Question about expectation value.

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Discussion Overview

The discussion revolves around the time independence of the energy expectation value in quantum mechanics, particularly in the context of systems with a time-independent Hamiltonian. Participants explore the implications of this concept and its generalization to other operators that commute with the Hamiltonian.

Discussion Character

  • Exploratory, Technical explanation, Conceptual clarification, Debate/contested

Main Points Raised

  • Some participants propose that the energy expectation value is independent of time, particularly in the case of an infinite square well, noting that time evolution cancels out during integration.
  • Others argue that this independence is a manifestation of energy conservation, applicable as long as the Hamiltonian remains time-independent, and extends to any operator that commutes with the Hamiltonian.
  • A participant emphasizes that energy conservation is a fundamental result in both classical and quantum mechanics when dealing with a time-independent Hamiltonian.
  • One participant reiterates their initial claim about the time independence of the energy expectation value and expresses a desire to prove it in a more general context, referencing the Ehrenfest theorem as a related concept.

Areas of Agreement / Disagreement

Participants generally agree on the time independence of the energy expectation value under certain conditions, but there is some debate regarding the implications and generalizations of this concept.

Contextual Notes

The discussion does not resolve the broader implications of the time independence of expectation values or the specific conditions under which these results hold. There is also a lack of consensus on the necessity of further proof or generalization.

cragar
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It seems that the energy expectation value is independent of time.
I did it for an infinite square well. And when you time evolve your wave function
the time evolution cancels when you complex conjugate it and then do the integral.
<E>=<ψ|E|ψ> it seem like this might always independent of time, What do you guys think.
I guess I could try to prove it in general.
 
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cragar said:
It seems that the energy expectation value is independent of time.

That's called energy conservation and should not be so surprising. It's true as long as the hamiltonian is time independent. And it generalized to the expectation value of any operator that commutes with the hamiltonian.
 
If you have a time-independent Hamiltonian, energy is always conserved in your system. This is a very fundamental result and it works both in classical and quantum mechanics.
 
cragar said:
It seems that the energy expectation value is independent of time.
I did it for an infinite square well. And when you time evolve your wave function
the time evolution cancels when you complex conjugate it and then do the integral.
<E>=<ψ|E|ψ> it seem like this might always independent of time, What do you guys think.
I guess I could try to prove it in general.

A more general result is the Ehrenfest theorem: http://en.wikipedia.org/wiki/Ehrenfest_theorem
 

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