In atoms, electron orbital and spin dynamics are coupled to the electric field of the protons in the atomic nucleus according to the Dirac equation. An electron moving in a static electric field E sees, according to the Lorentz transformations of special relativity, a complementary magnetic field B ≈ ≈ ( v / c ) E in the electron frame of reference. However, for slow electrons with v / c ≪ ≪ 1 this field is weak and the effect is small. This coupling is known as the spin–orbit interaction and gives corrections to the atomic energies about the order of the fine-structure constant squared α α 2 , where α α = e 2 / ℏ ℏ c ≈ ≈ 1 / 137 . However, this constant appears in combination with the atomic number Z as Z α α , and this product is larger for massive atoms, already of the order of unity in the middle of the periodic table. This enhancement of the coupling between the orbital and spin dynamics in massive atoms originates from the strong attraction to the nucleus and the large electron speeds. While this mechanism is also expected to couple electron spin to the electric component of electromagnetic fields, such an effect has been probably never observed in atomic spectroscopy.