# Negative mass

1. Nov 17, 2004

### Matrixman13

Ive heard that its believed that a particle with a negative mass, could "in theory" travel faster than light.

my question is, what does negative mass mean? how does it differ from a positive mass, or having no mass at all?

2. Nov 18, 2004

### mathman

Negative mass (gravity repels) and faster than light travel are theoretical solutions to relativity equations. There is no evidence for either.

3. Nov 19, 2004

### ohwilleke

A fairly heretical view is found here:
http://users.primushost.com/~ewall/

More common than negative mass as theoretical proposals (again, no evidence for such things) is negative energy. We are all familiar with the trade off in electrical fields and graviational fields between kinetic and potential energy. We also know that it is possible to design a coordinate system for measuring potential energy which produces negative energy in that coordinate system.

For example, suppose you have a system of a ball and a fictionless valley. It you assign zero potential energy to the bottom of the valley, a ball rolling from the rim of the valley has a high positive potential energy at the rim of the valley, that converts to a high kinetic energy and no potential energy at the bottom of the valley, and then returns to high potential energy and no kinetic energy at the rim on the other side of the valley.

But, nothing in Newtonian physics prevents you from declaring that the point of zero potential energy is at the top of the valley, in which case a ball rolling to the bottom of the valley has a large negative potential energy to match its kinetic energy.

In GR, there are real problems to having a Newtonian style relative potential energy scale, as opposed to an absolute potential energy scale. This follows because in GR E=mc^2, and energy as well as mass gravitates. You can't determine how much space is bent by matter and energy, creating gravity, unless you have an absolute value for energy in a particular volume.

But, there is no obvious mechanism in GR for setting a ground state, and GR does not expressly require that E always be positive.

For a long time, the idea of negative energy pretty much went to the waste bin of nonphysical alternate solutions of the GR equations. But, the apparent discover that the universe is not only expanding, but expanding at an increasing rate, has renewed that discussion. The empirical result, interpreted naiively, at least, makes it appear that there is some repulsive fifth force at work in the world counteracting gravity.

One way to explain this result is that the vacuum is filled with "dark energy" which is equivalent to the "cosmological constant" in the GR equations. The trouble is that it has proven difficult to reconcile local quantum mechanical ideas about "zero point energy fields" with experimental results in current theoretical frameworks (which a deeply related to the idea of a "ground state" for calculating energy). A dark energy approach is essentially a "pull approach" to the apparent repulsion. The mass of the dark energy in the vaccuum halo around the universe pulls it apart.

A negative energy scenario would be basically a "push approach" to the same result. If energy is negative in the core of the universe then it would have a repulsive effect pushing the expansion of the universe.

Last edited by a moderator: Apr 21, 2017
4. Nov 20, 2004

### Mk

Also, particles with imaginary rest mass, as a value of i, are called tachyons and would travel faster than light. Exotic matter is a form that has negative gravity, meaning objects are repeled rather than sucked in by its gravity.