The Emergent Multiverse Part I

I have been, slowly and painfully, reading

Wallace, D. (2012). The Emergent Multiverse: Quantum theory According to the Everett Interpretation. Oxford: OUP.

Slowly, because it is a complex subject, and painfully because while I am a fully paid up quantum mechanic, or at least I was, my skills are a bit rusty.

Still, Wallace’s claims so far (I have read Part One of the book) are reasonably clear, even to a rusty quantum physicist. The main claim is that the multiverse interpretation of quantum mechanics is the only one that does not require additional bits to be added to the formalism of quantum mechanics. Hence, the argument goes, it is the best interpretation of quantum mechanics that we have.

Now, as an argument, this is valid, but it is a bit weak, in my view. Firstly, the interpretation produces nothing measurable, it seems (perhaps it will in due course). Secondly, just because we cannot think of a better interpretation of quantum mechanics than the emergent multiverse view does not mean it is right, it simply means that we cannot think of anything better. So much the worse for us.

A post or two ago I produced a couple of questions about the emergent multiverse view. So far, the book has addressed one of the. I wondered about the multiverse view and Occam’s razor. That is, we are strongly encouraged by Occam’s razor not to multiply entities unnecessarily. Wallace’s reply is that Occam’s razor is valid, but the key word is unnecessarily. That is, his view is that as the formalism of quantum mechanics requires it, creating additional universes does not break Occam’s razor. We multiply entities but not unnecessarily.

This may or may not be true, of course. Within the assumptions that Wallace makes it is reasonable, but I think that worries do remain and are not answered by this assertion. This relates to the over concern I raised, which is where the energy comes from to create another universe, exactly the same as this one, every time something happens to require that to happen in the quantum mechanical formalism. Creating anything, including universes, takes energy, and quantum mechanics, while it can, temporarily, play fast and loose with conservation laws, has to obey them in the final analysis. Quantum mechanics may be weird, but it does not break the conservation of energy.

Without getting too technical, Wallace seems to be arguing, so far as I have followed him, that each quantum transition does not cause a new universe to emerge. Hence, if I have an atom in its ground state and it collides with another atom, and it has a probability of gaining energy to promote its electron to one of two other states, two new universes are not created for the two possible outcomes. It is only when we have (to use the standard example) a cat in a box which may be in one of two states that there are two universes, one with a dead and one with an alive cat. So the number of universes that are created is not so big as it might be, but it is still uncountable.

I still think that there are further worries. Firstly, as an extension of the above, there is still the underlying question of what the quantum formalism means. The normal interpretation of the wave function is that it gives the probability of finding a particle in a given state. While Wallace raises concerns as to what probability means in this case (and, indeed, in wider cases) the wave function is not the particle. That is, the quantum formalism is not a full description of what is going on at the micro-scale. The quantum scale has little to do with our intuitions and understandings, which is based on our macro-scale experience. What we get out of the equations is not reality, or a description of it, but something we can work with.

It seems to me that there is a danger here of taking the relatively clean quantum mechanical description of the world available through the mathematics of Hilbert space and it associated entities, and thinking that this is the description of the world and all that is in it. That seems unlikely. Quantum mechanics is not, despite its success in the twentieth century, the be-all and end-all of physics, let alone science as a whole, or human experience. The status of scientific theories, especially in physics is disputed. Wallace seems to think that because we do not dispute our theories of dinosaurs and take their description of the world to be pretty well true, we should do the same with quantum mechanics, otherwise something is wrong with our conception of science.

However, we need not take the quantum mechanical description of sub-atomic particles to be true in the same sense that we take dinosaurs to be true. Both theories (no doubt; I am not an expert on dinosaurs) have problems in interpretation, but we are not constrained to take both theories as having the same sense of truth about them. Dinosaur theories are true because we can find bones and reconstruct what the creatures may have looked like. Quantum theories are true because we can make predictions about what will happen under certain circumstances which turn out to be valid within experimental error. These are not scientific theories on the same level as each other, it seems to me.

Theories in physics, therefore, are somewhat odder than those in other subjects in science. Mostly, this is because they rely on two aspects that Wallace seems to rather (so far at least) ignore. Firstly, of course, they are based on mathematics. This is not, of course, a problem (except for people who have to learn how to do the mathematics in the first place) but there are questions about what the mathematical entities thus created mean. Secondly, most physics, including quantum mechanics, is the outcome of a process of modeling some aspects of reality. Again, there arises, perhaps more pressingly, questions about what the models mean as representations of reality. We certainly do not have to take them literally. If we do, then perhaps we do land up with an emergent multiverse from quantum mechanics, but the move is not forced.