Last Friday’s IAP “blockbuster” seminar speaker was the renowned cosmologist George Ellis. Ellis talked to us about the “multiverse”, a topic which has gained an enormous amount of coverage in the popular press. Countless books have been written about it: during his talk he gave us a healthy selection of quotes from many of these texts.
The “multiverse” idea is essentially that the the Universe we observe is really only a universe, with a small u, and that outside the bubble of our past “light cone”, those photons which can reach us within the lifetime of the Universe, there are in fact an infinite number of other Universes. And, this is the important part, these Universes many not resemble anything like our own Universe at all: they could be Universes completely different. Radically different, with a different set of physical parameters, leading to them being utterly barren of planets, stars and structures. God does not only play dice, but he does it with the whole Universe, infinite numbers of times. There could be a Universe exactly like the one we are living in with the only difference being that my espresso this morning was slightly stale. The reason this idea is attractive is that helps us to explain why there are complex structures like human beings and stars — we just happen to live in one of these multiverses where everything was tuned just the right way. Otherwise, of course, we wouldn’t be around to observe any of it — the anthropic principle.
Or, does it really explain anything at all? The point is that we can never, ever observe any of this. Since the multiverse are too far away from us, photons from these Universes can never reach our Universe. They are in a region of space which is not casually connected to our own (or maybe — Ellis talked about the mind-bending idea that Multiverses could actually collide — if that happened in our Universe it would lead to the formation of a ‘ring’ on the cosmic microwave background. No rings have been spotted so far ;- ).
And that is really his criticism — multiverse theories aren’t really science. One of the most important aspects of any scientific theory is that it should falsifiable, which is to say that there must be some observation which you can make to exclude the theory. Since there is no casual link between these other universes and our own there is no way we could ever make an observation to rule out the theory. It’s not testable: these universes don’t ever interact with our own. But, it is certainly presented as a scientific theory by its practitioners. Fair enough — to me, as someone who spends a lot of time on galaxy surveys and observations, I appreciated very much his blast of Popperian realism, which is something sometimes you don’t get too much of in the more speculative branches of cosmology (I think all most of my observer colleagues would agree with Ellis). And, warned Ellis, once you accept the theories of the multiverse as a real scientific theory, the gates are soon opened to all other kinds of questionable theories…
Is this really any different from what has happened before? In the past, there have been lots of crazy theories which either turned out to be consistent with observations and not falsifiable (quantum mechanics is a pretty good example of that — the theory is so counter-inutitive, and people have gone to enormous lengths to try to “catch the universe out”, like Serge Haroche with his single atom traps — but no-one has succeeded so far). Or, they have been ruled out by observations, like the “luminiferous aether”, instantly killed by Michelson and Morley and their interferometer experiment. These “bad” theories just fall away, because they don’t help us understand the Universe any more. However, the difference with the ideas around the “multiverse” is that they have a strong philosophical attraction to us — they claim to provide (at least a partial) answer the question, “why are we here?”. But, for the moment, they’re not science.
Reflecting on this I see one possible course of action for the multiverse practitioners — start a church!