My original intention for the Freehauler Alcione ‘Verse (created universe) was to combine realistic science (say 8 on the Mohs Scale of scifi hardness) with esotericism, new weird, and similar crazy things that would rank at say 3 on that same scale (like Warhammer 40,000). I didn’t want to write a pure hard science story, but an imaginative and imaginal crossover.
Anyway, since space opera always will be central, I figured it’s important to get the spaceships right. Sure I have hyperspace FTL, a classic trope which ranks at say 5 or 6 on the scientific realism scale, but for the rest I wanted everything to be kosher. I consulted the excellent atomic rockets site, to get all the stats on a realistic high performance spaceships. Only the ship kept getting bigger as I added more modules, until by no stretch of the imagination could it be a privately owned (freehauler) tramp spacer! It was megacorp fleet hauler size.
Another problem is such ships have to be insanely powerful (just check out the figures required on the Atomic Rockets torchships page, this for a tiny ship of a mere one thousand tons!). Assuming classic newtonian rocket science only, if you have a million ton ship you need about half a hiroshoma bomb / second of output for even milligees or tens of milligees of acceleration. I mean, talk about a Weapon of Mass Destruction drive! Also, how to stop the engine, and for that matter the rest of the ship, vaporising, even with magnetic plasma handwavium containment and exhaust focussing.
Then the other day I came across a brief essay on negative mass at Science News Org. Apparently, physics says you can have mass with a negative value . Not antimatter, but a sort of counter matter, that is identical except opposite in effects. So if you push it, instead of moving away, it moves towards you. Like charges attract. Gravity is repulsive.
Which tweaked my curiosity and imagination, so I looked up more on this subject (of which there isn’t much). David Darling negative mass propulsion gives a short and useful summary; Esther Inglis-Arkell Negative Mass: The Nuttiest Thing Never Seen describes in more detail, and Ask a mathematician Q: What’s the difference between anti-matter and negative-matter? has more discussion on all this.
What really got my attention, negative mass falls towards positive mass. But positive mass is repelled by negative mass. Put a bit of negative mass near an equal amount of positive mass in a zero gee environment, and they two will fly off, the former chasing the latter, forever. Moreover, this doesn’t contradict the laws of physics, because the net kinetic energy and momentum is always zero (the negative mass has negative kinetic energy, the positive mass positive.
Wow. A space drive.
Actually this was originally shown by the physicist and science fiction writer Robert Forward in a paper some years ago . There’s a neat drawing, which appeared on Atomic Rockets, that I have reposted here.
This made me think, if you have a ship that weighs N tons, and you have a dense plate of negative mass of weight N tons at the back, you have reactionless, inertialess drive.
Most importantly, you can go up to relativistic speeds, but net kinetic energy always remains zero. So no planet busters. (Unlike the conventional scifi reactionless drive, in which anyone can make a bathtub into a planet cracker, but for some strange reason no one does. This is how you do it. Stick your handy Acme Agrav Acceleration Unit on the back of any small object, say, a fridge, or a bathroom sink. Accelerate it to high relativistic velocity, and aim it a planet. Boom, no planet).
Like the Alcubierre drive it doesn’t contradict the laws of physics. It’s just, unobtanium. But unlike the Alcubierre drive, you don’t have to worry about being fried by hawking radiation, or frying whatever your destination is with the same (note, this only happens with afaster than light Alcubierre drive ). But if you have this amazing unobtanium stuff, you can just stick it on the back of your ship, and off you go.
Or, according to John Cramer Anti-Gravity and Anti-Mass Alternate View Column AV-14, you don’t. Because by tethering the positive ship to the negative mass, the two forces cancel each out. It seems this only works if the two are not physically tethered.
So the only way around this (other than having the negative mass as a sort of big floating ball following the ship) is to add still more handwavium, and say hey no worries we have a special hyperspace thingie in which the joint momentum isn’t cancelled out! (science fiction is good because whenever you are in a whole, you can always cheat). Alternatively, use the Alcubierre metric but only go slower than light  (will save this for the next blog post).
There are still a few questions that need answering.
Given equivalence of normal matter and negative mass (say 10,000 tons and -10,000 tons), how fast would the ship accelerate? Or is acceleration determined by how close the two are?
Is it necessary to have equivalence. If you have a 10,000 ton ship, and you add (subtract, whatever) 5000 tons of negative mass, will it still be a reactionless drive, but only accelerate half as fast? Because the negative mass would contribute less momentum? Or would it accelerate twice as fast, because negative mass attracted to positive moves faster the smaller it is?
Also minor technical questions. How do you steer? How do you switch it off? I guess if you can couple and decouple the negative and positive mass with some special gizmo, you can switch it off, or tilt it like sails on a sailboat. Or whatever.
Incidentally, one thing negative mass won’t do. As the Science News Org essay points out, it won’t float just above the ground. You can’t have ships hovering on a planet, or a cool hover bike like Rey’s on The Force Awakens. When it comes to a planetary gravity well, negative mass behaves exactly the same way positive mass does. No negmass newtonian apples falling upwards. 
So, just like an atomic rocket, a negmass ship can only be used in deep space. No standard space opera trope of rusty tramp freighters or sleek warships effortlessly climbing up and down the gravity well, a la StarWars or Firefly. Just as well, because it means I can keep space habitats, not as peripherals, but as central to the entire setting as the ships are.
There’s something to be said for being not too imitative of standard tropes.
 Bondi, H. “Negative Mass in General Relativity,” Reviews of Modern Physics, Vol. 29, No.3, July 1957, pp. 423-428.
 Forward, R. L. “Negative Matter Propulsion”, Journal of Propulsion and Power (AIAA), Vol. 6, No. 1, Jan.-Feb. 1990, pp. 28-37. See also Winterberg, F. “On Negative Mass Propulsion,” International Astronautical Federation, Paper 89-668, 40th Congress of the International Astronautical Federation, Malaga, Spain, Oct., 1989.
 Finazzi, S., Liberati, S., & ́, C. B. Semiclassical Instability of Dynamical Warp Drives, Physical Review D 79, 124017 (2009).
 Hammond, RT 2015, ‘Negative mass’, European Journal of Physics, Volume 36, Issue 2, accessed 24 Jan 2015.