Proof of orthonormality of the hamiltonian when not real

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Discussion Overview

The discussion revolves around the proof of orthonormality of the Hamiltonian when it is not real, contrasting it with the case when the Hamiltonian is real. Participants explore the implications of this distinction in the context of eigenstates and self-adjoint operators.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • One participant expresses difficulty in proving orthonormality for a non-real Hamiltonian, having previously understood it for a real Hamiltonian.
  • Another participant questions the use of the term "orthonormality," suggesting it may be confused with "Hermitian" or "self-adjoint," and notes the importance of clearly stating the problem.
  • A participant clarifies that the discussion pertains to exam revision rather than homework and attempts to explain their understanding of the proof involving eigenstates and the Hamiltonian.
  • Further elaboration includes a reference to the relationship between eigenstates of self-adjoint operators and orthogonality, emphasizing that distinct eigenvalues lead to orthogonal eigenstates.
  • One participant provides a brief proof outline for the orthogonality of eigenstates of a self-adjoint operator, indicating that the proof is straightforward once the concepts are understood.
  • There is a request for clarification regarding the textbook used by the lecturer, suggesting a potential resource for further understanding.

Areas of Agreement / Disagreement

Participants express differing views on the terminology used and the clarity of the problem statement. There is no consensus on the correct approach to proving orthonormality for a non-real Hamiltonian, and the discussion remains unresolved regarding the specific proof methodology.

Contextual Notes

Participants note the distinction between "Hermitian" and "self-adjoint" operators and the implications for eigenvalues and eigenstates, but the discussion does not resolve the specific assumptions or definitions that may affect the proof.

NicolaNorth
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Hi,

I know how to prove the orthonormality of the hamiltonian when it is real but am struggling to work out how to prove it when the hamiltonian is not real.

When proving for a real hamiltonian the lefthand side equals zero as H(mn)=H(nm)complexconjugate. but if the hamiltonian is not real, i don't know how to proceed.

Can anybody help?

Thanks
 
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NicolaNorth said:
[...] orthonormality of the hamiltonian when it is real but am struggling to work out how to prove it when the hamiltonian is not real.
I suspect you're using the term "orthonormality" incorrectly in this context. Did you perhaps mean "Hermitian", or maybe "self-adjoint"? (Try looking up these terms on Wikipedia if they're not familiar...)

It's a bit hard to help when the problem is not clearly stated. (BTW, if this is homework it belongs over in the homework forums... :-)
 
Thanks for the reply.

It is not homework, but exam revision. Is that ok in this part of the forums?

I think maybe i did not correctly explain the problem?

We proved orthonormality for the eigenstates of the hamiltonian when assuming H is real and then the lecturer said that we would need to be able to do this when H is not real.

To prove it we considered

<ψm|H|ψn> = En<ψm|ψn> and <ψn|H|ψm> = En<ψn|ψm>

and then took the complex conjugate of the second equation and subtracted it from the first eqn.

The LHS equaled zero as Hmn=(Hnm)complex conj

Does that make more sense?

I am aware of how to prove that the Hamiltonian is Hermitian as this is something else we have covered, but maybe i misunderstood the lecturer?

Thanks
 
NicolaNorth said:
It is not homework, but exam revision. Is that ok in this part of the forums?
Not sure. I'll leave it to the moderators to move this thread if they wish.

We proved orthonormality for the eigenstates of the hamiltonian when assuming H is real and then the lecturer said that we would need to be able to do this when H is not real.
A slightly more correct (and generalized) version of the theorem is that eigenstates of a self-adjoint operator A corresponding to distinct eigenvalues are orthogonal. "Hermitian" is the finite-dimensional (matrix) version of "self-adjoint". The latter applies to more general operators (e.g., derivatives) which are not ordinary matrices.

The proof is a one-liner, once you understand the (abstract) meaning of "adjoint" and "self-adjoint", and also that eigenvalues of a self-adjoint operator are real (which also has a 1-line proof).
(Textbooks such as Isham, or Ballentine can probably help there.)
[tex] \def\<{\langle}<br /> \def\>{\rangle}<br /> (a-b) \, \<\psi_a|\psi_b\><br /> ~=~ a\,\<\psi_a|\psi_b\> - b\<\psi_a|\psi_b\><br /> ~=~ \<A\psi_a|\psi_b\> - \<\psi_a|A\psi_b\><br /> ~=~ \<\psi_a|A\psi_b\> - \<\psi_a|A\psi_b\><br /> ~=~ 0 ~,[/tex]
implying [itex]\<\psi_a|\psi_b\> = 0[/itex] since a,b were assumed distinct.

Which textbook(s) is your lecturer using?
 

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