For those who do not know hard sci-fi is where the author sticks to known science, mostly things that we do not know how to do now but are probably possible, or things that we can’t say are impossible but don’t really know much about (such as practical nanobots).
Hard sci-fi is limited by our current understanding of physics. If modern physics says something can’t happen then you can’t use it in your book. At least in theory, part of being an artist or a good writer is learning how to break the rules after all.
The practical problem with this is that its a paradox. Our understanding of physics is ever changing and growing. If you are writing a book set in 2,600 AD you have to account for a greater understanding of physics. This means that you have to acknowledge that scientists are almost certainly wrong about a great many things today. A great many things that we think are impossible today might very well be thought of as possible or even easy in the future.
One of the basic things that your average physicist will say is impossible is faster then light travel. However I doubt they are right.
Humans are very good at thinking around problems. Even today people are working on the Alcubierre drive, an FTL system that may skirt around relativity. I am not saying I know they are going to get it to work of course, they have massive problems to solve. What I am saying is that we will find a way, even if it takes centuries.
All of which is why almost all hard sci-fi is set in the middle future, a couple centuries at most. Far enough to have all the tech you want to go out into space but not enough to have that tech violate physics as we understand it.
So how do you do it? How do you have hard sci-fi set five hundred years in the future? How do you have FTL in a close to hard sci-fi setting?
If there is one thing I have learned it is that just because something is impossible to get perfect, does not mean you can’t get close, and close is better then nothing.
The first thing to know is that people will be more likely to buy it if its hard. One of the first rules of tricking people is that people trust what they had to work hard to get. If someone had to bite and claw to get a piece of information then it will be considered more trustworthy then something they are given for free.
So you make FTL hard, which means slow. In my setting it takes years for the first ship to get to a system. Then months for normal ships to make the trip.
Another way is to limit access to it. Meaning it can only be used under certain conditions, so it can’t be used to ignore problems. Just because you have FTL does not mean you don’t want to have plots that involve getting away from things.
Their are several ways to do this, one way would be the way Babylon 5 does it. If your not a huge military grade ship you have to use a jump station to get into hyperspace. Outside of those you have to go the long way round. You could be more generous then that and require your ships to be in certain places to go FTL, such as outside gravity wells.
I believe that you need to try and maintain the spirit of the hard sci-fi as much as possible. Taking months or years to travel between stars feels more real then taking days, even if its just as unrealistic scientifically. I believe it is the same thing for other fields. Curing all cancer with one injection and no side effects is unrealistic (almost boring), curing cancer with a series of painful injections and procedures that might kill you? This feels right even if its just as unrealistic.