photons are bosons

Two trapped charged bosons for different densities and temperatures;
PIMC simulation by Jens Böning

Physicists in the US have carried out an extremely precise test of the one of the cornerstones of modern physics – the idea that the two types of fundamental particle, bosons and fermions, follow two distinct kinds of statistical behaviour.

Physics tells us that fundamental particles come in two basic varieties: bosons, which have integer values of intrinsic angular momentum or "spin", and fermions, which have half-integer spin. Bosons include force-carrying particles such as the photon, W and Z and follow Bose–Einstein statistics. An important consequence of this is that many identical bosons are free to occupy the same quantum state, leading to phenomena such as Bose–Einstein condensates and lasing.

Fermions include the fundamental matter particles such as quarks and electrons and obey Fermi-Dirac Statistics. Identical fermions can never exist in the same quantum state, giving us the shell structure of atoms and, with it, chemistry.


Dmitry Budker and Damon English of the University of California at Berkeley decided they would test this principle, known as the spin-statistics theorem, as precisely as they could.

The researchers fired two green laser beams from opposite directions into a beam of barium atoms contained within an optical cavity, with the combined energy of a photon pair (made up of one photon from each of the beams) equal to the barium absorption energy. They found that when the frequencies of the two beams were very slightly different to one another this absorption took place, which they observed by measuring the photons given off by the barium's subsequent de-excitation. But they observed no such absorption when the frequencies were identical – demonstrating that photons really are bosons.

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