The Multiple Universe

 Quantum mechanics is weird, counter-intuitive. The ‘true meaning’ of quantum mechanics is the subject of many late-night undergraduate discussions, preferably fuelled by a few beers, but, so far as I know, most physicists and practitioners of allied subjects such as mathematics and chemistry simply and pragmatically decide not to worry about the meaning, get on, and calculate stuff that is useful. What it means more widely than the result to apply to the case in hand is not really a problem.

Despite the pragmatic approach of most physicists, there are a few, theoretical physicists and philosophers of science, who continue to worry about what quantum mechanics means for our view of the universe. The problem which most concerns them is that quantum mechanical weirdness seems to potentially have macroscopic effects.

The principal example of these macroscopic effects is the widely known ‘Schrödinger's cat’ paradox. Suppose we have a cat in a box (no cats were harmed in the writing of this post). Also in the box is a radioactive source. These sources decay randomly, and, it being in the box, we do not know when it is going to decay, but when it does, the cat will die. From outside the box, we have no idea as to whether the cat is alive or dead. In quantum mechanics terms the wavefunction of the radioactive source and the cat is in a superposition of states:

|cat> = a|cat alive> + b|cat dead>

The odd-looking brackets are standard quantum mechanics notation denoting the quantum wave functions. The letters ‘a’ and ‘b’ are the probabilities of the cat being either alive or dead. In standard quantum mechanics, these are complex numbers; the modulus squared of the complex number giving the probability of that state.

The only way to establish the health of the cat is to open the box and look. At this point the wave function |cat> collapses, or at least the superposition collapses, and the observer sees either |cat alive> or |cat dead>. The superposition of states is not observed. The question arises of how, exactly, this works. What is going on to collapse the superposition of the wavefunction?

This leads us to at least one conceptual problem. We can view the collapse as a purely instrumental affair, and ignore the human questions of what is going on. The important thing is the answer and, to boot, that it is the right answer. While this works for quantum mechanical problems, it leaves the understanding of the universe behind. Quantum mechanics does not describe the universe as it is, but is a tool to describe the results of experiments.

Conceptually, instrumentalism means that all we are interested in is the observational output of the theory of quantum mechanics. How such results are achieved is unimportant. But quantum mechanics is very successful at predicting and explaining the unexpected, such as superconductivity. The idea that results such as this are simply the consequences of some sort of formalism unconnected with reality is uncomfortable, to say the least.

There are, of course, other options. Another approach suggests that there are hidden variables that represent the state of the system but are unknown until we choose to measure it. However, from a physics point of view, the hidden variables simply add complexity to the theory of quantum mechanics without adding any physics. Making, possibly, the theory intelligible is not sufficient pay-off to make these theories popular.

Another approach is the quantum many-worlds theory. Here, the idea is that that there are two parallel, but non-interacting worlds created at the point of a quantum fluctuation. The argument is something like this:

The cat is either alive or dead, but we do not know which. When I make an observation, I go from being ignorant of the fate of the cat to knowing it. I do not see an indefinite cat, but a cat in a given state. But then if you ask me ‘Is the cat alive or dead?’ you are in a state of ignorance until I tell you. And so on. The point here is that the wavefunction of the cat, the observer, and observers of the observer are not separable. Macroscopic forces act of the cat, me, you, and everything else. Before I look the whole universe is in two states, one with a live cat, one with a dead one.

Upon looking the wavefunction collapses into one or the other case. But the argument of the many-worlds approach is that collapse does not happen. The universe splits into two, one with a live cat and one with a dead cat. There is no collapse but simply we find ourselves in a world with one outcome or another. We might have found ourselves in a world with another outcome.

Given that quantum interactions occur many times a second in the universe, that universe is splitting into incredible numbers of universes all the time. This is a very strange idea (much exploited by science fiction writers) but it can be claimed, with some justification, that it is a natural result of the quantum mechanics theory.

I do have my doubts, however. Quantum mechanics is well known for breaking the conservation of energy over short time periods, but the energy required to create many universes, each a copy of the current one seems to be prohibitive. I have never seen an argument to suggest that this can be achieved within our understanding of physics as it is. Admittedly I have not yet looked very far, so it might exist.

My second objection is a bit more philosophical. The creation of many, many, universes which do not interact ever again seems to me to be multiplying objects to no avail. This seems to be a clear break of Occam’s razor. If we can never tell if the universe has just bifurcated, why bother having it so. This also suggests that the many-worlds interpretation is heading away from being science – we can never test the idea because, by definition, ‘our’ universe and its parallel never interact past creation.

That is not to say that such ideas are in principle untestable or pointless. I may simply be expressing my ignorance and I certainly need to read a bit more about them. But quantum mechanics and its implications are unsettling enough for most people. Multiplying universes at such speed seems to be a bit beyond the pale.