You can't really talk about space travel without addressing gravity.
Not only have I written a short story called "Gravity" (included in the anthology Aquasynthesis--but the story isn't what you might think), I've noticed that gravity is dealt with rather poorly in standard science fiction universes...for example, in Star Trek, no one ever floats in microgravity when the ship pulls into orbit around a planet. In reality that had at least as much to do with the fact that the series was filmed on planet Earth than any futuristic talk of "gravity plating" or the generation of artificial gravity on board Enterprise.
Actual weightlessness is dealt with a little bit in the extended Star Trek films (notably VI) and series (in several episodes of Enterprise), but isn't it interesting that every planet seems to have the same gravity? Just like almost all aliens speak English, there are rarely any planets in which a human would be much lighter or much heavier than on Earth...even though in our own Solar System you have a wide range of potential gravity situations—most places humans can actually walk have in fact less gravity than Earth (e.g. Mars and Mercury share about 40% Earth standard surface gravity). And in science fiction deep space, gravity is effortlessly generated by some machine that works so smoothly and efficiently that no one scarcely mentions its existence. Star Wars is even worse, by the way—no explanations of gravity at all in any of the main films.
Why should I care? Well, gravity generation would bring up some interesting circumstances if it existed. For example, if you use artificial gravity to compensate for acceleration, what happens if you suddenly jump up to light speed (as is done often enough in the “Star” shows) and your gravity machine is on the fritz? Wouldn’t that cause sudden massive acceleration that instantly crushes you and the rest of the crew into a fine red mist? Or what if it’s just a little off, so the compensation hits you unequally?…in that case, you might only get torn in two…
Just as some plots in Star Trek feature the engines going bad and you have to get some more dilithium or you wind up going backward in time or something rather silly like that—or the transporter is acting wacky and now you’re cranking out evil duplicates of everthing that beams up—there logically should have been some stories centered around problems with the gravity system. Bones swears it's heavier in sick bay than everywhere else, perhaps. And when the Klingons or other baddies target enemy ships, the artificial gravity (or the “inertial dampeners” as what may be the same system is occasionally called in Star Trek) should be high on the target list…sure the ship still would be able to accelerate after losing gravity, but not faster than the human crew can survive…which isn’t all that fast--a great move in preparation for capturing an enemy vessel. Plus, if a “tractor beam” is based on gravity (and why wouldn’t it be?) sweeping the beam rather randomly through an enemy ship would be a fine tactic…assuming the ship would stand up to the strain (it might not) the crew certainly would not--red mist on every deck. And would the shields block the gravity/tractor beam? I don’t think so…but maybe you could try to compensate by reversing the pull on everything with your own gravity beam pulling the other way...God help you if you failed to exactly counter the other beam...
Of course, plots that involve artificial gravity really cross over into the realm of tech-flavored fantasy, since there isn't a theoretical means to generate gravity from a machine. The stuff works—why? Just because it does, I can’t complain too much about fantasy technology because I use it myself at times. In my novel The Crystal Portal, an elf carries a sword that can cut through anything, just because it can. But I carry that impication forward and have people use it to slice through walls and doors and such, as opposed to just bad guys, which only makes sense given the properties the sword is supposed to have. I’d like to see the same sort of thing happen in stories that feature artificial gravity--that the implications of such technology would be explored at least sometimes in the story universe that employs them.
More sober science fiction shows "artificial" gravity being produced in a realistic way. More technical science fiction must deal with it, because human beings can’t live forever in weightlessness without experiencing seriously damaging side effects. Examples of greater realism on the generation of gravity include Rendezvous with Rama or 2001 by Arthur C. Clarke, or the spinning Ringworld created by Larry Niven. Each of these tales feature so-called centrifugal force to create a sensation of gravity (by the way, gravity goes unexplained in the crystal world of my own novel). In more realistic stories, acceleration is limited to what a human being can endure…just so you know, the amount of time it would take for a person to get up to 90% speed of light while accelerating enough to push you back into the seat with a force equivalent to standing on Earth (a.k.a. one gravity), would take around 300 days, depending on how fast you were going when you started…and wouldn't that change Han Solo’s getaway just a bit? “Light speed” he orders, but it’s not until next year he actually gets there…Obi Wan needs to trim his beard more than once during the build up...
There have been realistic proposals to use artificial gravity using actual mass with some form of very heavy matter such as neutronium. A ship could be built with a crew pod that moves closer to and further away from a very heavy capsule in the nose. When accelerating hard, the crew pod would move closer to the heavy nose, which would cause the force of being pushed back in the direction of acceleration to be balanced by being pulled forward by gravity. When not accelerating, the pod could be pulled away from the heavy nose to simulate an approximation of Earth gravity (a small difference in distance could make a big difference in gravity experienced because the pull of gravity increases at the square of reduced distance--so half the distance equals four times the force of gravity). My favorite version of this involves miniature black holes that are kept in place by loading them with an electric charge and employing a strong magnetic field.
A problem, of course, comes with using real matter to produce artificial gravity. Anything you put into your starship you have to have to expend energy to move, and it takes a lot of energy to get up to any speed near the speed of light--please see my previous post on Nanite Space Weapons.
Unless you had a Higgs field suppressor , as I made up from thin air in a previous post. In that case, you could cancel out all your mass for the sake of easier movement. Would that cause you also to lose gravity? According to a theoretical physicist's blog post on "What if the Higgs Field were zero?", it would not (he addresses gravity at the end of the post). According to him, mass and gravity are linked by coincidence...gravity really pulls on energy...
However, his post also makes it very clear the fundamental nature of matter would change if the Higgs Field were zero, so there would be no atoms, i.e. you would not survive the field being shut down...for story purposes then, let's be sure to imagine we can reduce the field greatly without going all the way down to nothing...so you could build a massive starship with black holes in an electrical field to produce gravity and to counteract inertia...while at the same time you'd be able to reduce the mass of the ship to something light enough (well, with low enough mass) to easily accelerate as fast as light speed would allow you to go.
Or even faster, if recent experiments with neutrinos prove to show that the speed of light is not the ultimate limit, the highest speed anything can travel (as I talked about in e does not equal mc2, neutrino story technology).
New takes on science fiction starships await!