Quantum Randomness and Many Worlds

Most recent answer: 05/09/2016

When I'm reading or hearing an explanation of ideas such as the multiverse it's often said that somewhere there is another me that decided to watch a film instead of reading/listening to that explanation, or something of that sort. My understanding is that we live in a mostly cause and effect universe, so how is it possible that in a world exactly like this one things ran in exactly the same way for all this time and then suddenly the same causes have a different effect? It doesn't seem possible to me that things were slightly different in an unnoticeable way that only manifested millions of years into the Earth's history. Does this have something to do with quantum mechanics, where I think I'm right in saying truly random events occur?
- Sean (age 25)

You've put your finger on the key point. In a fully deterministic universe you can't just change one detail without unraveling the whole pattern. On the other hand, quantum mechanically a range of different outcomes are possible even for a completely described past. The violations of the Bell Inequalities show convincingly that our universe does have that sort of pure chance element to it. That allows one interpretation of quantum mechanics to be that all of the possible outcomes do occur, losing communication with each other. That's called the Many Worlds interpretation.

You used the word "multiverse", which has a somewhat different meaning. Extending back to the Big Bang, we don't know what sort of manifold our spacetime is part of. There are some reasons to suspect that it has many branches, maybe an infinite number. If so, there would be branches very similar to this one, including you-ish beings. That's different from the you-ish beings of Many Worlds, who were in fact you up to the point at which some quantum event split things into different versions.

 One of the most important questions in philosophy concerns the meaning of "causation". Probably the clearest definition is based on "counterfactuals". We say "A1 caused B" if  B wouldn't have happened under some default condition A0, but replacing A0 with A1, not changing anything else, gives a world with B happening. Classically, that runs into the problem you describe. In a deterministic world its impossible to replace A0 with A1 without changing anything else.  Quantum mechanics allows that sort of change for at least some events, so that allows the counterfactual definition of causation to have meaning. (Oddly, an epidemiologist friend and I are writing up something about this for a dispute about causation in public health issues.)

Mike W.

(published on 05/09/2016)