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Question about expectation values.

by cragar
Tags: expectation, values
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cragar
#1
Dec11-12, 09:12 AM
P: 2,469
Is it possible to define operators to find the expectation value of position for a Gaussian wave packet. Similar to finding raising and lowering operators for the harmonic oscillator in terms of position and momentum and then using those to find <x> and <p>. But I was just wondering if this could be done for a Gaussian wave packet.
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mfb
#2
Dec11-12, 11:03 AM
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P: 12,113
Here is one: X

If your wave packet is expressed as function of position, ##<X> = \int \psi \psi^* x dx##.
cragar
#3
Dec12-12, 08:35 AM
P: 2,469
I don't completely understand what you are doing? Is X my new operator.
are you starting with the definition of expectation value and then going from there.

jtbell
#4
Dec12-12, 08:46 AM
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Question about expectation values.

The position operator (in the position representation) is simply ##x##. So the general definition of expectation value:

$$\langle A \rangle = \int {\Psi^* A_{op} \Psi dx}$$

becomes

$$\langle x \rangle = \int {\Psi^* x \Psi dx}$$

Plug in your wave function and grind out the integral.
cragar
#5
Dec13-12, 08:05 PM
P: 2,469
ok i understand that. I was trying to think of a way to compute that with out doing an integral. Like how you can do that for the harmonic oscillator with a+ and a -
like [itex] <x>=<U|a^+ + a^-|U> [/itex]
u is the wave function and a+ is the raising operator.
can I do this for a Gaussian wave packet.
mfb
#6
Dec14-12, 08:11 AM
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P: 12,113
If you define appropriate operators.
For a gaussian wave packet, the expectation value of the position is the central value of the distribution. If you know that, you are done.


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