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In this Stanford lecture (YouTube, 15m 40s), Prof. Susskind claims that with the emission of a photon, reverse-aligned particles flip from down to up.

But if that were the case, then...

If a beam of spin-down particles were fired through a Stern-Gerlach apparatus the magnetic field of which is oppositely oriented to that of the particles, they would not all exit the SG spin-down!

There would be two exit beams, one strong, one weaker (depending on the strength of the SG's magnetic field). But this doesn't happen (or does it?).

So what is Suskind talking about? Do spin-1/2 particles flip, or don't they?

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  • $\begingroup$ Susskind is talking about EPR: en.wikipedia.org/wiki/Electron_paramagnetic_resonance, but I also applies to NMR and MRI . $\endgroup$
    – my2cts
    Commented Jul 5 at 18:21
  • $\begingroup$ what would help is if you reminded us which alignment was lowest energy--I think it's the spin points to the S pole; and a definition of "reversed-aligned"...does that mean "in the higher energy state" (pointing to N, if my original supposition is correct). Also: I don't understand the quip about the strength of SG's B field..the field strength drives the spin-state selection, while the gradient drives the spatial separation. $\endgroup$
    – JEB
    Commented Jul 5 at 18:28
  • $\begingroup$ @jeb If the magnetic field of SG1 is aligned in the opposite direction to that of SG2, and the spin UP (with respect to the magnetic field of SG1) beam of particles coming out of SG1 is passed through SG2, then since all of these particles are anti-aligned with the magnetic field of SG2, they should, according to the literature, ALL of them exit SG2 spin DOWN (with respect to the magnetic field of SG2). But Suskind seems to suggest that SOME of them will flip from anti-aligned to aligned, after an average time T which is dependent on the strength of SG2's magnetic field. $\endgroup$
    – questing-monkey
    Commented Jul 9 at 17:30
  • $\begingroup$ To begin with, the rate of spontaneous magnetic dipole transition is very low. To also conserve energy and momentum, I believe this particular process must be at least two-photon, making it even rarer. You probably won't see a single event for the duration of a typical Stern-Gerlach experiment. $\endgroup$
    – T.P. Ho
    Commented Jul 20 at 2:57
  • $\begingroup$ It's not clear what Susskind's discussion of photon emission is supposed to have to do with Stern-Gerlach experiments. There are no photons emitted or absorbed during SG experiments. $\endgroup$
    – ACuriousMind ♦
    Commented Jul 20 at 6:56

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