In our last exciting episode we considered the possibility that the dictum Causality, Relativity, FTL travel: chose any two may not be strictly true. Consult the comment thread for discussion, particularly some thought experiments offered by commenter Luke.
I cannot vouch for the validity of these arguments, only that they are either a) valid, or b) extremely high quality bullshit, and I choose a as my working hypothesis.
Einsteinian street-legal FTL is, as the discussion takes note, subject to certain constraints. Its use in a story is, in turn, subject to some meta-level contraints. So far as I can tell, it is consistent only with the 'jump' style of FTL, in which you pre-select your destination before hitting the big red button. FTL in which you navigate freely seems ruled out. And you are taking your chances with drawing (or narratively describing) a 'subway map' of FTL jump routes, unless you can ensure that your map meets the requirements for a directed acyclic graph.
You have been warned.
Having said that, the whole subject is pretty much moot unless you are a physicist, or can credibly impersonate one. Nearly all your readers will ignore your validation, and assume either that you are jiving them as part of the story, or else that you are simply a crank. (Which will not help them buy into the story.) Within the SF world, FTL is more or less universally understood to be a pure dodge, dumping physics for the sake of story. And it is just as universally accepted on that basis.
A point made in comments to last post was that there are also STL workarounds, even for such tropes as interstellar empires. Said commenter Horselover Fat: "For FTL, you need to dump Einstein. For STL empires, you need to dump your cultural assumptions. And you choose to dump Einstein?"
Very good question. My answer would be that cultural assumptions tie in closely to characterization, right at the heart of fiction. Change them greatly and your story pretty much has to be about those changes. Dumping Einstein, not so much. (Note: I am not saying that you need to ignore the laws of physics, only that - in space opera - you can.)
You do still have to fake it convincingly. As I suggested last post, the less you say about your specific handwaves the better off you will probably be. You also have to adhere to some internal consistency. If FTL jumps require generating Stupendous Energy on demand, you need to deal with the implications of a technology that can do this. Similarly, if you need to travel 100 AU in normal space to reach jump points, you need a normal-space drive capable of doing so in convenient time. (And if you stick with relativistic STL, you need a beaucoup powerful drive.)
All of which produces, or can produce, its own awkward complications. If your interstellar tramp freighter has a drive engine capable of slagging a continent, the movement of such ships anywhere near an inhabited planet will be very strictly regulated. This may spoil some otherwise charming tropes. (Think Firefly.)
There are a host of other possible complications to bear in mind. If FTL jumps can be made by small spacecraft just above planetary atmospheres, you've opened the door to bomber-mission nuclear strikes, or even interstellar ICBM strikes. But if it takes too long to reach jump points in normal space you could end up at least partly defeating the purpose of FTL.
Another criticism I have seen is mildly meta: settings in which there is FTL for convenient star travel, while the rest of the technology pretty much resembles the Plausible Midfuture [TM]. The problem here is that you have supposedly had a fundamental revolution in physics, yet with no technological consequence other than FTL itself. How remarkably convenient ....
A reasonable counter-argument might be that relativity itself gave us the atomic bomb and nuclear subs to deliver it, but no atomic motorcycles or force-field steak knives. Nuclear power plants put out the same kind of juice as coal-fired plants, and for most story purposes are pretty much invisible. I imagine that modern physics is implicated in a host of everyday gadgets, but not in dramatic ways.
So, the presumed future physics that gives us FTL might also give us antigravity drive and other Cool Stuff, or - for different desired values of coolness - might have few other obvious effects besides fast interstellar travel.
For that matter, a Pretty Strong argument could be made that all of this is tech geek navel gazing, irrelevant to the practical problems of SF world building. Heinlein's Starman Jones remains a favorite of mine in spite of an FTL that requires you to violate relativity in normal space, before you even get to make the FTL jump.
And to take a more modern example, I thoroughly enjoyed Elizabeth Moon's Heris Serrano books, though the stuff that reads like hard SF is really almost pure bluff. The fact of the matter is that if onboard instruments indicate the approach of an enemy ship, and the characters respond to this situation in a persuasive way, we as readers do not insist that they stop to calibrate their instruments for us.
On the other hand, this blog is more or less dedicated to the art of faking details convincingly.
Discuss. (As if you needed an invitation.)
The subway-esque hyperspace image comes, via Google Images, from a screensaver website.