In high-energy physics, however, one is faced with processes where particles are produced and absorbed and this demands a more general field theoretical approach called second quantization.
However this is true only in a first approximation: If one considers the possibility that a positive and a negative pion are virtually related in the sense that they can annihilate and transform into a pair of two neutral pions (or two photons), i.e. a pair of identical particles, we are faced with a more complex situation, which has to be handled within the second quantisation approach.
[5] These effects illustrate the superiority of the field theoretical second quantisation approach as compared with the wave function formalism.
They also illustrate the limitations of the analogy between optical and particle physics interferometry: They prove that Bose–Einstein correlations between two photons are different from those between two identically charged pions, an issue which had led to misunderstandings in the theoretical literature and which was elucidated in.
[7] While initially it was used mainly to explain the functioning of masers and lasers, it was soon realized that it had important applications in other fields of physics, as well: under appropriate conditions quantum coherence leads to Bose–Einstein condensation.
As the names suggest Bose–Einstein correlations and Bose–Einstein condensation are both consequences of Bose–Einstein statistics, and thus applicable not only to photons but to any kind of bosons.
Almost in parallel to the invention by Robert Hanbury-Brown and Richard Twiss of intensity interferometry in optics, Gerson Goldhaber, Sulamith Goldhaber, Wonyong Lee, and Abraham Pais (GGLP) discovered[8] that identically charged pions produced in antiproton-proton annihilation processes were bunched, while pions of opposite charges were not.
[note 3] One reason for this interest is the fact that Bose–Einstein correlations are up to now the only method for the determination of sizes and lifetimes of sources of elementary particles.
[20] This experiment has also a particular significance because it tests in quite an unusual way the predictions of quantum statistics as applied to Bose–Einstein correlations: it represents an unsuccessful attempt of falsification of the theory.