I'm not sure if this is a question for philosophers or for physicists, but I'll ask it here anyway. Do you think it is possible that there are other universes? I mean "other universe" in a very physical sense: any group of objects that have no past, present or future physical relations to the objects in our universe. For example, they don't originate in the Big Bang. And it is physically impossible that a photon leaves one of such objects and hits one of the objects in our universe. And those objects aren't at any distance from the objects in our universe (it cannot be said truthfully that those objects are or were n light-years away from any star in our universe). But I mean real, actual objects, and not merely "possible objects" (there is a previous answer on this subject in AskPhilosophers, but that's not what interests me)! Do you think that there can exist other universes in this sense?

Let's use a phrase from the philosopher David Lewis: concrete worlds. Let it mean complete, concrete universes. Lewis thought that there are concrete worlds other than our own, and that there is at least one for each way our world could be. Lewis also characterizes these words in the way you do: they aren't in our space-time, so they're not at any spatial or temporal distance from our concrete world, and they don't interact causally with our world. As Lewis understand things, they wouldn't be other worlds if these conditions didn't hold.

Lewis thought there were such things. He thought that making sense of ordinary truths about what's possible calls for them. My admittedly unpopular view is that this is the wrong way to think about possibility. Even if such worlds do exist, there may be (would be, I'd say) non-trivial modal truths about them. Any particular such world might not have existed, for example. Far as I can see,here's no home in Lewis's account of modality for these sorts of possibilities. In fact, I'm tempted to resurrect an old term and say that the Lewis-style way of thinking about possibility amounts to a category mistake.

But that's just me, and at least some of my colleagues would hoot in derision.

Be that as it may, it's one thing to say that these worlds would be the wrong sorts of things for the job Lewis assigns them (and I realize that's not your issue); it's another to say they couldn't exist. I can't think of any good reason to believe that such worlds are flat-out impossible. And though I'm open to persuasion, my instinct is to suspect that any "proof" to the contrary would be sophistical.

This might sound like a simple "yes" to your question, but it's not so simple as that. What's possible and what I seem to be able to describe or imagine may not be the same thing. It may be that, for reasons I'm not grasping, utterly other concrete worlds are not possible. All I can say is that if I were making philosophical bets, I'd go with saying they are. I'm just not sure what my odds would be...

Read another response by Allen Stairs
Read another response about Physics