Negative Mass: Possibility of FTL Travel?

[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.

 

[selfAdjoint]

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.

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 $$i = \sqrt{-1}$$.

 

[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

 

[hellfire]

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.

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.

 

[selfAdjoint]

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.

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.

 

[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.

 

[hellfire]

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.

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?

 

[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.

 

[matt.o]

it is imaginary, meaning it has no physical meaning.

 

[hellfire]

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.

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 [itex]p/c^2 < - \rho[/tex]. The arguments I have seen in some references are as follows: either,

(a) you take $dp/d\rho > c^2$ 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).

 

[pmb_phy]

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.

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

 

[eNathan]

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

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

 

[selfAdjoint]

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

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

 

[pmb_phy]

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

Its a prediction made using general relativity.

Pmb

 

[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

 

[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

 

[MeJennifer]

Its a prediction made using general relativity.

How does GR theory predict that?

Or do you consider mathematically possible equivalent to prediction?

 

[pmb_phy]

How does GR theory predict that?

Or do you consider mathematically possible equivalent to prediction?

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!

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 referred to as negative mass or anti-gravity.

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

Best wishes

Pete

 

[MeJennifer]

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!

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 referred to as negative mass or anti-gravity.

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

Best wishes

Pete

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.

 

[pmb_phy]

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

Okay. How would you phrase it?

Until we can demonstrate that there is something like negative mass we should question GR like any other scientific theory.

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

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.

That's just nuts! unicorns are white!

Pete

 

[MeJennifer]

That's just nuts! unicorns are white!

:rofl:

And you believe that the universe is accelerating at an acccelerating ratre because...?

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.

 

[JesseM]

This acceleration is not unique to the current expansion. This negative pressure/mass is what drove inflation

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 (written by two physicists who have 'collaborated on negative energy issues for over a decade' according to their bio at the bottom) says:

One should also not confuse negative energy with the energy associated with the cosmological constant, postulated in inflationary models of the universe [see "Cosmological Antigravity, by Lawrence M. Krauss; SCIENTIFIC AMERICAN, January 1999]. Such a constant represents negative pressure but positive energy. (Some authors call this exotic matter; we reserve the term for negative energy densities.)

 

[pmb_phy]

Isn't negative pressure different from negative energy?

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

 

[pmb_phy]

Point is, we don't know for sure until we can demonstrate the existense of dark matter.

That's alrady been done. WE can observer the presence of dark matter by observing the gravitational effects it has on galaxies

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.

What is it you believe would constitute evidence of the existence of dark matter?

Best wishes

Pete

 

[lightarrow]

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

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

 

[MeJennifer]

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

That is correct.
So either the theory is incomplete or wrong or there is something we have not taken into account, for instance something like dark matter.

 

[MeJennifer]

That's alrady been done. WE can observer the presence of dark matter by observing the gravitational effects it has on galaxies

No we don't, we infer the presence of dark matter by accounting for the differences between what GR predicts and what we measure.

 

[JesseM]

The active gravitational mass density is a function of the energy density as well as pressure.

But what is the pressure a function of?

The pressure could be negative to the extent that the active gravitational mass would be negative.

You're saying the physicists who wrote that article are wrong, and that it's not possible to have negative pressure with a positive energy density?

 

[JesseM]

No we don't, we infer the presence of dark matter by accounting for the differences between what GR predicts and what we measure.

In fact there was recently some more direct evidence for the presence of dark matter, involving a cluster of dark matter (seen by gravitational lensing) that had overshot the matter it was associated with and was thus in a somewhat different position. See here for a good summary.

 

[pmb_phy]

No we don't, we infer the presence of dark matter by accounting for the differences between what GR predicts and what we measure.

Almost all observation is accomplished in such a manner, i.e. we don't directly observer something, we indirectly observe it. And we do so in as many ways that we can. When our observations are consistent with our theory of what is going on then it is said that evidence has been provided. Something important to understand in physics is that it is not the goal to prove things in physics.

Have you ever observed an electron? If not then how do you know they exist?

Pete

 

[pmb_phy]

But what is the pressure a function of?

I wish I knew. I'd be rich!

You're saying the physicists who wrote that article are wrong, and that it's not possible to have negative pressure with a positive energy density?

No. That is not what I'm saying. The active gravitational mass density = energy density + 3*pressure (c=1). For the the active gravitational mass to be negative you must have


energy density + 3*pressure < 0

So the energy density can be positive and if the pressure density is far enough into the negative then it will overwhelm the energy density and make the active gravitational mass density negative.

Pete

 

[JesseM]

No. That is not what I'm saying. The active gravitational mass density = energy density + 3*pressure (c=1). For the the active gravitational mass to be negative you must have


energy density + 3*pressure < 0

So the energy density can be positive and if the pressure density is far enough into the negative then it will overwhelm the energy density and make the active gravitational mass density negative.

Pete

Ah, thanks, I hadn't caught the distinction you were making between the energy density and the active gravitational mass. So in the case of dark energy, it's thought that the pressure is negative while the energy density is positive, which I guess would mean the active gravitational mass could be either negative or positive depending on the balance between them.

 

[pmb_phy]

Ah, thanks, I hadn't caught the distinction you were making between the energy density and the active gravitational mass. So in the case of dark energy, it's thought that the pressure is negative while the energy density is positive, which I guess would mean the active gravitational mass could be either negative or positive depending on the balance between them.

Yup. In a vacuum domain wall the active gravitational mass is negative. For an infinitely long straight cosmic string the active gravitational mass is zero!

Pete

 

[taylaron]

aside---
a negitive mass object would revolutionize weight limmited objects such as aircraft and many many other applications.
this would definately be usefull. if we can produce a working model though,
until then... happpy theorizing...
you'll never fully know if it will work until you actually do it.

 

[pmb_phy]

aside---
a negitive mass object would revolutionize weight limmited objects such as aircraft and many many other applications.
this would definately be usefull. if we can produce a working model though,
until then... happpy theorizing...
you'll never fully know if it will work until you actually do it.

That there is a negative active gravitational mass at work in the cosmos cannot be taken to imply that it can be applied to a finite body. No hovercrafts on this account.

Pete