**Was Galileo Wrong?**

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

It\'s may be the most famous physics experiment ever done: Four hundred
years ago Galileo Galilei started dropping things off the Leaning Tower
of Pisa and timing their fall. In those days, it was widely thought that
heavy objects fell faster than light ones. Instead, he found, everything
hit the ground at the same time. Canon balls, musket balls, gold, silver
and wood: gravity accelerated each downward at the same rate, regardless
of mass or composition.

Nowadays this is called \"The Principle of Uniqueness of Free Fall,\"
and it is a cornerstone of modern physics. Isaac Newton made his own
tests of the Principle using pendulums and, satisfied, he devised his
famous laws of motion. Later, Einstein crafted his theory of gravity,
i.e., the theory of General Relativity, assuming the Principle was 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. If so, there would be a revolution in physics. Relativity,
and perhaps many other theories, would have to be re-written.

A group of NASA-supported researchers plan to test the 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 theory of relativity,\" says Slava
Turyshev, a scientist at NASA\'s Jet Propulsion Laboratory 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 retroreflectors
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.

It\'s a modern version of the Leaning Tower of Pisa experiment. Instead
of dropping balls to the ground, though, 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 Principle holds. If not, let the
revolution begin.

A violation of the 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 Uniqueness of Free Fall\--has
held up to a precision of 5 parts in 10\^12. But that\'s not good enough
to test all the theories vying to overthrow General Relativity.

Current lunar laser ranging can measure the distance to the
Moon\--roughly 385,000 km\--with an error no greater than 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 picoseconds, or just
a trillionth of a second (10\^-12). The speed of light is known\--it\'s
about 186,000 miles 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.

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 (Austin, 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 10\^17 photons!); the APOLLO telescope will catch
about 5 photons from each pulse, which greatly improves the statistical
strength of the results.

Several potential disturbances also 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 shimmer and twinkle. And
tiny tectonic motions of the ground beneath the APOLLO observatory,
typically a few centimeters 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 superconducting
gravimeter and precision GPS sensor alongside the observatory to detect
slow ground movements, and an array of precision barometers will map out
the state of the atmosphere.

Turyshev\'s team has recently received a grant from NASA\'s Office of
Biological and Physical Research to improve JPL\'s range 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 sub-cm level,\" notes Turyshev.

Through careful accounting of such small effects, the Principle of
Uniqueness 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. They describe the Universe in fundamentally
different ways. (Read the Science@NASA story \"Evicting Einstein to
learn more about this.) Finding a flaw in the underpinnings of
relativity could lead to a new \"Theory of Everything\" finally
combining quantum physics and gravity.

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