Testing Gravity with Lunar Laser Ranging James Battat August 9, 2005

Point Strong Laser at Moon Apache Point Observatory 3.5 meter dish 20 Hz pulses ~ 2.5 second round trip travel time. ~ 50 pulses in air simultaneously

Aim for Retroreflectors 3.5 meter dish Thank you Buzz Aldrin… 36 years and counting.

Motivation for LLR Test fundamental physical phenomena. –Push GR tests by 10x –Possibility that GR cannot explain data. –Unification of gravity with other forces –New clues to explain existing paradigm.

LLR is the best test of Strong equivalence principle Gravitomagnetism dG/dt Geodetic precession (de Sitter) Long range forces

10x Better Range Accuracy

APOLLO’s Capabilities Weak Equivalence Principle to  a/a ≈ Strong Equivalence Principle to  ≈ 3×10 -5 Gravitomagnetism (frame dragging) to dG/dt to G per year Geodetic precession to ≈ 3×10 -4 Long range forces to × the strength of gravity Timescales to achieve stated 1-σ results vary according to the nature of the signal.

Equivalence Principles Weak Equivalence Principle –Objects follow the same trajectory in a gravitational field irrespective of their internal structure and composition. –Testable in the lab Strong Equivalence Principle –WEP is valid for self-gravitating bodies –Gravitational self-energy has mass –Does this mass accelerate the same as other mass? –M g /M i = ?

Damour, Class. Quantum Grav., 13, A33, 1996 Push the Equivalence Principle Test Thibault Damour

Gravitational Self-Energy Objectm SE /m 1 kg sphere, 6” diam5 x Moon2 x Earth5 x

Nordtvedt Effect Visualized Nordtvedt, Phys Rev, 169, 1014, 1968 Nordtvedt, Phys Rev, 169, 1017, 1968 Nordtvedt, Phys Rev, 170, 1186, 1968

APOLLO is at APO In New Mexico Elevation: 2,788 m 32° 46’ 49” N 105° 49’ 13” W 3.5 m diameter dish Median Seeing: <1” Sloan 3.5 m Solar

The APO 3.5 meter

The Action is at the Back Telescope boresite

The Action is at the Back Timing Electronics Laser APD Detectors Control Computer Thermal Control Telescope boresite

The Laser Firing

APD Detectors MIT Lincoln Labs Single  detectors 50% efficiency 50 ps timing jitter (7 mm one-way)

Some Challenges Lunar crustal tides (~4 m) Earth crustal tides (~20 cm) Solar radiation pressure (~4 mm)

Some Solutions Geophysical metrology –High precision GPS –Gravimetry Detailed ephemeris modeling –Harvard, JPL –Included manifold geophysical effects Signal at known frequency

F I R S T L I G H T

APOLLO Summary LLR is an incisive probe of gravity. Any known GR alternative violates SEP. 1mm ranging improves constraints by 10x. Geo-metrology and models account for contaminating effects. APOLLO data is imminent.

APOLLO Collaboration Harvard: Christopher Stubbs James Battat UCSD: Tom Murphy Eric Michelsen Adam Orin U. Washington: Eric Adelberger Erik Swanson C. D. Hoyle JPL: Jim Williams Jean Dickey Slava Turyshev Lincoln Labs: Brian Aull Bernie Kosicki Bob Reich Northwest Analysis: Ken Nordtvedt