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I have very little knowledge of physics, so I apologize if this is a wrongheaded question, or phrased poorly.

I'm trying to get my head around an aspect of negative mass. I understand that, as explained in this earlier Phys.SE question, there is a theoretical category of exotic matter that is called negative mass, and that a behaviorally effective version of this was recently created. I also understand that while this matter pushes contrary to all acceleration it still naturally tends down toward the center of gravity, just like regular matter, because gravity pulls on weight, not mass, and the two are distinct. Please correct me if I've gone wrong.

Otherwise, I'll get to the meat of my question: I've read that mass does work into buoyancy. If that is the case, wouldn't negative mass released on earth still rise up like a lift gas, even if this was not true antigravity? Or have I become confused along the way?

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  • $\begingroup$ Oh, and a quick shout-out to the user @knzhou who answered the related linked question, just in case they have the time and inclination to help with this one. $\endgroup$ – Random May 26 '17 at 8:03
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Firstly, if we're allowing negative mass, then Einstein's equivalence principle might work differently. It seems that you're talking about negative inertial mass (accelerates the opposite direction in response to force), but positive gravitational mass (still falls toward other objects with mass). For ordinary matter, as per Einstein, these two are the same, but with negative mass, things might be different.

Regarding bouyancy, what you must remember is that the bouyant force is applied by the surrounding gas, and therefore, inertial mass is relevant here. Since we have decided that our matter has negative inertial mass, it will indeed behave strangely. The bouyant force now points downward, which means that everything sinks to the ground, even things that are lighter than the surrounding fluid.

Now, if we have negative inertial and gravitational mass, things get a little easier to intuit, since it's just a complete reversal of the normal situation. Light objects sink to the ground, as the bouyant force pulls them down more than gravity pushes them away. Likewise, heavy objects float.

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  • $\begingroup$ Wouldn't both dense negative mass and less dense negative mass behave in the same direction since densities of -2 < -0.5 < 1? $\endgroup$ – 0xFFF1 Jun 3 '17 at 13:17

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