Time partial derivative of a wave function

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Discussion Overview

The discussion revolves around the time partial derivative of a wave function and its implications for the conservation of observables in quantum mechanics. Participants explore the nature of the time derivative as an operator and its relationship with Hermitian operators, particularly in the context of expectation values and the Hamiltonian.

Discussion Character

  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant claims that the time derivative operator is anti-Hermitian and suggests that this leads to the conclusion that every observable is conserved, questioning where their reasoning may have gone wrong.
  • Another participant clarifies that the time derivative of the expectation value of an observable is time-independent if the commutator between the Hamiltonian and the observable is zero, noting this as a standard result in quantum theory.
  • A different participant argues that the anti-Hermitian nature of the time derivative implies conservation of observables, seeking further clarification on this point.
  • One participant challenges the assumption that the time derivative is an operator on the Hilbert space, stating that time is an external parameter and not associated with the Hilbert space, suggesting a re-evaluation of the argument using spatial variables instead.

Areas of Agreement / Disagreement

Participants express differing views on the nature of the time derivative as an operator and its implications for observable conservation. There is no consensus on the correctness of the initial claims regarding the anti-Hermitian nature of the time derivative and its consequences.

Contextual Notes

Participants highlight the distinction between time as an external parameter and the functions defined in the Hilbert space, indicating potential limitations in the assumptions made regarding the time derivative's role.

NeroKid
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the probability of finding particle is a constant with time <ψ|\partialψ/\partial(t)> = -<\partialψ/\partial(t)|ψ> , the equation holds for all ψ so the time derivative operator is an anti-hermitian operator, but then consider any hermitian operator A, the rate of change of A is d(<ψ|Aψ>)/dt = <\partialψ/\partial(t)|Aψ>+<ψ|\partial(Aψ)/\partial(t)> , interchange the anti hermitian operator for the first one the equation equals to 0 which means every observable is conserved in all situation and it's clearly wrong, so where did I think wrong? Please show me
 
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What your derivation is trying to say is that the time derivative of the expectation value of an observable in an arbitray pure state is time independent iff the commutator between the hamiltonian and the observable in that pure state is 0, thing which is a pretty standard result of the theory.
 
yes but that is what after you change frome time derivative to hamiltonian , what i was trying to say is that because the time derivative is anti-hermitian it has the form < f|Ag> = - <Af|g> so any observable is conserved
 
NeroKid,
In your attempt to prove that time derivative is an anti-hermitian operator on the Hilbert space, you tacitly take for granted that time derivative is at least an operator on the Hilbert space. However, that is wrong. The time derivative is not an operator on the Hilbert space. This is because states in the Hilbert space are functions of x, not functions of x and t. The parameter t is an external parameter, not associated with the Hilbert space.

If you are still confused, try to work out your argument with x instead of t. Do you see a difference?
 
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ok I see now tks a lot
 

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