In the late nineteenth century, Scottish mathematician James Clerk Maxwell devised a set of four fundamental equations that describe the behavior of both electric and magnetic fields. These equations provided the basis for unifying electricity and magnetism into a single force called electromagnetism.

Einstein used Maxwell's equations to develop his special theory of relativity (1905), which shows how light -- an electromagnetic wave phenomenon -- behaves in different frames of reference. He later used the equations to develop his general theory of relativity (1915), which explains gravity and the geometry of the universe. In both theories, time is not completely separate from what is happening in space. Indeed, observers in different frames of reference (that is, moving with respect to one another) find that their measurements of the rate at which time passes do not agree. In his general theory of relativity, Einstein also established that time and space are bound together in a four-dimensional space-time continuum. Einstein used "space-time" to explain why Newton's understanding of gravity was incomplete.

Einstein concluded in his general theory of relativity that an object's motion due to gravity could be understood not as a response to a force, but as free and unforced movement in a region of space and time that had been distorted by the presence of a mass. We feel gravity on Earth because Earth's mass causes a curvature of space-time. Our bodies respond to that curvature by accelerating toward Earth's center. What's more, the larger the mass, the greater the curvature. The obvious curved path we see a tossed object take at Earth's surface is not the curvature of space-time Einstein is talking about. His curvature is in a four-dimensional space, with time as the fourth dimension, and is more subtle than that. Still, that more-subtle curvature does explain the path in three-dimensional space we see before us.