Particles decay via force carrier particles. But in some cases a particle may decay via a force-carrier particle with more mass then the initial particle. The intermediate particle is immediately transformed into lower-mass particles. These short-lived high-mass force-carrier particles seem to violate the laws of conservation of energy and mass -- their mass just can't come out of nowhere!

A result of the Heisenberg Uncertainty principle is that these high-mass particles may come into being if they are incredibly short-lived. In a sense, they escape reality's notice. Such particles are called virtual particles.

Virtual particles do not violate the conservation of energy. The kinetic energy plus mass of the initial decaying particle and the final decay products is equal. The virtual particles exist for such a short time that they can never be observed.

Most particle processes are mediated by virtual-carrier particles. Examples include neutron beta decay, the production of charm particles, and the decay of an eta-c particle, all of which we will explore in depth soon.