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Department of Physics Brown APJC – August 2008 Recent Precision Tests of General Relativity Thomas P. Kling Brown Astrophysics Journal Club August 2008
Department of Physics Brown APJC – August 2008
Recent Precision Tests of General Relativity
Thomas P. KlingBrown Astrophysics Journal Club
August 2008
Weak Equivalence Principle
Department of Physics Brown APJC – August 2008
Some local frame
&
Should fall with the same acceleration
Strong or EinsteinEquivalence Principle
Department of Physics Brown APJC – August 2008
some local framein free fall
&
should fall freely along straight lines, or all observationsconsistent with SR
Implications for gravity
• Gravity is “curved space-time”• Gravity is a metric theory• Freely falling observers
= locally flat region• Freely falling test bodies fall along
geodesics
Department of Physics Brown APJC – August 2008
Department of Physics Brown APJC – August 2008
EEP + Ocham’s razorleads directly to . . .
1 8 G
2ab ab ab abR R g g T
Schiff’s conjecture (1960):
“Any complete, self-consistent theory of
gravity that embodies WEP necessarily embodies
EEP.”
Department of Physics Brown APJC – August 2008
Combine gravity with standard model?
Violations of•Equivalence Principle•Inverse square law
Department of Physics Brown APJC – August 2008
Two Projects
• Lunar ranging at Apache Point Observatory
• Rotating Torsion Balance of Univ. of Washington
Department of Physics Brown APJC – August 2008
Eöt-Wash Group
Department of Physics Brown APJC – August 2008
Careful Torsion Balance!
Department of Physics Brown APJC – August 2008
15 2
15 2
Be Ti 0.6 3.1 10 m/s
Be Ti 2.5 3.5 10 m/s
N N
W W
a a
a a
Department of Physics Brown APJC – August 2008
< 100 m – local hill
< 10 km – bedrock
>1000 km – Earth
Yukawa Potential
Towards galactic center
15 2
hypothetical (dashed):
20 10 m/sa
Department of Physics Brown APJC – August 2008
15 2
Best fit (pink):
2.1 3.1 10 m/sa
Classical Equivalence Parameter
13Be Ti 0.3 1.8 10
Department of Physics Brown APJC – August 2008
5, 4 7 10DM Be Ti
APOLLO Lunar Ranging
A pacheP ointO bservatoryL aserL unar-rangingO peration
Department of Physics Brown APJC – August 2008
Department of Physics Brown APJC – August 2008
Basics of LLR
• Apollo missions & others left Moon reflectors.
• Measure time of flight.
• Past 15 years: about 1-3 cm precision.
Department of Physics Brown APJC – August 2008
All about Numbers
• Previous return rates: 0.002 to 0.01 photons return per pulse . . . Typically get 15 to 40 photons return per pointing.
• To get mm precision, need about 225 to 1300 photons returning.
Department of Physics Brown APJC – August 2008
Department of Physics Brown APJC – August 2008
APOLLO Contributions
• Larger aperture (3.5 m), better seeing (1.1 arcsecond) means higher return rate.
• Higher energy laser:– 90 ps FWHM Nd:YAG– 20 Hz & 115 mJ/pulse
• Best return rate: Apollo 15 at 1.33 photons per shot
Department of Physics Brown APJC – August 2008
Department of Physics Brown APJC – August 2008
Stuff making you worry
• Tides change surface height ~ 350 mm
• Crustal loading from atmosphere, ground water, etc. ~ 2-5 mm
• Atmospheric propagation delay must be modeled
Department of Physics Brown APJC – August 2008
Murphy et al. 0710.0890v2
Department of Physics Brown APJC – August 2008
Department of Physics Brown APJC – August 2008
