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There are one or two questions on this site regarding the complex conjugate of Schrodinger equation but they do not clear my doubt.

Question : The Schrodinger equation is $$iħ \frac{∂Ψ}{∂t} = HΨ $$ How to take the complex conjugate of it, I think there are two ways to do it.

  1. $$\left( iħ \frac{∂Ψ}{∂t}\right)^* = \left(\frac{-ħ^2}{2m}\frac{\partial^2Ψ}{\partial x^2}+VΨ \right)^*$$ $$\implies \frac{∂Ψ^*}{∂t} = \frac{1}{-iħ}\left(\frac{-ħ^2}{2m}\frac{\partial^2Ψ^*}{\partial x^2}+VΨ^* \right) \tag{1}\label{e1}$$

  2. $$\left( iħ \frac{∂Ψ}{∂t}\right)^* = \left(H Ψ \right)^*$$ $$\implies \frac{∂Ψ^*}{∂t} = \frac{1}{-iħ} Ψ^* H^* = \frac{1}{-iħ} Ψ^* H $$ $$\implies \frac{∂Ψ^*}{∂t} = \frac{Ψ^*}{-iħ}\left(\frac{-ħ^2}{2m}\frac{\partial^2}{\partial x^2}+V \right) \tag{2}\label{e2}$$ Which one of the above two is correct? It seems to me that first one is correct as H is nothing but a double derivative with respect to x(in 1D). However when I was trying to prove Ehrenfest Theorem(see description), the second one worked and gave me correct result, but I was unable to prove it with first one. So is the $\eqref{e2}$ correct and $\eqref{e1}$ wrong?

Description of question : I was trying to prove Ehrenfest Theorem, $$\frac{d\langle p\rangle}{dt} = \langle \frac{-\partial V}{\partial x}\rangle$$ for that I did, $$\frac{d\langle p\rangle}{dt} = \frac{ħ}{i}\frac{d}{dt}\int\left( Ψ^*\frac{\partial Ψ }{\partial x}\right)dx = \frac{ħ}{i}\int\frac{\partial}{\partial t}\left( Ψ^*\frac{\partial Ψ }{\partial x}\right)dx = \frac{ħ}{i}\int\left( \frac{\partial Ψ^*}{\partial t}\frac{\partial Ψ }{\partial x} + Ψ^*\frac{\partial^2 Ψ}{\partial x \partial t}\right)dx$$ In the next step I substituted $\frac{∂Ψ}{∂t} $ and $\frac{∂Ψ^*}{∂t} $ from schrodinger equation. Now I have two options, to proceed according to $\eqref{e1}$, or $\eqref{e2}$.

According to $\eqref{e1}$, I am getting, $$ \int\left( \left(\frac{-ħ^2}{2m}\frac{\partial^2Ψ^*}{\partial x^2}+VΨ^* \right)\frac{\partial Ψ }{\partial x} - Ψ^*\frac{\partial }{\partial x }\left( \frac{-ħ^2}{2m}\frac{\partial^2Ψ}{\partial x^2}+VΨ \right)\right)dx \tag{3}\label{e3}$$ After that I am unable to prove the theorem, I don't know if I am lacking in the math part or if the use of $\eqref{e1}$ is incorrect.

Going according to $\eqref{e2}$, I am getting, $$ \int\left( Ψ^*\left(\frac{-ħ^2}{2m}\frac{\partial^2}{\partial x^2}+V \right)\frac{\partial Ψ }{\partial x} - Ψ^*\frac{\partial }{\partial x }\left( \frac{-ħ^2}{2m}\frac{\partial^2Ψ}{\partial x^2}+VΨ \right)\right)dx \tag{4}\label{e4}$$ This easily reduces to $\langle \frac{-\partial V}{\partial x}\rangle$, proving the theorem.

Hence it appears $\eqref{e2}$ is correct, but why is that so? Why is $\eqref{e1}$ incorrect? I got to know about $\eqref{e2}$ from here

Edit - This question was closed because it looked like homework question or check my work question, however my question was which of $\eqref{e1}$ or $\eqref{e2}$ is the correct way to take complex conjugate. It is a Physics/Math conceptual question that what is the correct way and why so. According to the comment under the question, I got the result from $\eqref{e1}$ as well, but this raises even more questions are $\eqref{e1}$ and $\eqref{e2}$ equivalent, because they do not look so, in one $\frac{\partial ^2}{\partial x^2}$ is operating on $Ψ^*$ and in other $Ψ^*$ is merely multiplied to $\frac{\partial ^2}{\partial x^2}$ which will operate on something else.

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  • $\begingroup$ IBP in the first term of (3) twice gives you the result. Note that the second one doesnt need IBP (or the fact its an integral) since you actually use * as an operator on the algebra of operators. Thats why the order of Psi and H also change in your second method (IBP = integration by parts) $\endgroup$ Commented Jul 22 at 12:29
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    $\begingroup$ (2) can be correct, when you interpret both sides as linear forms (bras). However, the way you wrote it, as ordinary equations with complex terms, it is not correct. With ordinary functions, you can't equate $(H\Psi)^* $ to $\Psi^* H^*$, because these are different objects; the first one is a function of coordinates, the second one is a linear operator that acts on such functions, and these two things act differently, and have different numerical representation. To do this properly, you can use the Dirac notation and take a corresponding linear form (bra) for the given function (ket). $\endgroup$ Commented Jul 22 at 22:00
  • $\begingroup$ For example, for vector $H|\psi\rangle$ we can define the corresponding linear form $\langle \psi | H$. $\endgroup$ Commented Jul 22 at 22:03
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    $\begingroup$ There seems to be some confusion here about the meaning of complex conjugate and hermitian adjoint. See also this thread $\endgroup$
    – Roger V.
    Commented Jul 23 at 7:54
  • $\begingroup$ @JánLalinský , I get it that the second one can be correct as well if you work in dirac notation, but then is converting it back to the normal form and using it in (4) correct? Because in (3) and in (4) the integrand is coming out to be different. I understand that when integrating from -infinity to infinity the values will turn out to be the same, but what if the limits of integration were finite? then which of the integrand is correct and why? $\endgroup$
    – Param_1729
    Commented Sep 2 at 6:32

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The first approach is correct.

$H$ is an operator that acts on the wave function, so it doesn't commute with it. It seems like in the second approach you (incorrectly) mixed up the hermitian conjugate (which also involves transposing the equation) with the complex conjugate.

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  • $\begingroup$ I know the difference between hermitian conjugate and complex conjugate. It seems that the second approach is correct as well, and it is used in this wikipedia page. $\endgroup$
    – Param_1729
    Commented Sep 2 at 6:20
  • $\begingroup$ @Param_1729 that is an abuse of notation. Writen as it is, that equation is nonesense. If you properly understand that in the cited Wikipedia equation, the wavefunction isn't a function, but a bra (on which the Hamiltonian is operating), then you might write it as it is. Edit: I just saw that Jan Lalinsky pointed the same thing to you in the comments. $\endgroup$ Commented Sep 2 at 7:16

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