[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.