Revolutionary Concepts in Physics

[NeutrinoWave]

So LHC has shed new light over Matter imbalance over Anti Matter

I am curious if any of the following stacks up ... please let me know your thoughts

[1] Could AntiGravity be considered as a candidate for Dark Energy

Where in the Universe could you find physical candidates for

[2] Negative Pressure

[3] Negative Energy

[4] Negative Mass?

[5] AntiMatter Repulsion

[6] is AntiMatter Repulsion influenced by Antigravity

[7] how many of the above are attributes of a traverseable wormhole

[8] how many of the above are attributes of time travel

intrigued by the minds of the wacky physicists on the forum ...

[JulianO]

So LHC has shed new light over Matter imbalance over Anti Matter

Oh has it - I must have missed that

I am curious if any of the following stacks up ... please let me know your thoughts

[1] Could AntiGravity be considered as a candidate for Dark Energy

Well a) what is anti-gravity I don't think we've seen anything like it so far. If it was anti-gravity then it would tend to cancel out gravity. Gravity comes from matter, but anti-matter doesn't generate anti-gravity - just regular gravity.

Where in the Universe could you find physical candidates for

[2] Negative Pressure

[3] Negative Energy

[4] Negative Mass?

So far only in wild theories.

[5] AntiMatter Repulsion

Not sure I understand what this is.

[6] is AntiMatter Repulsion influenced by Antigravity

[7] how many of the above are attributes of a traverseable wormhole

[8] how many of the above are attributes of time travel

intrigued by the minds of the wacky physicists on the forum ...

As [6] is based on stuff we haven't seen being influenced by something else we haven't seen its difficult to extrapolate.

[George Jones]

In Newtonian gravity, the gravitational field depends on the amount of matter, but not on what the matter is doing. In general relativity, roughly because of the equivalence of energy and matter, the strength of the gravitational does depend on what the matter is doing.

For example, consider the gravitational field of a massive sphere. According to Newton, the strength of the gravitational field outside the sphere does not depend on the whether or not the sphere is spinning. According to Einstein, the strength of the gravitational field outside the sphere doe depend on the rate of spin of the sphere (roughly, mass + energy of rotational motion).

Also, according to Einstein, the pressure in matter contributes to gravity.

[1] Could AntiGravity be considered as a candidate for Dark Energy

Dark energy has positive energy/mass density and negative pressure. Negative pressure acts as repulsive gravity that drives stuff apart, which some folks might term antigravity.

Where in the Universe could you find physical candidates for

[2] Negative Pressure

[3] Negative Energy

[4] Negative Mass?

[2], [3], and [4] are somewhat related. For example, if some stuff has, as measured by observer A, positive mass (or energy density) and negative pressure with

|pressure| > energy density (or mass density times c^2)

then there will be another observer B (moving with respect to A) that measure the same stuff to have negative mass (or energy) density!

Quantum field theory predicts the possibility of negative energy density. See

Casimir Effect.

Stuff that, for some observer, has negative matter or energy density is called exotic matter.

[5] AntiMatter Repulsion

I'm not sure what you mean by this. Normal antinatter generates attractive gravity. Exotic antimatter generates repulsive gravity.

[6] is AntiMatter Repulsion influenced by Antigravity

[7] how many of the above are attributes of a traverseable wormhole

A traversable wormholes requires exotic matter or energy to hold the wormhole open.

[8] how many of the above are attributes of time travel

None. There are theoretical universes that:

1) allow time travel;

2) have zero energy and matter content;

3) have zero spacetime curvature.

[edit]While I was thinking and typing, both of which I do painfully slowly, I didn't notice that JulianO also posted a reply.[/edit]

[JulianO]

Dark energy has positive energy/mass density and negative pressure. Negative pressure acts as repulsive gravity that drives stuff apart, which some folks might term antigravity.

In one sense it might be considered antigravity, but I think unless it has the same form as gravity with the signs reversed it isn't really. Its like saying hydrogen is anti gravity because it opposes gravity when contained.

I'm not sure what you mean by this. Normal antinatter generates attractive gravity. Exotic antimatter generates repulsive gravity.

I haven't heard of this - can you give a reference? All the forms of matter I've heard of have positive gravity.

[George Jones]

To start, a couple of (supposedly) popular-level books, both of which give accurate technical details, and both of which have useful reference lists that include research literature.

For wormholes:

The Physics of Stargates: Amazon.co.uk: Enrico Rodrigo: Books

For time travel:

Time Machines: Time Travel in Physics, Metaphysics, and Science Fiction: Amazon.co.uk: Paul J. Nahin: Books

The best comprehensive (very) technical reference for wormholes (though now somewhat out-of-date) is

Lorentzian Wormholes: From Einstein to Hawking AIP Series in Computational and Applied Mathematical Physics: Amazon.co.uk: Matt Visser: Books

Dark energy has positive energy/mass density and negative pressure. Negative pressure acts as repulsive gravity that drives stuff apart, which some folks might term antigravity.

In one sense it might be considered antigravity, but I think unless it has the same form as gravity with the signs reversed it isn't really. Its like saying hydrogen is anti gravity because it opposes gravity when contained.

Personally, I wouldn't call the repulsive effect of dark energy "antigravity", but in some articles these terms do appear together.

What do you mean by "the same form as gravity with the signs reversed"? Our best theory of gravity is Einstein's theory general relativity, so "the same form as gravity" would seem to mean governed by Einstein's equation of general relativity. The source of gravity in Einstein's equation is the stress-energy tensor, and the stress-energy tensor for dark energy does have negative signs where positive signs normally are found, i.e., for the pressure terms.

Also, dark energy (and also the field that caused inflation) violates the strong energy condition. This means that a (non-rotating) bundle of initially parallel worldlines for massive particles can have a positive expansion scalar. Without dark energy, the cosmological expansion scalar is negative, i.e., gravity is attractive, even in expanding universes.

What about Newtonian gravity? With suitable approximations, general relativity reduces to Newtonian gravity. Keeping the dark energy term (Lamda) while working through the example of a sphere of mass M results in gravitational acceleration (page 186 of General Relativity: An Introduction for Physicists by Hobson, Efstathiou and Lasenby)

g = - GM/r^2 + r Lambda c^2 /3.

The first term is standard attractive Newtonian gravity, but the second term is repulsive gravity that grows as distance increases. Lambda is so small, though, that this term only make a contribution on large cosmology scales for which the Newtonian approximation fails.

I'm not sure what you mean by this. Normal antinatter generates attractive gravity. Exotic antimatter generates repulsive gravity.

I haven't heard of this - can you give a reference? All the forms of matter I've heard of have positive gravity.

I didn't write this very well.

Exotic matter/energy is stuff that violates the weak energy condition (and, by continuity, the null energy condition). This means:

1) at least some observers will measure the density of the stuff to be negative;

2) a (non-rotating) bundle of initially parallel worldlines for photons can have a positive expansion scalar (again, repulsive gravity.

Normal matter and antimatter generate attractive gravity. Exotic matter and antimatter can generate repulsive gravity. The Casimir effect is an example of energy that violates the weak energy condition. How likely is it that larger amount of exotic matter/energy can be found? Opinions vary from: impossible; widely speculative; maybe. See

Twilight for the energy conditions? by Carlos Barcelo and Matt Visser

http://arxiv.org/PS_cache/gr-qc/pdf/0205/0205066v1.pdf

Exotic solutions in General Relativity: Traversable wormholes and “warp drive” spacetimes by Francisco S. N. Lobo

http://arxiv.org/PS_cache/arxiv/pdf/0710/0710.4474v1.pdf

Even though wormholes are highly speculative, dozens of serious scientific research papers are published every year. For several years off-and-on, I have played around with wormholes with great enjoyment. I enjoy:

that this give me a pleasant environment and motivation to learn advanced technical aspects of general relativity which are relevant for less speculative areas;

looking at the maths in pure maths books;

the sci-fi aspect of this.

Currently, I am trying to learn how to join two spacetimes along a common null hypersurface, so that I can better understand the technical details of

Our universe at home within a larger universe? So suggests physicist's wormhole research

http://arxiv.org/PS_cache/arxiv/pdf/0902/0902.1994v3.pdf