virtual particle

A short-lived subatomic particle whose existence briefly violates the principle of conservation of energy. The uncertainty principle of quantum mechanics allows violations of conservation of energy for short periods, meaning that even a physical system with zero energy can spontaneously produce energetic particles. The more energy a virtual particle has, the shorter its existence. Interactions between normal particles and virtual particles play a crucial role in quantum field theory analyses of interactions between real particles.

See also Casimir effect