I’m a fan of the Mathematical Universe idea. Or rather, I was. I think I came up with the idea independently of (and before) Max Tegmark, based on one of my old LiveJournal blog-post dated “2007-01-12” (from context, I think that’s YYYY-MM-DD, not YYYY-DD-MM).
Here’s what I wrote then, including typos and poor rhetorical choices:
Ouch, my mind hurts. I've been thinking about The Nature of Reality again. This time, what I have is the idea that from the point of view of current science, the universe can be described as a giant equation: each particle obeys the laws of physics, which are just mathematical formula. Add to this that an mathematical system can exist before anyone defines it (9*10 was still equal to 90 before anybody could count that high), and you get reality existing because its underlying definitions do not contradict each-other.
This would mean that there are a lot of very simple, for lack of a better word, "universes" along the lines of the one containing only Bob and Sarah, where Sarah is three times the age of Bob now, and will be twice his age in 5 years' time. But it would also mean that there are an infinite number of universes which are, from the point of view of an external observer looking at the behaviour of those within them, completely indistinguishable from this one; this would be caused by, amongst other things, the gravitational constant being represented by an irrational number, and the difference between the different universes' gravitational constants varies by all possible fractions (in the everyday sense) of one divided by Graham's number.
Our universe contains representations of many more simple ones (I've described a simple one just now, and you get hundreds of others "universes" of this type in the mathematics books you had at school); you cannot, as an outside observer, interfere with such universes, because all you end up with is another universe. The original still exists, and the example Sarah is still 15. In this sense of existence, the Stargate universe is real because it follows fundamental rules which do not contradict themselves. These rules are of course not the rules the characters within it talk about, but the rules of the Canadian TV industry. There may be another universe where the rules the characters talk about do apply, but I'm not enough of a Stargate nerd to know if they are consistent in that way.
The point of this last little diversion, is that there could be (and almost certainly is) a universe much more complex than this one, which contains us as a component. The question, which I am grossly unqualified to contemplate but tried anyway (hence my mind hurting), is what is the most complex equation possible? (Apart from "God" in certain senses of that word). All I feel certain of at the moment, is that it would "simultaneously" (if you can use that word for something outside of time but containing it) contain every possible afterlife for every possible subset of people.
Tomorrow I will be in Cambridge.
Since writing that, I found out about Boltzmann brains. Boltzmann brains are a problem, because if they exist at all then it is (probably) overwhelmingly likely that you are one, and if you are one then it’s overwhelmingly likely that the you’re wrong about everything leading up to the belief that they exist, so any belief in them has to be irrational even if it’s also correct.
Boltzmann brains appear spontaneously in systems which are in thermal equilibrium for long enough (“long enough” being 101050 years from quantum fluctuations), but if you have all possible universes then you have a universe, an infinite number of universes, where Boltzmann brains are the most common form of brain — Therefore, all the problems that apply to Boltzmann brains must also apply to the Mathematical Universe.