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: Question if expectation value is considered a measurement?

  1. Sep 13, 2012 #1

    I was just curious about expectation values. One of the postulates of quantum mechanics state:
    The only possible results of a measurement is an eigenvalue of the operator.

    Now, is the expectation value considered a measurement, thus considered an eigenvalue?

  2. jcsd
  3. Sep 13, 2012 #2
    An expectation value is a weighted average of all of the eigenvalues. So it may or may not actually be a valid eigenvalue itself.

    As an example, the X component of the spin operator has two eigenvalues: -1/2 and 1/2. However, if you take the X expectation value of a Z state (which is a linear combination of both of those eigenstates), the expectation value will be 0, since both -1/2 and 1/2 states appear in equal combination. 0 is not actually a valid eigenvalue of the X operator, but it's the average of the two actual eigenvalues.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook