There was a young lady named Bright
Whose speed was much faster than light;
She set out one day,
In a relative way
And returned on the previous night.
— Arthur Henry Reginald Buller, 1923
FTL stands for Faster-Than-Light. It's most commonly used in Speculative Fiction to refer to a form of stardrive, an engine capable of exceeding the speed of light. Such a device is a pretty necessary element if you want to have Space Opera, or just characters who can travel from one stellar system to the next in less than a year.
According to Special Relativity, it is impossible to travel faster than light locally, as accelerating even a particle of mass to such a speed would require infinite energy.
That said, it's theoretically possible for regions of spacetime to become so distorted that objects entering them may seem to move faster than light in relation to objects (and light) that is moving outside such a region. This potential forms the basis of concepts such as warp, wormhole, and hyperspace. It's also possible that massless particles may exist which constantly move faster than light.
Another interesting element of FTL in regards to Relativity is the fact that any movement at FTL speeds is, by definition, a form of Time Travel. Several hypothetical Time Machine concepts have been proposed that exploit loopholes in Einstein's field equations of relativitistic spacetime. Most would be unstable, uncontrollable, unfeasible (requiring immense energies or exotic matter), inaccessible, and/or are only capable of folding time back to the moment they were first turned on, but still, for the sake of a fun campaign or story…
Subtypes and applications of FTL technology:
- Jumpgate - a fixed point that is like a wormhole (see below), or like a cannon that propels ships passing through it. Allows non-FTL ships that pass through it to enter hyperspace or travel faster than light.
- Krasnikov Tube - a one-way artificial curvature of spacetime, vaguely similar to a worm hole, not so much a stardrive as a permanent short-cut maker
- Alcubierre Drive - a spherical spacetime bubble that warps space to allow for FTL travel
- Jump Drive - folds space to relocate a ship instantly
- Shortcut Drive - creates a temporary wormhole
- Alderson Drive - utilizes naturally occurring wormholes near suns - a lot of time is spent traveling in-system
- Hyper Drive - uses Hyperspace or Subspace, an exotic layer of space, or possibly even another dimension, where the speed of light is not a barrier to movement
- Warp Drive - warps the laws of physics, to enable you to accelerate more readily and exceed the speed of light.
- Quantum Tunneling - a form of teleportation evidenced by subatomic particles
- Teleportation - like Quantum Tunneling, but on a larger scale and possibly explained by Magic or Psionics instead of Alternative Science
- Wormhole - a portal between two points in spacetime, may be natural or man-made. See also Rotating Black Hole
- Tachyon - hypothetical subatomic particle that moves faster than light
- Time Machine
"Damn! There goes my image again. I keep telling them not to go 186,000 miles-per-second when I'm shaving."
— Einstein for Beginners by Joseph Schwartz & Michael McGuinness
Tropes related to FTL:
- Acceptable Breaks From Reality
- Cool Starship
- FTL Radio
- FTL Travel
- Hyperspace Is A Scary Place
- Old School Dogfighting
- Our Wormholes Are Different
- Portal Network
- Rule of Cool
- Sci Fi Writers Have No Sense Of Scale
- Space Friction
- Space Is An Ocean
- Space Opera
- Stern Chase
- Subspace or Hyperspace
- Teleporters and Transporters
"There are some fundamental truths in the universe. We cannot travel faster than light, and we cannot make a copy protection system that is uncrackable."
Ways to justify FTL technology in your game:
- We don't need no stinking Einstein: You can just throw science out the window for the sake of plot and the rule of cool.
- Higher Science: Principles such as Casimir Vacuum, Quantum Tunneling, Hartman Effect, wormhole, EPR Paradox and Hyperspace suggest there may be specific circumstances under which some form of FTL is possible. You could pick a method, and expand upon it, having just one or two specific versions of FTL being possible in your setting.
- Phlebotinum or Unobtainium: Some sort of special material, element, or superscience in the setting somehow suspends or bends the laws of physics
- A Wizard Did It: Magic and Powers violate relativity, and since it's magic, we can hand wave it.
- Do away with Causality: Causality is an underlying assumption of Relativity, but you could imagine a setting where it is merely a construct of the human mind. In such a setting, the past is mutable, and things can happen before their cause. You still have the "infinite energy to accelerate" problem, but wormholes or the like would allow for specific types of FTL and Time Travel, along with all the dangers of Temporal Paradox and Collapse of Causality.
- Higher dimensionality may provide the key to FTL. It may be possible to step out of our reality entirely, take a shortcut in some dimension we don't have, and return to normal space. See Hyperspace or Subspace, Riemann's Bookworms and Fourth Dimensional Lifeform for ideas on how this might work.
Game and Story Use
- Unless you're really at the hard end of Mohs Scale Of Sci-Fi Hardness, you're probably gonna need some form of FTL in your sci-fi game.
- Research into FTL travel might seem like just a simple way to visit the universe. But since it's also Time Travel, it could lead to Temporal Paradox or even a horrific Collapse of Causality.
- If the game is set far enough in the future, Einstein's theories could have been replaced/refined by newer Doubly-Special Relativity, Quantum Gravity, or Variable Speed Of Light theories. This gives at least a plausibly scientific-sounding veneer to the idea that Albert was wrong.
- The GM could also decide Einstein was wrong and it's figured out by a Mad Scientist or Twenty Minutes Into The Future, but the passage of a greater amount of time before the campaign means it's more plausible that someone's worked out an actual law of physics that allows us to build technology based on the things Albert had wrong. Relativity being wrong doesn't do us any good unless there's been time for science to explore the new theories.
- Of course, since FTL = Time Travel, it's possible this gets figured out in the future, and then the info is brought back into the past.
- The consensus amongst physicists is that you just can't move faster than light, unless you are a massless particle that always moves faster than light, such as a Tachyon. So, an FTL communication device might take far less energy or effort than building an FTL engine. This provides for some variety between settings and technologies, as a society might have access to only certain subsets of FTL tech.
- The advice at Atomic Rocket  is really good. They suggest working backwards. First, figure out what kind of story you're telling, and what type of FTL travel it will require. Then concentrate on effects and limitations, the nuts and bolts of how works and all the implications thereof. Then, as the final step, come up with some light technobabble to explain it.
- What type of FTL you have in your setting will have lots of ripple effects and implications for the rest of the setting.
- If any ship can fold space or travel faster than light, you end up with actual stealth in space, and sneak attacks are the main form of combat. In fact, they're possibly the only form of space combat, because once the enemy knows you're there, they can flee, and you won't be able to see where they went. So combat becomes nuclear ambush and instant retreat. Fleets are kept spread out, and war is held off by mutual assured destruction.
- On the other hand, if jump gates exist, but otherwise all travel is slower than light, space becomes tactical and politics subtle. The gates are a thing that can be conquered, and the space around them can be held. If you're willing to cut your planet off from the rest of the galaxy for a few decades, you can trash your gate to keep the enemy away. Conversely, the enemy can send a bomb through your gate to isolate you. Fleets are probably massed somewhere between the local gate and the populated planets, with a few ships at the gate to deal with bombs and emergencies. A smart civilization keeps an extra gate in-system but powered-down or contained in some way, so they can use it in emergencies but the enemy can't sneak in or blow it up.