"Fictional" was a poor word choice on my part. "Hypothetical" or perhaps even "conjectural" might have been a better choice.
I understand that the gravitron as a hypothetical mechanism for gravity has been a popular idea with a good pedigree, but I've heard other propositions as well with their own interesting implications.
Speaking as an uneducated layman, the concept that seems most intuitive to me is that gravity is an emergent phenomenon where mass distorts spacetime, causing otherwise "flat" space to curve, sending objects in their straight-line inertial path in an objectively curved course. It also slows down time--the closer to the mass, the slower time flows. "Down" is where time runs slower. The idea of this working because of photon-analog particles doesn't make sense to me (of course I concede that people smarter than me think it makes sense just fine).
My recollection, though it's vague, is that it's possible to describe electromagnetic fields in terms of pure geometry/topology in the same way Einstein describes gravity, and we know that photons exist. Often in physics, it's possible to use more than one mathematical model to describe different facets of the same thing. So just because gravitation can be described topologically doesn't exclude the possibility of a quantum interpretation.
After all, all physics is fundamentally quantum physics. Classical physics is just an approximation of quantum physics for large ensembles of particles. That, I think, is the reason it's so hard to construct a quantum theory of gravitation -- because it's a force that's only measurable on large scales, between large masses where the particle-level quantum effects are swamped by noise and average out to classical behavior. Gravitational interactions of individual particles are so feeble that it's extremely hard to gather the measurements that would confirm quantum gravity.
I've come across one or two hypotheses claiming that gravitation is actually a fictitious force and gravitons don't exist, but they both define it as a manifestation of quantum effects. I actually used one of those theories in my Hub novelettes in
Analog -- that gravitation is a property of particles' wavefunctions to tend toward their most probable location in an ensemble, which statistically speaking would be toward its center of mass. The other one, IIRC, is that gravitation is basically the Casimir Effect writ large -- that massive objects shield each other from the background vacuum energy of the cosmos and thus get pushed toward each other by the pressure of the vacuum energy, which looks like they're being pulled toward each other by gravity. I'm pretty skeptical of both of those theories, though. (I used one because it was interesting from a story perspective, not because I really believed it. And The Hub is a comedy series, so sometimes Rule of Funny prevails.)
The idea that gravity effects time is something that I feel hard sci-fi has not explored enough of. There are some interesting ideas in there for sure.
Gravitational time dilation is an extremely weak effect, so it would only come into play extremely close to a black hole, which limits its applicability.
Gene Roddenberry's Andromeda used time dilation from a close passage near a black hole as a mechanism to suspend its protagonist in time 300 years, although it fudged the physics and massively exaggerated how severe the time dilation was (handwaving it as an interaction with the
Andromeda Ascendant's AG fields). Fredrik Pohl's classic novel
Gateway used a similar plot device, though I don't know the specifics.
Stargate SG-1's "A Matter of Time" and
Doctor Who's "Worlds Enough and Time"/"The Doctor Falls" also used gravitational time dilation from black holes, again exaggerating it enormously for story purposes.
Although if you want to talk more generally about gravitation affecting time, most forms of time warp in fiction are consequences of gravitation affecting spacetime, tilting the space and time axes so that travel through space results in travel through time. Wormholes are one example, and there's also the closed timelike curve resulting from passage through a black hole's ergosphere (which TOS: "Tomorrow is Yesterday" managed to depict 7 years before Frank Tipler's paper formally describing the phenomenon). The time warp in TNG: "Yesterday's Enterprise," described as "a Kerr loop of superstring material," was based on the physics of a Kerr black hole, a rotating one where the singularity becomes a ring that would function basically as a spacetime portal if you could survive passage through it. (They mistakenly said "superstring" for "cosmic string," but otherwise it was much better physics than later Trek productions where they just made up gibberish words.)
I have wondered if gravitons are like Ptolemy's epicycles: the math works, but it just ain't the real deal. If they are experimentally confirmed soon, I'll eat my words.
The thing about epicycles is that they made the theory more complicated than it needed to be, piling on arbitrary ad hoc assumptions rather than just admitting a mistake and starting over with something more straightforward. I don't think that applies here. Gravitation following the same quantum physics as everything else in the universe seems straightforward and elegant to me; gravitation being the only purely classical phenomenon in an otherwise quantum universe seems arbitrary and clunky to me, something people want to believe because they can't think of an alternative rather than something that actually makes sense if derived from first principles. (Although as I mentioned, there are a couple of fringe theories explaining gravitation as a quantum effect without gravitons.)
But it's functional, and at the end of the day, it's better than Centripetal Artificial Gravity.
It's not wrong to call it centrifugal. Centrifugal force is "fictitious," but that doesn't mean it doesn't exist at all, as laypeople often misinterpret the term; it means that whether you observe it depends on what frame of reference you measure it in. What an outside observer will measure as a centripetal force counteracting an object's inertia, an observer within the rotating frame will measure as an acceleration directly outward from the center, hence centrifugal. And only an observer inside the rotating frame will feel the force as an equivalent of gravity, so from that perspective it should be called centrifugal. (Or one could parse it more simply as "pertaining to a centrifuge.")
