# Spin (physics)

• Protons behave like quantum spinning tops with spin ½ in units of Planck's constant ${\displaystyle \hbar }$. This spin is responsible for many fundamental properties of nature including the proton's magnetic moment, the different phases of matter in low temperature physics, the properties of neutron stars and the stability of the known Universe. One of the main questions in particle and nuclear physics the last 20 years has been: How is the proton's spin built up from its quark and gluon constituents?
• In dealing with problems about electrons according to quantum mechanics, one finds one does not get agreement with experiment if one assumes the electrons to be simply point charges repelling one another according to the Coulomb law of forces. It is necessary to make the assumption that each electron is spinning and so has an internal angular momentum, and also that it has a magnetic moment. To make the theory agree with experiment we must assume that the eigenvalues of the Cartesian component of the spin angular momentum in any direction are ½${\displaystyle \hbar }$ and –½${\displaystyle \hbar }$, and that the magnetic moment of the electron (with its sign reversed) always lies in the same direction as the spin angular momentum ...