When analyzing faster than light travel, we can first look at what physics currently says on the subject, and then look at how authors defy that. For an extra helping of details, see Rockets and Propulsion Methods.
FTL in Real Life
Introduction
Using conventional propulsion techniques (rockets of one sort or another: chemical, nuclear, anti-matter, etc.), even approaching lightspeed (from the perspective of your origin point) would require more energy per second than all of Earth's industries use per year. While this may be explained by advances in technology, the general formulas for velocity and acceleration are such that as you approach the speed of light, the energy needed to accelerate anything with non-zero mass increases asymptotically. In other words, you need an infinite amount of energy (and an infinite amount of time) to accelerate to the speed of light. It isn't just an engineering challenge — it is fundamentally impossible.
But why exactly is this the case? To understand, we have to explore relativity: specifically, Einstein's special theory of relativity. General relativity is another beast, to be discussed later.
Special relativity
The speed of light is 299,792,458 meters per second, approximated as 300,000 kilometers or 186,000 miles per second. Within the laws of physics as they are currently understood, it is the speed that all massless particles must travel and the speed at which information itself propagates throughout the universe. Nothing with mass can travel at the speed of light through space: at best it can only approach this speed in certain reference frames.
Reference frames are crucial in physics. An object floating in space without acceleration is at rest in its own frame of reference no matter how fast it is moving relative to other objects. You can easily understand this if you have ever ridden in a car or flown in a plane. Even though the vehicle may be moving at 100 or 1000 kilometers per hour, you don't feel that motion. You can look out the windows and see things moving past you, but if you had no way of observing them, you would feel as if you were still. You can only feel motion when you accelerate: by speeding up, slowing down, ascending, descending, or turning.
The same is true even if you're traveling at a hundred million kilometers per hour (0.09 c). Everything would seem to be moving past you very quickly, but you would feel yourself to be at rest unless you accelerate. Light, on the other hand, is always going the same speed in all reference frames. It doesn't matter how fast you go relative to Earth, or to the galaxy, or to any other object: in your frame of reference, the speed of light is always 300,000 km/s in all directions, and thus you are always infinitely far away from it.
This measurement has been done countless times by countless scientists, starting with the famous Michelson-Morley experiment
in 1887 that disproved the idea of the luminiferous ether. No matter how fast any object or person is going, they observe the speed of light to be the same in all directions. Our modern technology relies on this fact: as just one example, the satellite navigation systems (GPS, Magellan, etc.) that we use to get directions only work because they take special relativity into account.
To summarize, the reason it takes infinite energy and infinite time for anything with mass to accelerate to the speed of light is that no matter how fast you go, the speed of light is always the same relative to yourself. There is no velocity at which your cosmic "speedometer" crosses a line and you're going at lightspeed.
Causality
If we discover that, or write a story in which this postulate of special relativity isn't quite accurate, and it is possible to accelerate to the speed of light with enough energy, another problem rears its head: causality.
To illustrate the issues with causality: according to relativity, whenever two people (say, Alice and Bob) are moving relative to each other, time is passing at a different rate for each of them — but the speed of light is still always the same relative to both. If the consequences of this are explored, it turns out that if Alice were to send a faster-than-light signal to Bob, from some points of view Bob would actually receive the signal before Alice sends it.
It should be noted this isn't merely an "illusion" caused by light-speed "lag" — from the perspective of some observers, Bob really would have gotten a signal that wasn't sent yet! In fact, it would be possible for Bob to then send out an FTL reply — that would reach Alice before she ever transmitted the original signal! This causality-violating reply might then change the original signal Alice was going to send, or might even prevent it from being sent in the first place — and then the situation results in a Grandfather Paradox.
This is the reason why some say that FTL travel implies time travel, but that is sloppy language. What actually happens is that causality is thrown out of whack, allowing some observers to see events preceding their own causes. Since causality is at the logical foundation for our entire understanding of the universe, this means in effect that in special relativity, going faster than light cannot be possible.
For the more mathematically inclined, going faster than light causes the "gamma" — the relativistic correction factor for time and mass dilation — to become imaginary (the square root of a negative number), and the spacetime interval — a formula that corrects for relativity to establish the proper distance between any two frames of reference — to flip signs. This is what leads to the implication of time travel in pop culture.
A pithy way of explaining the paradox is thus: "FTL, relativity, causality: pick any two".
General relativity
But what about more exotic forms of propulsion: wormholes, warping space-time itself, and the like? These ideas are discussed in Einstein's much more comprehensive Theory of General Relativity, where matters are considerably more complex and FTL travel is not known to necessarily imply causality violations.
No-one actually knows how these techniques could be accomplished, however. In the math of general relativity, solutions that lead to faster-than-light movement all require things that are believed to be impossible within this universe, like objects of infinite length, "exotic matter" with negative values of mass-energy, and traveling through the singularity of a rotating black hole. note
It is worth noting that relativity does not prohibit two objects from having faster-than-light velocities with respect to each other. In fact, we know this to be happening in our own universe thanks to the expansion of space, and such expansion of space has been theorized to have happened at a extreme rate just after the very Big Bang itself during the period of cosmic inflation. Objects more than a few tens of billions of light-years apart are moving away so rapidly that the light they emit will never reach each other. This is not because they are moving through space, though. Rather, space itself is expanding, carrying galaxies along with it.
The concept of the Alcubierre warp drive involves making use of this kind of stretching and contraction of spacetime to allow patches of space to move FTL with respect to each other. However, as mentioned above, all mathematical formulations of such a drive require exotic matter to work. They also run into other problems, like horizon radiation and collisions with objects in the path of the warp field.
Quantum mechanics
While quantum mechanics and general relativity don't get along very well, another problem with FTL travel emerges when we apply the law of conservation of quantum information. Because information propagates through quantum systems at the speed of light, moving faster than the speed of light would, in principle, reverse the direction of entropy and lead to information being spontaneously generated out of nothing.
Time dilation
Even if we can't go faster than light, in relativistic physics it is possible to travel an arbitrary distance within an arbitrarily short time from the perspective of the person doing the traveling. Time itself is relative, and stationary observers see the flow of time for the person travelling slow down, so that in the time it takes the traveler to get to their destination, he or she experiences less time passing than the stationary observers do.
The person making the journey sees the distance to their destination contracted along their direction of travel (Lorentz contraction), so they have less distance to cover than they did when they were stationary. This makes it theoretically possible for an astronaut to cross great distances in a single lifetime, but FTL travel is still required if you don't want the journey to take aeons from the perspective of those the traveler left behind on Earth. See also Time Dilation.
Conclusion
Any concept of FTL travel will encounter some kind of paradox or require ignoring certain things that we know to be true of our universe. None of this should be meant to discourage its use in works. Rather, it is meant to establish that any such use will require some serious hand waving by the author, no matter how well it is justified internally.
FTL in Fiction
Introduction
The fact that we can't go faster than light in real life presents a challenge for writers in that it rather kills the pacing of most sci-fi stories (especially science fantasy or space opera) if the characters set off for distant stars and arrive hundreds or thousands of years later. They resolve this problem by creating a device or system of some kind that allows for travel faster than the speed of light.
The "realism" of such a system can vary widely, from a well-thought-out device that uses as much of modern science as possible, or something that Runs on Nonsensoleum but gets the characters from Point A to Point B. Since it's such a common device, faster-than-light travel has developed a set of conventions that writers repeatedly use, essentially allowing several broad categories of technology with their own capabilities and limitations. This is useful for building the viewer's Willing Suspension of Disbelief, because even though FTL travel is theoretically impossible, as long as the story sticks to the conventions, the audience will know what to expect.
Types of propulsion
FTL travel requires an "FTL drive", a device of some sort that holds the laws of physics in abeyance and, when activated, allows a ship to travel faster than light. FTL drives tend to fall into these categories:
- Hyperdrives: The ship leaves local space and enters Another Dimension where it can go faster. This allows the ship to circumvent the limitations of physics by moving to a place where physics no longer applies, at least in a way that we know it. Travel through this other dimension still takes time, allowing for the middle ground between instantaneous travel and impractically long journeys. The alternate dimension is often called "subspace" or "hyperspace", and the device that allows you to enter it is often called a "hyperdrive".note The other dimension is often characterised by Alien Geometries and has decent odds of being a scary place with its own hazards to manage. One of the best known "hyperspace" regimes is Star Wars.
- Jump drives: The ship instantly disappears and reappears elsewhere. This is often accomplished by changing space itself — instead of moving faster than light, it reduces the distance that must be travelled. A common analogy is with a piece of paper; if you have to draw a line from one dot to the other, the best way is to fold the paper so that the dots are right on top of each other. (Indeed, jump drives might be called "fold drives" to reinforce the idea.) Jump drives tend to be the fastest FTL drive, but occasionally there are limitations — most often, they're range-limited, so you'd need to make a series of jumps to go longer distances, perhaps requiring recalibration between jumps. One of the best known "jump drive" regimes is Dune.
- Warp drives: The ship doesn't move fast, but shortens the distance it has to travel by forcing space itself to move. Think of it like an ocean, on which the ship rides a wave to move faster (sometimes called a "warp bubble"). Unlike other methods, with this one the ship keeps travelling within our ordinary universe, rather than through another dimension or a wormhole, and thus is still subject to all the usual hazards that travel would normally entail. To an outside observer, the ship will appear to be moving impossibly fast. Of all methods listed, this is probably the closest to reality — the Alcubierre Drive works this way, compressing space in front of the ship and stretching space behind it. One of the best known "warp drive" regimes is Star Trek.
- Portal networks: FTL travel is only possible between certain pre-defined points. Sometimes it can work with naturally occurring wormholes; other times, it's a system of Cool Gates that someone built. Given that it would take forever to build them manually, these are often leftover technology from an ancient galaxy-spanning civilization. The advantage of a portal network is that you can still use the chase-related tropes, as ships use their conventional drives to reach the specific point at which they can escape. Within the portal network, all three of the popular "drives" are possible; if hyperspace is involved, this leads to the creation of Hyperspace Lanes. It's also entirely possible for no "drives" to be used at all, with the ships simply using their conventional drives and allowing themselves to be catapulted from place to place. A prominent example of this regime is the eponymous ring-shaped devices in the Stargate-verse.
- Phlebotinum drives: It doesn't matter, let's just break physics! The ship is moving from one place to another, and no one needs to lose their hair over how exactly it's doing it. In some cases, the mechanism by which the ship is travelling faster than light is not mentioned at all. In other cases, it's only mentioned in the context of a Phlebotinum Breakdown — what's important isn't what's powering the ship so much as what the characters are doing to keep it that way. It's the softest sci-fi setting, used when interstellar travel is a backdrop for something else in the story. The Hitchhiker's Guide to the Galaxy features a variety of methods of FTL travel, each sillier than the last.
FTL conventions
No matter which kind of FTL drive you've got, there are a lot of other FTL conventions that tend to crop up frequently in works of this kind:
- You need a Cool Spaceship. Not every ship can have an FTL drive. Only the best, most specialised ships will bother to install them. Certainly, they're not always visually impressive, but they're always well-equipped for the journey. In many cases, FTL is just so cool that any ship that has an FTL drive is automatically a Cool Spaceship. However, some special characters, such as Space Whales or certain Flying Brick superheroes, may be capable of dispensing with ships altogether and using Shipless Faster-Than-Light Travel.
- You can't just go FTL wherever you want. In some cases, you're restricted by the Portal Network. In other cases, you can't go too close to a planet's surface because there's a No Warping Zone; most FTL drives are so destructive to the space around it that bad things happen when you use them next to a planet. In still other cases, you need to be pointed in the right direction before activating FTL; if you make a Blind Jump, you could crash into something that you can't even see. In any case, it allows for Stern Chases and Standard Starship Scuffles to occur while a ship tries to reach a point where it can safely activate its FTL drive. This also reserves FTL for very long trips; an Interplanetary Voyage can only be accomplished by a conventional drive.
- It's easy to escape. Once you've activated FTL, odds are good that no one's going to catch you, even if they have FTL drives themselves. In some cases, this is just because they won't have any clue where you've gone once you've made the jump. In others, the FTL mechanism necessarily means that you'll never catch a ship that's gone FTL even if you know where they're going. It's a useful device that allows for an easy end to a Stern Chase. On the other hand, it's not always the case, especially when you have Hyperspace Lanes where the ships can continue the fight.
- You'll find Space Friction. If a convention drive stops working, a spaceship should keep going at the same speed through the void of space even if unpowered. If an FTL drive stops working, though, the FTL travel will stop. A busted hyperdrive will kick the ship out of hyperspace; a busted jump drive will maroon the ship between jumps; a busted warp drive will cause space to reassert itself. Thus, the ship might still be drifting forward, but not nearly as fast as it was before, and it becomes vulnerable to other ships with FTL drives.
- Communication is FTL. Ships travelling faster than light can communicate with entities who are not travelling nearly as fast. FTL-enabled ships will often have an Everything Sensor that allows them to detect, in real time, things happening millions of lightyears away. All of this allows ships to know that they need to move somewhere faster than light, even if it should take the information that prompts them to do so a long time to get there. In some cases, this is accomplished with a Subspace Ansible, which allows ships to send messages faster than light even when not going FTL themselves; it's particularly useful for Distress Calls. FTL communication, however, opens up some nasty cans of worms with respect to causality; the way real-life physics works, a communication sent faster than light will be observed to have been received before it was even sent. Some works have FTL travel but not FTL communications, like Andromeda, Honor Harrington, and the Vorkosigan Saga, which means that their ships are pretty much flying blind.note
- It's not always safe. Sometimes it's because Hyperspace Is a Scary Place; it's dangerous to be wandering around in there, but it's necessary if you want to go anywhere in any reasonable length of time. Sometimes it's because the technology is unstable; one small wrong move, and everything gets blown to smithereens. And in still other cases, travelling that fast can have negative effects on the ship's occupants.
- Terminology will differ. The fact that we call something a "hyperdrive", "warp drive", or "jump drive" doesn't necessarily mean that any given work will use those terms in the same way. For instance, Babylon 5 uses "jump drive" for a hyperspace-like system, and Warhammer 40,000 uses "warp drive" for its particularly scary conception of hyperspace. Out here, we're going for substance over form; what the work calls something is not as important as what it actually does.
Conclusion
These conventions are by now well-established Acceptable Breaks from Reality. Since it's practically impossible to tell a story about interstellar travel with physics and technology as we know them now, instead we rely on a story falling into one of the above categories and call it a day. After all, these days it's better than finding aliens on Venus or Mars, like we used to, and it's also better than the Conveniently Close Planet. Writers also appreciate these conventions because Most Writers Are Writers, not scientists or mathematicians — they know there's no way they can accurately conform to science as it is, so they conform to the FTL conventions. All in all, Tropes Are Tools, and the unrealistic nature of FTL travel doesn't have to detract from the story at all.
