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

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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 whichmatter 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.
Tom Murphy and his colleagues fire laser beams at reflectors on the moon with the aim to
选项 A、measure the distance between the moon and the Earth.
B、calculate the speed of light.
C、check how long it takes light to cross it.
D、prove the equivalence principle more accurately.
答案D
解析 事实细节题，考查目的细节。根据Tom Murphy定位到第四段。该段紧承第三段，从该段首句A moreprecise test requires a taller tower可知，下文所说的Tom Murphy和他的同事所做的激光实验是为了更精确地论证等效原理，故D项正确。A项“测量地月之间的距离”和C项“检测激光穿过月球所需要的时间”不是文中提到的test的目的。而是其步骤；B项“计算光的速度”和文中“光速已知”不符。
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