# Negative mass

1. Dec 1, 2004

Anyone tried to imagine a world with negative mass? A mixed world of normal mass and negative mass does not seem so bad in the sense that a negative mass moon would surprisingly still orbit the earth! Might be problems in making atoms though......

2. Dec 1, 2004

### pmb_phy

Negative mass has been discussed in the physics literature. Some describe the accelerating expansion of the universe to be due to negative mass (and that mass is called "cosmological constant" matter or "dark energy").

There was a well known article on this subject published back in 1957

"Negative Mass in General Relativity," H. Bondi, Reviews of Modern Physics, Vol. 29 ,No. 3, pp 423-428 (1957)

Pete

3. Dec 1, 2004

### ceptimus

If the Moon had a negative mass, but the Earth a positive one, then the force of gravity between them would be a repulsive one - the Moon would be pushed farther and farther away. I don't see why the O.P. said it would still orbit - unless he meant that the Earth would have a negative mass as well.

4. Dec 1, 2004

### chronon

The force would be repulsive, but acting on a negative mass it would accelerate the moon towards the Earth.

5. Dec 1, 2004

### ceptimus

Hmmmm... I get it. But it would still push the Earth away from the moon. Right?

In fact this leads to a seeming absurdity. If two bodies had no initial relative velocity, then the negative mass one would fall towards the positive mass, while the positive mass would 'fall' away from the negative one.

If the two masses had the same magnitude, but opposite signs, they would maintain their initial separation, while accelerating away across the universe.

6. Dec 1, 2004

### chronon

Right.

Yes, but of course their total kinetic energy would remain zero, (since their total mass would be zero).

I had a look on the web and found the following page on this subject

http://www.concentric.net/~pvb/negmass.html [Broken]

Last edited by a moderator: May 1, 2017
7. Dec 1, 2004

### pervect

Staff Emeritus
Yes, though you would need perfect balance to keep one object from accelerating more than the other. As others have pointed out, both momentum and energy are conserved, because the positive energy and momentum of the positive mass is exactly balanced by the negative energy and momentum of the negative mass.

There are more/similar absurdities in sight, though. Gas made out of negative mass matter (usually called exotic matter) would have a negative temperature. This would mean, thermodynamically, that a gas made out of exotic matter interacting with a gas made out of normal matter would lead to a runaway situation - the gas with the normal matter would gain a large amount of positive energy, the gas with the exotic matter would gain a large amount of negative energy.

See for instance the sci.physics.faq on negative temperature

here

(it doesn't talk about negative mass, just negative temperature).

For a while I thought this situation meant that negative mass would violate the second law of thermodynamics, but that's not quite right. It does lead to the runaway situation I described, but this sort of runaway doesn't violate the second law, if you read the fine print...
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