String Theory
Physicists believe there to be four fundamental forces. Three of these—the electromagnetic, the strong force, and the weak force—are amalgamated in the standard model of elementary particle physics, a family of quantum field theories that has enjoyed stupendous empirical success. Gravity, the fourth and feeblest fundamental force, is the subject of a stupendously successful nonquantum field theory, Einstein's general theory of relativity (GTR). Desiring to fit all of fundamental theoretical physics into a quantum mechanical framework, and suspecting that GTR would break down at tiny ("Planck scale," i.e., 10−33 cm) distances where quantum effects become significant, physicists have been searching for a quantum theory of gravity since the 1930s. In the last quarter of the twentieth century, string theory became the predominant approach to quantizing gravity, as well as to forging a unified picture of the four fundamental forces. A minority approach to quantizing gravity is the program of loop quantum gravity, which promises no grand unification. Both attempts to quantize gravity portend a science of nature radically different from the Newtonian one that frames much of classical philosophical discourse. They also present gratifying instances of working physicists actively concerned with recognizably philosophical questions about space, time, and theoretical virtue.
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