On Tue, Jul 29, 2025 at 7:26 PM Brent Meeker <[email protected]> wrote:
*>> I was talking about an electron, not a beam of electrons. * > > > *> Which makes me think you don't know how an SG works. Typically it's a > beam (although one at a time is fine) of neutral atoms with a magnetic > moment, e.g. silver atoms.* > *Stern and Gerlach used neutral silver atoms in their original 1922 experiment, however in 2015 **Hosein Majlesi used their magnet to separate spin up electrons from spin down. * *Stern-Gerlach experiment by free electrons <https://arxiv.org/pdf/1504.07963>* *Electrons are simpler than atoms and I like to use the simplest thing possible in thought experiments. In fact I could make the same point if I got rid of both atoms and electrons and replaced them with photons of light, and junked the Stern Gerlach magnet and replaced it with a simple pair of polarizing sunglasses. * *>> 2) You say "an SG doesn't make a record; you need a detector for that". >> But after the electron (singular) encounters the magnetic field of the SG >> the electron is moving on a trajectory that is different from the one it >> would've had if it had no such encounter. And that is a classical record.* > > > *> Ok, I take your point. If the atom (not electron) comes out the up > channel you've measured it to be spin UP in the sense of an ideal > preparation. But you have to block the down channel or otherwise know it > is in the up channel. * > *Regardless of if I or anybody else knows it's in the up channel, the electron (or if you prefer the neutral silver atom) will behave differently because of its encounter with the Stern Gerlach magnet, and that is a classical record; although it would be possible to erase that record by sending it through a Stern Gerlach magnet again that had been rotated by 90° into the left right direction. That is quantum erasure. * *> if you leave both channels open after the x+ measurement, then the > z+ measurement recombines them and produces z+ * *As I said, measuring for up/down and then measuring for left/right is quantum erasure, you no longer have any idea of the electron's (or the neutral atom's or the photon's) up/down orientation, that information is no longer in the observable universe. * *John K Clark See what's on my new list at Extropolis <https://groups.google.com/g/extropolis>* 3b1 6nk -- You received this message because you are subscribed to the Google Groups "Everything List" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To view this discussion visit https://groups.google.com/d/msgid/everything-list/CAJPayv355oDtNhGO8wmanq_osTfi90D9p%3Dw1fUw67uPV2LWPWg%40mail.gmail.com.
