# Where does the position operator come from?

In quantum mechanics the momentum and energy operators appear in Schroedinger's equation. In fact in the derivation of Schroedinger's equation from the classical wave equation the momentum operator comes from the space derivative, and the energy operator comes from the time derivative, which has nice links to Noether's theorem.

My question is where does the position operator come from? What does it mean for a wave to have a position anyway? What I mean is what were the reason for choosing the position operator?

• Suppose you Fourier transform to the momentum representation, the FT of ψ(x) being φ(p). The position operator will now be a gradient with respect to momentum, and, the momentum operator simply multiplication by the variable p. Noether's theorem would give you conservation of x if only your hamiltonian did not depend on p. This is, of course, hardly unusual for conventional hamiltonians with stock kinetic terms, but you can contrive highly bizarre mirror systems where such things happen, if only for formal mathematical convenience. Jun 6 '16 at 21:46
• What do you mean "where does it come from"? The very definition of canonical quantization is that position and momentum become operators. Jun 7 '16 at 13:18

Purely formal considerations suggest that the Lie bracket of quantum observables should be proportional to the Poisson bracket of their classical counterparts. (In particular, the classical Poisson bracket satisfies$\lbrace u,wx\rbrace v +u \lbrace v,wx\rbrace =\lbrace uv,w\rbrace x + w \lbrace uv,x\rbrace$, and the proportionality follows from this and a little algebraic manipulation (after dropping the commutativity assumption.))
Classically, the Poisson bracket of position and momentum is a constant (namely $1$), so in the quantum setting we want the Lie bracket of position and momentum to be a constant (in this case purely imaginary so that $p$, $q$ and $[p,q]$ can all be Hermitian). That is, we want to identify position and momentum with Hermitian operators satisfying $[p,q]=i\hbar$ for some real constant $\hbar$.