My name is Alexander FufaeV and here I write about:
What is an Observable in Quantum Mechanics?
An observable is a measurable quantity (e.g. momentum, location, energy). It is represented by a hermitian operator \( \hat A \) and its eigenvalues \(a\). This means that if you apply the operator \( \hat A \) to its eigenstate \( \Psi_a \), you get back a possible value \( a \) of the observable and the eigenstate:
$$ \begin{align} \hat A \, \Psi_a ~=~ a \, \Psi_a \end{align} $$