Thousands of atoms across: that is a macroscopic state. It is not so easy to break that pair of electrons apart, that is why the critical temperature is high. It is very hard for just one electron to be scattered by an irregularity in the lattice, since the electrons are averaging over billions of atoms in the lattice. We saw before in normal metals that the mean free path, and the conductivity, go to infinity except for scattering. There is no scattering in the superconducting state, so the resistance is zero. Experiments have looked for decay of currents for years and seen none in good superconductors.
Thousands of atoms across: that is a macroscopic state. It is not so easy to break that pair of electrons apart, that is why the critical temperature is high. It is very hard for just one electron to be scattered by an irregularity in the lattice, since the electrons are averaging over billions of atoms in the lattice. We saw before in normal metals that the mean free path, and the conductivity, go to infinity except for scattering. There is no scattering in the superconducting state, so the resistance is zero. Experiments have looked for decay of currents for years and seen none in good superconductors.