Virtual particle
A virtual particle is a short-lived particle, similar to an ordinary particle, but it cannot be directly detected. Virtual particles can suddenly appear in the vacuum and then quickly disappear. They can also be emitted by real particles and interact with other particles. In both cases their lifetime is limited by the Heisenberg's uncertainty principle. They interact also with macroscopic objects and, for example, cause the attraction of two metal plates in a vacuum. This phenomenon is called the Casimir effect (predicted by Hendrik Casimir in 1948)