Moonbeams Shine on Einstein, Galileo and Newton

MEDIA RELATIONS OFFICE
JET PROPULSION LABORATORY
CALIFORNIA INSTITUTE OF TECHNOLOGY
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
PASADENA, CALIF. 91109 TELEPHONE (818) 354-5011
http://www.jpl.nasa.gov

Jane Platt (818) 354-0880
Jet Propulsion Laboratory, Pasadena, Calif.

News Release: 2005-039
March 4, 2005

Moonbeams Shine on Einstein, Galileo and Newton

Thirty-five years after Moon-walking astronauts placed special
reflectors on the lunar surface, scientists have used these
devices to test Albert Einstein's general theory of relativity to
unprecedented accuracy. The findings, which also confirm
theories from Galileo Galilei and Isaac Newton, may help to
explain physical laws of the universe and benefit future space
missions.

"Our research with the Lunar Laser Ranging experiment probes the
equivalence principle, a foundation of Einstein's general theory
of relativity, with extreme accuracy," said Dr. James Williams, a
research scientist at NASA's Jet Propulsion Laboratory, Pasadena,
Calif. Galileo established this principle in 1604 when he
dropped objects of various weights and composition from Italy's
Leaning Tower of Pisa. All the objects were affected equally by
gravity, so they fell at the same rate.

Newton published a supporting explanation in 1687 in his
Principia, and Einstein extended the principle nearly 100 years
ago. Einstein's premise, called the strong equivalence
principle, holds that all forms of matter accelerate at the same
rate in response to gravity. This principle became a foundation
of Einstein's general theory of relativity.

The Lunar Laser Ranging experiment confirms that the Moon and
Earth "fall toward" the Sun at the same rate, even though Earth
has a large iron core below its rocky mantle, while the Moon is
mostly rocky with a much smaller core. The findings by Williams
and Drs. Slava Turyshev and Dale Boggs, also of JPL, have been
published in the Physical Review Letters.

"Lunar laser ranging can conduct very accurate tests of gravity
and fundamental physics," said Williams, who pointed out that
small variations in gravity are difficult to study because the
force is weak, unless very large masses are used. The new
results of this experiment provide a bonanza for modern physics.

"An important property of gravity is its universal effect on
massive objects, despite their size and composition. This is
why, as we understand more about gravity in the solar system, we
learn a lot about gravitational and cosmological processes in the
entire universe," said Turyshev.

"In addition to providing the most accurate test yet of the
strong equivalence principle, our experiment also limits any
possible changes in Newton's gravitational constant," said
Turyshev. The gravitational constant deals with the attraction
between objects in space, and some theories suggest that this
attraction would change over time. If so, the general theory of
relativity would need modification.

"This latest research shows no evidence of such a change. Both
findings -- about the strong equivalence principle and the
gravitational constant -- boost Einstein's theory," added
Turyshev.

Great strides have been made over the past decade in refining the
theories of Einstein, Galileo and Newton. The latest findings
are twice as accurate as any previous results on the strong
equivalence principle, and 10 times as accurate as anything
previously published on the variation of Newton's gravitational
constant

The JPL team tested the theories by beaming laser pulses to four
Moon reflectors from McDonald Observatory in western Texas, and
an observatory in southern France. The lunar reflectors bounced
the laser beams straight back to Earth, where the roundtrip
travel time was measured. Three of the reflectors were installed
by the Apollo 11, 14 and 15 astronauts, and one built by France
was carried on the unmanned Soviet Lunokhod 2 rover.

The current Moon reflectors require no power and still work
perfectly after 35 years. As NASA pursues the vision of taking
humans back to the Moon, and eventually to Mars and beyond, new,
more precise laser ranging devices could be placed first on the
Moon and then on Mars. To guide a spacecraft to a precise
location on the Moon and to navigate trips on its surface, the
Moon's orbit, rotation and orientation must be accurately known.
Lunar laser ranging measurements are helping future human and
robotic missions to the Moon.

More information about the research is available online at
http://arxiv.org/abs/gr-qc/0411113 or
http://funphysics.jpl.nasa.gov/physics/index.html .

The research was conducted under NASA's Astronomy and Physics
Research and Analysis program, part of the agency's Science
Mission Directorate, Washington, D.C. JPL, is a division of the
California Institute of Technology, Pasadena.

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