Saturday, November 27, 2021

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?


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.

Saturday, November 13, 2021

Realism and Physical Laws

Are you a realist about the laws of nature? Most people, I would warrant, would not know how to answer the question, which is fair enough. It is only physicists and philosophers of science who tend to worry about such things. But it is, at least, a bit interesting.

The question is whether we think that a physical law is correct and really describes the world, or whether a law is a relation inhering in the world. The difference is in the words ‘inhere’ and ‘describe’. A view in which the laws describe the world can be traced back the David Hume, the Scottish Empiricist. A law which inheres in the world is a metaphysical entity which controls the world.

Most people, including most physicists, do not worry about the difference too much. But taking the laws of nature as inhering in the world can lead to some odd sorts of effects in our thinking. Hence, we can start to believe that every time a quantum event occurs in a way that affects the macroscopic universe (a cat in a box with a phial of poison which opens when a radioactive decay occurs) the universe splits into two, and we go down one branch or another (the cat is, when we look at it, alive, or dead, but not both).

If we take the view that laws of nature only describe the universe to us, there is less of a problem with unknowable branching universes. In a sense, it seems to me, the Humean version of physical laws are a bit more humble than metaphysical laws of nature. In the Humean case, what matters is the description of the universe as it appears to us. We might be grasping at each advance of science towards a better understanding of what is going on, but each step is not getting at a final truth, but simply a better understanding.

I think there might be an analogy with theology here; indeed, some authors about the problems surrounding quantum mechanics reach for God-like beings to give a total explanation of the theory. But that is not accessible to us as limited humans. Ian Ramsey once noted that the early Church Fathers were giving the best descriptions they could of the Godhead, but they were not dialling Chalcedon 451 for updates on the Trinity every week and writing it down. Human descriptions of the Trinity are exactly that: human descriptions of the indescribable in human terms.

We can see, perhaps, that we might be over-egging our quantum pudding by claiming that quantum mechanics gives a complete description of the quantum scale. Quantum Mechanics gives rise to some paradoxes, grated, but the paradoxes are human paradoxes, grounded on our ignorance of what is really going on at the quantum scale (the very, very small), in a similar way to Trinitarian theology, which is grounded on our ignorance of what is going on in the Godhead.

Therefore, when physicists or philosophers of science come up with ideas like the multiple emerging universes of branching quantum mechanics and argue (quite coherently, admittedly) that this is all fine and no counterargument can really stand against it, we have the right to be a little sceptical. Quantum mechanics might be the best theory we have which describes the micro-world, but that does not mean it is giving us the constraints that the micro-world acts under. Just because it is correct (i.e., gives accurate predictions) does not mean that quantum mechanics is a metaphysical entity controlling the world.

Again, caution is suggested by the analogy with Trinitarian theology. Just because something is a deduction from an idea about the Trinity, does not mean that the deduction is right. Theology works with a host of images and metaphors for God and the Godhead. Running one of them to death is liable to land the theologian in heresy. Similarly, running one idea of quantum mechanics to death is liable to land the philosopher of science in a but of a muddle, claiming that, for example, each time the cat is placed in a box and is later observed, the universe splits into two identical universes, one with a live cat and the other with a dead one.

The quantum world is a great deal weirder than we can imagine, I think. We do not know what it is like to ‘be’ and electron. How does an electron experience a magnetic field and emitting photons by moving through it? We have no idea. To claim that we do have is to argue that quantum mechanics is an absolute, cast-iron physical law. In fact, I would suggest, quantum mechanics is simply a human language construct (that language being mathematics, of course) which predictions which can be experimentally verified as being reasonably (and certainly usefully) accurate.

If this is correct, then arguments about multiple emerging universes start to look a little superfluous. The problems which such theories claim to solve – the relation between the Schrodinger equation and the Born rule, one being a solvable wave equation and the other being probabilistic, dissolves. It is simply the result of having to use two bits of a human construct to explain what we see.

Similarly, the problem of wavefunction collapse vanishes as well, I think. This is the issue that before we look in the box, the cat exists in a superposition of states – it is either alive or dead. When we make an observation, it is either one or the other, the wavefunction as suddenly collapsed into one state. The question is then asked about how the collapse happens, physically. The answer is that it does not happen, because the collapse of the wavefunction is something that emerges from the human level description of the universe. It does not necessarily happen in the quantum level because the quantum level is inaccessible to us.

Quantum mechanics is quite weird enough, it seems to me, not to require additional counter-intuitive ideas to further complicate matters. While emergent universes might not be ruled out specifically by the theories of quantum mechanics, they are an unnecessary and, perhaps, rather arrogant addition to the laws of nature. We simply do not know what is happening at the quantum level; that knowledge is unavailable to us.


Contemporary Theology

What, you might well ask, is contemporary theology and why does it matter? I have been reading MacGregor, K. R., Contemporary Theology: A ...