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 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.
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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 in Sunspot, New Mexico (pictured).
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.
Dicke was a member of the Apollo science advisory committee. He listened as the astronauts complained that many of the proposed experiments were too fiddly to be performed when they were wearing their spacesuits. So he suggested that they simply set down some mirrors, angle them roughly at Earth and let astronomers do the rest.
The Brans-Dicke theory became an early victim of laser-ranging’s success. The measurements were precise enough to show that gravitational mass and inertial mass are indeed equivalent, to an accuracy of one part in 1013. That severely constrains how strong a fifth force of nature could be. Still, new theoretical approaches to gravity such as string theory, and antigravity theories such as quintessence, all seem to imply that the equivalence principle must break.
“We’re already in the regime where violations might be expected. Any push to greater accuracy is theoretically relevant,” says Murphy.
Millimetre precision
To that end, ground-based improvements in technology have allowed the team to move from a precision of a few centimetres to just a few millimetres. The trouble is that the analysis has not kept pace with the wealth of measurements.
At the millimetre scale, there are a number of new effects to deal with, such as solar radiation pressure, which pushes the moon’s entire orbit from its calculated path by about 4 millimetres. These must all be included in the analysis. On top of this, general relativity has to be gone through with the mathematical equivalent of a fine-tooth comb to determine whether there are subtleties that have been neglected so far, yet are relevant at millimetre scales.
“The important thing is that we are collecting data. That is money in the bank to us,” says Murphy. “The analysis will follow once we are satisfied with our new moon model.”
As for that proposed 20-year observation of Newton’s gravitational constant, which sounded so impossible in the 1960s, Murphy’s team have pinned down any changes in the constant to less than one part in 1012 per year. This has provided another powerful constraint on new physics and cosmological theories.
“It’s amazing to think that it’s been such a technical success and that we can keep pushing the technology to greater accuracy,” says Nordtvedt, now emeritus professor at Montana State University in Bozeman. “I would not have counted on a 40-year observation. It is a tremendous bonus.”
Read more: Apollo 11: Why the moon still matters
- Stuart Clark’s latest book is Galaxy (Quercus)
