Quantum mechanics is extraordinarily powerful at describing microscopic systems. With its relativistic generalisations, it is able to explain all the results of the experiments carried out at the present time. Of course, for complicated systems with more than three particles, one can generally only rely on approximate calculations. On the other hand, the experiments themselves have limited accuracy. There are, however, extremely precise experiments that aim to measure observable quantities for which extremely precise calculations are possible and have been carried out. The results agree on all figures, which are not affected by experimental or theoretical errors. For example, the deviation between the experimental measurement of the gyromagnetic factorGyromagnetic factor  \(g_e\) of the electron [Eq. ( 12.6.24 )] and the value of 2 predicted by the Dirac equationDirac equation (Sect. 14.4.5 ) is explained with a precision better than \(10^{- 12}\) by quantum electrodynamicsquantum electrodynamics, a relativistic