Its astonishing predictions have proven correct in numerous applications. For example, when placed on high-speed aircraft, ultraprecise cesium clocks lag by the precise amount Einstein predicted. Mammoth accelerators boost elementary particles to near light speeds by faithfully timing their actions to relativity’s rhythms. Nuclear reactions generate energetic offspring that—invigorated by time dilation—live longer lives than their slower cousins.

Given such a fantastic achievement, why didn’t Einstein stop there? Why did he wrestle with nature’s laws for another decade, until he could mold special relativity into a far more mathematically intricate theory, known as general relativity? The reason stems from two critical omissions: acceleration and gravity.

An avid reader of Mach, Einstein knew that special relativity failed to answer Mach’s question, “How does it come about that inertial systems are physically distinguished above all other coordinate systems?” That is, what makes constant velocity the favorite type of motion in nature?

Einstein also realized that this question was deeply linked to the mysteries of gravitational attraction. Why, without air resistance, do light feathers and heavy stones drop toward Earth at the same rate? Clearly, he surmised, gravity’s pull cannot just depend on the bodies in question but must be seated in space-time itself.

In similar fashion to his earlier theory, gravity came to Einstein’s attention in the form of a thought experiment. He imagined someone falling off the roof of a house while simultaneously dropping an object (say a box of tools). As the unfortunate man descends, he notices that it remains right next to him. Although the dropped object is falling independently, he can reach out and grab it whenever he wants. Except when he hazards to look down, his situation