Quantum choices and Universe splitting

Question

Multiverse theory says that when a choice is made between two quantum states, the Universe splits. What happens if there are 3 or more choices that need to be made? Will that state make the Universe split into 3 new Universes?

Answer 1

The Everett Interpretation of quantum mechanics (also known as the Many Worlds Interpretation) does not actually say that any new universes are created. It says that according to the ordinary rules of quantum mechanics, if two initially isolated systems interact - an observer and a system being observed - then the quantum state of the observer system becomes correlated with the observed system, moving to a sum of orthogonal states each of which looks like one classical observer observing one classical measurement outcome. (Although Everett didn't say so, this is just the phenomenon of 'normal modes of vibration' of coupled oscillators which is well-known in classical physics.) 'Orthogonal' means that they don't interact with one another. Each component state behaves as if none of the others existed, as if each alternative was in its own separate world.

It's like dropping two pebbles into a pond at either end - the ripples from one pebble pass through the ripples from the other undisturbed, as if they couldn't 'see' one another. But there is only one pond, like there is only one universe.

If the observation experiment has three or more possible outcomes, then the result will be a sum of three or more orthogonal states, each of them acting (as far as the quantum observer is concerned) as if it was in its own separate world. It will look to the observer as if the universe splits into three new universes, of which he/she only gets to see one.

The idea of splitting universes comes from John Wheeler's early attempts to explain the idea to a lay audience in non-technical terms, and is really unfortunate because it has caused decades of confusion and misunderstanding. There are no alternate universes. There is no physical 'split' process. Everett simply pointed out that quantum mechanics already implied that to a quantum observer it would look as if only one outcome happened, even though they all do, and we simply cannot see the other alternatives.