I find it amazing that the scientific method (or methods, if you prefer) is powerful enough to start classifying different types of multiverse. Max Tegmark has had a go at it, as described here by Tony Padilla in another Sixty Symbols video:

So, four types, in order of increasing generality, controversy and wildness, which I’d sumarise as

1. Stuff which is so far away that even travelling at the speed of light, nothing from there can have got here yet, and vice versa. These disconnected universes will nevertheless have the same physical laws and constants as ours. In fact, as the universe’s rate of expansion seems to be increasing, parts of the universe which we can currently see will eventually recede into another universe, in this sense.
2. Stuff which had a different “big bang”. That is, the current expansion of these universes started at a different point from ours in some kind of eternally-inflating manifold. They will in general have different physical laws.
3. The “many worlds” interpretation of quantum mechanics. That is, every time a set of quantum possibilities resolves into a definite event, then rather than a mysterious “wavefunction collapse” occurring, whole new universes are split off, in each of which one of the possibilties is borne out. Amongst other things, this means that every particle collision at the LHC creates multiple new universes. That sounds like a proper “big bang machine”, until you realise that the same is true for the little positronium beam upstairs from my office at UCL, and indeed for every radioactive decay, ever. And much more. Oddly, apart from the fact that such profilgate universe-creation seems to violate conservation of energy, all these universes would have the same physical laws as ours, if I understand correctly.
4. The mathematical multiverse. Why should a subset of mathematics correspond to the physical universe, while other bits don’t? Perhaps all mathematical structures actually correspond to a physical reality? (I love the bit in the video where Tony confidently dismisses the possibility that none of them do.) This is Tegmark’s particular speculation, and it is indeed very speculative, interesting and fun.

Some thoughts on all this from a humble experimental physicist…

I’m with Tony when he talks at the end of the video about the idea that you can have a democracy of quantum outcomes, or of mathematical structures, without actually having them all happen somewhere. Maybe they do all have a probability of occurring, but why would that mean they all have to do so?

It seems to me that there is a huge amount of symmetry in multiverse number 3 (many worlds, all with the same physical laws). I wonder what conservation law, or possible even Goldstone boson, is associated with that?

Also, despite the claims of Tony and Tegmark, I’m not sure level 4 is the end. It may be true that the set of mathematical structures is itself a mathematical structure, but where does that leave non-mathematical structures? Is there a universe, for example, where astrology and homeopathy work? Where they form the basis of an effective healthcare system? Where snowballs really do carry more information about climate science than decades of measurement and modelling? Or where murdering people gets you a divine pat on the head and a long-term ticket to paradise? (No link for that one, too many options, sadly.) These are surely not mathematical structures; logic has nothing much to do with any of it, so surely they aren’t contained in Tegmark’s multiverse? Maybe we need a level five to contain all the really stupid stuff?

But I would say that, I’m a Scorpio.

Jon Butterworth has written a book about being involved in the discovery of the Higgs boson, Smashing Physics, available here, and in North America as “Most Wanted Particle”. Some interesting events where you might be able to hear him talk about it etc are listed here. Also, Twitter.