# The determination of a particle's spin

Randall71284
If I am unable to distinguish the spin of a particle in an absence of an electric field or magnetic field, how am I able to determine whether there is an electric or magnetic field in a real-life context?

How is it that we can be sure of the uncertainty of the spin of particles if we are unable to create a situation in which there is an absence of an electric or magnetic field?

If a small enough field could be ignored (as there still are fields, however small in magnitude, acting on every particle in the universe), is there a level of magnitude in which the electromagnetic fields can be deemed insignificant that the particle’s spin is unaffected?

Gold Member
2022 Award
I don't understand what you mean. How else do you want to measure the spin of your particle than with applying some electromagnetic field?

Randall71284
I mean that if we could only measure the spin of a particle by applying an electromagnetic field, and since there must be an effect on the particle by some electromagnetic field, then is there a point in saying that we must apply some electromagnetic field to measure it? Will there be any uncertainty in a particle's spin?

Homework Helper
Gold Member
2022 Award
I mean that if we could only measure the spin of a particle by applying an electromagnetic field, and since there must be an effect on the particle by some electromagnetic field, then is there a point in saying that we must apply some electromagnetic field to measure it? Will there be any uncertainty in a particle's spin?
I'm also not sure what you're asking. QM says that a particle's spin (relative to any given axis) is indeterminate unless you measure its spin about that axis. If you do, you get one of a set of possible discrete values. In the case of an electron, for example, you get ##\pm \dfrac \hbar 2##.

What are you asking in relation to that?

vanhees71