So, somehow, I have discovered a process that makes an objects mass negative. Not its weight, its mass (although, I guess that the weight would also become negative). The thing is, before I try it, I want to know if this is a bad idea.

A number of scary words spring to mind, such as "strange matter", "white holes", and maybe even "naked singularity". However, I am not really the sort of guy who can do special relativity, so I have no idea which of these might happen. They never taught that at school.

Why might it be a good idea? Well, everyone loves themselves a good space opera. To get a space opera, you need FTL. A number of theoretically possible FTL drives need some parts to have negative mass to work. Another thing that could use this might be airships or planes. Just stick a lump of processed material in it, and you don't need to worry about going down because of engine failure in the case of planes. Or, in the case of airships, you don't need to have such a large balloon, which makes potential tourists nervous. You can even make floating islands in the sky! (tie them down though, unless you want them floating away into space). Oh yes! you can also use this method to get to space and back for almost free! Just collect enough mass while you're up there, and you can come back down again. If you feel like you are going down too fast, you can always use a parachute.

So, yeah. a lot of reasons that it might be useful.

But, this matter is going to be quite strange. Does it qualify as strange matter? If so, will it cause a chain reaction, like shown in a certain Kurzgesagt video?

Basically, what I'm asking is the following:

If a process* can multiply the mass of any** object by negative 1, does special relativity or quantum mechanics, or basically any other sub-field of physics cause this process to have drastic consequences?

*: the process, while the setup takes a long time, works through about one cubic meter of any material per millisecond. (you can change the rate for your answer)

**: This is something artificial, and I am no supervillain. I won't do it to anything bigger than, say, a brick.


  • negative mass gets repulsed by gravity. I guess that means that it should also be repulsed by any acceleration?
  • The mass-reversal either works completely, or not at all.
  • this actually happens.
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    $\begingroup$ Which 1-3 of the three types of mass listed in that wikipedia article are you inverting? Regardless, we run into Jon's law: if there is a method of space travel that lets you get to your destination in an acceptable amount of time, then it can, and will, be used as a superweapon. $\endgroup$ Feb 15, 2020 at 15:36
  • $\begingroup$ In your fantasy world, are inertial mass and heavy mass the same thing, or are they not? $\endgroup$
    – AlexP
    Feb 15, 2020 at 16:24
  • $\begingroup$ Have you created negative inertial mass or negative gravitational mass objects or objects with negative inertial mass and negative gravitational mass? The results depend critically on the type of negative mass. $\endgroup$
    – a4android
    Feb 15, 2020 at 23:48
  • $\begingroup$ I guess that any and all types of mass are turned negative. $\endgroup$ Feb 16, 2020 at 17:57

1 Answer 1


Even before getting to the consequences of easily created negative mass, remember that FTL by itself, even without considering how it is implemented, implies time travel. Note that it does not require time travel, and the existence of privileged reference frames or some form of chronology protection mechanism would prevent everything just disintegrating into a sea of paradoxes.

With that aside, even with privileged frames (or whatever) negative mass that has the the "runaway motion" that you want is hideously dangerous and breaks all sorts of bits of physics.

Even if you limit it to something brick-sized, you can end up with something that takes a pair of brick-equivalents and accelerates them to arbitrarily high relativistic speeds with no input of energy. Such a device, especially if multiple such devices were used, is the sort of thing that can be used to smash planets. It puts relativistic kill vehicles in the hands of anyone capable of building one of your levitation, reactionless drive or FTL devices. No input of energy is required, so there's no launch signature. They'll be undetectable, unstoppable, and incredibly destructive.

Any projectile that misses its target will presumably keep getting faster until it is either vapourised by the blue-shifting of the cosmic microwave background, or it turns into a miniature black hole and promptly and violently evaporates.

Further more, you've also created a perpetual motion machine of the first kind (just put some of your negative mass on the rim of a wheel) so everyone has as much free energy as they could ever want or need, and then some. You can create tanks of gas which will spotaneously heat up, turning into instant free-energy bombs.

I'm sure there's more I haven't thought of.

It might be worth considering that negative mass isn't the only thing you can make FTL with... there are more subtle distinctions, like negative energy and negative pressure that might help you build warp drives and wormholes. Reactionless warp drives are also hideously dangerous (because anyone can use them to make relativistic kill vehicles) but at least they don't break thermodynamics at the same time as destroying everywhere intelligence has ever lived.

  • $\begingroup$ If you launch something with negative mass at something with positive mass, it will repulsed by the thing it's aimed at. Probably enough to stop it from hitting. Besides, the thing will have negative momentum and kinetic energy. $\endgroup$
    – Spencer
    Feb 15, 2020 at 21:44
  • $\begingroup$ @Spencer the repulsion effect may not be strong enough, and there's a chance that it will induce additional interesting effects in both the target and the negative-mass projectile (eg. everything explodes). It may not be enough to deflect any positive-mass projectile it was dragging along with it, so the damage will still be done. $\endgroup$ Feb 15, 2020 at 22:01
  • $\begingroup$ Just FYI, physicists have already created matter with negative mass: bbc.com/news/science-environment-39642992 $\endgroup$ Feb 16, 2020 at 12:27
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    $\begingroup$ @Astrid_Redfern not necessarily; you'll note that the paper talks about "a region of negative effective mass", which isn't quite the same as converting the mass of some of the rubidium to negative mass. $\endgroup$ Feb 16, 2020 at 13:02
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    $\begingroup$ @MarkGardner I'm not sure what part of my answer implied that this wouldn't be apocalyptic. Anything that can reach arbitrary speeds with no input of energy is world-ending. $\endgroup$ Feb 16, 2020 at 18:31

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