Gravity: Ultimate free fall

 作者:涂铖氇     |      日期:2019-03-15 01:10:00
By Dana Mackenzie According to legend, it was Galileo Galilei who first performed one of science’s most famous experiments. After dropping a pair of cannonballs of different weights off the top of the Leaning Tower of Pisa, he heard the two balls hit the ground at the same instant. In this way he proved what physicists would later call the principle of equivalence: all objects fall at the same rate, regardless of their mass or composition, provided there are no forces acting on them other than gravity. Whether or not Galileo really performed this experiment, it is the prototype of an iconic demonstration in physics, requiring only a pair of weights and a vacuum chamber to eliminate air resistance. The equivalence principle that it demonstrates has meanwhile become a cornerstone of Einstein’s general theory of relativity, the model of gravity that has reigned for nearly a century. Now two physicists at Stanford University in California, Mark Kasevich and Savas Dimopoulos, are preparing to challenge this whole premise, with a modern Tower of Pisa. Rather less picturesque than the Tuscan original, theirs is a shaft dug 10 metres down from a basement laboratory. Instead of dropping cannonballs, they will launch rubidium atoms upward and watch them fall down again. To measure the rate of descent, they will use an exquisitely sensitive timing method. The result: a test of the equivalence principle and general relativity far more stringent than any that has gone before. Why would they want to do that? Because, says Dimopoulos,