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| rambler |
 Posted: Tue Feb 15, 2005 3:48 am |
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Joined: 10 Sep 2004
Posts: 11
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In the Stern-Gerlach experiment, a magnetic field splits electrons (or equivalently, silver atoms) in two beams which collide against a photographic plate.
What happens if one single electron in a spin superposition state (|up> + |down>) is sent through the SG device? Does the electron go both possible paths and then collide against the plate at only one point? Or does it collapse into one of both possible spins when interacting with the magnetic field, goes then only one of both paths and collides afterwards at one point with the plate?
It seams to me that the second situation takes place (otherwise an interference pattern would be seen in the plate when performing the experiment with multiple electrons), but it is unclear for me why the electron spin collapses when interacting with the magnetic field and not afterwards, when colliding with the plante… Why isn't is possible to have still a superposition of spin states until the collission with the plate? |
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| Bjoern Feuerbacher |
| Posted: Tue Feb 15, 2005 4:52 am |
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Guest
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rambler wrote:
[quote:107fcc1199]In the Stern-Gerlach experiment, a magnetic field splits electrons (or
equivalently, silver atoms) in two beams which collide against a
photographic plate.
[/quote:107fcc1199]
Not equivalent. Electrons are charged, silver atoms are not. So for the
electrons, you additionally have a Lorentz force.
[quote:107fcc1199]What happens if one single electron in a spin superposition state
(|up> + |down>) is sent through the SG device? Does the
electron go both possible paths and then collide against the plate at
only one point?
[/quote:107fcc1199]
Neglecting the Lorentz force: yes. (if we want to use Feynman's "sum
over paths" idea here)
[quote:107fcc1199]Or does it collapse into one of both possible spins
when interacting with the magnetic field, goes then only one of both
paths and collides afterwards at one point with the plate?
[/quote:107fcc1199]
No.
[quote:107fcc1199]It seams to me that the second situation takes place (otherwise an
interference pattern would be seen in the plate when performing the
experiment with multiple electrons),
[/quote:107fcc1199]
Huh? Where should the interference come from??? You get interference
only when two paths "cross" somewhere. This does not happen in this
thought experiment.
[quote:107fcc1199]but it is unclear for me why the
electron spin collapses when interacting with the magnetic field and
not afterwards, when colliding with the planteďż˝ Why isn't is possible
to have still a superposition of spin states until the collission with
the plate?
[/quote:107fcc1199]
There is.
I *heavily* recommend the book "The strange world of Quantum Mechanics"
by Styer to you, where he discusses thought experiments like the one you
proposed here at length. |
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