# Graviphoton - Repulsion between Matter and Antimatter

1. Mar 28, 2013

### Johnleprekan

Gravitons are suppose to mediate the gravitational force, but there is also the graviphoton which is the graviton's symmetry partner. The graviphoton causes repulsion between matter and antimatter but this is not confirmed. Can someone explain in layman's terms how this particle could do this?

2. Mar 28, 2013

### Staff: Mentor

Neither gravitons nor supersymmetry are confirmed, so how could an effect of a particle which requires both be observed?
If you can find a layman's description how photons can attract opposite and repel same charges, it is probably easy to transfer this. In other words: I don't think this is possible.

3. Mar 28, 2013

### lpetrich

Symmetry partner? I don't get what you mean. Could you please link to whatever you are using as a source?

The likes of supergravity and Kaluza-Klein compactification can produce several graviton relatives:

Spin 2: the graviton itself
Spin 3/2: the gravitino, a result of supergravity -- supersymmetric gravity
Spin 1: the gravivector or graviphoton, a result of "extended supergravity" and Kaluza-Klein
Spin 1/2: (?), likewise
Spin 0: the graviscalar or radion, likewise

4. Mar 28, 2013

### Johnleprekan

I looked it up here http://en.wikipedia.org/wiki/Antigravity

By symmetry, I mean supersymmetry where fermions have an opposite boson partner and vice versa.

I am aware it has not been observed, but thought there might some theories as to how it could work.

5. Mar 28, 2013

### fzero

That wikipedia article is somewhat misleading. What is true is that, if the graviphoton exists, it mediates a new interaction with a strength that is related by supersymmetry to the strength of the gravitational interaction. However particles will still have some charge under this new interaction. If it is analogous to the U(1) electromagnetism, then an antiparticle will have an opposite charge with respect to the corresponding particle. As mfb wrote, opposite charges will attract, while like charges will repel. So the graviphoton will lead to a repulsive matter-matter interaction and an attractive matter-antimatter interaction, as long as we are considering particles of the same type. In this way, there will be a new repulsive force comparable to gravity between two protons, but a proton and antiproton will still attract both gravitationally and via the new interaction..

6. Mar 28, 2013

### Johnleprekan

Thank you.

7. Mar 29, 2013

### lpetrich

A gravitino won't produce any "fifth force" interactions, for two reasons.

Its mass is at least a TeV because of supersymmetry breaking, making its interaction range at most about 10-18 m.

Emitting or absorbing a gravitino would change a particle into its supersymmetry partner.

If two particles continuously interact by gravitino exchange, they would get put into a mixed state with their supersymmetry partners: (a,b) + (a',b') where a <-> a' and b <-> b'.

The same goes for gravispinors, as they might be called, spin-1/2 particles related to the graviton by extended supersymmetry.

8. Mar 29, 2013

### lpetrich

There are other problems, like how one constructs gravitational-strength interactions for gravivectors and graviscalars.

One can see where the problem will be by doing dimensional analysis of possible interactions. One sets hbar = c = 1 and finds what powers of mass the coupling constants are. Mass = 1/length in these units. I'll be doing the calculations for 4D space-time, but they can be extended to additional dimensions.

The field is given by D2(field) = (coupling constant) * (source density)
and the resulting potential energy is (field) * (source density) integrated over 3-volume.

If the source current has dimension L-s or Ms, then the coupling constant has dimension M6-s.

For electromagnetism and similar theories, the current has dimension L-3, since its integral over volume is a dimensionless number in hbar = c =1 units. That means that the electric charge and other gauge "charges" are dimensionless.

For gravity, the current is the energy-momentum tensor, an energy/momentum density/flux. Its dimension is M*L-3 = M4. The coupling constant has dimension M-2, in agreement with
(gravitational constant) = 1/(Planck mass)2

So one has to construct currents with dimension M4 for the gravivector and the gravivector.

For the graviscalar, there's a current that's very easy and gravity-related. Contract the energy-momentum tensor over its indices, giving
(mass/energy density) - 3*pressure.

For the gravivector, it's much more difficult. I've done a lot of searching, and I can't find what would be a sensible sort of source current.

One can construct (some mass) * (gauge-theory vector current)
where (gauge-theory vector current) is what one gets out of electromagnetism, for instance. For an elementary-fermion field, that current is
$j^\mu = {\bar\psi} \gamma^\mu \psi$

What would be an appropriate mass here?

But this sort of term has the property that matter-antimatter interchange will reverse its sign, unlike the case for gravity and my graviscalar interaction. It will also yield the "mass"
$M_{gravivector} = \sum_{flavor\ i} m_{(i)} N_i$

where Ni is the number of elementary fermions with flavor i (ordinary particles - antiparticles), and m(i) is the mass value associated with flavor i.

For an atom with Z protons and N neutrons, this gives us
Mgravivector = m(electron)*Z + m(up quark)(2Z+N) + m(down quark)(Z+2N)

Photons and gluons don't enter, because they are their own antiparticles.

The overall mass is approximately mnucleon(Z+N)

The average N/Z is 0 for hydrogen-1, 1 for the lighter elements, and greater than 1 for the heavier elements, going up to 1.5 near uranium.

So the ratio of these masses is about the ratio of m(flavor) for electrons, up quarks, and down quarks to mnucleon.

9. Mar 29, 2013

### shreyakmath

Graviphotons are superpartners of gravitons. They can produce both attractive and repulsive gravitational forces. This is a completely theoretical concept and hasnt been validated experimentally. Graviphoton in some models are considered a part of dark energy