Two weeks before death, Hawking submitted a paper on parallel universes

It's not unreasonable because everyone understands the state things are in. Once upon a time atom was meant to be descriptive, and time has proven that original description insufficient. We could call atoms something else to address this, but it seems like we've decided "atom" stays, its meaning has just evolved over time in specific usages.

This reminds me of how the prefixes we use in computing hardware aren't right either. Your PC might have 8 gigabytes of RAM, but it doesn't literally have 8,000 megabytes or 8 billion bytes, and the same problem with mega- and kilo- too. Somehow we manage and when we're talking gigawatts giga means 10^9, and when we're talking gigabytes in computing we know it has a special meaning, 2^30.

Same prefix, two different usages depending on context. The only thing I can promise you though is we're never going to adopt gibi, mebi and kibi, at least not in my lifetime anyway.

Understanding the history and evolution can be interesting or informative as to why things are the way they are. So yeah, one upon a time "sub-atomic" would make no sense, it makes sense not of course. We're familiar and used to the idea. Wasn't always the case though.

In physics it's absolutely true. You can only disprove theories. You can never prove them.

In math it's relatively true, because you can only prove theorems given a set of axioms and other assumptions you take as true.

> If there are truly separate "universes", then by definition

Sometimes words "truly" and "by definition" don't play nicely together, if the established definition happens to be built on mistaken assumptions.

The "atoms" were named like that because they were thought to be elementary and indivisible. Later we learned better, but the name stuck; it would be confusing to try to change it now.

Similarly, the "universe" was believed to be all there is. Maybe it is not. But we already have a thing which has this name, and it would be confusing to rename it, just because we found later that some other things exist too.

The current definition, intuitively, is something like: If you can hypothetically fly there using a rocket (ignoring the light speed limit), it is a part of the universe. If there is no way to get there (even hypothetically ignoring the light speed limit), then it is not a part of the universe... and it is probably wrong to even say that it exists, because how could you know that?

But this is based on an assumption that you can either travel somewhere (ignoring the light speed limit) or there is absolutely no way you could interact with it. And maybe that assumption is true. But maybe it is not... and in such case the reasoning above would fall apart. If there are, uhm, places you could interact with in certain ways, but not in other ways, would they "by definition" be in our universe or not? Well, the definition does not speak about such cases either way, because they were not assumed to exist.

How could you possibly "interact with a place in certain ways, but not in other ways"? I can imagine a few ways, and the nature may still surprise us by doing something unexpected.

Imagine that there are different "regions" such that certain kinds of particles can travel from one to another, but other kinds of particles can't. So for example, we could send there light, and with a bit of luck have some of that light reflected back, but we couldn't travel there. And if in certain direction does not happen to be anything that would reflect the light back from that specific direction, there is nothing we could do about it.

Or imagine that different "regions" interact for certain amount of time, and then stop interacting. So while the parallel universe is connected, we can scientifically prove it, but after it disconnect, we cannot prove the existence of this specific parallel universe anymore. Or maybe the disconnection does not depend on time, but on the number of particles exchanged, so by the mere act of experimentally proving the existence of the parallel universe we are also pushing it away, irrevocably.

I haven't read Hawking's paper, so I don't know if his idea is similar to any of this. It could be something completely different.

Yes, and that’s why such theories aren’t popular with theoretical physicists. If you can’t falsify a theory, it isn’t scientific.

The other extreme end, where there is significant interaction between parallel universes isn’t popular, either, because we would have noticed such universes if that were the case.

So, instead, theoretical physicists that think parallel universes may make sense look for mathematical models that either only have tiny interactions between those ‘separate’ universes or at least only have tiny interactions in the neighborhood (in time and space) where we live (the latter might even be a win; a theory that explains dark energy, dark matter or the Big Bang as detectable effects of paralllel universes would be a winner)

In the end, however, it all boils down to educated guessing, with good theoretical physicists being better (or luckier; the difference can be hard to spot if you need only one or two bright ideas/lucky guesses to build a career) at guessing and at building mathematical structures with some desired set of properties.