(adsbygoogle = window.adsbygoogle || []).push({}); 1. The problem statement, all variables and given/known data

compute [itex]\left\langle {{{(\Delta {S_x})}^2}} \right\rangle \equiv \left\langle {{S_x}^2} \right\rangle - {\left\langle {{S_x}} \right\rangle ^2}[/itex], where the expectation value is taken for the S[z] +state.

2. Relevant equations

3. The attempt at a solution

Wait...how can we be speaking of the expectation value for the S[z] state when we are computing the expectation value of the S[x] operator? Is this problem statement saying that the system is in the |+> eigenstate, that is, the state that gives 1/2-hbar from the S[z] operator with 100% certainty? The |+> eigenstate that is a linear combination of the |x;+> and |x;-> eigenstates with a common coefficient of sqrt(2)/2? If so, I sure can evaluate the dispersion in S[x], fo' sho....

**Physics Forums | Science Articles, Homework Help, Discussion**

Dismiss Notice

Join Physics Forums Today!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# Homework Help: Help me understand the problem statement

**Physics Forums | Science Articles, Homework Help, Discussion**