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Name: Unknown
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Date: Around 1993


Question:
Two electrons with the same spin are not allowed to occupy the same orbital. How does one electron "know" what the other's spin is?



Replies:
This is another of those mysteries involving spin in quantum mechanics. A single electron can have a spin that points in any direction, unless there is some external effect (like a magnetic field) that forces an energy difference between two opposite spin directions (either in the direction of the field, or opposite to it). When you bring two electrons together, they have an effect on each other similar to the effect of the magnetic field - the energy eigenstates either have both electrons spins pointing in the same direction (a so-called "triplet" state) or they have them always opposite (a "singlet" state) although the actual direction that both point (either parallel or opposite to) is again arbitrary. So, there is actually a direct interaction between electrons that depends on their spins - this is called the "exchange" interaction -resulting in different energy states depending on their relative spin orientations.

A. Smith



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