OT 4 More Things Atheists Should Stop Saying

To clarify this issue, there are many different types of possible parallel universes. All of them without exception are consequences of other theories/models being true. Certain cosmological models predict multiple universes if those models are true. It's true that we can't directly test/observe those other universes, but most cosmological models are, at least in theory, testable in other ways, and if they turn out to be right then we should take multiple universes seriously. 

One specific case of other "universes" being actually observable is the Many-Worlds interpretation of quantum mechanics. When we perform the double-slit experiment we literally see these "worlds" interfering with each other in the experiment. The weird thing is that one of these worlds (or states) go away when we start to measure these states beforehand. The last century of quantum mechanics has mostly been about how to make sense of this. Many interpretations say the other world/state collapses or goes away, but this creates all kinds of physical and logical contradictions. Other interpretations say there are hidden variables we're missing that would make the process deterministic; but Bell's Theory has shown no hidden variables could achieve that while maintaining locality, which is a major thing in physics as it would make QM incompatible with General Relativity. Many-worlds states both worlds/state still exist, but they're now entangled with the multiple states of the observer and have decohered into the environment, never to interact again. If that interpretation is right then we're literally seeing other worlds every time we perform such experiments in quantum mechanics, but we simple can't see those other worlds any more after we interact with them.