I 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?

 

[vanhees71]

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?

 

[PeroK]

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?

 

[Meir Achuz]

"How else do you want to measure the spin of your particle than with applying some electromagnetic field?"
Most particle spins are measured without an electrical electromagnetic field, especially neutral particles.
The neutrino spin was measured using conservation of angular momentum.