But aside from that and the whole aliens technology thing the plot revolves around, it is very realistic. They certainly get a lot closer than anybody else.
Power = m v a = 1e9 x 1e5 x 1e1 = 1e15 W
While petawatt is a lot, a megaton ship is also huge. Epstein drive is a fusion drive of sort, the easiest aneutronic reaction would be
B+p -> C + 16 MeV
So 2.5e-12 Joules per fusion
Which means you need to burn
1e15 J/s /(2.5e-12 J/atom)*(2e-26 kg/atom) = 8 kg/s
Suppose your engine runs at 10% efficiency, it means you are powering a megaton ship on less than a 100kg/s of material. So if your megaton ship starts as 10% fuel you have ~115 days of continuous thrust.
But that gets to what the Epstein drive basically is, in-universe: a 100% efficient fusion drive. No physical laws being broken, technically, just the nuclear rocket equivalent of a perpetual motion machine.
They do in fact go around at 1G (or 1/3 G) thrust for months at a time in the books and show, accelerating full speed for half the trip, then turn around and decelerate for the other half, for trips to/from the outer solar system. In the inner solar system it's only a few days to weeks to get anywhere. This lets them operate in artificial gravity without spinning the spacecraft, which is convenient for filming (although it's in the source novels too).
EDIT: Well, I should say that the propellant mass/delta-v is really high and that most of the fusion energy will can end up as kinetic energy of the propellant and ship. But the vast majority of that kinetic energy will be the propellant rather than the ship. To maximize ship kinetic energy over fuel energy you want the propellant to exit the ship at the same velocity the ship is traveling which is very far from what you see in an Epstein drive. But fusion provides tons of energy so that isn't going to be a figure of merit we care about.
Re: accelerating for many months at 1G is definitely bad physics, not only are you relativistic, you start getting into the kind of speeds where a minor meteorites will wipeout your ship. 1 gram meteorite with a relative velocity of 0.5c has 1.4e13 J of kinetic energy = 3.5 kton of TNT, so a small nuke.
For me, good sci-fi does just that, ie limit the magic to one thing or technology, and use that to explore something interesting.
Same with Altered Carbon. What would happen to society if we could swap bodies but keep our minds? That one piece of technology ("the stack") is magic, but from it follows lots of interesting exploration of the possible impacts on society etc.
What I hate is when they try to explain the magic. Don't do that. It never works. Just assume it's there, someone discovered it, invented it whatever.
For example, it matters how is it the ships can casually sustain a 1G burn for a week at a time. If it's efficient nuclear fusion, then the ships get bulky, energy is no longer a problem in the society at large, and the exhaust of said engine is a weapon of mass destruction. If it's subspace field shenanigans, then aforementioned impacts don't necessarily apply, but there is an open question of how small such subspace field combobulators can get, and what else could you do with them.
Technologies have second-order consequences, and societies shape to accommodate them. Half the fun of science fiction is in exploring those.
If you just call it magic and define it by a set of arbitrary rules, then your fictional universe will look shallow and inconsistent.
Star Trek goes into intricate detail trying to explain its warp drives and comes off looking far less grounded than the vast majority of sci-fi properties with FTL or jumpgates or whatnot that either don't explain anything, or just offer a few details. Whether it breaks your universe depends on the themes you're trying to explore, such as the nth order effects of radical technologies on society. Most of the time, the story isn't about the technology (in the same way that Westerns aren't always about animal husbandry and gun manufacturing), and it's sufficient that you push some buttons, pull a lever and spaceship go brrr.
everyone is entitled to define their own categories, but to me a key element of sci-fi is that the science and its implications drive the story as much as the characters. if the science merely enables the story, it's space fantasy.
The science in that case is necessary to bring the plot elements together and constrain the action under a set of known rules and behaviors regarding hyperspace and related technology, but little of it is explored in detail. It's just there for the same reason that boats have to be there for a story set in the Age of Sail.
a common trope in sci-fi is that jumpgates (or something similar) were built by some ancient super-advanced civilization, while the current civilization understands them to the minimum extent necessary to actually use them. this is a good way to sidestep the issue, since any inconsistencies can plausibly be the result of missing knowledge in-universe.
as to "whether it's still sci-fi", imo it depends a bit more on the setting. if FTL has already existed for 100+ years, it makes sense for it to be a background detail. on the other hand, if you have a story where FTL was discovered recently and it doesn't profoundly impact the plot, I would consider that to be pretty bad sci-fi.
like I said, this is just how I personally categorize things. to me, sci-fi is not just lasers in space. it's a genre where science/technology drives the plot, rather than being made up after the fact to justify the plot you want to have. it can still be sci-fi if the tech is totally implausible from a modern physics perspective; it just needs to be consistent.
star wars is usually my go-to example of "not sci-fi". there are no consistent rules for how anything works in-universe. all the technology works in whatever way is necessary to justify the current scene and will likely contradict itself later for a different scene. it's still a very enjoyable franchise, just not what I would consider sci-fi. a show like the expanse is almost the opposite. there's some unrealistic stuff in there, but it's internally consistent. once you nail down what's possible given "the rules" in-universe, the plot almost writes itself.
For the record, Babylon 5 was explicitly designed to be The Lord of the Rings in space. It's fantasy in sci-fi dress-up. Similar things could be said for Star Trek, Star Wars, and other sci-fi staples.
Hard science fiction like The Expanse, or most of the collected works of Arthur C Clarke, Alistair Reynolds, Kim Stanley Robinson, and Robert Forward present entirely self-consistent universes set in the future with logical extrapolations of technical capability governed by physical law.
Some people don't care for the distinction, but there are also those of us that have very strong preferences for ONLY readying hard science fiction. I like to feel that I learned something, or that the book describes societies that might someday come into being, or alien cultures that might realistically exist.
As much as I love Star Trek, it isn't really explaining all that much (it would be, if the writers could be arsed to read the technical manuals created for the series, but even the manuals themselves say they aren't mandatory reading, so here we are). It's DDoSing your disbelief with technobabble to allow for whatever plot the episode's writer had in mind.
(Though it's not impossible to make something relatively consistent out of Star Trek's universe; /r/DaystromInstitute is a subreddit essentially dedicated to doing just that. It's an exercise in function interpolation, but having lots of data points does constrain possibilities.)
> Most of the time, the story isn't about the technology (in the same way that Westerns aren't always about animal husbandry and gun manufacturing), and it's sufficient that you push some buttons, pull a lever and spaceship go brrr.
There's very little point in choosing sci-fi as your setting if your story is setting-independent. The westerns whose story is entirely interchangeable with action movies set in ancient times or present days don't tend to be very interesting movies (YMMV). Or at least I tend to avoid those, because I find them boring. With sci-fi, if the story is all about setting-independent human drama, I can have that with any other genre. I like my sci-fi when it extrapolates and elaborates on science and technology, and its impact on societies.
That's it for me too, but what I'm trying to get at is that I vastly prefer it without "higgs reactor", "tachyon transmitters" or similar nonsense.
But don't try to explain the inner workings of the magic by spouting nonsense. That just takes me out of the story. Take it for granted, aliens made it, something other than semi-randomly assembled sentences sprinkled with fashionable pop-sci terms.
You need magic to describe shit that doesn't exist. Otherwise, then nothing makes sense? How do you have FTL when any possible explanation of how it works might as well be words chosen at random from a textbook because none of it will be plausible.
> If you just call it magic and define it by a set of arbitrary rules, then your fictional universe will look shallow and inconsistent.
Literally doesn't make any sense. All your complaints are second order issues. Good Sci-Fi places rules around its magic and is consistent about it. What "good" sci fi that relies on tech that doesn't exist is there that DOESN'T have its magic bounded by in universe rules.
Sci-Fi breaks down when you start to ignore or make up new rules around the magic, especially if its just "stupid"
You don't always have to. Vernor Vinge explains it as, you are allowed one central conceit which frees you to deeply explore some core theme. That single wild assumption has the interesting property of being both inconsequential to the themes you wish to explore and the single thing that makes it all work. A lot of good hard science fiction has this property: there is internal consistency and broad agreement with real world physics but with at most a single unexamined wild assumption from which the consistently handled deviations flow from. The Zones in his Zones of thought or the Bobbles in Across Real-time are examples of this principle applied for excellent effect. Orthogonal by Egan is an interesting one when viewed in this lens, what with it being physically unrealistic but mathematically driven.
A lot of would be hard scifi authors, in the hopes of increased realism, mistakenly add too much unnecessary detail to the detriment of their story or core big idea. On the other hand, if you manage your central conceit well, you might sometimes be able to enlist the imagination of your readers (some of whom will have more technical expertise in the relevant area) to fill in the blanks.