Analysis / Faster-Than-Light Travel

If we're talking about "conventional" propulsion techniques (i.e. rockets of one sort or another, whether chemical, nuclear, anti-matter, or whatever else), even approaching lightspeed 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.

The fundamental reason that it is fundamentally impossible to accelerate to the speed of light in a finite time (let alone surpass it) is this: the speed of light is the same in all frames. Suppose you, as a spacecraft pilot, accelerate to .99c with respect to the earth. You measure the speed of light (with respect to yourself) and you discover the speed of light is still 186,000 miles/second. The speed of light is just as far ahead of you as it was near the earth before you ever began your journey. Try it again, and you find yourself in the same position. The speed of light is always 186,000 miles/second relative to yourself. (The infinite-energy, infinite-time requirement is a consequence of this fact.) If we discovered that this postulate of special relativity isn't quite accurate, we might find a way to surpass the speed of light. However, one would still have to address the causality problem of the following paragraph.

But what about more "exotic" forms of propulsion — wormholes, warping space-time itself, and the like? Aside from the fact that no-one actually knows how any such thing could ever be done (most theories that even touch on such subjects have the power requirements be, literally, beyond astronomical — just for starters), there is in Albert Einstein's theory of Special Relativity still another, fundamental reason why FTL travel cannot be possible regardless of the exact method used: in it FTL travel simply cannot work, because it would imply violations of causality itself. (If you're about to point out that Relativity is "just a theory," please do note that it has been repeatedly tested and confirmed with experiments, and in fact GPS systems have to take it into account in order to work properly!) According to Relativity, whenever two people (let's call them "A" and "B") 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 A were to send a faster-than-light signal to B, from some points of view B would actually receive the signal before A sends it. It should be noted this isn't merely an "illusion" caused by light-speed "lag" — from the perspective of some observers, B really would have gotten a signal that wasn't sent yet! In fact, it would be possible for B to then send out an FTL reply — that would reach A before A ever transmitted the original signal! This causality-violating reply might then change the original signal A 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. And causality is at the logical foundation for our entire understanding of the universe. In effect, this means that, in special relativity, going faster than light cannot be possible. In Einstein's much more comprehensive theory of General Relativity, however, matters are considerably more complex, and FTL travel is not known to necessarily imply causality violations (see Real Life examples on the Main Page)... All in all, though, suffice it to say that having the protagonists' space-ship outrun a beam of light will always require some serious hand waving by the author.

Even outside of Einstein's flavor of Relativity note , other theories tend to impose limitations just as strict.

It should be noted that in relativistic physics it is, however, possible to get arbitrarily far within an arbitrarily short time from the perspective of the person doing the moving. 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 traveller 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, 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 life-time, but FTL travel is still required if you don't want the journey to take aeons from the perspective of those the traveller left behind on Earth. See also Time Dilation.