A stationary state is a quantum state with all observables independent of time. It is an eigenvector of the Hamiltonian. This corresponds to a state with a single definite energy (instead of a quantum superposition of different energies). It is also called energy eigenvector, energy eigenstate, energy eigenfunction, or energy eigenket. It is very similar to the concept of atomic orbital and molecular orbital in chemistry, with some slight differences explained below. == Introduction == A stationary state is called stationary because the system remains in the same state as time elapses, in every...

In differential geometry, the four-gradient (or 4-gradient) ∂ {\displaystyle \mathbf {\partial } } is the four-vector analogue of the gradient ∇ → {\displaystyle {\vec {\mathbf {\nabla } }}} from vector calculus. In special relativity and in quantum mechanics, the four-gradient is used to define the properties and relations between the various physical four-vectors and tensors...