2 4 4 D O C U M E N T 3 1 5 S T E R N - G E R L A C H E X P E R I M E N T and specifically, for about half of the atoms, in line with the field for the other half, opposing it.[3] The question of how the atoms arrive at this orientation naturally poses itself. § 2. It is, above all, observable that atoms do not experience any collisions as they enter the deflective magnetic field—the last collisions they experience are in the vaporizing chamber of the small melting furnace.[4] We initially ask ourselves how magnetic atoms change their orientation at all under the influence of a magnetic field. As long as one ignores the emission and absorption of radiation, the collisions, or other similar influences, the atoms exe- cute a precession (Larmor rotation) in the magnetic field around the direction of the field.[5] If the direction of the field changes slowly compared to the rapidity of the precessional motion, then the angle of the precessional motion remains unchanged.[6] Accordingly, an adjustment to the tilts required by quantum theory (O and for the silver atom from the experiment by Stern and Gerlach)[7] cannot take place without external influences of the type of radiation or collisions. § 3. The most obvious explanation for the experimental finding seems at first instance to be that the atoms’ adjustments take place upon entry in the electromag- net’s field, namely through an exchange of radiation. Then not only a release of energy is necessary but also an intake of energy from the radiation field, namely the latter for the atoms that set themselves antiparallel to the force lines. How quickly does the repositioning of the atoms’ moments now happen under the influence of the radiation (from room temperature)?—The required time is relatively reliably estimable for the case of transitions from quantum state to quantum state. For we know that in cases of this kind this time for the transition of an assemblage of atoms agrees—at least in order of magnitude—with that of a corresponding classical model. In our case of a precessing atom with a magnetic moment this would be a radiating magnetic dipole with its conic rotation. The adjustment time (for a field strength of 10,000 gauss) would be of the order of sec, provided the emission of the precessive motion alone was effective. But if one takes into account the influ- ence of the surrounding room-temperature radiation [“positive and negative radia- tion influx”(1)], it shortens to about sec.[8] (1) Comp. A. Einstein, Zur Quantentheorie der Strahlung. Phys. Z[eit]s[chrift] 18, 121, 1917, § 2.[10] π [p. 32] 1011 109 [p. 32]