Another way of describing the difference between bosons and fermions is that bosons have symmetric wave functions while fermions have antisymmetric wave functions. Now, it turns out that there are two sub-categories of particles: those with “integer” spin, which are known as bosons, and which include photons, gluons, W and Z bosons and hypothetical gravitons and those with “half-integer spin”, which are known as fermions, and which include electrons, neutrinos, muons, and the quarks which make up composite particles like protons and neutrons. Spin is therefore a completely quantum mechanical property of a particle, and cannot be explained in any way by classical physics.Īrtist's representation of the spin and charge of an electron The spin of composite particles (such as protons, neutrons and atomic nuclei) is just the sum of the spins and orbital angular momentum of the constituent particles, and is therefore subject to the same quantization conditions. it could only take certain discrete values. When spin was first discovered in 1922 by Otto Stern and Walther Gerlach, however, their experiments indicated that the intrinsic angular momentum, or spin, of a particle such as an electron was quantized i.e. In general, though, spin obeys the same mathematical laws of angular momentum as do spinning objects in classical physics (such as the Earth, for instance), and there are really only two important aspects to consider: the speed of rotation and the direction of the axis it rotates about (referred to as “up” and “down”). ![]() To return to the property of spin of fundamental particles, mentioned briefly in the previous section, spin can perhaps be most easily thought of as a rotation of particles around their own axis, although this is in fact something of a simplification, and in reality it is impossible to tell whether something as small as an electron is spinning at all.
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