Maxim 24: Any sufficiently-advanced technology is indistinguishable from a really big gun.
Wait, where was THAT specified?
Neutronium. Unfortunately, unlike some fan wikis, the episode of first use is not cited.
Who’s the Doper whose sig goes something like this:
“I can imagine a society of peaceful violence free-people. I can also imagine us invading them.”
I think gravity-alterers are worse than impossible - they’re also implausible. When black holes merge, the resulting gravity waves temporarily release more power than the rest of the universe combined, and its gravitational effects are not felt appreciably past 1 AU or so. And the gravity is only being altered a few hundred times a second.
So even if you could create directed gravity, you’d have to also make sure its range is limited. Even if it is directed only to a small slice of the horizon and its strength is several orders of magnitude less than the black hole mergers, every time you turn it on or off you’d have to expend a tremendous amount of energy, since you’d be propagating a gravitational wave across the rest of the universe.
1950s-style Rocket ships! 
But acceleration reminds me of ‘inertial dampers’, the devices that keep you from becoming a flat wet spot when your ship suddenly goes to light speed. (I think CDR Riker actually mentioned what would happen if the inertial dampers failed in one episode of Star Trek: The Next Generation.) I think in Dark Star the crew were encompassed by a ‘stasis field’ that held them immobile, but I’ve never seen any explanation of how inertial dampers work.
Inertial dampers are exactly the same problem as artificial gravity. Not that that’s much of an answer, of course.
How about gravity drives? (That is assuming you have enough handwavium and unobtainium to make one)
I’ve seen them described as giving an acceleration vector towards various nearby masses, which sounds kind of like periodically throwing out “grappling hooks” to “nearby” planets.
How would a gravity planer like the Kzinti had in Niven’s known space propel you in interstellar space? Or help you in taking off from or hovering over a planets surface?
BTW, IIRC, my first personal exposure to gravity manipulation was a Tom Swift book, Tom Swift and his Cosmic Astronauts. (This is NOT the “repeller”, which he uses to lift his Flying Lab, amongst many other uses.)
http://www.tomswift.info/homepage/cosmic.html
“This technology is unrealistic!” said Tom heavily.
God damn it, why did you have to type that out? I’ve got my filters ready when watching Star Trek, where I automatically file anything related to biology into the “unforgivably stupid nonsense” part of my brain. But when I read actual words like that, those words enter my brain, and pollute the parts that are dedicated to understanding real molecular biology.
Ahem. Repeletron.
As James Nicoll said “in science fiction biology is the redheaded stepchild that comes to school covered in bruises”
Yah, sure.
Although I’m sure Tom Swift made me the man I am today*, a LOT of water has gone under the bridge since I’ve last read one of those stories, and my google-fu was too weak to find that word.
*(My friend had a whole bookshelf, I was so jealous!)
Andy L, that may be true… but in that case, physics is the brown-haired stepchild who comes to school covered in bruises, and chemistry is the blond one. All of the sciences get pretty badly mangled by Star Trek.
This dismissal seems almost as handwavy as SF antigrav. I mean, a supernova releases visible light in amounts equivalent to a whole galaxy, but that doesn’t make lightbulbs impossible.
I’m not saying your conclusion is wrong, I’m just curious about how you’re actually determining the energy requirements of a anti-grav unit.
LSL didn’t find it either. “Repeletron” comes up with unrelated stuff. Plugging in repeletron Tom Swift, however, gives links to the slightly different correct spelling, repelatron.
You say “Reh-Pel-Ah-Tron”, LSL says “Rep-El-Eh-Tron”; let’s call the whole thing off! 
Back to the Cosmic astronauts, the device in question focused Earth’s gravity (or presumably any other body’s gravity), to form a virtual tether to allow the space craft to sail the cosmic wind like a kite. I think this “concentration” or focusing of Earth’s gravitational field was virtually “free”, meaning not energy intensive.
He applied the technology to stabilize a pleasure sailboat (mostly as a lark?), using a pair of such devices as sorts of anti-outriggers, IIRC. There was some fainting involved, I think, when one of the girls was accidentally positioned in the fringe of the field and experienced reduced gravity.
I don’t think any of this tech was used outside this book. It probably could have made a nice perpetual motion machine. A circular chain falls down in the region of focused gravity, and rises in the fringe of reduced gravity. (Now that I’ve said that out loud, would that work, or would Maxwell’s demons jump on for the ride up and even things out?)
The vast majority of SF doesn’t deal at with real physics around gravity, especially relativity, and pretty much just handwaves that ‘the generator works so we don’t have to do low-G special effects in every slot’ or ‘the ship can move at the speed needed for the plot’. Actually generating gravity the way it’s done in SF, especially TV, would be really hard to reconcile with the understanding of gravity that modern science has. It really don’t make a lot of sense if you probe to deeply into fictional artificial gravity setups, and if you do make a sensible ‘theory’ of how they operate, they almost always end up either absurdly inefficient at creating ‘walk around on this spaceship’ gravity or so useful and far-reaching that ‘walk around the spaceship’ should hardly even be noticeable.
It’s a lot like the way that SF tends to ignore that having a lot of fast spaceships tooling around the solar system means a ton of individuals have dinosaur-killer level weapons of mass destruction at their fingertips.
The point, though, is that even with those inconceivably vast amounts of energy, the results are still only barely perceptible. When a supernova outshines a galaxy, you know it. When black holes merge, though… you generally don’t.
Sounds a bit like the “gravity lens” technology used in Piers Anthony’s Bio of a Space Tyrant series. Been a long time since I read the books, but I googled up this good summary at a site dedicated to a love for pregnant furries, of all places. ([DEL]Not that there is anything wrong with that[/DEL] For the love of all that is holy, please don’t think I’m a regular at that site.) Description in third post.