Do Parallel Universes exist?

Dominic - That's an interesting one because there's certainly a lot of scientific talk about parallel universes, but they're impossible to observe because by definition, everything we see around us is in our universe. The reason why theorists sometimes talk about them is because you can make neater, simpler theoretical models of the universe we have, if those models also predict the existence of other universes that maybe have different laws of physics. For example, our universe has complexity in it as a result of galaxies and stars collapsing at a balance between gravity and gas pressure, and that means those two forces have to be in a very fine balance. If they weren't in fine balance, we wouldn't have complexity in our universe. We wouldn't be here. So why is that balance there? Well, perhaps it's because there are other universes that have much simpler and less interesting laws of physics.

Chris - So there are lots and lots of different universes where the rules or the parameters may be slightly different and we're here, just because we happened to be in the one for which that physics works for us.

Dominic - That's a very controversial idea, but that would certainly be what Martin Rees for example would say.

Chris - So Catherine, Dominic has explained that we may just be one of many. I guess you'd also quite like to know whether or not we can detect them if they are there.

Catherine - Yes, definitely, yes.

Dominic - That's really quite difficult because all of the light that we see is traveling through our universe. There have been some bits of theory that have predicted that perhaps there could be interactions between parallel universes objects like white holes and worm holes that the problem is those are really speculation at the moment. So I don't think there's any imminent prospect of us getting any evidence for them.

Chris - Michio Kaku has got quite a good book called Parallel Universes and in there, he talks about the fact that because we believe gravity should be able to propagate between these different universes if they exist, that detecting the gravity waves coming from one into the other may be actually the way to infer their existence. There are various experiments that scientists are doing to look for gravity waves, aren't there?

Dominic - That's right. There are several experiments looking for gravity waves and I think it is likely we will detect them in the next 5 to 10 years. And that will be very exciting because we have many competing theories of gravity all extending Einstein's general theory of relativity and it will be gravitational waves that will really distinguish between those and tell us what we're looking for.