virtual particle

History

The idea emerged in the 1920s-30s with the development of quantum electrodynamics and was formally described in the 1940s by Richard Feynman and others.

How it works

Two ships exchange swift rowers who burst from the water and return. The rowers are unseen but push the ships. Similarly, virtual particles carry forces between ordinary particles: for example, virtual photons make a magnet attract.

History

The idea emerged in the 1920s-30s with the development of quantum electrodynamics and was formally described in the 1940s by Richard Feynman and others.

How it works

Two ships exchange swift rowers who burst from the water and return. The rowers are unseen but push the ships. Similarly, virtual particles carry forces between ordinary particles: for example, virtual photons make a magnet attract.

History

The concept grew from work in quantum electrodynamics (QED). Paul Dirac introduced the theory of radiation and absorption in 1927, and the idea of virtual particles was formalized in the late 1940s by Richard Feynman, Julian Schwinger, and Sin-Itiro Tomonaga, for which they received the Nobel Prize. The term was likely coined by Wheeler. John Wheeler also investigated quantum foam, where virtual particles account for the foam-like geometry at Planck scales.

How it works

According to the uncertainty principle, vacuum fluctuations of fields give rise to virtual pairs. In QED, virtual photons mediate the electromagnetic interaction between charged particles: each vertex in a Feynman diagram represents an act of emission or absorption of a virtual particle. They may be 'off mass shell' (their energy-momentum does not satisfy E² = p²c² + m²c⁴), unlike real particles. Effects of virtual particles manifest in vacuum polarization, the Lamb shift, the anomalous magnetic moment of the electron, and, macroscopically, in the Casimir effect, where an attractive force arises between two closely spaced conducting plates due to the difference in density of virtual photons outside and inside.

Open problems & fun facts

How to explain the cosmological constant if the vacuum energy from virtual particles is predicted to be 120 orders of magnitude larger than observed?; What is the role of virtual particles in quantum gravity, especially near black holes, where they may contribute to Hawking radiation?; Can virtual and real particles be experimentally distinguished at ultra-short time scales?

💡 In strong electromagnetic fields (e.g., near nuclei with large charge), virtual electron-positron pairs can become real — this is the 'Schwinger mechanism' of particle creation, predicted in 1951.