Using lasers to ping the Moon, researchers are testing a fundamental assumption of modern physics.

by Dr Tony Phillips and Patrick L Barry

Four hundred years ago - or so the story goes - Galileo Galilei started dropping things off the Leaning Tower of Pisa: Cannon balls, musket balls, gold, silver and wood. He might have expected the heavier objects to fall faster. Not so. They all hit the ground at the same time, and so he made a big discovery: gravity accelerates all objects at the same rate, regardless of their mass or composition.

Nowadays this is called "Universality of Free Fall" or the "Equivalence Principle," and it is a cornerstone of modern physics. In particular, Einstein crafted his theory of gravity, i.e., the general theory of relativity, assuming the Equivalence Principle is true.

But what if it's wrong?

"Some modern theories actually suggest that the acceleration of gravity does depend on the material composition of the object in a very subtle way," says Jim Williams, a physicist at NASA's Jet Propulsion Laboratory (JPL). If so, the theory of relativity would need re-writing; there would be a revolution in physics.

A group of NASA-supported researchers are going to test the Equivalence Principle by shooting laser beams at the Moon.

"Lunar laser ranging is one of the most important tools we have for searching for flaws in Einstein's general theory of relativity," says Slava Turyshev, a research scientist at JPL who works with Jim Williams and others on the project.

Their experiment is possible because, more than 30 years ago, Apollo astronauts put mirrors on the Moon - small arrays of retro reflectors that can intercept laser beams from Earth and bounce them straight back. Using lasers and mirrors, researchers can "ping" the Moon and precisely monitor its motion around Earth.

more A sketch of Galileo Galilei's legendary experiment.

It's a modern version of the Leaning Tower of Pisa experiment. Instead of dropping balls to the ground, the researchers will watch the Earth and Moon drop toward the Sun. Like musket balls and cannon balls dropped from the Tower, the Earth and Moon are made of a different mix of elements, and they have different masses. Are they accelerated toward the Sun at the same rate? If yes, the Equivalence Principle holds. If not, let the revolution begin.

A violation of the Equivalence Principle would reveal itself as a skewing of the Moon's orbit, either toward or away from the Sun. "Using masses as large as the Earth and Moon, we may be able to show this subtle effect, if it exists," notes Williams.

Scientists have been pinging the Moon since the Apollo days. So far, Einstein's theory of gravity - and the Equivalence Principle - has held up to a precision of a few parts in 10¹³. But that's not good enough to test all the theories vying to overthrow Einstein.

Current lunar laser ranging can measure the distance to the Moon - roughly 385,000 km - with an error of about 1.7 cm. Beginning this fall, a new facility funded by NASA and the National Science Foundation will boost this accuracy 10-fold to within only 1 to 2 mm. This jump in accuracy will mean that scientists can detect deviations from Einstein's theory 10 times smaller than currently possible, which may be sensitive enough to find the first evidence of flaws.

To achieve that accuracy, the facility, called the Apache Point Observatory Lunar Laser-ranging Operation (APOLLO), must time the laser pulses' roundtrip flight to the Moon within a few pico seconds, or just a trillionth of a second (10^-12). The speed of light is known - it's about 300,000 km per second - so measuring the time of flight for the laser pulse tells scientists the distance between the APOLLO telescope and the mirror sitting on the surface of the Moon.

more A retro reflector array left on the Moon by Apollo 14 astronauts. Similar mirrors were emplaced by Apollo 11 and Apollo 15 astronauts, and by a pair of Soviet-era Lunokhod rovers.

How does APOLLO's design achieve this 10-fold improvement? First of all, it uses a larger telescope than the older facility at the McDonald Observatory in Texas - 3.5 meters vs. 0.72 meters. The larger mirror lets the APOLLO facility catch more of the photons of light returning from the Moon, explains Tom Murphy, a professor at the University of California, San Diego, and the mastermind behind the design of APOLLO. The smaller telescope catches, on average, only one returning photon for every 100 out-going laser pulses (each pulse contains more than 10¹⁷ photons!); the APOLLO telescope will catch about 5 photons from each pulse, which greatly improves the statistical strength of the results.

Several potential disturbances had to be reckoned with. Earth's atmosphere, for one, can distort the path of the pulse of laser light, in the same way that it causes starlight to twinkle and shimmer. And tiny tectonic motions of the ground beneath the APOLLO observatory, typically a few centimetres per year, could skew the long-term results. So the project leaders chose a mountaintop near White Sands, New Mexico, that enjoys a particularly calm overhead atmosphere and ground that is relatively stable. In addition, they are installing a super conducting gravimeter and precision GPS sensor alongside the observatory to detect slow ground movements, and an array of precision barometers will map the state of the atmosphere.

more Lunar laser ranging works by firing pulses of laser light at reflectors on the Moon's surface and catching the returning photons. Shown here is the laser ranging experiment at the University of Texas McDonald Observatory.

Williams and Turyshev have recently received a grant from NASA's Office of Biological and Physical Research to improve JPL's lunar laser ranging analysis software by an order of magnitude to match the capability of the New Mexico site. "It will be necessary to deal with many small effects at the millimetre level," notes Turyshev.

Through careful accounting of such small effects, the Universality of Free Fall … could fall.

Many physicists would welcome the news. They've been puzzled for years by a curious incompatibility between general relativity and quantum mechanics. The two theories, so successful in their own realms, are like different languages describing the Universe in fundamentally different ways.

Finding a flaw in the underpinnings of relativity could lead to a new "Theory of Everything," finally combining quantum physics and gravity in one harmonious framework.

From Pisa, Italy, to the Moon, to White Sands, New Mexico: this is a far-flung experiment spanning hundreds of years and hundreds of thousands of miles. Soon, perhaps, we'll have the answers.