1
$\begingroup$

Some quantum mechanics books tell us that the definitions of hermitian are

If $\langle\psi|A\phi\rangle=\langle B\psi|\phi\rangle$ for linear operators, then $B=A^\dagger$

If $\langle\psi|C\phi\rangle=\langle D\psi|\phi\rangle^*$ for anti-linear operators, then $D=C^\dagger$

  1. Why are definitions different between linear operators and anti-linear operators ?
  2. Are they equivalent to the definition $Q^\dagger=(Q^T)^*$ in linear algebra, where $Q^T$ is the transpose of $Q$ ?
$\endgroup$
1

1 Answer 1

3
$\begingroup$

Why are defintions different between linear operators and anti-linear operators?

Let $\lambda$ be any complex scalar. Then $$\langle \psi|C(\lambda \phi)\rangle = \langle\psi|\overline \lambda C\phi\rangle = \overline \lambda\langle\psi|C\phi\rangle$$

by the definition of anti-linearity. If we use the definition we normally use for linear operators, then we would find $$\langle \psi | C(\lambda \phi)\rangle = \langle D\psi|\lambda \phi\rangle$$ but also that $$\overline \lambda \langle\psi|C\phi\rangle = \overline \lambda\langle D \psi|\phi\rangle = \langle D\psi|\overline \lambda \phi\rangle$$

which is inconsistent. Using the slightly modified definition fixes this problem.

Are they equivalent to the definition $Q^\dagger=(Q^T)^∗$ in linear algebra, where $Q^T$ is the transpose of $Q$?

The first one is, yes.

If you imagine two arbitrary column vectors $\mathbf x$ and $\mathbf y$ and matrices $A$ and $B$, then the first definition becomes $$\mathbf x^\dagger A \mathbf y = (B\mathbf x)^\dagger \mathbf y \implies B=A^\dagger$$

where the dagger denotes conjugate transposition. But this should be clear, since $$(B\mathbf x)^\dagger = \mathbf x^\dagger B^\dagger$$ so $$\mathbf x^\dagger A \mathbf y= \mathbf x^\dagger B^\dagger \mathbf y$$ If that equality holds for all $\mathbf x$ and $\mathbf y$, then $A=B^\dagger \iff A^\dagger = B$.

The case of anti-linear operators is a bit more subtle because an anti-linear operator cannot be written as a matrix all by itself; it has to be a matrix plus a complex conjugation, and therefore cannot be realized if we restrict ourselves to standard matrix algebra.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.