Virtual particle

A virtual particle is a theoretical particle that takes part in interactions as a force carrier but is not observable due to the uncertainty principle, which limits its existence and permits the breakage of fundamental principles of physics below certain time scales. A particle that is observable is called a real particle.

The time-energy formulation of the principle states

The interpretation of what $\Delta t$ is varies with context, but it implies some lower bound below which a duration becomes too short to be well-defined. This time scale becomes smaller as the energies involved in the process become higher. The duration of a fundamental interaction falls beneath this range: thus, we can assume (but not verify!) that the interaction is mediated by a virtual particle which exists for a shorter time than $\Delta t$, then vanishes.

The inherent uncertainty of the particle's existence leads to some remarkable consequences. Namely, it is allowed to temporarily break conservation laws, such as conservation of energy, provided that the conservation is restored within the time period $\Delta t$ of uncertainty. This is a key point, because this temporary violation of laws is required by several physical properties that we otherwise know well, such as beta decay¹.

The concept of a virtual particle originates from quantum field theory, where the fundamental interactions between ordinary particles are represented through exchanges of virtual particles. This interaction is visualized using a Feynman diagram.