London created a number of conceptual difficulties. The very first of these is the question of what happens to the kinetic energy of these particles at temperatures below TB, since, as each one drops into the ground state, it carries with it an energy ≈0.77kTB. Thus, at any temperature T < TB, the particles should make a contribution U0(T ) = 0.77N(1 − (T/TB)3/2)kTB to the internal energy. Expression (11) then becomes