Bosons are particles with integer spin (1 ħ, 2 ħ, 3 ħ, etc.). They function as particles that transmit interactions, they are therefore often referred to as force carriers. The most famous boson is indisputably the photon. Bosons also include the W and Z bosons, which are accountable for weak nuclear force (the interaction that causes radioactive decay), gluons, accountable for strong nuclear force (the interaction that holds particles inside of the nucleus of an atom together), and the famous Higgs boson.
Bosons do not obey the Pauli exclusion principle, since they are described by symmetrical wave functions, which means that more bosons can occupy the same quantum state. Bosons basically “crave” to be in the same state as other bosons. This property is responsible for the existence of multiple fascinating phenomena.
Let us start with a laser beam. Laser is a device emitting an immensely narrow beam of light, whose photons have the same frequency and are in phase. This is completely different from classical lightbulbs, which produce light of dozens of frequencies in all directions.
Lasers exploit the fact that photons belong to bosons. Within a laser, there are millions of atoms whose electrons are exited from the ground state to a higher energy level using electric current. Some of these electrons consequently emit energy in the form of photons and jump back to the ground state. The emitted photons then fly around the remaining excited electrons and stimulate the emission of other photons, while all of these photons enter the same quantum state (i.e. have the same frequency and are in phase). Once there are enough emitted photons, they leave the laser in the form of a laser beam.
Another mesmerizing instance of bosons in action can be observed when cooling a group of helium-4 atoms to extremely low temperatures – no more than two degrees above absolute zero. Every helium-4 atom is composed of an even number of fermions. This, however, makes the atom itself a boson, which means that it does not obey the Pauli exclusion principle.
All helium-4 atoms therefore behave just like other bosons – they wish to be in the same quantum state. Unfortunately, they cannot achieve that under normal conditions, as their wave functions look nothing alike. Nevertheless, once they reach temperatures just above absolute zero, their wave functions start spreading and overlapping. Eventually, they enter the same quantum state and the wave functions join into a single unified wave function which describes the entire group as a whole. In other words, quantum behaviour starts to transform into the macroworld!
Such a state of matter, in which atoms enter the same quantum state, is called the Bose-Einstein condensate. In some cases, this condensate behaves unlike any other state of matter. For instance, if one fills a vessel with cooled helium-4 atoms, they gradually creep along the walls of the vessel and escape, seemingly defying gravity.
Keishinrei 