## Me versus Multiverses

I don’t like the idea of a multiverse. I think it’s bad science. This might sound odd coming from someone who has just recently blogged about how all discrete universes simpler than are own are real. But I see a difference. In fact, the term ‘multiverse’ makes me groan each time I hear it.

Why? Because in order for the idea of a ‘multiverse’ of the sort that’s commonly envisaged to be correct, it requires that we buy into the existence of a very large amount of stuff, the existence of which we can never prove or disprove. (Just to make it clear exactly what kind of multiverse I don’t like, it’s the kind that invokes the ‘string-theory landscape‘ and asserts that a very large number of independent universes that share some kind of physical reality with our own.) This notion strikes me as unscientific.

One might argue that I have done exactly the same thing with my assertions about mathematical reality. However, the fact that I can *count* demonstrates that the integers exist, at least up to the value at which I’ve counted. Because the act of counting provides a complete implicit description of each integer, I have duplicated that pattern within my own universe. Hence, the pattern is ‘real’. The same cannot be said for vast tranches of hypothetical spacetime, each requiring eleven smooth dimensions for their description.

The most eloquent defender of the multiverse notion is, in my opinion, Max Tegmark, the same man who proposed the Mathematical Universe Hypothesis. I quote (via Wikipedia):

A skeptic worries about all the information necessary to specify all those unseen worlds. But an entire ensemble is often much simpler than one of its members. This principle can be stated more formally using the notion of algorithmic information content. The algorithmic information content in a number is, roughly speaking, the length of the shortest computer program that will produce that number as output. For example, consider the set of all integers. Which is simpler, the whole set or just one number? Naively, you might think that a single number is simpler, but the entire set can be generated by quite a trivial computer program, whereas a single number can be hugely long. Therefore, the whole set is actually simpler. Similarly, the set of all solutions to Einstein’s field equations is simpler than a specific solution. The former is described by a few equations, whereas the latter requires the specification of vast amounts of initial data on some hypersurface.

He’s dead right that a rule can be simpler than a result. However, as we’ve seen in previous posts, exactly the logic he invokes here to justify the multiverse rules out the existence of universes any more complex than the minimum needed to describe our own. That same logic also makes it clear that those universes are mathematically *disjoint* from each other. So while they may share a mathematical reality, sharing a physical reality they almost certainly do not.

So if it’s easy to reach the conclusion that a multiverse is an ugly idea, why is it so frequently invoked? Because, I would propose, it makes it easier to justify the usage of models that are otherwise hard to support.

This is the second reason I don’t like the notion of a multiverse. Not only does it require an unscientific abundance, but it smells of a kind of theoretical cosmology that is slowly bankrupting itself. Requiring that we believe in a very large number of things we can never witness is fine, so long as it’s the only viable explanation. (Sherlock Holmes springs to mind.) However, when it comes to theoretical cosmology, there are plenty of options out there that have been barely explored.

This is not to say that I have something better to replace the current favorite models, because I do not. I’m not a theoretical cosmologist. However, as an engaged citizen scientist, it’s my job to exercise skepticism about any explanation I’m presented with that I either don’t understand, or which appears to break in the face of simple logic.

If I become better informed and change my mind, that’s okay, because exercising doubt is the best way to know what questions to ask.