1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
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

  1. Sep 25, 2010 #1
    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....
     
  2. jcsd
  3. Sep 25, 2010 #2

    fzero

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Yes, they want you to compute [tex]\langle z;+ | (\Delta S_x)^2|z;+\rangle[/tex]. The rest of your intuition seems on track.
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook