Advent calendar door #15: Playing games in many worlds
Would you stake your fortune on a 100 to 1 outsider? Probably not. But what if, somewhere in a parallel universe, the straggling nag does come in first? Would the pleasure you feel in that universe outweigh the pain you feel in the one in which you've lost?
Questions not dissimilar to this one occupy physicists and for entirely respectable reasons. Quantum mechanics suggests that reality is fuzzy, at least at very small scales. Particles can be in a state of superposition, simultaneously possessing properties we would normally deem mutually exclusive. For example, they can be in several places at once (see the previous article for more on this).
The big question is why we never see superposition in everyday life. Traditional interpretations of quantum mechanics say that when we make a measurement (such as looking where a particle is) the superposition somehow collapses and only one of the superposed states remains real. We don't know which one that will be but quantum mechanics provides us with probabilities. The method which extracts those probabilities from the maths of quantum mechanics is known as the Born rule.
The many-worlds interpretation takes a different approach. It suggests that when the measurement is made the world splits into separate branches. In each of the branches you see one of the possible outcomes.
But then, what of the Born rule? If every possible outcome happens, it makes no sense to talk about the chance of it happening — that chance is 1. If the many-worlds interpretation is to be taken seriously, then the Born rule needs a new explanation. What do the numbers it attaches to different outcomes, to different branches of the world, mean?
Find out more in this article.