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Old 01-13-2020, 10:27 AM
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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?
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Old 01-13-2020, 10:33 AM
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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; 01-13-2020 at 10:34 AM.
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Old 01-13-2020, 11:07 AM
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In before the real physicists.

Pick any equation with an "m" in it. Mangetout has capably handled f = ma and f = G(m1m2)/r2.

How about e = mc2? Now we have negative energy. look at every equation with an e ...
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Old 01-13-2020, 11:20 AM
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Mangetout has capably handled ... f = G(m1m2)/r2.
Now I'm not so sure. If m1 is the mass of the Earth and m2 is the mass of my negatively massed red rubber ball, then G(m1m2)/r2 produces negative force which is applied to a negative mass causing it to fall? Perhaps my discomfort in using f = G(m1m2)/r2 and e = mc2 in the same post was warranted.

m1 still has that weird habit of falling out of the calculation when you calculate acceleration.
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Old 01-13-2020, 11:24 AM
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m1 still has that weird habit of falling out of the calculation when you calculate acceleration.
That should be m2. I'll quit pretending I know what I'm talking about now.
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Old 01-13-2020, 11:25 AM
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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; 01-13-2020 at 11:26 AM.
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Old 01-13-2020, 11:30 AM
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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.
I light of this, my earlier posts should be understood to be the projectile case where m2 is enough smaller than m1 (in absolute value?) that the force on m1 can be ignored.
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Old 01-13-2020, 11:58 AM
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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.
Why would the positive mass be attracted to the negative mass? It seem that both should be repelled from each other. Plugging in a negative mass to the gravitational force equation for both masses will result in force vectors in opposite directions.
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Old 01-13-2020, 12:04 PM
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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?
https://www.npl.washington.edu/AV/altvw14.html by physicist John Cramer discusses negative mass.

" 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."
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Old 01-13-2020, 12:31 PM
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I'll just note that the relativistic result described there happens to be the same as the Newtonian result, and that the infinitely-accelerating behavior of the two particles, while extremely weird and counter-intuitive, 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 mass-dipole 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.
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Old 01-13-2020, 01:39 PM
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Such dipoles could potentially be made into perpetual motion machines. I see I'm not the only with this idea

Last edited by dtilque; 01-13-2020 at 01:39 PM.
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Old 01-13-2020, 01:47 PM
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How about e = mc2?
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 previously-held beliefs. Specifically, it might be more accurate to say that e=|m|c2 (energy equals the absolute value of the mass, times c squared)
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Old 01-13-2020, 01:53 PM
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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 high-school 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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Old 01-14-2020, 10:58 AM
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Why would the positive mass be attracted to the negative mass? It seem that both should be repelled from each other. Plugging in a negative mass to the gravitational force equation for both masses will result in force vectors in opposite directions.
Plugging in two negative masses does result in them repelling each other. But plugging in one negative and one positive mass results in the runaway acceleration described above.

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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Old 01-14-2020, 11:50 AM
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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 high-school 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.
Yeah, that (the former) just like describing my overdrawn bank balance as negative money isn't it? Not the same thing as would be actual physical money with the properties that are like money, but negative.
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Old 01-14-2020, 01:08 PM
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Quote:
Quoth Babale:

Plugging in two negative masses does result in them repelling each other.
We have to be careful with the terminology, here: Does "repel" refer to the direction of the force, or the direction of the acceleration? With ordinary masses, the distinction doesn't matter, because they're both the same direction. But with negative mass, the force and acceleration will be in opposite directions.

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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Old 01-14-2020, 01:24 PM
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We have to be careful with the terminology, here: Does "repel" refer to the direction of the force, or the direction of the acceleration? With ordinary masses, the distinction doesn't matter, because they're both the same direction. But with negative mass, the force and acceleration will be in opposite directions.

Two negative masses will each have a force pointing toward the other, but will each have an acceleration pointing away from each other.
This was my confusion in my original post. I quickly ran through the equations and found the force vectors going in opposite directions. I was not considering that acceleration and force would no longer be in the same direction. Thank you for the clarification Chronos.
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Old 01-14-2020, 04:35 PM
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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 previously-held beliefs. Specifically, it might be more accurate to say that e=|m|c2 (energy equals the absolute value of the mass, times c squared)
Nah, if negative mass exists, its kinetic energy needs to be negative as well. Because if you had an object composed of 50% positive mass and 50% negative mass, the net mass of the object is zero, and it takes an infinitesimally small force to accelerate it. So it makes sense that the net kinetic energy of this object is zero.
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