Transcript Next-Generation Lunar Laser Ranging

Next-Generation Lunar Laser
Ranging
Tom Murphy
UCSD
Designing the Perfect Gravity Test
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Gravity must be the dominant influence
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Make test bodies large
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Negligible frictional, electrostatic, radiation forces
Gravity dominates
Test Strong Equivalence Principle (SEP) by
having appreciable “self-energy”: how does
gravity pull on gravity?
Place in vacuum environment
Precision Requirements
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Order-of-Magnitude improvement over
current measurements/tests of gravity
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~10-5 test of General Relativity
~10-14 measurement precision needed
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Good clocks are 10-12
Need leverage somehow
Earth-Moon System Fits the Bill
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Earth “self-energy” is ~0.5×10-9 of total mass
Moon is large, but only 0.02×10-9 in grav. energy
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Sun dominates for both bodies
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Non-gravitational forces very small
Self-energy small compared to that of earth
Can test differential motion/acceleration to Sun
Leverage from proximity of moon to earth
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Rearth-moon = 1/400 A.U. → test motion with respect to sun at
1 A.U. via much shorter (differential) measurement
Historical Accuracy of Lunar Ranging
Weighted RMS Residual (cm)
30
25
20
15
10
5
0
1970
1975
1980
1985
1990
1995
2000
2005
Current PPN Constraints on GR
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Lunar Laser Ranging
Mercury Perihelion Shift
1.002
Mars Radar Ranging
• Is the Parameterized
Post-Newtonian (PPN)
formalism still relevant?
Basic phenomenology:
1.001
VLBI & combined
planetary data
1
Spacecraft range
& Doppler
0.999
 measures curvature of
spacetime,  measures
nonlinearity of gravity
• What fool would want
to push this further?
Isn’t GR obviously right?
0.998
0.998 0.999
1
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1.001 1.002
Why Push Further: Aristotelian Analogy
Newtonian Gravity
Einsteinian Departures
at ~10-8 level of precision
Trajectory Polynomial Order
2.5
2.0
Aristotelian
“Gravity”
1.5
1.0
0.5
0.0
-1.0
-0.5
0
0.5
Trajectory Asymmetry
1.0
“Real” Rationale for Pushing Further
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Gravity is incompatible with the Standard Model
Cosmological departures from old GR model
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Fine structure constant, , possibly varying?
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What about gravitational constant, Equivalence Principle
Scalar Field modifications to GR
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Acceleration of expansion of Universe
Predictions of PPN departures from GR
Brane-world cosmological models
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Gravitons leaking into bulk, modifying gravity at large scales
APOLLO: Next-Generation LLR
recipe for success:
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Move LLR back to a large-aperture telescope
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Incorporate modern technology
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Detectors, precision timing, laser
Re-couple data collection to analysis/science
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3.5-meter: more photons!
Scientific enthusiasm drives progress
Devise brilliant acronym:
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Apache Point Observatory Lunar Laser-ranging Operation
APOLLO Goals*:
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One millimeter range precision
Weak Equivalence Principle (WEP) to a/a ≈ 10-14
Strong Equivalence Principle (SEP) to  ≈ 3×10-5
Gravitomegnetism (frame dragging) to 10-4
dG/dt to 10-13•G per year
Geodetic precession ( ) to ≈ 3×10-4
Long range forces to 10-11 × the strength of gravity
* These 1 errors are simply ~10 times better than current LLR limits. In each
case, LLR currently provides the best limits. Timescales to achieve stated
results vary according to the nature of the signal.
The APOLLO Apparatus
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Uses 3.5-meter telescope at
9200-ft Apache Point, NM
Excellent atmospheric
“seeing”
532 nm Nd:YAG, 100 ps,
115 mJ/pulse, 20 Hz laser
Integrated avalanche
photodiode (APD) arrays
Multi-photon capability
Daylight/full-moon capability
APD Arrays
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We have a working
prototype courtesy MIT
Lincoln Labs
4×4 format (LL has made
much larger)
30m diameters on 100m
centers
Fill-factor recovered by
lenslet array
~30 ps jitter at 532 nm,
~50% photon detection eff.
Multiple “buckets” for photon
bundle
Millimeter Range?!!
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Seven picosecond round-trip travel time error
Half-meter lunar reflectors at ±7° tilt → up to 35 mm
RMS uncertainty per photon
95 ps FWHM laser pulse → 6 mm RMS
Need ~402 = 1600 photons to beat down error
Calculate ~5 photon/pulse return for APOLLO
“Realistic” 1 photon/pulse → 20 photons/sec →
millimeter statistics achieved on few-minute timescales
APOLLO Random Error Budget
Expected Statistical Error
RMS Error (ps)
One-way Error (mm)
Laser Pulse (95 ps FWHM)
40
6
APD Jitter
50
7
TDC Jitter
15
2.2
50 MHz Freq. Reference
7
1
APOLLO System Total
66
10
Lunar Retroreflector Array
80–230
12–35
Total Error per Photon
105–240
16–37
APOLLO Systematic Errors
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Various contributions to systematic error:
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Will implement supplemental metrology on-site
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Atmospheric refractive delay (2-meter signal)
Ocean, atmosphere, and ground-water loading
Thermal expansion of telescope & reflectors
Barometric transducer array
Superconducting gravimeter (<1 mm vertical displacements)
Precision GPS (0.5 mm horz., 2.3 mm vert. in 24 hr)
IMPORTANT: Science signals are narrow-band
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Environmental factors will not mimic new physics
Laser Mounted on Telescope
Mounted June 2003
In thermal enclosure (“fridge”)
Timing Electronics Built/Verified
Timing System in Operation
APOLLO Command Module
Timing/APD control, CPU interface
CAMAC Crate Inhabitants
Calibration/Frequency Board
Future Laser Ranging Tests of GR
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Interplanetary is next logical step
Laser transponder on Mars measures  to 4×10-6,
and  to 10-5
Mercury orbiter (Messenger: launched) could be
used to measure perihelion shift → 
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May perform test-ranging from Apache Point this summer
LATOR (stay tuned for Turyshev talk) uses interspacecraft laser ranging to measure curvature of
spacetime to unprecedented precision:  to 10-8
APOLLO Collaboration
UCSD:
Tom Murphy
John Goodkind
Eric Michelsen
JPL:
Jim Williams
Jean Dickey
Slava Turyshev
U. Washington:
Eric Adelberger
Jana Strasburg
Larry Carey
Lincoln Labs:
Brian Aull
Bernie Kosicki
Bob Reich
Northwest Analysis:
Ken Nordtvedt
Harvard:
Chris Stubbs
Joint NASA/NSF funding