The True Science of Parallel Universes
Parallel universes are not a theory, but deductions from the « Many Worlds » interpretation of quantum mechanics. And there is at least on other theory that predicts not alternate universes but parallel universes where the universes are just like ours, only slightly different, where different versions of us could exist. One is the idea of Infinite universes. The reason space appears to be only 13.7 billion light years in radius is because That limit is set by the distance that light has had the ability to travel since the instant of the Big Bang. The universe is likely larger, and perhaps infinitely larger. If space is infinite, then it must start repeating at some point, because there are a finite number of ways that any set of particles can be arranged.
In the 1950’s Hugh Everett proposed the Many Worlds interpretation of quantum mechanics. It is so logical in hindsight but with a bias towards the classical world, experiments and measurements to guide their thinking, it’s understandable why the founders of quantum theory didn’t come up with it. Rather than proposing different dynamics for measurement, Everett suggests that measurement is something that happens naturally in the course of quantum particles interacting with each other. The conclusion is inescapable. There is nothing special about measurement, it is just the observer becoming entangled with a wave function in a superposition. Since one observer can experience only their own branch, it appears as if the other possibilities have disappeared but in reality there is no reason why they could not still exist and just fail to interact with the other branches. This is caused by environmental decoherence.
Is it possible we live in a multiverse? What is the Multiverse, and does it even exist? Some scientists believe that we live in a parallel universe that is closely related to our known observable universe. But this isn’t just a wild idea that someone thought up. There is some science behind this idea. And while some physicists disregard the idea that a multiverse exists, there are others who believe that it is possible that we live in a parallel universe.