In his lecture on symmetry in physical law, Feynman said:

Using a very strong magnet at a very low temperature, it turns out that a certain isotope of cobalt, which disintegrates by emitting an electron, is magnetic, and if the temperature is low enough that the thermal oscillations do not jiggle the atomic magnets about too much, they line up in the magnetic field. So the cobalt atoms will all line up in this strong field. They then disintegrate, emitting an electron, and it was discovered that when the atoms were lined up in a field whose B vector points upward, most of the electrons were emitted in a downward direction. Therefore if we were to put it in a corresponding experiment in a “mirror,” in which the cobalt atoms would be lined up in the opposite direction, they would spit their electrons up, not down; the action is unsymmetrical. The magnet has grown hairs! The south pole of a magnet is of such a kind that the electrons in a β-disintegration tend to go away from it; that distinguishes, in a physical way, the north pole from the south pole.

This doesn’t make sense to me.

I understood that the Wu experiment found electrons were emitted in the direction of Cobalt atom spin and, therefore, in the direction of the B vector, not opposite it as Feynman writes. Where have I gone wrong?


1 Answer 1


The electron charge is negative, so the magnetic moment is anti-aligned with the spin.

  • $\begingroup$ I made a typo in the final sentence where I mistakenly wrote "Cobalt atom spin" as "electron spin"- I have changed this now. Feynman talks about the direction of electron emission being opposed to the magnetic field (and therefore to the Cobalt atom spins which are aligned with the field), and doesn't refer to the magnetic moment of the electrons at all. All other sources have told me that the direction of emission in our universe should be in the same direction as the Cobalt atom spin and, thus, in the same direction as the magnetic field set up. This is where my confusion lies. $\endgroup$
    – P0W8J6
    Commented Sep 4, 2021 at 1:05

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