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: Dirac Notation

  1. Apr 22, 2008 #1
    Why is it true that
    < x | f > = f ( x )
    shouldn't it be the result of the integral from minus infinity to infinty on xf(x)?
    but then it can't even be a function of x...:confused:
  2. jcsd
  3. Apr 22, 2008 #2

    George Jones

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Here, x is an "eigenvalue" of the position operator. What you need to use in the integral is not the eigenvaule, but the state that is the "eigenfunction" that corresponds to this eigenvalue.

    What states are eigenstates of the position operator?
  4. Apr 22, 2008 #3
    detla functions... and it certainly works that way, thanks:smile:

    but I'm still confused: the definition of the bra-ket is simply the inner product of the functions you put there:
    < f | g > = integral from minus infinity to infinity on f*g (f*=the complex conjugate of f).
    So the product < x' | f > should be written as follows:
    < delta (x-x') | f >...
    why do they write the eigenvalue in the bra, and not the function itself?
  5. Apr 22, 2008 #4


    User Avatar
    Science Advisor
    Homework Helper

    They write the eigenvalue in the bra because they are too lazy to write the eigenfunction. Really, it's just a notational abbreviation.
  6. Apr 22, 2008 #5
    everything makes much more sense now, thanks a lot!:smile:
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook