Those who have read about quantum mechanics have heard about the Many Worlds Interpretation (MWI) proposed by Hugh Everett in 1957. It is an idea seems unbelievable when one first hears of it because it implies the existence of many, a huge number in fact, of unobservable worlds that exist in parallel to our own but of which we are unaware. One needs to get over the initial feeling of incredulity before one can judge it properly on its merits.
Everett proposed his solution to a conceptual problem in quantum mechanics in that as quantum states evolve undisturbed, they can consist of a superposition of distinct observable states. According to the standard Copenhagen interpretation of quantum mechanics, when we try and observe what specific state it is in, i.e., we make a measurement, the very act of measurement disturbs that superposed quantum state and forces it into one of the observable states. The specific one it ends up being in is unpredictable except in terms of the probability of it occurring. This is the basis of the famous Schrodinger’s Cat, where the cat is both alive and dead until we look at it, and then we always observe that it is either alive or dead.
The poorly understood process by which the act of measurement ‘collapses’ the full quantum superposed state into one observable state in the Copenhagen interpretation has been used to challenge that interpretation. Everett proposed an alternative interpretation, that an act of measurement does not collapse the wave function at all but that multiple universes are created, each corresponding to one particular observable outcome of the measurement. We observe the one that we are in, but there are other unobservable universes in which ‘we’ also exist but where the outcome of the measurement was different. In the Schrodinger cat example, the act of measurement results in two universes being created, one in which the cat is alive and one in which the cat is dead. In one universe, ‘we’ (being devotees of the Copenhagen interpretation) will interpret the wave function as having collapsed to give a dead cat, while in the other universe ‘we’ will say that the wave function collapsed to give a live cat. Since the ‘we’ in one universe cannot observe the ‘we’ in the other, this interpretation explains the observed result just as well as the one where the superposed state collapses to one state.
As one can imagine, the idea of multiple universes ever-multiplying seems somewhat extravagant and for that reason alone some people reject the idea. But Sean Carroll argues that a lot of people reject it for the wrong reason, that they hink that the splitting of the universes is an extra postulate of the Everett’s when in fact there are no extra postulates at all but the splitting is predicted to occur purely as a consequence of the same postulates of quantum mechanics that all physicists agree on. So there is no a priori reason to prefer the Copenhagen interpretation over the MWI.
I tend to agree with Carroll but I think we need to be a little circumspect about the use of the word ‘predict’ in this context. That word seems to imply that the outcome is a necessary consequence of our information about the initial state. There is a difference between a prediction of a theory and a prediction by an individual about a possible outcome.
For example, the search for Higgs boson was based on a prediction of the Standard Model of particle physics. Not finding it would be a problem for the Standard Model. But that is not what we mean with the MWI. This situation is more like when we toss a coin and one person calls heads. That is a prediction about the future based on the situation now and the prediction can turn out right or wrong but it does not challenge the underlying theory that there was an equal chance of it being heads or tails.
In this case, we have a situation in which everyone agrees that before the measurement, we had a superposed state and after the measurement we will have a single observable state. The question is how we get from the before state to the after state. Both pathways (Copenhagen and MWI) are possible and there is no reason as yet to prefer one or the other. Either one is compatible with the basic ideas of quantum mechanics. So I prefer to stick with the word ‘interpretation’ rather than ‘prediction’.
Of course in science the way we judge between different models is to devise a test that can distinguish the two. But despite the hard work of many scientists, no way has yet been found to distinguish the Copenhagen and MWI pathways. That does not mean it will not be found. After all, recall that for a long time people thought that one could not distinguish between the local reality model of quantum mechanics preferred by Einstein and the non-local ‘action at a distance’ of the Copenhagen model, as sharply illustrated by the Einstein-Rosen-Podolsky paradox published in 1935. But John Bell showed in 1964 that such a test could be devised and the difficult experiments started producing results the mid-1980s, supporting the Copenhagen model. So it took about 50 years to get some insight into which might be correct.
Solving difficult questions takes time and we should be patient and not reject ideas simply because they seem preposterous on the surface. As Arthur Eddington said “Not only is the universe stranger than we imagine, it is stranger than we can imagine.” (There are different versions of this quote with J. B. S. Haldane saying “The universe is not only queerer than we suppose, it is queerer than we can suppose.”)