New posts in Hilbert Space

Matrix elements of the free particle Hamiltonian

Does any basis belong to a set of eigenfunctions of some observable?

In quantum mechanics, why is $\langle T\rangle=\frac{\langle p^2 \rangle}{2m}$ rather than $\langle T\rangle=\frac{\langle p \rangle^2}{2m}$?

How to derive annihilation and creation operators?

In quantum mechanics, is $|\psi\rangle$ equal to $\psi(x)$?

If spontaneous symmetry breaking only occurs in infinite systems, why do we observe similar effects in finite systems?

Definition of an operator in quantum mechanics

Do the ladder operators $a$ and $a^\dagger$ form a complete algebra basis?

Why are $SU(2)$-generators interpreted as *spatial* components of spin?

Does "natural" superposition of particles exist?