2. 考研
  3. Gravity is a slippery beast. We don’t know how strong it is, how it works or how fast its effects move. But this year we made pr

Gravity is a slippery beast. We don’t know how strong it is, how it works or how fast its effects move. But this year we made pr

admin2010-01-13  68
问题    Gravity is a slippery beast. We don’t know how strong it is, how it works or how fast its effects move. But this year we made progress.
   October saw the most accurate measurement yet of Newton’s gravitational constant(引力常数 ), G, a measure of the strength of the gravitational interaction between two objects. A Swiss team calculated G’s value by measuring how the gravitational pull of two huge tanks of mercury affected the weights of test masses.
   However, there are discrepancies between measurements of G made in different labs. This year a highly contentious(有争议的) explanation for this was proposed. A group of string theorists proposed that gravity is subtly affected by magnetic fields, and that G should be larger near Earth’s poles where the magnetic field is stronger. Sure enough, this fits with the measurements so far. So G’s varying values might just be the first proof of the hidden dimensions predicted by string theory.
   Equally tantalising is possible evidence for the existence of gravitational waves, the ripples in space-time supposedly caused by abrupt, violent cosmic events. An Italian team reported that two massive aluminium bars, one at CERN(欧洲粒子物理研究所) near Geneva, the other in Italy, had once vibrated in unison(一致)—perhaps as a result of a passing gravitational wave, they suggest.
   The claims will be closely scrutinised by gravity researchers in Washington state. They got to turn on a very expensive toy this year. LIGO, one of the biggest scientific instruments ever built. Its twin sets of intersecting 4-kilometre-long laser beams should be very sensitive to any waves. But so far the $400-million machine has not seen anything.
   At least one gravitational mystery has (hopefully) been wrapped up this year. when you move something, how long before its new position will affect its gravitational pull on surrounding objects? In other words, what is the speed of gravity? Newton thought the effect instantaneous, but Einstein said it could travel no faster than the speed of light.
   Astronomers have finally devised a way to test which one of them was right, based on the way gravity bends radio waves from a distant quasar(类星体). They finished the experiment in September. We don’t yet know the answer but our money is on Einstein.
Until now LIGO has ______.
选项 A、never been turned on yet
B、been the biggest scientific instrument
C、done nothing to look for the gravitational waves
D、found no gravitational waves
答案D
解析  答案可以从第5段的最后一句But so far the $400-million machine has not seen anything中推出。
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