Emergent Multiverses

I have now finished reading:

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

In keeping with my self-proposed rule of reading things you disagree with, I managed to get through to the end of this lengthy and in places very technical book. You do need a background in quantum physics (which I have) as well as probability (sort of) and philosophy of science, especially physics (umm…). Anyway, we got there.

The question then is ‘was I convinced?’ Do I now see universes (even ‘local’ universes, whatever that means) popping into existence whenever a cat is placed in a box with a radioactive source?

No.

I think there are questions, some of which I outlined before. To take the answered one first: what about Ockham’s razor? This states that entities should not be multiplied unnecessarily. The answer from Wallace is that this is true but according to the multiverse interpretation of quantum mechanics each extra universe is necessary, therefore Ockham’s razor does not apply.

As an argument that is acceptable, of course, but it does rather beg the question as to the reality of the universes popping into existence on a very frequent basis, even if the frequency is uncountable. If the interpretation is correct, the razor is not applied, but the ‘if’ is a big one.

The unanswered question is about where the energy comes from to create a universe. A universe, even a local one, is a big thing with a lot of matter and energy. To duplicate it exactly except for one particular facet will take an equal quantity of energy, and so on for all the uncountable multitudes of universes which appear. Somewhere this uncountable quantity of energy has to be found. The conservation of energy is a well known law of science and even quantum mechanics managed to obey it, only bending the rule slightly and temporarily.

The question of energy, therefore, is unanswered and I think it needs an answer before the emergent multiverses idea could be accepted.

The rest of the book seems to be mainly a defence of the emergent multiverses idea along the lines that it does not do any worse than any other theory of the interpretation of quantum mechanics. For example, one of the bones of contention over the multiverse theory is something to do with probability. I suspect it is something like that in the multiverse theory there is no objective probability because all outcomes occur, just with greater of lesser numbers of branches available. Someone who experiences a branch will land up in one of the subsequent branches with a probability given by the number of branches which are possible with that outcome.

There may well be problems with the philosophy of probability. I am sure there are, but just to argue that the multiverse universe does no worse when subject to those problems than any other interpretation is hardly a strong argument in favour of the theory. It might remove an objection, but that does not make it correct.

The consequences of the choice of metaphysics that one takes to be behind quantum mechanics and its interpretation are rather few. Semi-conductors, for example, work weather or not you think every time a P-N junction triggers gives another parallel universe where it does not. One argument put forward for the usefulness of multiverses is it is helpful in calculations (although I have not seen one where it is used) but that is not evidence for reality. In solid state physics it is useful to calculate using electrons of negative mass. That does not mean that anyone believes they exist. Even if multiverses are helpful computationally or conceptually, that does not mean they exist either.

There is a question floating around as well as to whether or not emergent multiverses are science or metaphysics or mere speculation. The problem here is that scientific theories, in order to be counted as such, need to be testable. That is, you need to be able to conduct an experiment to verify (or not) the hypothesis. So far as I know there is no suggested verification experiment possible for emergent multiverses. There is a lot of speculation about finding wormholes and practicing time travel (really) but none of these seem to be a practical experimental set-up. Towards the end of the book Wallace does seem to rely a bit heavily on science fiction for ideas and explanations of what he is discussing. I am sure that there are good scientific bases for it (to a point) but quoting fiction, no matter how ‘serious’ it is, is hardly going to prove the point.

As I mentioned before, the most likely explanation for the issues we find around quantum mechanics are likely to be found, in my opinion, in the fact that we are limited, macro-scale, beings and quantum mechanics deals with the unimaginably small. At the scale of atoms and electrons, things are different from what we experience ourselves. Our expectations are not going to be met, no matter what they might be. We can verify an awful lot of the theory of quantum mechanics, granted, to perhaps surprising levels of accuracy, given the probabilistic nature of the theory. But that does not and cannot mean that we can take things like wavefunction collapse and the Schrödinger's cat thought experiment as being seriously real. They are parts of our discussions and conceptual equipment around quantum physics. Taking the collapse of a superposed wavefunction to mean that the entire universe splits into two seems to be taking realism and faith in our models of the very, very small far too much for granted.

As wise physicist (I forget whom) once commented that not only is quantum physics stranger than we imagine, it is stranger than we can imagine. Talk of multiplying universes at the drop of a quantum hat really does not add any clarity, I suspect. Quantum mechanics is a human level theory, which is extremely accurate in what it does (most of the time). To claim that it is so good that the universe has to multiply itself repeated to follow it seems to be imbuing it with a great metaphysical burden which I doubt it can carry.