Rμv– 1/2 gμv R = 8πTμv The left side of the equation starts with the term Rμv, which is the Ricci tensor he had embraced earlier. The term gμv is the all-important metric tensor, and the term R is the trace of the Ricci tensor called the Ricci scalar. Together, this left side of the equation-which is now known as the Einstein tensor and can be written simply as Gμv-compresses together all of the information about how the geometry of spacetime is warped and curved by objects. The right side describes the movement of matter in the gravitational field. The interplay between the two sides shows how objects curve spacetime and how, in turn, this curvature affects the motion of objects. As the physicist John Wheeler has put it, "Matter tells spacetime how to curve, and curved space tells matter how to move."83 Thus is staged a cosmic tango, as captured by another physicist, Brian Greene: Space and time become players in the evolving cosmos. They come alive. Matter here causes space to warp there, which causes matter over here to move, which causes space way over there to warp even more, and so on. General relativity provides the choreography for an entwined cosmic dance of space, time, matter, and energy.84 At
( Walter Isaacson )
[ Einstein: His Life and ]
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