Quantum seminar | 07 February 2024

The Stern-Gerlach experiment confirms directional quantization of spin angular momentum, but what exactly causes the dynamics of spin quantization is a mystery. Einstein and Ehrenfest postulated that a radiative process was at work, and modeled the silver atom as a radiative magnetic dipole. Einstein and Ehrenfest’s model resulted in an alignment time on the order of 100 years. Given that the atom has a flight time of ≈ 10−5 s, their failure ended the search for a classical physical explanation of the alignment process. The force or torque, random or otherwise, which causes the atoms to align, either parallel or antiparallel, with the external magnetic field is still unknown within the realms of both classical and quantum mechanics. In this work, we identify the nature of the physical mechanism responsible for alignment. We have also found the error in Einstein-Ehrenfest’s model and corrected it. By identifying the power absorbed by the magnetic dipole as mainly responsible for alignment rather than the radiated power, we calculate an alignment time in fractions of nanoseconds.