the sky, confirming one prediction of this approach. Then when those ideas also suggest something else that may seem more far out, like multiple universes, we're compelled to take that idea seriously.

Ben - What do you think will be the next stage? What do we need to do to get a bit further with this work?

Brian - Well I think there are two major directions: One, we need to understand the mathematical underpinnings of all of these ideas with yet, greater precision. That is vital in order that we can make more precise statements of what experimenters and observers of astronomy should find. Then, on the experimental and observational front, we need to keep pushing onward. I mean, the Large Hadron Collider is a device that may give us a lot of insight in the coming years. Some of the parallel universe proposals do come out of string theory and we need to see whether string theory is right or wrong. There's at least a chance that the Hadron Collider could give us some insight looking for supersymmetric particles, a class of particles that string theory says should be out there, but we've not seen. The idea of extra dimensions which comes from string theory - the Collider actually has a chance of finding them. How? Slam two protons together, the equations show that some of the debris created in that high energy collision can be ejected out of our dimensions into the others. How would you notice that? The debris would take away some energy which means our detector would measure less energy after the collision than before. So there's a real possibility for some interplay between experiment, observation, and theory and they all need to go forward hand in hand.

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