Stuart Clark writes:
EACH clear night when the moon is high in the sky, a group of astronomers in New Mexico take aim at our celestial neighbour and blast it repeatedly with pulses of light from a powerful laser. They target suitcase-sized reflectors left on the lunar surface by the Apollo 11, 14 and 15 missions, as well as by two Russian landers.
Out of every 300 quadrillion (1015) photons that are sent to the moon, about five find their way back. The rest are lost to our atmosphere, or miss the lunar reflectors altogether.
From this small catch, the team can assess the movement of the moon to an accuracy of a millimetre or two - a measurement so precise that it has the potential to show up any cracks in Einstein's general theory of relativity. If that's what it does, this lunar laser-ranging experiment will become Apollo's greatest scientific legacy.
Lunar laser ranging has a long history. "I wasn't even born when the first reflectors were left on the moon," says 39-year-old Tom Murphy from the University of California, San Diego, who heads the experiment at the Apache Point Observatory in Sunspot, New Mexico.
In the mid-1960s, when NASA asked for suggestions for experiments that could be carried out on the moon, laser ranging was mooted but no one really knew what to do with it. There was a suggestion to look for gradual changes in Newton's gravitational constant, but this would have meant running the experiment for over 20 years - something no one was prepared to commit to. Then a young researcher called Ken Nordtvedt had an idea.
Through a fiendish piece of mathematics, he showed that, with just a few years' worth of data, lunar laser ranging could be used to test a cornerstone of general relativity known as the equivalence principle. It starts from the idea that a body has two kinds of mass. The first, called gravitational mass, is the mass that produces and feels the pull of gravity. The second is inertial mass, which describes how hard it is to move an object out of its current state of motion - or lack of it. The equivalence principle asserts that the two are exactly equal.
The equivalence principle holds in general relativity, but in the mid-1960s, a rival theory developed by American physicists Carl Brans and Robert Dicke was gaining ground. By postulating a fifth force of nature, the Brans-Dicke theory of gravitation broke the equivalence principle and predicted a 13-metre perturbation in the moon's orbit. Nordtvedt showed that analysing light signals reflecting from the moon could prove the existence of such a disturbance.
continue at the link: Apollo special: Mirrors on the moon - space - 12 July 2009 - New Scientist
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