Is Antigravity Possible with Negative Energy Density Matter?

[Anael]

Hi, I am a first year physics student and i was just wondering, why is anti Gravity impossible? (iv'e read it many times without clear explanation). if an anti graviton would exist (IF a graviton) would it have negative mass.

 

[ansgar]

who has claimed it?

 

[tom.stoer]

What you mean by anti-gravity? Something like a repulsive force as in electromagnetism?

A simple explanation is that gravity is always attractice as there is no "negative mass". In terms of gravitons (which should be massless particles as the range of the force is infinite; or, as the potential is ~1/r) this is due to the fact that gravitons should be spin-2 particles which corresponds to the fact that classical gravitational waves are quadrupol waves. Compare this to electromagnetism: photons are spin-1 particles and el.-mag waves are dipol waves.

 

[Anael]

has it been experimentally or mathematically proven that there is no possibility of a "negative mass"??

 

[mate0]

to understand why there is no possibility of antigravity you have to remember that gravity is mass warping space, not attracting normally as a magnetic field would. All matter and antimatter warps space the same way.

 

[Anael]

ok, that makes a lot of sense thanks :P

just being curious (and stubborn), there is not even a minute possibility that something might warp space differently?? so far in our universe there is no indication to such a thing?

 

[mate0]

as far as i know there has never been the slightest indication of antigravity from a credible source.

 

[Anael]

thats too bad, i already wasted my money on a Delorian :P Lolz

 

[tom.stoer]

there are a lot of assumptions ...

If you look at the cosmological constant you see that it warps space differently which leads to accelarated expansion of the universe - instead of slowed down expansion as you expected for ordinary matter. Now there are attempts to interpret the cosmological constant as "dark energy", which means energy that obeys a rather strange equation of state (you will find phantom energy, Chaplygin gas etc. in the literature).

But this interpretation as "dark energy" is equivalent with "antigravitating energy".

 

[Anael]

there are a lot of assumptions ...

If you look at the cosmological constant you see that it warps space differently which leads to accelarated expansion of the universe - instead of slowed down expansion as you expected for ordinary matter. Now there are attempts to interpret the cosmological constant as "dark energy", which means energy that obeys a rather strange equation of state (you will find phantom energy, Chaplygin gas etc. in the literature).

But this interpretation as "dark energy" is equivalent with "antigravitating energy".

interesting, thanks! ill read into that. (here's hoping i can understand 1% :P)

 

[Nabeshin]

ok, that makes a lot of sense thanks :P

just being curious (and stubborn), there is not even a minute possibility that something might warp space differently?? so far in our universe there is no indication to such a thing?

People have probably been considering the possibility ever since Einstein wrote down his field equations. In case you're not familiar, in words they say the curvature of spacetime is caused by the distribution of mass, energy, momentum, etc. This is the way we typically think about it: a given distribution of mass (etc.) causes space to curve into some shape.

However, we can imagine looking at it the other way around and asking: what distribution of mass, energy, etc. will produce a given curvature of spacetime? In essence, this is exactly what Alcubierre did when he invented his famous "warp drive", a spacetime which allows objects to travel at many multiples of c (the details aren't important, but suffice it to say that they don't go faster than c locally, but to an outside observer). Unforunately, we also find that to produce such a spacetime geometry, we need highly exotic forms of mass, energy, etc. Specifically, the matter distribution that creates this geometry violates what are known as energy conditions which we believe to be true of our universe (they include things like mass is always positive).

So, people certainly think about it, and it's possible that in the future we may come to understand these subjects better and the energy conditions might not be the final story (some quantum mechanical discoveries already suggest this may be the case).

Links:
http://en.wikipedia.org/wiki/Alcubierre_drive
http://en.wikipedia.org/wiki/Energy_conditions

 

[Antiphon]

There's a big difference between nullifying gravity and having a propellantless propulsion system. Both ate coloquially referred to as antigravity.

As a physics problem, all you need is a negative energy density.

The "simple" way to visualize this is like so: reverse time. Then gravity becomes repulsive just like electric charge but weaker. If you then have positive energy density and negative energy density you can work out what antigravity would be like. Negative energy-density matter would be repelled by ordinary matter but be attracted to other negative energy density matter.

We don't know how to create matter with a negative energy density.