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“Galileo Galilei ( GG )” A test of the Equivalence Principle

“Galileo Galilei ( GG )” A test of the Equivalence Principle. Suresh Doravari (University of Pisa & INFN) for the GG/GGG collaboration http://eotvos.dm.unipi.it/nobili/. The motivation: GENERAL RELATIVITY NEEDS TESTS of the EQUIVALENCE PRINCIPLE.

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“Galileo Galilei ( GG )” A test of the Equivalence Principle

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  1. “Galileo Galilei (GG)” A test of the Equivalence Principle Suresh Doravari (University of Pisa & INFN) for the GG/GGG collaboration http://eotvos.dm.unipi.it/nobili/

  2. The motivation:GENERAL RELATIVITY NEEDS TESTS of the EQUIVALENCE PRINCIPLE The difficulties we encounter in merging gravity withquantum mechanics suggest that the pure tensor structure of GR needs modification or augmentation. The most promising scenario for the quantization of gravity and the unification of all natural interactions is the superstring theory. However, superstring theory predicts the existence of long range scalar fields (in addition to the pure tensor field of GR) which are composition dependentand therefore violate the Equivalence Principle (EP) (see for eg. PRL, 89, 081601)

  3. The Observable Consequence:Universality of Free Fall The most direct experimental consequence of the Equivalence Principle is the Universlaity of Free Fall (UFF): in the gravitational field of a source mass all bodies fall with the same acceleration regardless of their mass or composition The quantity to be measured is the relative acceleration of test masses of different composition in the gravitational field of a source body (i.e. Earth, Sun..): a/a=0 The Eötvös parameter: A measure of the violation of the Universality of free fall.  = 0if the Equivalence Principle holds.

  4. EQUIVALENCE PRINCIPLE TESTS: WHAT’s ON The best ground tests (with slowly rotating torsion balance) provide:   9.310-13 Proposed and ongoing experiments for EP testing :   10-17 , 10-18 GG (I) 250 kg; STEP (USA) 1000 kg- LEO   10-14 , 10-15 GREaT (I-USA) -balloon, SCOPE (F) 200 kg -LEO   10-12 Torsion balances (USA)

  5. GG: configuration for EQUATORIAL ORBIT 1m Configuration for equatorial orbit (VEGA launch; operantion from ASI ground station in Malindi)

  6. GG: the SPACE EXPERIMENT DRIVING CONCEPTS (I) • The satellite chassis and the test masses are concentric cylinders -- avoids the classical tidal effects • The experiment is housed in a Pico-Gravity Box -- vibration isolated and drag-free • The masses coupled by weak springs and constrained to move opposite to each other in the orbital plane – common mode rejection • Capacitance sensors measure relative displacements in 2-D and are sensitive only to the differential motion – a further attentuation of the common mode

  7. GG: the SPACE EXPERIMENT DRIVING CONCEPTS (II) • The satellite and the test masses are together set into rotation about the symmetry axis  Spin • Passive stabilisation by supercritical rotation • . The spin modulates the EP violation signal at a frequency much higher than the orbital frequency -- separates out many systematic effects • Two identical experiments in one satellite: one with masses of same composition and another with different composition) -- provides a zero-check

  8. GG ACCELEROMETERS: SECTION ALONG THE SPIN AXIS GG inner & outer accelerometer (the outer one has same composition test cylinders for systematic checks) Accelerometers co-centered at center of mass of spacecraft for best symmetry and best checking of systematics…

  9. GG ACCELEROMETERS CUTAWAY Design symmetry is extremely importnat in small force gravitational experiments….. Note the azimuthal symmetry of the accelerometers around the cylinders’ axis –which is also the spin axis- as well as the top/down symmetry. The rest of the spacecraft around the accelerometers preserves both these symmetries too.

  10. AUTOCENTERING of GGG TEST CYLINDERS vs SPIN FREQUENCY Experimental evidence of autocentering of the test cylinders in supercritical rotation: relative displacements of the test cylinders in the rotating frame (X in red, Y in blu) decrease as spin frequency increases and crosses the resonance zones (shown by dashed lines) ….. See next slide….

  11. In supercritical rotation (defined by spin frequency > natural frequency) whirl motion arises at each natural frequency whose growth is determined by the Q of the system at the SPIN frequency (not at the natural frequency …..) Q in SUPERCRITYICAL ROTATION Integration time available until whirl of period Tw grows by factor k High Q means slow whirl growth, and Q at higher frequencies is larger …. ok In supercritical rotation thermal noise also depends on Q at the spin frequency (not at the –low- natural one) and this is a crucial advantage..

  12. Q MEASUREMENTS @ NATURAL FREQUENCIES Q measured from free oscillations of full GGG system at its natural frequencies –see blu lines- with system not spinning: 0.0553 Hz (18 sec) 0.891 Hz (1.1 sec) 1.416 Hz (0.7 sec) Q MEASUREMENTS @ NATURAL FREQUENCIES Q of GGG apparatus at frequencies other than the natural ones (e.g. at 0.16 Hz) can be measured (during supercritical rotation at that frequency) from the growth of whirl motion….

  13. GROWTH of WHIRL MOTION Spin period 6.25 sec (0.16 Hz), whirl period 13 sec (O.0765 Hz), whirl control off Measurements of whirl growth made with 2 different read-outs give the same value of Q at 0.16 Hz: this is the relevant Q for operation at that spin rate

  14. DIFFERENTIAL MOTION of ROTATING TEST CYLINDERSfrom Rotating Capacitance Bridges: improvements since 2002 GGG operation in INFN lab started in 2004: • Gained by 2 orders of magnitude in residual noise • Long term stable continuous operation without instability demonstrated

  15. ETA in GGG: In the field of the Earth from space (GG orbit) with natural differential period of TMs

  16. The sensitivity to differential accelerations between the test masses (sensitivity to EP tests), is inversely proportional to the square of their natural differential period: The GREAT ADVANTAGE of WEIGHTLESSNESS The natural differential period is inversely proportional to the stiffness of their coupling: In space, thanks to weightlessness, the stiffness of coupling can be weaker than on Earth by many orders of magnitude… From GG Phase A Study (ASI 1998; 2000), as compared to GGG, we see that the factor gained in absence of weight is:

  17. GGG in INFN lab GGG lab 2005 (March) 1m

  18. Satellite: —spin axis stabilized; ADVANCED DRAG COMPENSATION by FEEP thrusters (ASI) — FEEP thrusters: 150 N thrust authority; built in Pisa, already funded by ESA for SCOPE and LISA-PF to be availbale 2008-2009 GG MISSION PROGRAMMATICS Payload: —differential accelerometer similar to GGG, incorporating all what has been learned in the lab (INFN) —PGB enclosing accelerometr (noise attenuation + test mass for drag-free control (ISRO-Indian Space Research Organization) Launch: —VEGA (qualification launch…multiple launch since GG is MICRO) Operation: —MALINDI GG included in ASI National Space Plan recently approved – VEGA launch foreseen Data archiving and analysis: —University of Pisa

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