I have read about the Stern-Gerlach experiment in Wikipedia, this experiment proved that electron spin could only take two discrete values. The experiment consist in sending neutral charged atoms through a inhomogeneous magnetic field so it would change the particle’s direction a quantity that would depend on the direction of the spin in relation with the magnetic field’s direction. Before the experiment, it was thought that the magnetic field would scatter the particles in different directions in a continuous way, while the result was that the particles were only scattered in two “opposite” directions. It also says that the experiment was done in a concatenated way changing the direction that was measured and found the [now] well known result that when you measure the spin in one direction you lose all knowledge about the particle’s spin in other previous direction. My question is know: if the particles initially have random spin directions. What causes them to collapse in the new direction? I suspect that it’s the magnetic because although the particle must be detected in some “box”, the box where it ends it’s a consequence of the effect of the magnetic field over a well-defined spin so it must have collapsed before… I have searched quite for the internet but I have found no evidence.