*Originally posted by Metal Brain*

**Are you referring to the hypothetical graviton?**

Yes, and another one called the gravitino. I imagine there are more in the cookbook but nothing detected as of yet.

Here is a bit from Wiki about the graviton:

In physics, the graviton is a hypothetical elementary particle that mediates the force of gravitation in the framework of quantum field theory. If it exists, the graviton must be massless (because the gravitational force has unlimited range) and must have a spin of 2.

This is because the source of gravitation is the stress-energy tensor, which is a second-rank tensor, compared to electromagnetism, the source of which is the four-current, which is a first-rank tensor.

Additionally, it can be shown that any massless spin-2 field would be indistinguishable from gravitation, because a massless spin-2 field must couple to the stress-energy tensor in the same way that the gravitational field does.

[4] This result suggests that if a massless spin-2 particle is discovered, it must be the graviton, so that the only experimental verification needed for the graviton may simply be the discovery of a massless spin-2 particle.[5]

And a quickie from wiki on the gravitino:

The gravitino, symbol G͂, is the supersymmetric partner of the graviton, as predicted by theories combining general relativity and supersymmetry; i.e. supergravity theories. If it exists it is a fermion of spin 3/2 and therefore obeys the Rarita-Schwinger equation.

The gravitino field is conventionally written as ψμα with μ = 0,1,2,3 a four-vector index and α = 1,2 a spinor index. For μ = 0 one would get negative norm modes, as with every massless particle of spin 1 or higher.

These modes are unphysical, and for consistency there must be a gauge symmetry which cancels these modes: \delta\psi_{\mu\alpha} = \partial_\mu\epsilon_\alpha where \epsilon_\alpha(x)\, is a spinor function of spacetime. This gauge symmetry is a local supersymmetry transformation, and the resulting theory is supergravity.

Thus the gravitino is the fermion mediating supergravity interactions, just as the photon is mediating electromagnetism, and the graviton is presumably mediating gravitation. Whenever supersymmetry is broken in supergravity theories, it acquires a mass which is directly the supersymmetry breaking scale.

As a proposed solution to the fine tuning problem of the Standard Model, and in order to allow grand unification, the supersymmetry breaking scale needs to be pushed down to the TeV range.

Therefore the gravitino mass needs to be of this order (unless we have intermediate scale SUSY breaking), much lower than Planck scale, which is the natural scale for gravitation interactions. This difference in energy scales is known as the hierarchy problem.