Virtual particle

In quantum physics, the uncertainty principle implies that particles can come into existence for short periods of time even when there is not enough energy to create them. In effect, they are created from uncertainties in energy. One could say that they briefly "borrow" the energy required for their creation, and then, a short time later, they pay the "debt" back and disappear again. Since these particles do not have a permanent existence, they are called virtual particles.

Even though we can't see them, we know that these virtual particles are "really there" in empty space because they leave a detectable trace of their activities. One effect of virtual photons, for example, is to produce a tiny shift in the energy levels of atoms. They also cause an equally tiny change in the magnetic moment of electrons. These minute but significant alterations have been very accurately measured using spectroscopic techniques. The Casimir effect is an attraction between two uncharged parallel metal plates because less virtual particles can be created between the plates than in the surrounding space.

Virtual particles are often created as a pair of particle-antiparticle.