That is, the rules of science fiction.
In a totally shameless theft, I stole the majority of this entry from Fearsome Pirate over at metalutheran.blogspot.com but please don’t hold that against me, as I’ve added a few extra links and text to try and make my offering a little more educational and relevant to myself. (8+)
I am frequently insulted by the stupidity of Hollywood but I totally understand that what it does it does in the name of ‘entertainment’, as such I can (more or less) forgive such inaccuries as those listed below.
These things are tolerable
1. Sound in space.
Everyone knows there’s no sound in space, but silent explosions are also pretty boring. Try watching a dogfight in Star Wars with the sound turned off. In general, you can put everything here that violates physics for the sake of visual and audio effects, such as visible laser beams, colorful nebulae, and so on.
Space.com has a really interesting article about sound in space and how it depends on the size and sensitivity of your ear.
2. Faster-than-light travel.
You really can’t do much in the way of interplanetary adventuring if no one has hyperspace or a warp drive. I think everyone who likes science fiction is okay with this one, because we know that even if physics says we can’t, we sure wish we could.
We already have devices that can make things travel faster than the speed of light.
I think it highly unlikely that engineers will be building space craft with FTL capabilities in the foreseeable future. Heck, we took long enough to agree on the space shuttle successor, and that’s not even set to fly for a few years yet.
Lightsabers are impossible, but they’re awesome. In general, you can do “impossible” things as long as they’re awesome. So you want a handheld particle accelerator gun that instantly vaporizes flesh and leaves smoldering skeletons behind? Sure, that’s fine.
There’s an excellent little discussion on the physics of Lightsabers here.
For what it’s worth it’s one of the coolest gadgets in sci-fi, in my opinion.
4. Humanoid aliens.
We all understand your actors are going to be human.
I like the entry on tvtropes.org regarding this particular subject.
These things are not okay
1. Aliens that basically can’t be killed.
2. Humans living on a planet with no vegetation.
3. THERE ARE NO MORE USEFUL ELEMENTS. NONE.
4. Just because it’s the future doesn’t mean no one will be using iron, steel, copper, wood, etc any more.
I have deliberately not included the explanation for the second section. If you’re that interested you can click on through to the original article, it deserves a read.