[antognini]

Thinking of a virtual graviton or photon or gluon as a particle is somewhat misleading. It is better to think of it as an excitation of the underlying field.

It is possible to show (with fairly elementary techniques) that when the excitations have a spin of 2, these excitations always reduce the energy of the system, and so produce an attractive force. If the excitations have a spin of 1, then they increase the energy of the system and so produce a repulsive force. This is why the gravitational force attracts and like charges repel each other.

[AnimalMuppet]

> If the excitations have a spin of 1, then they increase the energy of the system and so produce a repulsive force. This is why the gravitational force attracts and like charges repel each other.

But then why do unlike charges attract? The force mediator is still a spin-1 particle...

[antognini]

The charge is in essence a measure of how strongly the particle couples to the field. If the charges are opposite then the coupling gets reversed and so the excitation reduces the energy.

Likewise if a particle somehow had a negative mass, it would gravitationally repel particles of positive mass.