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Goddard Space Flight Center Engineering Colloquium

Date: Monday, May 14, 2001

Title: Laser Ranging to Retro-Reflectors on the Moon as a Test of Theories of Gravity: Does Gravitational Binding Energy Gravitate?

Speaker: Carroll O. Alley


The first Laser Ranging Retro-Reflector (LR3) was deployed during the Apollo 11 mission and has been used continuously for nearly 32 years.  Additional LR3s were placed on the Moon by the Apollo 14 and15 astronauts and by the Soviet roving vehicles Lunakhods1 and 2.  The concept of the experiment originated in the research group of the late Professor Robert Henry Dicke of Princeton University.  The measurements require the detection and accurate timing of single light quanta because of the very weak returned signals. 

The accuracy has improved from about 30 centimeters initially to a current value of about 3 centimeters.  Important new results in the dynamics of the Earth-Moon system and in geophysics and selenophysics have been obtained, including the probable existence of a small molten iron core in the Moon. 

The most significant results relate to gravity theories.  Einstein's general relativity is not adequate to describe the motion of the Earth-Moon-Sun system.  It is the curved spacetime gravity theory of Yilmaz that treats gravitational energy correctly.  Following a review of the history and experimental techniques, including video clips of the deployment of the Apollo 11 LR3 and of ground station ranging operations, the gravity theory situation will be explained.

The Yilmaz theory and the problems with general relativity are discussed in the recent paper, "Energy Crisis in Astrophysics: Black Holes vs. N-Body Metrics", by C. O. Alley, D. Leiter, Y. Mizobuchi, and H. Yilmaz.


Carroll O. Alley is a professor of physics at the University of Maryland College Park campus where he has taught and conducted research as director of the quantum electronics research group since 1963.  He was the principal investigator for the Apollo 11 Laser Ranging Retro-Reflector experiment and served as the first chairman of the Lunar Ranging Experiment team. 

He received B.S. degrees in mathematics and physics from the University of Richmond and earned the Ph.D. degree from Princeton University, where he studied theoretical and experimental physics and electrical engineering.  His experimental thesis developed new optical detection techniques for the hyperfine ground state resonance in optically pumped rubidium gas cell atomic clocks.  This type of atomic clock, which was invented at Princeton by Professors R. H. Dicke and T. R. Carver, is now the preferred choice for the Global Positioning System satellites.

In 1973 he received the NASA Exceptional Scientific Achievement Medal for his work on the lunar laser ranging experiment.

Colloquium Committee Sponsor: Dr. Eugene Waluschka

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