Dreams of a Final Theory: The Scientist's Search for the Ultimate Laws of Nature

by Steven Weinberg

Mach’s hint was picked up by Einstein and made concrete in his general theory of relativity. In general relativity there is indeed an influence exerted by the distant stars that creates the phenomenon of centrifugal force in a spinning merry-go-round: it is the force of gravity. Of course nothing like this happens in Newton’s theory of gravitation, which deals only with a simple attraction between all masses. General relativity is more complicated; the circulation of the matter of the universe around the zenith seen by observers on the merry-go-round produces a field somewhat like the magnetic field produced by the circulation of electricity in the coils of an electromagnet. It is this “gravitomagnetic” field that in the merry-go-round frame of reference produces the effects that in more conventional frames of reference are attributed to centrifugal force. The equations of general relativity, unlike those of Newtonian mechanics, are precisely the same in the merry-go-round laboratory and conventional laboratories; the difference between what is observed in these laboratories is entirely due to their different environment—a universe that revolves around the zenith or one that does not. But, if gravitation did not exist, this reinterpretation of centrifugal force would be impossible, and the centrifugal force felt on a merry-go-round would allow us to distinguish between the merry-go-round and more conventional laboratories and would thus rule out any possible equivalence be...