Physics Forums (http://www.physicsforums.com/index.php)
-   Quantum Physics (http://www.physicsforums.com/forumdisplay.php?f=62)
-   -   expected value of Q(x,p) (http://www.physicsforums.com/showthread.php?t=243554)

 nicksauce Jul5-08 01:24 PM

expected value of Q(x,p)

To find the expected value of Q(x,p) we evaluate $<\psi|Q(x,-i\hbar \frac{\partial }{\partial x})|\psi>$. But what do you do if you want to find, say, <p^(3/2)>. How do you raise the derivative operator to the three-halves?

 Count Iblis Jul5-08 07:03 PM

Re: <p^(3/2)>

Quote:
 Quote by nicksauce (Post 1791485) To find the expected value of Q(x,p) we evaluate $<\psi|Q(x,-i\hbar \frac{\partial }{\partial x})|\psi>$. But what do you do if you want to find, say, . How do you raise the derivative operator to the three-halves?
Expand the state in momentum eigenstates.

 Euclid Jul5-08 07:04 PM

Re: <p^(3/2)>

You would make like you do for angular momentum. Since you can't take square roots of operator, you have to study the square of the quantity you are interested in. So in your case, you would have to settle for $$\sqrt{ \langle p^3\rangle}$$, but you would have to put absolute values somewhere.

...edit: go with count iblis' suggestion instead

 clem Jul5-08 07:05 PM

Re: <p^(3/2)>

Fourier transform the wave function to the momentum representation, getting
$$\phi(p)$$. Then p is just like a c number.

 nicksauce Jul5-08 08:17 PM

Re: <p^(3/2)>

Okay, thanks for the replies.

 Count Iblis Jul5-08 09:55 PM

Re: <p^(3/2)>

Next problem:

How would you handle:

<x^(1/2)p^(3/2)>

 per.sundqvist Jul6-08 02:19 AM

Re: <p^(3/2)>

Quote:
 Quote by Count Iblis (Post 1791982) Next problem: How would you handle:
Formally if f(x) is a nice function you could always calculate the n'th derivative using Fourier transforms, like

$$\frac{d^nf[x]}{dx^n}=\frac{1}{2\pi}\int_{-\infty}^{\infty}dk(ik)^ne^{-ikx} \int_{-\infty}^{\infty}dxf[x]e^{ikx}$$

also when $$n=\sqrt{pi}/i^5.8$$, for example. So the <x^(1/2)p^(3/2)> involves a tripple integral.

 clem Jul6-08 05:41 AM

Re: <p^(3/2)>

Quote:
 Quote by Count Iblis (Post 1791982) Next problem: How would you handle:
Fourier transform $$\psi(x)$$ and $$x^{1/2}\psi(x)$$ separately.

 Count Iblis Jul6-08 08:45 AM

Re: <p^(3/2)>

Ok, that was too easy for you two. :smile: Let me think of a more difficult problem. Well, why not just consider <f(x,p)> where f is some arbitrary function, like e.g.:

$$f(p,x)=\sqrt{x^2 + p^2}$$

Now, you can just modify the Fourier transform method and write this too as a triple integral. However, that may not be the simplest way, particularly not in the case when f is given as above. :smile:

 clem Jul6-08 06:58 PM

Re: <p^(3/2)>

For that you may need a Taylor expansion.

 Count Iblis Jul7-08 11:52 AM

Re: <p^(3/2)>

Diagonalizing the operator x^2 + p^2 is easier. The eigenstates are the harmonic oscillator eigenstates, let's denote them by |n>. You can thus compute the average as:

<psi|sqrt(x^2 + P^2)|psi> =

sum over n of <psi|sqrt(x^2 + p^2)|n><n|psi>

sqrt(x^2 + p^2)|n> = sqrt[(n+1/2)C]

were C follows from the usual H.O. algebra (i'm too lazy to compute it right now) So, the average is:

sum over n of sqrt[(n+1/2)C] |<n|psi>|^2

 clem Jul8-08 04:58 PM

Re: <p^(3/2)>

Expanding in SHO states may or may not be easier than Taylor expansion.
This would depend on the original wave function and operator.
SHO works for the particular operator x^2+p^2, but TE works for most operators.

 All times are GMT -5. The time now is 09:47 AM.