bosons

Bosons are particles whose joint wavefunction is symmetric under exchange: swap two of them and the state comes back exactly as it was, with no change of sign. Their defining feature is integer spin — spin zero, one, two, and so on, measured in units of the reduced Planck constant. The photon, the particle of light, is the most familiar boson, and so are the gluon, the W and Z particles, and the Higgs.

The symmetric rule makes bosons sociable. Far from avoiding one another, identical bosons are actually more likely to pile into the same quantum state than chance alone would suggest. Any number of them can share a single state, and the more that are already there, the more eagerly the next one joins. This gregarious tendency is the engine behind the laser, where vast numbers of photons march in step, and behind the Bose–Einstein condensate of ultracold atoms.

Composite objects can be bosons too, as long as they are built from an even number of fermions so that their spins add up to a whole number. A helium-4 atom, made of an even tally of protons, neutrons, and electrons, behaves as a boson and can become a superfluid that flows without friction. Whether elementary or composite, what unites all bosons is that simple plus sign hiding in their exchange symmetry.