If in 1M years we have a different view about the universe it won't be because we were wrong about physics today. It will be about something underlying or outside physics?
Maybe they'll turn out to be boring and useless. Maybe they have even more complex interactions than ordinary matter does. About all we can say today is that there's a mystery 20x more massive than all known matter.
Or do you only mean classical, newton physics?
Because as far as I know, (quantum) physic is far from explained. Shure we do know a LOT more than we did 100 years ago, but I doubt we have fewer questions. I even suspect it can never be fully understood. Some basic principles, yes. But if the universe is infinitely big and small and it seems it is, there is no full understanding nor resolving all questions. I thought that mindset died out allready. Because before quantum mechanics etc. physicist actually did thought they allmost solved all questions. And then: oh. There might be more ...
If that's the case, then we can be as wrong as they were.
It doesn't seem likely that everything we've observed, discovered and experimentally verified about the universe since the beginning of recorded history is simply wrong, or that our level of ignorance about the nature of reality remains constant regardless of the data amassed.
An easy, however unlikely, example of what could change it again is to find out we are in a simulation.
It does not remain constant. But it remains indeterminable. Ie we can say we know something about the universe. But we cannot say we know x % of the universe simply because we have no way of knowing what 100% is...
It's just as likely that there are constraints on what is possible that we are not aware of. Take Newtonian physics for example. It turns out physical systems are significantly more constrained by relativity than Newton would have supposed. Relativity makes less possible, not more.
We might assume that super advanced civilizations can do things based on our understanding of physics, but we might be wrong. A Newtonian physicist might assume an advanced civilization could accelerate rockets to arbitrarily fast speeds and get to the nearest star in less than 4 years, but would have been wrong.
The big impact of relativity is that less time passes on the ship.
That's not strictly true. Before relativity, time dilation was not know to be possible.
Right, but since we have no scientific basis for figuring out which capabilities those would be, the reasonable guess is that EM is the best means of communication. It's more likely that we've figured out some basic physical limitations, even if there exists technologies within those limitations far beyond what we've though up so far.
Agreed that it's a flight of fancy to speculate on these yet unknown capabilities, but that doesn't mean they aren't highly likely to exist.