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#1




Negative mass
I see negative mass invoked for wormholes and warp drives, but does such a substance actually exist? What sort of properties would large quantities of it exhibit? Would a ball of it fall up?

#2




AFAIK, it doesn't exist, and it would do strange things; falling up being among the lesser of them.
Pushing on it in the direction of motion, when it was in motion would make it slow down, for example. Last edited by Mangetout; 01132020 at 10:34 AM. 
#3




In before the real physicists.
Pick any equation with an "m" in it. Mangetout has capably handled f = ma and f = G(m_{1}m_{2})/r^{2}. How about e = mc^{2}? Now we have negative energy. look at every equation with an e ... 
#4




Now I'm not so sure. If m_{1} is the mass of the Earth and m_{2} is the mass of my negatively massed red rubber ball, then G(m_{1}m_{2})/r^{2} produces negative force which is applied to a negative mass causing it to fall? Perhaps my discomfort in using f = G(m_{1}m_{2})/r^{2} and e = mc^{2} in the same post was warranted.
m_{1} still has that weird habit of falling out of the calculation when you calculate acceleration. 


#5




That should be m_{2}. I'll quit pretending I know what I'm talking about now.

#6




As I understand it, the equations work out so that a negative mass object accelerates away from a positive mass object. Meanwhile, said positive mass object is accelerated at the same rate towards the negative object. This would theoretically lead to both objects accelerating away forever, reaching arbitrarily high speeds for "free".
This all depends on negative mass actually existing. Which is by no means guaranteed. Last edited by Babale; 01132020 at 11:26 AM. 
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" But Newton's theory of gravity can't really be used as a reliable guide to the effects of negative mass, because we know that it is only an approximation to the best gravity theory we have, Einstein's general theory of relativity. Fortunately for this discussion general relativity was used in the late 1950's by the British physicist Sir Hermann Bondi to investigate the effects of negative mass. Bondi pointed out that when general relativity is considered purely as a theory of gravity, mass never actually appears. It first appears when the equations are solved in a way devised by the German physicist K. Schwartzschild. Then mass appears as a constant of integration. Bondi noticed that this mass constant could be made either positive or negative. He was able to show that when m is made negative, both the inertial and the gravitational mass effects are reversed. The results of Bondi's calculations can be summarized in a few words: a positive mass attracts all nearby masses whether positive or negative; an negative mass repels all nearby masses whether positive or negative. ... There is a curious corollary of this result, which Bondi pointed out in his paper. Consider a pair of equal and opposite positive and a negative mass placed close to each other. The negative mass is attracted to the positive mass, while the positive mass is repelled by the negative mass. Thus the two masses will experience equal forces and accelerations in the same direction (in violation of Newton's third law) and the system of two particles will accelerate, seemingly without limit. The negative mass will chase the positive mass with constant acceleration." 


#10




I'll just note that the relativistic result described there happens to be the same as the Newtonian result, and that the infinitelyaccelerating behavior of the two particles, while extremely weird and counterintuitive, does not actually violate Newton's 3rd Law. The accelerations of the particles are in the same direction, but the forces on them are in opposite directions, just as Newton would have predicted.
As to whether such a substance can exist, the smart money is no. If any did exist in the observable Universe, then we'd get dipoles as described above zipping around all over the place, and that would be the sort of thing that'd be hard to fail to notice. That still leaves the possibility that such objects can come into existence, but the process for it to happen is extremely difficult, such that the conditions for it are not right anywhere in the observable Universe... but then one must ask why such conditions would be so difficult. Typically, when objects can exist but are very rare, it's because a great deal of concentrated energy would be required to produce them, but the energy required to create a massdipole is the exact opposite of a great deal: Such a dipole could in fact spontaneously come into existence with no expenditure of energy at all. It's still conceivable, of course, that such objects can exist, and could be brought into existence, but that there is some difficulty in such a process other than energy. But like I said, that's not the way to bet. 
#12




My WAG is that if we do ever discover something with negative mass, and have the opportunity to examine its properties, we may have to revise some of our previouslyheld beliefs. Specifically, it might be more accurate to say that e=mc^{2} (energy equals the absolute value of the mass, times c squared)

#13




Eh, negative energy isn't actually all that big a deal, and in fact one often deals with negative energy in physics, even at the highschool level. For instance, if one says that a rock sitting on the ground has zero potential energy, then logically, that same rock at the bottom of a hole must have negative potential energy. And in fact, in a more detailed view, the most usual accounting for gravitational potential energy is that it is always negative.
Now, the total energy in some finite region of space, that's much harder to make negative. 
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Asking why this is doesn't make a whole lot of sense. That's just the result you get when you plug one negative and one positive mass into the equation that we have found best describes how objects interact through gravity based on observation. It is entirely possible that negative mass doesn't exist, and this is just an artifact of the math involved. It is also possible that negative mass can exist, and one day we will create some and find it behaves differently than we expected, which would mean we need to adjust our equations. 


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Two negative masses will each have a force pointing toward the other, but will each have an acceleration pointing away from each other. 
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