next-generation lunar laser ranging
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Next-Generation Lunar Laser Ranging. Tom MurphyUCSD. Designing the Perfect Gravity Test. Gravity must be the dominant influence Negligible frictional, electrostatic, radiation forces Make test bodies large Gravity dominates - PowerPoint PPT PresentationTRANSCRIPT
Next-Generation Lunar Laser Ranging
Tom Murphy UCSD
Designing the Perfect Gravity Test
Gravity must be the dominant influence– Negligible frictional, electrostatic, radiation forces
Make test bodies large– Gravity dominates– Test Strong Equivalence Principle (SEP) by
having appreciable “self-energy”: how does gravity pull on gravity?
Place in vacuum environment
Precision Requirements
Order-of-Magnitude improvement over current measurements/tests of gravity– ~10-5 test of General Relativity
~10-14 measurement precision needed– Good clocks are 10-12
– Need leverage somehow
Earth-Moon System Fits the Bill
Earth “self-energy” is ~0.5×10-9 of total mass Moon is large, but only 0.02×10-9 in grav. energy
– Non-gravitational forces very small– Self-energy small compared to that of earth
Sun dominates for both bodies– Can test differential motion/acceleration to Sun
Leverage from proximity of moon to earth– 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 RangingW
eigh
ted
RM
S R
esid
ual (
cm)
1970 1975 1980 1985 1990 1995 2000 2005
30
25
20
15
10
5
0
Current PPN Constraints on GR
1
0.998
10.999 1.001
1.002
1.002
1.001
0.999
0.998
Lunar Laser Ranging
Mercury Perihelion Shift
Mars Radar Ranging
VLBI & combined planetary data
Spacecraft range & Doppler
• Is the Parameterized Post-Newtonian (PPN) formalism still relevant?
• What fool would want to push this further? Isn’t GR obviously right?
Basic phenomenology:
measures curvature ofspacetime, measuresnonlinearity of gravity
Why Push Further: Aristotelian Analogy
1.5
-1.0 0-0.5 0.5 1.0
2.5
2.0
1.0
0.5
0.0
Trajectory Asymmetry
Tra
ject
ory
Pol
ynom
ial O
rder
Newtonian Gravity
Aristotelian“Gravity”
Einsteinian Departuresat ~10-8 level of precision
“Real” Rationale for Pushing Further
Gravity is incompatible with the Standard Model Cosmological departures from old GR model
– Acceleration of expansion of Universe
Fine structure constant, , possibly varying?– What about gravitational constant, Equivalence Principle
Scalar Field modifications to GR– Predictions of PPN departures from GR
Brane-world cosmological models– Gravitons leaking into bulk, modifying gravity at large scales
APOLLO: Next-Generation LLRrecipe for success:
Move LLR back to a large-aperture telescope– 3.5-meter: more photons!
Incorporate modern technology– Detectors, precision timing, laser
Re-couple data collection to analysis/science– Scientific enthusiasm drives progress
Devise brilliant acronym:– Apache Point Observatory Lunar Laser-ranging Operation
APOLLO Goals*:
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
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
We have a working prototype courtesy MIT Lincoln Labs
4×4 format (LL has made much larger)
30m diameters on 100m centers
Fill-factor recovered by lenslet array
~30 ps jitter at 532 nm, ~50% photon detection eff.
Multiple “buckets” for photon bundle
Millimeter Range?!!
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
Various contributions to systematic error:– Atmospheric refractive delay (2-meter signal)– Ocean, atmosphere, and ground-water loading– Thermal expansion of telescope & reflectors
Will implement supplemental metrology on-site– 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– 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 CAMAC Crate Inhabitants
APOLLO Command ModuleTiming/APD control, CPU interface
Calibration/Frequency Board
Future Laser Ranging Tests of GR
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 →
– May perform test-ranging from Apache Point this summer
LATOR (stay tuned for Turyshev talk) uses inter-spacecraft laser ranging to measure curvature of spacetime to unprecedented precision: to 10-8
APOLLO Collaboration
UCSD:Tom MurphyJohn GoodkindEric Michelsen
U. Washington:Eric AdelbergerJana StrasburgLarry Carey
Northwest Analysis:Ken Nordtvedt
JPL:Jim WilliamsJean DickeySlava Turyshev
Lincoln Labs:Brian AullBernie KosickiBob Reich
Harvard:Chris Stubbs
Joint NASA/NSF funding