# Negative mass

1. Mar 11, 2005

### eNathan

Is there any theories or something of that sort, which allows negative mass? Hence, v > c rather than v < c. This seems like the only logical possibility to FTL travel.

2. Mar 11, 2005

Staff Emeritus
It's not m < 0 that allows v > 0; because the energy dilation formula is a function of $$\frac {1}{\sqrt {1 -\frac{v^2}{c^2}}}$$, the requirement is m^2 < 0. This means that mass has to be a pure imaginary number, a multiple of $$i = \sqrt{-1}$$.

Last edited: Mar 11, 2005
3. Mar 12, 2005

### Garth

It has nothing to do with superluminal velocities, but Fred Hoyle postulated a mass field theory, a version of his conformal gravity approach, where the value of m varied from event to event. The mass field went negative beyond a zero mass field surface. ("On the Origin of the Microwave Background", Ap.J. 196:661-670 1975 March 15) He postulated that as photons went from a -mass field to a +mass field region they were thermalised and became the microwave background, which is simply the light from galaxies beyond that zero-mass field surface.

Garth

4. Mar 13, 2005

### hellfire

In the cosmological models which postulate a phantom energy (a fluid with $p < - \rho$) leading to a big-rip, the dominant energy condition in general relativity is violated. As far as I know this implies FTL effects.

5. Mar 13, 2005

Staff Emeritus
I don't know if it IMPLIES FTL effects, I believe it is true that FTL effects break the weak energy condition. FTL => ~WE does not prove DE => FTL.

6. Mar 13, 2005

### Chronos

My understanding is consistent with SA's explanation. Imaginary, not negative mass is required for FTL velocities. But those are unphysical results. You can play other mathematical games and come up with concepts like negative or imaginary gravity, or light speed, with similar results. Just because a mathematical solution exists doesn't mean it has any real world meaning.

7. Mar 14, 2005

### hellfire

One can prove that a violation of the dominant energy condition implies that the energy flow can be superluminal. But may be this is not the same as matter being superluminal (e.g. a sound wave could be superluminal), isn't it?

8. Mar 14, 2005

### X-43D

Note is that any superluminal effect (which could transmit information!) could be described by this equation: E = mc2/(1-v2/c2)1/2

If v>c, then either the energy is imaginary (for real mass) or the mass is imaginary (for real energy). (imaginary, mathematically, meaning you have taken the square root of a negative number). So we will have to figure out what imaginary mass or energy is.

Last edited: Mar 14, 2005
9. Mar 14, 2005

### matt.o

it is imaginary, meaning it has no physical meaning.

10. Mar 14, 2005

### hellfire

I think this is not always the case, but I am not sure of that and this is actually what I was trying to say in my previous posts: in general relativity one can have superluminal propagation of (at least) energy flows with real positive energy densities, but with a negative pressure such that $p/c^2 < - \rho[/tex]. The arguments I have seen in some references are as follows: either, (a) you take [itex]dp/d\rho > c^2$ to be the (squared) speed of sound of a sound wave, or,

(b) you consider $T^{\mu}_{\nu} U^{\nu}$ to be a energy flow, which is spacelike if the dominant energy condition is violated ($U^{\nu}$ was timelike).

(OTOH it can be easily proven that the condition [itex]p/c^2 < - \rho[/tex], implies a violation of the dominant energy condition).

Last edited: Mar 14, 2005
11. Mar 14, 2005

### pmb_phy

Yes. As for an example, look at what is know as a "vacuum domain wall". Its active gravitational mass density is negative (due to negative pressure). An object made of normal matter will be gravitationaly accelerated away from the wall.

Pete

12. Mar 14, 2005

### eNathan

Are you saying that negative mass, or "negative gravity" has already been demonstrated? What are the exact physical requirements for negative mass?

13. Mar 14, 2005

Staff Emeritus
None of this stuff has been demonstrated. It's all theory - various different theories.

14. Mar 18, 2005

### pmb_phy

Its a prediction made using general relativity.

Pmb

15. Oct 12, 2006

### Edmond-Miksch

Reply to question abaout negative mass

Hi.
You might like to take a look at the webv site:

www.negative-mass.com

Ed Miksch

16. Oct 12, 2006

### Edmond-Miksch

Reply to question abaout negative mass

Hi.
You might like to take a look at the webv site:

www.negative-mass.com

Ed Miksch

17. Oct 12, 2006

### MeJennifer

How does GR theory predict that?

Or do you consider mathematically possible equivalent to prediction?

18. Oct 12, 2006

### pmb_phy

Yes. Einstein used to assume that the universe was static. But in order to have a static universe there must be a repulsive component to gravity on a cosmological scale. For this reason he introduced the cosmological constant. Later when evidence came in that the universe was not static but expanding he said of his mistake "That was the biggest blunder of my life!" But he spoke to soon!! Recently observations of the universe have indicated that not only is the universe exanding, but it is doing so at an accelerating rate! :surprised

The only thing that can cause this acceleration is either a positive cosmological constant or a large negative pressure. I don't think they know which yet. In any case this repulsive source of gravity can be refered to as negative mass or anti-gravity.

See - http://www.iop.org/EJ/abstract/1742-6596/8/1/016

Best wishes

Pete

Last edited: Oct 12, 2006
19. Oct 13, 2006

### MeJennifer

Sorry but I do not see how you could call that a prediction of GR.

Within the theory of GR there is no logic to the addition of such a constant it simply a hack to match the experimental data.

Until we can demonstrate that there is something like negative mass we should question GR like any other scientific theory.
Otherwise we might as well make an axiom that GR is eternally true and if we find that some experimental data do not match it must be caused by invisible pink unicorns.

Last edited: Oct 13, 2006
20. Oct 13, 2006

### pmb_phy

Okay. How would you phrase it?
And you believe that the universe is accelerating at an acccelerating ratre because....?

This acceleration is not unique to the current expansion. This negative pressure/mass is what drove inflation
That's just nuts! unicorns are white!

Pete

21. Oct 13, 2006

### MeJennifer

:rofl:

It could be some dark matter or it could be that GR is not quite as exact as some wish to believe it is.

Point is, we don't know for sure until we can demonstrate the existense of dark matter.
And demonstrate to me does not mean to a priori assume GR is correct and then pointing to some experimental data that disagrees with the theory and thus, QED, concluding that there must be dark matter.

22. Oct 13, 2006

### JesseM

Isn't negative pressure different from negative energy? Maybe negative energy would also tend to have negative pressure, but I didn't think that negative pressure would necessarily imply the presence of negative energy. http://www.physics.hku.hk/~tboyce/sf/topics/wormhole/wormhole.html [Broken] (written by two physicists who have 'collaborated on negative energy issues for over a decade' according to their bio at the bottom) says:

Last edited by a moderator: May 2, 2017
23. Oct 14, 2006

### pmb_phy

The active gravitational mass density is a function of the energy density as well as pressure. The pressure could be negative to the extent that the active gravitational mass would be negative.

Note: Einstein's equations were constructed so as to describe nature. When Einstein added a cosmological constant all he was doing was what he had been doing - Creating equations which described nature. The equations are now in the most general form they can be with this constant. I don't see this as an ad hoc way of constructing them.

Best wishes

Pete

24. Oct 14, 2006

### pmb_phy

That's alrady been done. WE can observer the presence of dark matter by observing the gravitational effects it has on galaxies
What is it you believe would constitute evidence of the existance of dark matter?

Best wishes

Pete

25. Oct 14, 2006

### lightarrow

So, it's not possible at all, in GR, to explain those galactic effects , without assuming the existence of dark matter?