Newton was criticized for this-was he suggesting that some higher being was responsible for the force between stars and planets? But his law was extremely successful, and, for almost two centuries, this question was largely put aside. Neptune, for example, is so far away that it takes light from the sun four hours to reach it, but it would move immediately in response to the sun’s sudden motion. This despite the fact that the planets are far away. In Newton’s laws, if, say, the sun were suddenly to “jump” (for the moment, take the far-fetched possibility of some space invader attaching rockets to it), the effect on the planets in the solar system would be instantaneous. Newton-and perhaps more importantly his critics-were very troubled by a feature of his theory called action at a distance. This was actually related to a deficiency of Newton’s law of gravitation that was clear from the moment he promulgated it. In the course of this project, he confronted the question: How does Newton’s theory of gravity fit in with his principles? The simple answer: It doesn’t. In 1907, two years after putting forward his special relativity, Einstein was asked to write a review of the subject. But what about Newton’s law of gravity? How might it be purged of the notions of absolute space and absolute time? We’ve seen that the laws of electricity and magnetism, encoded in Maxwell’s equations, forced a rethinking of the most basic concepts of space and time. In 1905, physicists understood something of the laws governing two types of forces: those of electricity and magnetism and of gravity.
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