Multiple Worlds

Multiple worlds exist in theory and in fiction but do they also exist in reality?

First, for philosophical precision, we need some more general term than "exists." In the literal sense, fictions do not exist. However, and for good reasons, the meanings of words are extended when they are used in diverse contexts. Thus, we say that Sherlock Holmes exists in several books and films although not in reality. Children need time to learn this distinction. Holmes stories are neither truths nor lies but something else.

When analyzing Poul Anderson's Time Patrol series, we can discuss the relationships between timelines without considering whether or in what sense other timelines exist. Thus, timelines alpha and beta are related as cause and effect even if inhabitants of beta deny the existence of alpha. The Temporal language is supposed to have appropriate tenses.

I read somewhere that there are two theories of many worlds:

that the universe splits every time a particle makes a quantum jump;
that macroscopic events diverge after every human choice and/or historical turning point.

One thing is certain. If multiple worlds do exist, then they will be very different from any fictional representations of them. Compare From The Earth To The Moon or The First Men In The Moon with Apollo 9. I think that travel between alternative histories is regarded as theoretically impossible? And, even if it were possible, it would certainly not be as easy as stepping through a silver screen in SM Stirling's Conquistador.

Are alternative histories entirely a matter of probabilities? How probable was it that the Germans would win World War II? I do not know the answer to the second question but let us consider that question while asking another: in a world where the Germans did win World War II, how probable is that you or I would appear from nowhere complete with our memories of having grown up in a world where the Germans lost that war?

I would think that our appearance in that timeline, although logically possible, has the lowest possible probability level.