In his Principia, published in 1687, Newton recognized the importance of this question but acknowledged that his own law was disturbingly silent about the answer. Newton was certain that there had to be something communicating gravity from place to place, but he was unable to identify what that something might be. In the Principia he gibingly left the question “to the consideration of the reader,” and for more than two hundred years, those who read this challenge simply read on. That’s something Einstein couldn’t do.
