In quantum field theory, vibrations in the field that correspond to a force give rise to particles called force carriers. Particles that interact via a particular force do so by exchanging these force carrier particles. For example, the photon is a vibration of the electromagnetic field and the carrier of the electromagnetic force. Particles such as electrons, which have negative electric charge, repel one another by exchanging virtual photons. The carrier of the strong force is the gluon, and the carrier particles of the weak force are the W and Z bosons. Force carriers are always bosons, and may be either massless or massive.