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eNathan
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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.
eNathan said: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 [itex]p < - \rho[/itex]) leading to a big-rip, the dominant energy condition in general relativity is violated. As far as I know this implies FTL effects.eNathan said: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.
hellfire said:In the cosmological models which postulate a phantom energy (a fluid with [itex]p < - \rho[/itex]) leading to a big-rip, the dominant energy condition in general relativity is violated. As far as I know this implies FTL effects.
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?selfAdjoint said: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.
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,X-43D said: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.
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.eNathan said: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.
pmb_phy said: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 said: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.eNathan said:Are you saying that negative mass, or "negative gravity" has already been demonstrated? What are the exact physical requirements for negative mass?
How does GR theory predict that?pmb_phy said:Its a prediction made using general relativity.
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!MeJennifer said:How does GR theory predict that?
Or do you consider mathematically possible equivalent to prediction?
Sorry but I do not see how you could call that a prediction of GR.pmb_phy said: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
Okay. How would you phrase it?MeJennifer said:Sorry but I do not see how you could call that a prediction of GR.
And you believe that the universe is accelerating at an acccelerating ratre because...?Until we can demonstrate that there is something like negative mass we should question GR like any other scientific theory.
That's just nuts! unicorns are white!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.
:rofl:pmb_phy said:That's just nuts! unicorns are white!
It could be some dark matter or it could be that GR is not quite as exact as some wish to believe it is.pmb_phy said:And you believe that the universe is accelerating at an acccelerating ratre because...?
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:pmb_phy said:This acceleration is not unique to the current expansion. This negative pressure/mass is what drove inflation
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.)
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.JesseM said:Isn't negative pressure different from negative energy?
That's alrady been done. WE can observer the presence of dark matter by observing the gravitational effects it has on galaxiesMeJennifer said:Point is, we don't know for sure until we can demonstrate the existense of dark matter.
What is it you believe would constitute evidence of the existence 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.
So, it's not possible at all, in GR, to explain those galactic effects , without assuming the existence of dark matter?MeJennifer said:It could be some dark matter or it could be that GR is not quite as exact as some wish to believe it is.
That is correct.lightarrow said:So, it's not possible at all, in GR, to explain those galactic effects , without assuming the existence of dark matter?
No we don't, we infer the presence of dark matter by accounting for the differences between what GR predicts and what we measure.pmb_phy said:That's alrady been done. WE can observer the presence of dark matter by observing the gravitational effects it has on galaxies
But what is the pressure a function of?pmb_phy said:The active gravitational mass density is a function of the energy density as well as pressure.
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?pmb_phy said:The pressure could be negative to the extent that the active gravitational mass would be negative.
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.MeJennifer said: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.MeJennifer said:No we don't, we infer the presence of dark matter by accounting for the differences between what GR predicts and what we measure.
I wish I knew. I'd be rich!JesseM said:But what is the pressure a function of?
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 haveYou'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?
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 said: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
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!JesseM said: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.
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.taylaron said: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.