Going Interstellar, edited by Jack McDevitt, is a mix of SF stories and non-fiction articles related to interstellar flight, all written with the constraint that only “known physics” could be assumed. This means no warp drives or space-time worm holes, but fusion and anti-matter based propulsion and advanced AI are OK, even though a huge amount of “it’s just engineering” remains before such systems could possibly be built. And for anti-matter propulsion, even if you can build the reactors and starships, the energy costs and logistics of generating, collecting, and containing large amounts of anti-matter are non-trivial to say the least!
This limitation means that the starships in the fictional stories are limited to something like 0.2c (20% of the speed of light). While this is fast, a journey to Gliese 581 (20.1 light years) is going to take 100 or so years, so we are talking “generation ships,” or possibly some sort of hibernation technique to allow humans to survive the long journey (unless the travelers are technologically enhanced or even non-biological, advanced AI’s or hybrids of some sort). “Known physics” still allows a lot of room for imagination. The best story in the book is Michael Bishop’s Twenty Lights to “The Land of Snow,” in which a group of 990 Tibetan Buddhists sets off with the Dalai Lama to found a new homeland on a planet in the Gliese 581 system.
I also recently re-read Ender’s Game, where starships are still sub-light-speed, but there exists an advanced communication system captured from the enemy aliens. The “ansible” somehow (quantum entanglement? It is never explained) allows instantaneous communication across interstellar distances. This supports the key plot point of a fleet of starships sent toward the enemy’s home world years before anyone knew who would command them. When Ender Wiggin finally arrives at the Earth orbiting Battle School to learn how to save the day, there needs to be a way for his commands to reach Earth’s faraway fleet with no light-speed delay.
All of this got me thinking about dark matter and the question of “known physics.” Of course SF authors have always felt free to assume that new and convenient features of the universe will eventually be discovered so that faster-than-light travel and other amazing feats can be accomplished. The convenience of writing away the problem you need to solve is one of the luxuries of fiction. Quite possibly boring old Einstein is right and c is the ultimate speed limit. But once upon a time, there was phlogiston theory, which explained combustion in terms of a special essence contained within materials that burn. Chemistry and oxygen eventually provided a better explanation. And what about the ether? In the nineteenth century, physicists quite logically assumed that light waves would require a medium, much as do sound and water waves. Michelson, Einstein and others showed that no such medium existed nor was needed.
“Dark matter” really sounds like phlogiston or the ether to me. A placeholder until some new Einstein comes up with a better explanation. Will that explanation allow matter or energy to travel faster than the speed of light? Not necessarily. Einstein was a pretty sharp dude and plenty of experiments have shown that going faster than c is pretty damn hard if not impossible. I’m just saying that it’s quite likely that the physics we have now is nowhere near the final word on how the universe really works. It’s a very good approximation, as was Newton’s mechanics for most of the problems anyone could think to ask until around 1900, and still good enough for most purposes. Einstein’s special and general relativity theories are minor corrections for most mechanics problems even today (even for most orbital mechanics, but not quantum mechanics). Any “bigger and better” physics that may arrive in coming years will still have to reduce down to what we have now for most circumstances. But with any kind of luck, someone will figure out how to build some special circumstances into the back ends of starships, and when the new physics kicks in, we’ll be able to get to Gliese 581 in time for dinner.