A virtual particle is a particle that appears spontaneously and exists only for the amount of time allowed by the Heisenberg uncertainty principle. According to the uncertainty principle, the product of the uncertainty of a measured energy and the uncertainty in the measurement time must be greater than Planck's constant divided by 4. This means that a particle with a certain energy can spontaneously appear out of the vacuum and live for an amount of time inversely proportional to its energy. The force carriers exchanged in an interaction are virtual particles. Virtual particles cannot be observed directly, but their consequences can be calculated using Feynman diagrams and are verified experimentally.