# Thread: Is negative mass possible?

1. ## Is negative mass possible?

. . .Not anti-matter, but mass that would weigh negative on a scale. Is there anything in the math or the theories that would prevent such a thing?

According to F=GMm/d^2, negative mass would be attractive to other negative mass, but repellent to positive mass (hence a positive mass scale would register it as negative).

Is there anything in the math that would prohibit it interacting with photons (which are massless)? It would be tachyonic--moving ever faster the less energetic it became. It would also experience time backwards, so a negative world would perceive the universe heading toward the Big Crunch.

Could it be responsible for dark energy?

2. Well, a couple of answers that I'll reserve the right to revise as time goes on.

No. Negative mass is not possible. Certainly no such thing has ever been observed. Dirac predicted the existence of anti-matter as an interpretation of the negative energy eigenvalue solutions to the Schroedinger equation. I beleive ( and I need to verify this) that there are no such equivalent solutions to Einstein's GR equations or to Hamiltonian formulations of classical mechanics.

Although photons are massless, they do feel the influence of gravity. In GR, gravity couples not to an object with a non-zero rest mass, but to an object with momentum and energy. Photons may have a zero rest mass, but they do have momentum and energy, and feel the influence of gravity.

This isn't to say the idea of negative mass is totally off the wall (only partially off the wall). It might explain some aspects of dark energy (although this is outside my area of expertise). The main argument against it is that:
A) It would require a total reformulation of GR that is not obviously necessary.
B) There is no observational evidence that would suggest it.

3. A) It would require a total reformulation of GR that is not obviously necessary.
How so? Wouldn't space curve in some different way if you stuck a negative mass/energy pole in it?

4. I always liken gravity to E&M.

charges can both be negative and positive. I like to think that mass can have the same property.

why don't we see negative mass? it's because there ain't any around here.

5. Well, back when Zarkov was still with us, he claimed L1 and L2 points for the earth-moon and earth-sun system which were incompatible with both bodies having positive mass. There was some talk about whether one body having positive mass and the other having negative mass would make Zarkov's numbers possible. However, I think that was rather more in the category of silly thought experiments...

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I know that all mass is composed of positive energy. I have also heard gravity refered to as negative energy, so I suppose if you except that notion that would make a graviton negative mass if such exists.

7. We have to take on a positivist approach. Equations solutions yield negative mass at times. Take the famous formula for the energy of a particle:

E2 = m2*c4 + p2*c2.

You see that you could have a positive or negative mass if the momentum is zero. The problem is that negative mass has no physical meaning (they are just constructs) and the negative solution must be discarded.

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Originally Posted by crosscountry
I always liken gravity to E&M.

charges can both be negative and positive. I like to think that
mass can have the same property.

why don't we see negative mass? it's because there ain't any
around here.
I've been thinking exactly that since the mid-1970s.
If matter and antimatter repel each other antigravitationally,
then they would not accumulate in the same galactic cluster
together. The repulsion could account for the acceleration
of the cosmic expansion.

This implies that photons and antiphotons are not identical.
Whatever the difference is, it would be seen in gravitational
lensing of antiphotons by ordinary-matter galaxies or lensing
of ordinary photons by antimatter galaxies. Where the usual
lensing creates large, bright arcs, I predict small, dim radial
lines in antigravity lensing.

It also implies that anti-hydrogen atoms should fall upward.
I'm looking forward to the first results from the Athena
experiment at CERN to show the direction of "fall".

Unfortunately, I don't forsee any practical use of the
antigravity property of antimatter-- if it exists-- because
of the near-impossibility of containing large amounts of it.
Any container would weigh far more than the anti-weight
of the contained antimatter.

-- Jeff, in Minneapolis

9. Originally Posted by crosscountry
I like to think that....
This is Why People Believe Weird Things.

10. We have used the concept of negative matter in Orion's Arm, for reactionless drives and for wormhole stabilisation; however matter with negative mass is looking increasingly unlikely so we are slowly replacing the idea of negative mass with the idea of negative energy, which apparently does exist.

Negative energy manifests itself in the Casimir force, and also in the Cosmological constant; positive mass and energy are supposed to be equivalent, so I suppose that negative mass and energy would be also equivalent if negative mass really were physical.

Negative Mass (sorry, its Wikipedia)
Some constraints on negative energy Ford and Roman's useful essay on negative energy.
A reactionless drive in Orion's Arm using negative mass, based on a fictional concept by Robert Forward Diametric Drive
Last edited by eburacum45; 2005-Nov-30 at 06:09 PM.

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## negative mass

Every month it's a good thing to come up with 100 crazy ideas or inventions. Then as you sort through them, , if you find one or two good ones, each month, you've really done well. The tricky part always comes in what to throw away. This is one of them. Ciao. Pete.

12. Originally Posted by Jeff Root
I've been thinking exactly that since the mid-1970s.
If matter and antimatter repel each other antigravitationally,
then they would not accumulate in the same galactic cluster
together. The repulsion could account for the acceleration
of the cosmic expansion.

This implies that photons and antiphotons are not identical.
Whatever the difference is, it would be seen in gravitational
lensing of antiphotons by ordinary-matter galaxies or lensing
of ordinary photons by antimatter galaxies. Where the usual
lensing creates large, bright arcs, I predict small, dim radial
lines in antigravity lensing.

It also implies that anti-hydrogen atoms should fall upward.
I'm looking forward to the first results from the Athena
experiment at CERN to show the direction of "fall".

Unfortunately, I don't forsee any practical use of the
antigravity property of antimatter-- if it exists-- because
of the near-impossibility of containing large amounts of it.
Any container would weigh far more than the anti-weight
of the contained antimatter.

-- Jeff, in Minneapolis

my understanding is that antimatter actually has positive mass. thus it would be drawn to normal mass.

when the two contact.... photons pop up.

13. I suppose that anti-negative matter would also be possible, which would annihilate negative matter and produce a burst of negative energy...

negative matter would annihilate with ordinary matter to produce nothing; nada, zip.

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Originally Posted by crosscountry
my understanding is that antimatter actually has positive mass.
thus it would be drawn to normal mass.
It is known to have positive inertial mass. Application of the
principle of equivalence implies that it must also have positive
gravitational mass. A "+/-" symbol could be stuck onto the
principle of equivalence to keep it intact if it turns out that
antimatter and ordinary matter repel each other gravitationally.
It would twist the geometrical interpretation of general
relativity a bit, but I suspect that everyone would adapt very
quickly. It would extend GR, not destroy or even change it.

when the two contact.... photons pop up.
I'm suggesting that one is an ordinary photon and the other is
an antiphoton. The two might possibly be distinguished by their
opposite frequency shifts when launched vertically in Earth's
gravity, although I suspect that the original frequencies can't
be determined anywhere near precisely enough for that.

-- Jeff, in Minneapolis

15. Negative mass is currently considered possible. Same properties as mass, IIRC - inertia, etc. - but would generate repulsive rather than attractive gravitational fields.

16. Some conceptions of negative matter describe it as having negative inertia too; that is to say if you push it, it moves in the opposite direction to the way you push it, i.e. towards you.
Additionally negatively charged negative matter would be attracted to a negative charge; so the charge characteristics would be interesting.

17. Originally Posted by crosscountry
my understanding is that antimatter actually has positive mass. thus it would be drawn to normal mass.
Probably, but it hasn't been experimentally confirmed. From here:

http://www2.corepower.com:8080/~relf...ntimatter.html

If you believe that General Relativity is the exact true theory of gravity, then there is only one possible conclusion - by the equivalence principle, antiparticles must fall down with the same acceleration as normal matter.

On the other hand: there are other models of gravity which are not ruled out by direct experiment which are distinct from GR in that antiparticles can fall down at different rates than normal matter, or even fall up, due to additional forces which couple to the mass of the particle in ways which are different than GR. Some people don't like to call these new couplings 'gravity.'

So ... just maybe, but I wouldn't bet on it.

18. The stress-energy is positive definite for antimatter, but I'm not sure that anyone has proven that antimatter and matter are attractive gravitationally. GR predicts it, but there are other notions out there that predict the opposite.

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## anti-matter matter gravitational interaction

With the huge difference between electromagnetic effects, and gravitational effects, some decades of orders of magnitude, it is highly unlikely that the gravitational effects could be separated from the signal noise in an experiment with a few atoms, or baryons of each ....from a statistical population perhaps....LEP once found that tiny drifting of the beam, and it's subsequent corrections by the shift operator, was associated with the position of the moon, and introduced an algorithm to compensate therein. Pete.

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Originally Posted by trinitree88
LEP once found that tiny drifting of the beam, and its subsequent
corrections by the shift operator, was associated with the position
of the moon, and introduced an algorithm to compensate therein.
That was caused by the Moon's tidal effect on the storage ring
and the ground it was built on, rather than directly on the
particles circulating inside the ring. The whole ring flexed
by a couple of millimeters-- less than the beam thickness.

-- Jeff, in Minneapolis

21. Originally Posted by Jeff Root
That was caused by the Moon's tidal effect on the storage ring
and the ground it was built on, rather than directly on the
particles circulating inside the ring.
Indeed, the direct gravitational effect (i.e., not tidal) of the Sun would be vastly greater, so if there were any direct compensating to do, that would come first.

On a different point, eburacum45 made an interesting observation that if inertial mass is the same as gravitational mass, it would have the same sign. But what has not been recognized is the paradox that would result from this possibility. Negative mass objects would have repulsive gravitational fields to both positive and negative masses, while positive mass objects would attract both. Sound like a paradox? Sure is! It violates the law of conservation of momentum-- a pair of positive and negative masses would begin accelerating in the direction of the positive mass. This seems like an impossibility to a higher level than negative mass itself, so I would suspect that even if negative gravitational mass is possible, negative inertial mass is not. This also means you lose the equivalence principle. So in summary, if negative gravitational mass can exist, then either you have no equivalence principle, or you have no conservation of momentum.

22. pair of positive and negative masses would begin accelerating in the direction of the positive mass. This seems like an impossibility to a higher level than negative mass itself, so I would suspect that even if negative gravitational mass is possible, negative inertial mass is not.
How so? You're forgetting that negative mass has negative momentum. The two add to zero as they accelerate in the same direction.

23. Originally Posted by ASEI
How so? You're forgetting that negative mass has negative momentum. The two add to zero as they accelerate in the same direction.
Excellent point, I am indeed forgetting that. What I should have said is that the principle of conservation of momentum provides no useful constraint on a system comprising a positive and negative mass of equal magnitude, as they can move about willy nilly with no momentum requirements. The same holds for their energy and angular momentum-- the normal constants of motion simply have nothing to say about the motion of such a system, and that seems so contrary to what makes sense that it seems to me that positive inertia is a requirement we should be loathe to part with, I'd wager it is the equivalence principle that would have to go if the universe gives us particles with negative gravitational mass.

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Originally Posted by Ken G
... I would suspect that even if negative gravitational mass is
possible, negative inertial mass is not. This also means you lose
the equivalence principle. So in summary, if negative gravitational
mass can exist, then either you have no equivalence principle, or
you have no conservation of momentum.
I'm assuming that there is no such thing as negative inertial
mass. My unconventional hypothesis is that ordinary matter and
antimatter repel each other gravitationally. Like attracts like
and opposites repel. That would require a modification to the
principle of equivalence, and a shift in the way the geometric
interpretation of general relativity is described, but I think
that both would remain completely intact-- the existence of a
gravitational repulsion would require an extension to GR, but
not really a change in it. Similar to how the idea of negative
numbers in mathematics was an extension of the system of natural
numbers, not a fundamental change. Or how Riemann geometry was
an extension of Euclidean geometry.

-- Jeff, in Minneapolis

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