The Old FTL Problem

One of the big draws of SF as a genre is the whole ‘exotic places’ thing. You know, pretty double stars, masses of moons, deep-blue Rayleigh-scattering gas giants, that sort of thing. There’s a lot to be said for escapism, particularly given that the real world now seems to be getting more depressing rather than less (certainly here in Britain, anyway, where we now have clear empirical evidence that change for the better is apparently unpopular and politically-impossible).

But there’s a small problem, and that’s the distance one. Basically with regards to space travel, there’s a slight problem in that space is, umm, rather large. Even just the Earth-Moon system – a reasonably-compact arrangement, as these things go – is arranged so that the separation between the two bodies is such that you could line up 30-ish Earths, side-to-side, between here and the Moon. (Note that I need to check my maths on this … I don’t think that figure’s exactly right, but it’s on that order of magnitude…)

The further you go, the worse it gets. When you start looking at interstellar distances, the distances start looking, umm, prohibitive. (This, incidentally, is why stars look point-like. Enough light reaches your eye to trigger the cones-and-rods, but the object itself is so far away that its ‘disc’ appears smaller than the minimum size your eye can resolve. So you see a glowing point, which may or may not be smeared out depending on what the atmosphere is doing.)

But there’s a worse problem. The Universe imposes a speed limit. This is often, and perhaps somewhat-misleadingly, called the Speed of Light, or c. Strictly, it’s also the speed of gravity and the strong and weak nuclear forces, but light is what we tend to think of. (Also, very strictly, c is the speed light travels at in a ‘perfect’ vacuum – inside a medium, like water or an atmosphere, it will travel slower.)

If a material object is accelerated, it gains mass. But this is not evenly distributed over the acceleration – the closer you get to the speed of light, the faster your mass increases. Also your rulers shorten and your clocks run slow, relative to the outside universe. Theoretically, acceleration to c leaves you with infinite mass, your clocks frozen and your ruler contracted down to zero length. Basically, you turn into a black hole!

Obviously this put something of a crimp on trying to accelerate past, or even to, the speed of light. (Light has a get-out clause on the grounds that photons are massless – also photons don’t actually ‘accelerate’ to the speed of light, they just happen to exist at that speed. Yes, photons are weird.)

But even at the speed of light, stars are still rather a long way apart. Our nearest current known neighbour, Proxima Centauri (a truly feeble little red dwarf that’s so faint it can’t even be seen with the naked eye) is more than 4 years’ travel away at c. The further out you go, the worse it gets.

This is the main attraction for FTL in science fiction – it allows you the possibility of getting beyond the Solar Neighbourhood inside a human lifetime.

There’s a further problem, though. The classic ‘strong’ FTL – warp drives, hyperspace, etc. – is utter bunk. In physics terms, it’s complete and utter nonsense. If some sort of strong FTL – pop into Hyperspace, pop out of Hyperspace – existed, we’d have to put Relativity in the bin. Literally, in the bin. And there’s the small-but-significant problem that Relativity has survived every single experimental test that we’ve been able to throw at it. Pulsar rotation timings, Gravity Probe B, atomic clocks flown backwards around the world on Concordes*, deviation of starlight during eclipses … you name it, General Relativity has handled it all. It’s about as robust as scientific theories get.

So from a hard-science Point-of-View, strong FTL belongs in the bin. That said, a lot of writers simply choose to ignore this. It seems to be one of the loosely-accepted conventions of the genre – yes, we all know strong FTL is impossible, but just for once we all let that one slip by… And certainly, as a reader, it doesn’t bother me hugely. As long as the characters are sympathetic and the story’s good, I can tolerate science-howlers.

But there are a couple of loopholes in Relativity.

Strictly, the proscription on travelling FTL is a local one. Suppose that the distance you have to travel is somehow shorter than it should be, you can then produce the appearance of FTL travel, without actually accelerating to above (or even close to) c. This is the root of the wormhole work-around. A wormhole is a sort of extreme distortion in space, effectively a kind of ‘tunnel’ from A to B. The analogy often given is that a worm can either walk all the way around the surface of an apple – or it can bore through the middle. If it bores through the middle, it will get from A to B faster.

Better yet, wormholes are consistent with Relativity. In fact, they’re a prediction that emerges from it. They are, in a very limited sense, physically-possible. They also bring the time and energy costs of interstellar travel down to the ones we associate with interplanetary travel – in other words long and expensive, but not wholly beyond imagining. A wormhole network does allow for the possibility of interaction and trade between several different solar systems.

That said, there’s an awkward problem in that there’s no actual evidence that wormholes exist. If they do, they’re rare enough that, throughout the whole course of recorded human history, we’ve never had a visit from one here on Earth. Also, deep-sky microlensing searches have yet to turn up anything with a light curve that looks anything like what we would expect from a wormhole, so we haven’t seen any in space either.** Both of these facts put a ceiling on the maximum possible wormhole space density.

Could people build a wormhole? The answer is probably not. The energy costs of warping space to the required degree are the sort of things associated with big supernovae. (And no wonder – supernovae can knock out black holes, and there is a mathematical relationship between wormholes and black holes.) The idea of a human society ever wealthy enough or powerful enough to toss around ~10^45 Joules on a regular basis … well, per second, the Sun puts out about 4 x 10^26 J, so you’re looking at the equivalent of around 8 billion years of the Sun’s output, or 80% of its Main-Sequence lifespan.

Yeah. Just a small production-gap there!

But there’s one more possible get out for FTL.

One particularly-mad idea for quantum cosmology is Simulation Theory. In this idea, the universe we live in is actually a simulation, running on a computer in someone’s basement. (If they have basements in The Real Universe, of course.) In this case, the Speeds of Light, Gravity etc. are actually something a bit different – they’re computational limits, imposed on the simulation to slow it down and keep the information-exchange inside something that the software/hardware can actually handle.

Now, if we live in a simulated universe, then the person with kernel-level access can effectively do what they damn well want. So in this case, FTL means finding a way to ‘hack’ the system, effectively, and get it to designate you as a sort of internal administrator. At that point you can shuffle your bytes back and forth without any regard whatsoever to lightspeed limits, etc. etc.

Or alternatively, you can try asking the actual Users very, very nicely if they’d mind doing you a favour … But this has a slight problem that if a) it works and b) lots of other people start doing it, you run the risk of accidentally mounting a Denial-of-Service attack on God. Or possibly pissing off the Users enough that they delete you. (Though this could be one possible Fermi Paradox explanation … sufficiently advanced civilisations sooner or later manage to piss off the simulators, and get themselves bug-fixed.)

Of course, any FTL drive that’s dependent on asking favours from God is also one that has other issues … but that could easily be a post in its own right!
*Is it just me, or is that possibly the most awesome theoretical physics experiment ever? I mean, I know I shouldn’t be jealous, we astronomers get giant telescopes and beautiful mountaintops, but still…
** Of course, there is an assumption here: we’re assuming that we’d have any idea what a wormhole microlensing event would actually look like. I’m quite happy to believe that our ideas here could be laughably-wrong.


3 Responses to “The Old FTL Problem”

  1. Well, there is another slight problem. An FTL drive is more or less the exact same thing as a time machine. So suddenly you have causality problems.
    The old bromide is “Causality, Relativity, FTL travel: chose any two.”

    There is more discussion of this on my website in the “FTL” section. Link is in the reply header, I won’t put the link in this comment because wordpress tends to think this is spam.

  2. Which is why science fiction authors own dart-boards with a bulls-eye over a photo of Albert Einstein

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