Gravitational Physics via Lunar Laser Ranging: Optimizing Data Quality

University Of California-San Diego, La Jolla CA

Investigators

Abstract

Gravity, the most evident force of nature, is in fact the weakest of the fundamental forces, and consequently the most poorly tested. Einstein's general relativity, which is currently our best description of gravity, is fundamentally incompatible with quantum mechanics and is likely to be replaced by a more complete theory in the future. A modified theory would predict small deviations in the solar system that could have profound consequences for our understanding of the Universe as a whole. Lunar laser ranging (LLR), in which short laser pulses launched from a telescope are bounced off of reflectors placed on the moon by U.S. astronauts and Soviet landers, has for decades produced various leading tests of gravity by mapping the shape of the lunar orbit to high precision. The group proposes to continue conducting leading-edge observations with the Apache Point Observatory Lunar Laser-ranging Operation (APOLLO), in an effort to subject gravity to the most stringent tests yet. APOLLO, situated atop a 9,200 ft summit in Southern New Mexico, introduces a new regime of millimeter-precision in measuring the lunar orbit. However, incomplete models are thus far unable to confirm the accuracy. The group will therefore seek to build a calibration system to ensure that APOLLO meets its millimeter measurement goal, in addition to continuing the observation campaign. The proposed work will benefit the broader community in a number of ways. On the intellectual front, improving our knowledge of gravity informs a diverse range of cosmologists, astrophysicists, particle physicists, and string theorists. The effort would also contribute to Earth and planetary science, especially via measurements produced by the superconducting gravimeter. The APOLLO team will continue a track record of engagement in education and outreach activities, and will no doubt continue to attract public interest through print, web, radio, and television media. Among other attributes that contribute to APOLLO's superior observations, routine ranging to all five lunar reflectors on timescales of minutes dramatically improves our ability to gauge lunar orientation and body distortion. This information allows a more precise determination of the path for the Moon's center of mass, thereby facilitating tests of fundamental gravity. Simultaneously, higher precision range measurements, together with data from a superconducting gravimeter at the Apache Point Observatory and from a high-quality Global Positioning System (GPS) station 2.5 km away, will greatly improve our understanding of the instantaneous location of the Observatory with respect to the Earth's center of mass (needed for the gravitational tests) by exposing subtle Earth dynamics that must be incorporated into the model. LLR measurements provide the best available tests of the strong equivalence principle, the time-rate-of-change of Newton's gravitational constant, gravitomagnetism, the inverse-square law, and preferred frame effects. In addition to these classical gravitational tests, APOLLO will permit testing of new ideas in physics relating to dark energy, extra dimensions, and violations of Lorentz Invariance. A large part of the effort proposed here is the construction of an absolute calibration system based on a cesium clock standard, a low-jitter short-pulse laser, and a precision interval counter. This system will provide an independent check of APOLLO's fundamental measurement, potentially identifying faults and confirming their pursuant remediation.