Question: What do you think about the many worlds theory? Is there any way to prove it is true/untrue?

Arttu Rajantie answered on 15 Jun 2011:

The many worlds theory is an attempt to understand the probabilistic nature of quantum mechanics. In quantum mechanics you usually cannot predict the outcome of a measurement exactly. Instead, you can only calculate probabilities of different outcomes. The many world picture says that all of these outcomes correspond to different universes or “worlds”, which are all real but somehow one only observes one of them. In contrast, the original “Copenhagen interpretation” says that when you carry out a measurement ” the wave function collapses” and that essentially picks one of the outcomes according to the probabilities.

Both of these pictures give us an intuitive picture of what goes on when we carry out a measurement, but they do not actually predict anything more than quantum mechanics itself does, so they are not really theories in physical sense. Therefore they are usually called interpretations rather than theories. As long as the laws of quantum mechanics are correct, there is no way to prove that either of the interpretations is true or untrue.

Nevertheless, they many not be completely irrelevant because we don’t really understand how to apply quantum mechanics to the whole universe, so it is possible that these interpretations would lead to different predictions in some questions in cosmology.