# Nailing the physics of a negative mass planet

Another question about my exotic matter planet. I mentioned floating landmasses via anti-gravity and exotic matter mining, but I recently read this:

And was thusly educated. Turns out nothing will float, negative mass or not, unless you're on a negative mass planet. Thankfully, my story has a compromise. A planet whose crust contains a significant amount of negative mass exotic matter.

So if I want to obey the limitations laid out in that article, here are the rules and limitations I will have to have on my planet:

-Floating hills and mountains are not made of exotic matter themselves, they just float above vast underground reserves of exotic matter that act like the carpet described in the article, levitating the landmass.

-Safety protocols for handling exotic matter include never throwing it or dropping it from large heights so it can't build up enough speed to burrow through whatever material is in its way at a significant speed. If handled properly, and placed delicately on to a flat surface or container, it will not pierce the material for a long time. Conversely, if thrown or indeed fashioned into a bullet, it'll push through anything in its way at a rate relative to its velocity, eventually falling towards the planet, downwards, and exiting the other side of the planet at escape velocity.

-Never pair up a lump of negative mass with an equal but opposite lump of positive mass, as they will push against and accelerate each other at increasingly high speeds.

Does that sound about right as far as how I should realistically handle negative mass?

• Negative mass, while attractive from a story-telling POV, has several problems. Many people like to use it to make repulsive gravity, but, due to the equivalence description this fundamentally changes the interaction of the mass with any force. Sep 9, 2016 at 22:46

Negative mass was described in The Alternate View column in Analog, and some works by Robert L. Forward. In fact, that 1986 article blows away the new one you linked too and you should start there. Then continue with Forward's non-fiction essay (in his collection books) and scientific paper if available.

The force reversal thing has problems that are omitted from the descriptions.

For example, say you had a lump of the stuff on the floor. What supports it against the gravitational acceleration? Writeups just stop when it gets inconvienient and silent switch to normal behavior or some half-baked mixture.

Likewise, you could not have any solid mass of negative matter, since any bonding forces would blow it apart rather than holding atoms in place. In Hammond's paper you indirectly refer to, he states in the lower left on page 3, “in an immeasurably short time the object is traveling at essentially the speed of light.” so, electric charges are no good for trapping, say, a negative electron around a positive charge.

If you had negative mass trapped as you describe, it would need to be more than enough mass to cancel the pull of the planet. "Deposits" of gold or granite don't add enough mass to make you feel heavier, so reversing the sign would have the same sized effect! In fact, the paper linked to by the post you linked to concludes the carpet example with “Thus we could float an object of any mass, but it would take the (negative) mass of a small asteroid to make one square meter, so we may rule out this form of antigravity.” so I suppose you read the summary article only.

Edit: I suppose you could have negative mass particles in orbit, as gravity is the only thing that they behave with. They would need to be neutral in charge, and would have no interaction or effect other than to reduce the apparent mass of the system. Such rings would still leak over time due to multitbody gravitational interactions among the particles and any normal gas and dust particles that pass through.

• So basically it can only exist in particle form instead of as solid lumps or pebbles? That would eliminate the floating mountains entirely as well as any non-plasma weapons incorporating negative mass. Still though, I assume this doesn't forbid it existing as scattered particles dispersed throughout a larger lump of normal matter? That way it could still be mined assuming you had a way to refine the rock and extract just the negative mass particles. Am I correct? :) Jun 26, 2016 at 7:07
• How are you holding the negative mass particles in the lump of matter? What happens when you apply accelaration to that lump: it's transmitted as a force through to the individual atoms, causing them to accelerate and move with the solid because being displaced causes spring-like effects in the bonds. Now, sketch it out as just a few atoms with springs in between neighbors, but one in the middle is negative. Jun 26, 2016 at 7:12
• Basically, you ran right into my 3rd paragraph point again, forgetting all about the funny business with forces as soon as you changed context. Jun 26, 2016 at 7:13
• So a planet with negative mass particles in its crust is pretty much impossible then? Well that's... that's a bit of a stumbling block. Could they exist in orbit around the planet maybe? Like in a debris field? Or would they have to be emitted by something rather than just existing by themselves in the vacuum of space (say maybe a black hole?)? Jun 26, 2016 at 7:17
• Yeah, no floating islands, sadly. Still plenty of other interesting things on that planet though :) Jun 26, 2016 at 8:16