Some folks say that science fiction should be limited to what is possible according to current scientific theory. Others (and I count myself among them) are a bit more flexible.
Imagine it’s 1890 and you’re a physical scientist. Someone approaches you with the following:
“I have here two lumps of a material called Uranium 235. If you slam them together correctly, they will release energy with the explosive force of more than one hundred million sticks of dynamite.”
You’d laugh at him. The very idea is preposterous. First off, what’s this “235” business? Uranium is Uranium. It doesn’t come in types. You’re familiar with the atomic theory of matter, right? Atomic. From the Greek atomos. It means “indivisible. A Uranium atom is a Uranium Atom is a Uranium atom. And this ridiculous release of energy? Energy can neither be created or destroyed. You’ve can convert from one form to another but that’s about it. If there was so much energy, whether chemical or mechanical, in Uranium to do as you suggest, it would tend to go off at the slightest provocation–Like, say, sneezing anywhere in the same county. What you suggest is flat out theoretically impossible.
Now, instead, suppose someone approached you with the following instead:
“You know, if you applied a force to something, like say with a rocket, and continued applying it for long enough, there is no ultimate bar to how fast it could go. Enough force, for enough time, and one could travel between the stars in weeks, if not days. Of course that much acceleration would crush most things and the engineering challenges are probably prohibitive, but there’s no theoretical bar to it.”
You’d probably have to agree. After all speed is simply acceleration over time, and acceleration is simply force divided by mass. Enough force, applying enough acceleration, for enough time and any speed could be achieved without limit, at least theoretically. The engineering challenges might be prohibitive but there were no theoretical limits.
Now, instead of 1890 imagine it’s 1990. Now the possibility/impossibility of those two events have reversed. We’ve discovered the electron, neutron, and proton and learned that, far from being “indivisible” the atom is actually made up of components. We’ve discovered that there are differences among atoms–isotopes–of the same element. And we’ve discovered that matter and energy can be interchanged and very small amounts of matter can, in the right circumstances, be converted to very large amounts of energy. And we’ve demonstrated the very thing in the first example–slapping two pieces of Uranium 235 together to make whopping big explosions. (And using different materials we’ve made even bigger booms.) As for the other, we’ve found that force applied to an object will produce different accelerations depending on how fast one sees the object of moving and the faster it is moving–the closer it’s speed is to that of light–the less acceleration a given force will produce, with the result that it can never reach, let alone exceed, the speed of light.
Back in 1890 physical theory would declare certain things to be flat out impossible. Other things were theoretically possible but perhaps practically impossible (such as, say, focusing light so that it can burn through an inch of steel in the blink of an eye). Other things were readily achievable.
With the revolution in modern physics that came shortly thereafter, those categories got shuffled. Some things that were utterly impossible under the old theory were found to be possible and even achievable once you knew how. Other things that had been theoretically possible but difficult (which was why they had not yet been done) were found to be theoretically impossible.
The one constant was that things that had already been done clearly had to remain possible. Obviously, whatever has been done is possible. (What was done might not necessarily be what you think was done–ask any stage magician–but what was done remains possible.)
So what about 2090? Or 2190? or 9990? Will the things that the physical theory of that future day considers possible and impossible be the same as today?
I suggest that it is only hubris that would lead one to suggest that they will. Unless one believes that we have actually achieved the final answer to physical theory, that all our current answers to “how does the universe work” are right, then one must conclude that some things we think are possible will very likely turn out to be impossible. And some things we think are impossible (theoretically impossible) will turn out to be possible after all.
And nobody knows which things.
As a writer of science fiction set in the future (or in a present with alien cultures more advanced than our own), part of the job is to explore these possibilities. Now, most people don’t expect a science fiction writer to explore the detailed ramifications of “what if conservation of Baryon number can be violated?” or “what if it’s possible to alter the Pauli Exclusion Principle?” or even “What if Planck’s Constant isn’t actually a constant?” but limiting oneself to what we now “know” is probably the last likely future of all.