system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Where does the idea of multiple universes come from? Is there any scientific proof backing it? What exactly does the idea of "alternate/multiple universes" entail?
gpt
There are various places in physics where the idea of multiple universes arise, for a bit different reasons. I'll write a bit about two of the main ones. 


First, lets talk about the one coming from an interpretation of quantum mechanics, which roughly says that every time a quantum measurement is performed, the universe "splits" into multiple branches, where each branch has its own result of the measurement. This is called the many world interpretation, for obvious reasons. That somebody came up with this comes from how the math of quantum mechanics looks like: the state of a system is described by a wave function. The wave function "spreads out" over different classical values, and normally we think of this as giving probabilities of the various possible outcomes. When we then take a measurement, we see just one of the values, and the wave function "collapses" to the value we found. But the many world interpretation says that the wave function is actually real; so all the different outcomes actually all happen, they just happen in different "branches" that doesn't talk to each other. Then there is no collapse. So in a way this comes out of taking the math of QM very seriously. 






The other version of multiple universes comes from cosmology (the study of the very early universe). The common model believed today is that of inflation, which roughly means that the universe underwent a very rapid expansion closely after the big bang. This seems required to explain various observed features. Then some people observed that it is natural for this inflation to actually go on forever, something that is called eternal inflation. In this model, the rapid expansion only randomly stops in small "bubbles", that because of quantum fluctuations "freeze out" of the surrounding inflating universe. Each such bubble becomes its own universe, and the randomness of the freezing should give each new bubble some new random laws of physics. This also ties in with the anthropic principle, which is an attempt to explain why the universe looks the way it looks. The logic of this goes that of course the universe has to look this way and be such that it supports human life, since if it was any other way we wouldn't be here to ask the question. This combined with a multiverse is a possible answer for why the laws of physics look the way the do and so on. Of course many (probably most) physicists are not satisfied with this "answer". Some people also use the anthropic principle to argue for a multiverse without pointing to any particular physics model.