That's a good question. I'm still not completely understood the mass of neutrino and its oscillation between e, mu, and tau kind even though it can explain the missing solar neutrinos.
Time independence: [∂E(g)]²=[∂F(a)×∂r(a)]·[∂F(b)×∂r(b)] and Mass independence: ¶a(t)·¶r(t)=c²