So, what makes SF, even hard SF, is not so much what can or can't be possible but the storytelling approach which falls out of it.
This is genre fiction, and genres have rules. For example, Romance is a genre of happily ever after endings. If you write what is ostensibly a Romance novel, but the heroine realises six pages from the end that her new lover is cheating on her and so she walks away unhappily and that's it, that's not a Romance novel, a Romance imprint would reject it, if you're a big name author they'd tell you to take it to a literary fiction house - otherwise go away and rewrite with a happy ending.
In SF the rule is "What if ...?", so you absolutely can have anything, unicorns, magic spells, faster than light travel, God can be real, but the story is about what else if that was so? That's where science comes into it. OK, so there are unicorns, what's special about them, just horses with a weird horn or anything more? Do people... ride unicorns? Eat them? Or maybe the unicorns eat people.
A fantasy kingdom with a rich gold mine can be dirt poor and yet money is somehow measured in gold, the thing they have plenty of, in fantasy you needn't explain, but in SF that's either a massive error or the core thesis of a novel.
On FTL specifically. I'm not a fan for reasons Charlie Stross explained when he gave up on writing a sequel I'd kinda wanted to read some day in a setting where he'd tried to tame FTL. FTL is time travel. So, you need to either embrace that, and have arbitrary time travel in your story (good luck producing a narrative you can write down) or come up with a water tight reason nobody ever does this. That's just a high cost it usually isn't worth it.
That said, the Clockwork Rocket series by Egan does just do time travel, but it also has different spatial dimensional layout, it's set somewhere way stranger than the setting for Incandescence, which is our universe albeit not somewhere humans could ever go. Still, I wasn't enormously happy with the outcome, time travel still ends up being sort of cheating even in the framework Egan creates. They do, as hoped, solve their impending disaster by going very, very fast though. Also they fix the patriarchy, which is way harder in a world where women inherently die during the equivalent of childbirth...
This is genre fiction, and genres have rules. For example, Romance is a genre of happily ever after endings. If you write what is ostensibly a Romance novel, but the heroine realises six pages from the end that her new lover is cheating on her and so she walks away unhappily and that's it, that's not a Romance novel, a Romance imprint would reject it, if you're a big name author they'd tell you to take it to a literary fiction house - otherwise go away and rewrite with a happy ending.
In SF the rule is "What if ...?", so you absolutely can have anything, unicorns, magic spells, faster than light travel, God can be real, but the story is about what else if that was so? That's where science comes into it. OK, so there are unicorns, what's special about them, just horses with a weird horn or anything more? Do people... ride unicorns? Eat them? Or maybe the unicorns eat people.
A fantasy kingdom with a rich gold mine can be dirt poor and yet money is somehow measured in gold, the thing they have plenty of, in fantasy you needn't explain, but in SF that's either a massive error or the core thesis of a novel.
On FTL specifically. I'm not a fan for reasons Charlie Stross explained when he gave up on writing a sequel I'd kinda wanted to read some day in a setting where he'd tried to tame FTL. FTL is time travel. So, you need to either embrace that, and have arbitrary time travel in your story (good luck producing a narrative you can write down) or come up with a water tight reason nobody ever does this. That's just a high cost it usually isn't worth it.
That said, the Clockwork Rocket series by Egan does just do time travel, but it also has different spatial dimensional layout, it's set somewhere way stranger than the setting for Incandescence, which is our universe albeit not somewhere humans could ever go. Still, I wasn't enormously happy with the outcome, time travel still ends up being sort of cheating even in the framework Egan creates. They do, as hoped, solve their impending disaster by going very, very fast though. Also they fix the patriarchy, which is way harder in a world where women inherently die during the equivalent of childbirth...