On 7/28/2025 1:38 PM, John Clark wrote:
On Mon, Jul 28, 2025 at 3:50 PM Brent Meeker <[email protected]>
wrote:
/>>> So if I illuminate a piece of iron with some photons
that knock out electrons via the photoelectric effect,
then on passing them thru an SG magnet and detecting their
distribution, you say they must necessarily be 50/50 UP/DN?/
*>>Not necessarily. Electrons in magnetic iron are not
randomly oriented, so the electrons knocked out of the iron
may have some preferential spin orientation. Also because
angular momentum is conserved, circularly polarized light can
preferentially eject electrons of one spin orientation over
the other. But both the magnetic orientation of iron atoms and
the amount of polarization of the light are known quantities,
at least theoretically, *
/> What's that supposed to mean, besides "I need it to be right."?/
*Its meaning is self-evident. *
It's far from self-evident that you know the magnetic orientation of the
iron atoms before you measure anything? Even "theoretically".
*And I don't need to be right and often I'm wrong, but in this case I
am right. And I think you're mad because I didn't fall for your trap. *
>/You've just defined "unmeasured" to mean 50/50 distribution./
*That's the word I used to describe a single electron if I know
nothing about its history. If it's not a single electron but there is
a beam of them and I send many electrons through a Stern–Gerlach
magnet then I may conclude that whatever the origin of the beam was,
it's producing electrons in some distribution other than 50-50.*
Exactly my point. Your 50/50 just an epistemic assumption, which
measurement may prove wrong. Yet if you take the UP beam from the SG
and measure Left/Right the distribution will be 50/50, which is not just
epistemic.
/> What's your definition of "measurement"?/
*Rather than a definition of measurementI will give you something much
better, an example. Producing an electron with a known spin state, as
a piece of magnetized iron does when, thanks to the photoelectric
effect, it's exposed to circularly polarized light; or when an
electron passes through a Stern–Gerlach magnet.*
So which step is the example of measurement, producing an electron with
a known spin state, as when you first measure the magnetized direction
of a piece of iron before ejecting an electron from it, or when the
electron passes thru the SG and is detected as deflected up or down?*
*
/> A bit of iron found in the ground is unlikely to have
encountered an SG./
*True, but it's not unlikely that something equivalent could have
occurred.*
That it cooled in the Earth's magnetic field? Is that what you call
equivalent to being separated in an SG?
Let me help you out. A measurement is something that leaves a classical
record of a value. A classical record is one that can be read without
disturbing its state. So if you have piece of magnetized iron, its
direction of magnetization is already measured...even though you don't
know it; that's just because you haven't read the record. If it's a
single iron atom, it hasn't been measured.
Brent
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