““There is no suchthing as absolutely empty space. All space contains fluctuating fields and particles. Even in the emptiest space that the laws of nature permit, there are energy levels about which the energies of the fields and particles fluctuate; and these energy levels are never sharply defined. This is where the general theory of relativity comes in. To date, there has been ni successful quantum theoretical formulation of Einstein's theory, but we firmly believe that the fluctuating masses of the vacuum cause a curvature of space. We are ignorant of the mass level, if any, at which there is no curvature.””