# Info on anti-gravity

1. Aug 10, 2004

### marlon

Hi,

what are the latest developments on black and white hole-theory and how they can be viewed as timetravelling configurations???
I read that one should apply anti-gravity in order to make such a wormhole stable.

thanks
marlon

2. Aug 10, 2004

Staff Emeritus
Not anti-gravity, but negative energy. Two different concepts Worm holes and other extreme solutions of Einstein's equations violate the "weak energy condition" and that implies that they involve negative energy in some way.

3. Aug 10, 2004

### marlon

rrr, please feel free to elaborate as much as you want. I am not a specialist on this subject, though i find it very interesting. Isn't the concept of anti-gravity used to keep a wormhole open??? I thought this was invented by Kip Thorne from Caltech-institute ???

4. Aug 10, 2004

### meteor

There was the idea of using "exotic matter" to create a wormhole. Exotic matter if exist would have negative mass (whatever it means) and would repel matter with positive mass

5. Aug 10, 2004

### DW

The Misner-Thorne wormhole is not the same spacetime geometry as wormholes associated with black holes. They proposed that the following spacetime represents a stable transversable wormhole geometry,
$$ds^{2} = e^{\frac{2\Phi (w)}{c^{2}}}dct^{2} - dw^{2} - r(w)^{2}(d\theta ^{2} + sin^{2}\theta d\phi ^{2})$$
where r(w) reaches a nonzero minimum at some value of w.
The problem with constructing the hole is that according to Enstein's field equations, the energy density term of the stress-energy tensor goes negative. The exact stress-energy tensor corresponding to this exact spacetime is equation 12.2.10 at
http://www.geocities.com/zcphysicsms/chap12.htm#BM150
How to produce the negative energy density is what is primarily under debate.

Last edited: Aug 10, 2004
6. Aug 10, 2004

### pervect

Staff Emeritus
You might try looking at John Cramer's collected science-fact columns, from "Analog", online at

http://www.npl.washington.edu/AV/av_index.html

As others have commented, you don't need anti-gravity, just "exotic matter", which is matter with a negative energy density. Actually, you don't necessarily need matter, but you do need the negative energy density - gravitationally significant amounts of negative energy.

A small piece of exotic matter would fall to ground just like a small piece of normal matter if you dropped it in the Earth's gravitational field - while the exotic matter would experience a repulsive force from normal matter (F=GMm/r^2, and m has changed sign) it also has a negative inertial mass, so it would move towards the repulsive force.

A large piece of exotic matter would repel "test bodies" of both normal and exotic matter, this is the property that makes it useful and necessary to hold wormhole throats

It's still somewhat of an open question whether or not wormholes could become time machines. The mechanism for making a wormhole into a time machine is well known (just fly one around for a while at relativistic velocites, as in Robert Forwards SF novel "Timemaster"). The unknown issue is whether or not this process will destroy the wormhole. Current experts seem to favor the idea that the wormhole will self destruct if it ever attempts to cross the boudary between being "spacelike" and "timelike" because of ever-growing quantum vacuum fluctuations.

7. Aug 11, 2004

### Neo

Do you think the existence of anti-photons is theoretically possible?

8. Aug 11, 2004

### pervect

Staff Emeritus
The photon is its own anti-particle, so I guess the answer is "yes" :-)

9. Aug 11, 2004

### marlon

Thanks for the answers, they are very usefull.

On this concept of an anti-photon, i have my doubts. I am involved in theoretical fysics but I don't see how one would or could show the properties of a particle that has opposite quantumnumbers to that of a foton. Spin can be forgotten, right because it is integer. Besides bosons and anti-bosons should repel eachother, otherwise the bosonic nature is lost. I don't see that happening

10. Aug 11, 2004

### pervect

Staff Emeritus
There's no such thing as an anti-boson in physics that I've ever heard of (I don't know about theoretical fysics - that sounds like a question for "Theory Development).

Basically if we stick to three dimensions (and avoid anyons), all particles are either fermions, which obey the Pauli exclusion principle, or bosons, which don't.

The real issue involves the symmetry of the wavefunction under the "exchange operation" - fermions are antisymmetric, and bosons are symmetric.

Note that while bosons have iinteger spins and fermions have half-integer spins, this is a consequence of the spin-statistics theorem.

http://en.wikipedia.org/wiki/Boson

for the photon being its own antiparticle see
http://en.wikipedia.org/wiki/Antiparticle

11. Aug 12, 2004

### Neo

Interesting...
Are you familiar with Hegelianism?

Thesis-->Antithesis-->Synthesis

Specific Case:
neutral kaon-->anti-kaon-->photon

General case:
particle-->anti-particle-->photon

Could a photon be the synthesis of particle-antiparticle pairs? Or could particle-antiparticles be produced by the "fission" of light?