Testing Gravity with Lunar Laser Ranging

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Testing Gravity with Lunar Laser Ranging

• DoE Site Visit
• James Battat
• August 21, 2006

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Testing Gravity with Lunar Laser Ranging
3.5 m
New Mexico, near White Sands. Note SDSS
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APOLLO
Mapping the lunar orbit to 1 millimeter
UCSD Tom Murphy - PI Eric Michelsen Adam
Orin Harvard Christopher Stubbs James
Battat U. Washington Eric Adelberger Erik
Swanson C. D. Hoyle Larry Carey
JPL Jim Williams Jean Dickey Slava
Turyshev Lincoln Lab Brian Aull Bernie
Kosicki Bob Reich Northwest Analysis Ken
Nordtvedt
Ephemeris
APD detectors
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Why Push Gravity Tests Further?

• Order of magnitude in constraints
• Dark energy, w
• Scalar field modifications to GR
• Predict non-GR effects
• Brane-world cosmology
• Gravitons leaking into bulk modify gravity at
  large scales

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LLR is a Powerful Test of Gravity

• With one millimeter range precision
• Weak EP ?a/a 10-14
• Strong EP ? 310-5
• Gravitomagnetism 10-4
• dG/dt 10-13G/year
• Geodetic precession 310-4
• Long range forces 10-11 the strength
• of gravity

In each case, LLR currently provides the best
limit.
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LLR is a Powerful Test of Gravity

• With one millimeter range precision
• Weak EP ?a/a 10-14
• Strong EP ? 310-5
• Gravitomagnetism 10-4
• dG/dt 10-13G/year
• Geodetic precession 310-4
• Long range forces 10-11 the strength
• of gravity

In each case, LLR currently provides the best
limit.
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Nordtvedt Effect

• r 13 ? cos(D) meters

Nordtvedt, Phys Rev, 169, 1014,
1968 Nordtvedt, Phys Rev, 169, 1017,
1968 Nordtvedt, Phys Rev, 170, 1186, 1968
Synodic period 29.35 days
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Avalanche Photodiode Array
APOLLO Reaching 1 mm
3.5 meter telescope Good seeing
? 30mm

• -Courtesy of MIT Lincoln Lab

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Laser Ranging Apparatus Transmit
Corner cube
2.3 Watt NdYAG laser 20 Hz, l 530 nm lt 100 ps
pulse width 110 mJ per pulse
LASER
3.5m primary
APD array
START
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Laser Ranging Apparatus Receive
2.3 Watt NdYAG laser 20 Hz, l 530 nm lt 100 ps
pulse width 110 mJ per pulse
LASER
3.5m primary
APD array
STOP
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25 km
4 km
MOON
1017 attenuation
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Interpretation Requires Sophisticated Modeling
Measure telescope-to-reflector distance
Want center-to-center separation
Through the atmosphere
Need precise ephemeris information JPL
collaboration
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MLRS The Old Way
28 photons in 42 minutes
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Enter APOLLO
1,500 photons in 13 minutes 1 mm statistical
uncertainty
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Current Status

• Millimeter precision data
• Returns from all 3 Apollo arrays
• Remote operations
• Enter campaign mode (October)1-2 hours every 2-3
  nights
• Improve site coordinates
• New constraints on gravitational params

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The End
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THE END
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EXTRA SLIDES
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APOLLO Reaching 1 mm

• Large-aperture, good seeing
• Figure of merit goes like (D/?)2
• Incorporate modern technology
• Detectors, precision timing, laser
• Re-couple data collection to analysis/science

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LLR Targets
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Gravitational Self-Energy SEP
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APOLLO Random Error Budget
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Lunar Phase Coverage

• EP violation has a null at Quarter moon and
  maxima at Full and New Moons
• Same data, uniform coverage gives tighter ?

No measurements at max signal
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Current PPN Constraints

• Basic phenomenology
• measures curvature of
• Spacetime
• measures nonlinearity of gravity
• ?-1 (2.1 2.3) x 10-5
• ?-1 (1.2 1.1) x 10-4
• ? (4.4 4.5) x 10-4

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Solar System Parameters

• Separation
• Moon-Earth 0.38 million km
• Sun-Earth 150 million km
• Mass
• Sun 2 x 1030 kg
• Earth 6.0 x 1024 kg
• Moon 0.073 x 1024 kg
• Radius
• Sun 695,000 km
• Earth 6380 km
• Moon 1740 km
• Gravitational Constant
• 6.67 x 10-11 m3 kg-1 s-2

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Testing Gravity with Lunar Laser Ranging
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Historical Accuracy of LLR Data
30 cm
0 cm
1970
present
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Laser Ranging Apparatus
Corner cube
2.3 Watt NdYAG laser 20 Hz, l 530 nm lt 100 ps
pulse width 110 mJ per pulse
LASER
3.5m primary
APD array
START
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Laser Ranging Apparatus
2.3 Watt NdYAG laser 20 Hz, l 530 nm lt 100 ps
pulse width 110 mJ per pulse
LASER
3.5m primary
APD array
STOP