On 7/29/2025 12:17 PM, John Clark wrote:
On Tue, Jul 29, 2025 at 2:37 PM Brent Meeker <[email protected]>
wrote:
*
*
*>> If afterthe electron's encounter with the SG magnet it
ever runs into a magnetic field again then the electron's
behavior will be different than it would have been if it had
never had an encounter with that SG magnet. And that is a
classical record. *
/> No. That's why I included the diagram of the SG experiment in
which one blocks or doesn't block one side of a split beam. Until
the beam hits a detector the atoms (not electrons) are maintaining
their coherence between beams and can be recombined in a way that
is impossible classically./
*Two points:*
*
*
*1) In the above 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. Electrons are charged, so the Lorentz force
on them is very much bigger that the magnetic divergence. I don't know
that it's impossible, but using electrons would be like trying to
measure a small effect in the presence of much bigger orthogonal effects.
*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. That's the point of the diagrams I posted, see below.
/> Until the beam hits a detector the atoms (not electrons) are
maintaining their coherence between beams /
*I'm not sure what you mean by that. Neither an electron nor a beam of
them ever encounters the atoms in a Stern–Gerlachdevice, the electrons
only encounter a magnetic field. *
/>and can be recombined in a way that is impossible classically./
*Are you talking about quantum erasure?If so I don't see the relevance
except to say that's the only way you can make a detection without
also making a classical record.*
No. I'm talking about this. If you block one channel then you've made
a measurement. So first you measure (prepare) an atom in z+ then you
measure (prepare) it in x+ and then you measure it in z_+_ and find it
is 50/50. And this applies to one atom at a time as well as a beam.
But if you leave both channels open after the x_+_ measurement, then the
z_+_ measurement recombines them and produces z+. So simply putting an
atom thru the second SG was not a measurement, as it was in the above
example, because there was no determination of whether it went x+ or x-,
even though it went thru an SG.
*Brent*
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