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  3. Familiar as it may seem, gravity remains a mystery to modern physics. Despite several decades of trying, scientists have failed

Familiar as it may seem, gravity remains a mystery to modern physics. Despite several decades of trying, scientists have failed

admin2021-09-17  78
问题     Familiar as it may seem, gravity remains a mystery to modern physics. Despite several decades of trying, scientists have failed to fit Einstein’s general theory of relativity, which describes how gravity holds big objects together, with the quantum mechanics (an extension of statistical mechanics based on quantum theory) he pioneered, which describes the tiny fundamental particles of which matter consists and the forces by which they interact. Recent discoveries have highlighted further problems.
    Many physicists are therefore entertaining the idea that Einstein’s ideas about gravity must be wrong or at least incomplete. Showing exactly how and where the great man erred is the task of the scientists who gathered at the "Rethinking Gravity" conference at the University of Arizona in Tucson this week.
    One way to test general relativity is to examine ever more closely the assumptions on which it rests, such as the equivalence principle: that gravity accelerates all objects at the same rate, regardless of their mass or composition. This principle was famously demonstrated by Galileo Galilei some 400 years ago when he simultaneously dropped cannon and musket balls, and balls made of gold, silver and wood, from the Tower of Pisa. Each appeared to hit the ground at the same time.
    A more precise test requires a taller tower. In effect, researchers are sending balls all the way to the moon and back. Tom Murphy, of the University of California, San Diego, and his colleagues fire laser beams from the deserts of New Mexico at reflectors placed on the moon by American and Russian spacecraft in the late 1960s and early 1970s. They use a telescope to capture the small fraction of the light that returns. Because the speed of light is known, they can calculate the distance between the Earth and the moon from the time taken for light to pass through it.
    According to general relativity, because the Earth and the moon orbit the sun, they should "fall" towards it at the same rate, in the same way as Galileo’s balls fell to the ground. By repeatedly measuring the distance between them, scientists can calculate the orbits of the Earth and the moon around the sun relative to each other. If the equivalence principle were violated, the moon’s orbit around the Earth would not appear straight, either towards or away from the sun. So far, Dr Murphy told the conference, these experiments have merely confirmed the equivalence principle to one part in 10 trillion. Dr Murphy and his colleagues hope that even more precise measurements could ultimately show general relativity to be only approximately correct. This would usher in a new revolution in physics.
Galileo Galilei’s experiment at the Tower of Pisa around 400 years ago
选项 A、was one way of testing Einstein’s general theory of gravity.
B、illustrated the insignificance of mass and composition of an object.
C、acted as a well-known example to testify the equivalence principle.
D、showed that all objects on Earth were influenced by gravity.
答案C
解析 根据Galileo Galilei和400 years ago定位到第三段。其中提到大约400年前伽利略在比萨斜塔的著名实验证明了等效原理，C项与此意相同。
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