The complete microscopic theory of superconductivity was finally proposed in 1957 by John Bardeen, Leon N. Cooper, and Robert Schrieffer. This BCS theory explained the superconducting current as a superfluid of Cooper pairs, pairs of electrons interacting through the exchange of phonons. For this work, the authors were awarded the Nobel Prize in Physics in 1972. The BCS theory was set on a firmer footing in 1958, when Nikolay Bogolyubov showed that the BCS wavefunction, which had originally been derived from a variational argument, could be obtained using a canonical transformation of the electronic Hamiltonian. In 1959, Lev Gor'kov showed that the BCS theory reduced to the Ginzburg-Landau theory close to the critical temperature. Gor'kov was the first to derive the superconducting phase evolution equation 2 e V = ℏ ℏ ∂ ∂ ϕ ϕ ∂ ∂ t .