necessarily so. In general relativity the force of gravity results from the curvature of space-time by matter. But it does not explain where the mass in the universe comes from. Einstein’s theory merely assumes that the mass came into existence when the universe came into being and has since remained unaltered.
Einstein himself was dissatisfied by the dichotomy between the vibrant, flowing geometries on the left-hand side of his gravitational equations and the stultified stuff on the right. The mass terms seemed positioned on the right-hand side as necessary but awkward ballast. Like the counterweights on an elevator, they helped lift the geometric side to ethereal heights. It would be nice, Einstein felt, if everything were lofty and dynamic and there were no need for extra bulk. As he wrote in an essay, “Physics and Reality”:
[General relativity] is sufficient—as far as we know—for the representation of the observed facts of celestial mechanics. But it is similar to a building, one wing of which is made of fine marble (left part of the equation), but the other wing of which is built of low-grade wood (right side of equation). The phenomenological representation of matter is, in fact, only a crude substitute for a representation which would do justice to all known properties of matter.
Thus, one of Einstein’s principal goals in the latter half of his life was to perform the alchemy of turning wood into marble. His motivation for this effort stemmed largely from his strong belief in Machian ideals. Mass, Einstein felt, should draw its nature from the relationships between all objects in the universe. Pure geometry would be the proper mechanism for conveying such information.
Although Einstein first explored such notions in the 1920s, they were hardly new. In the early 1870s the British mathematician William Clifford caused quite a stir with his proposal that matter represents lumps in the carpet of space. His article, “On the Space Theory of Matter,” postulated that empty space is completely smooth