In theoretical physics, the Wheeler-deWitt equation is an equation that a wave function of the Universe should satisfy in a theory of quantum gravity. An example of such a wave function is the Hartle-Hawking state.
Simply speaking, the WDW equation says
where H is the total Hamiltonian constraint in quantized general relativity.
In general the Hamiltonian vanishes for a theory with time scaling invariance.
There is also a diffeomorphism constraint .