Status of “Galileo Galilei” (GG)

Test of the Equivalence Principle to 10-17

Anna M Nobili, Dipartimento di Fisica “E. Fermi” Universita’ di Pisa & INFN, Pisa – Italia

Gravitation and Fundamental Physics in Space – GPHYS “0”, October 2009

Some newsSome news

• GG Phase A-2 Study led by TAS-I in Torino, ASI funded: Drag Free Control & GG space experiment simulator based on GOCE expertise of TAS-I Torino team (assume VEGA launcher)

• GGG lab & experiment basic funds from INFN-CSNII as a national experiment

• Additional ASI contribution to GGG: for new vacuum chamber and new instrument to improve GGG sensitivity (by cardanic suspension) +

• TAS-I (Torino) to contribute a “GGG Experiment Simulator” also based on heritage from GOCE, to be compared with GGG measurement data (“Remote Ground Test”)

TAS-I is prepared to complete GG in 4 yrs from start of Phase B for a total cost of: 69.560 M€ (everything included except the cost of launch with VEGA)

Expression of interest from JPL to particpate in GG by contributing to the payload

State of the art (I)State of the art (I)

E. Fischbach et al.: “Reanalysis of the Eötvös Experiment” PRL 1986

(-0.2 ± 2.8)x10-12Be − Al

MaterialsSource massApparatusAuthors

(0.3 ± 1.8)x10-13Be − TiEarthRotating torsionbalance. 20’ modulation

Eöt-Wash, PRL 2008

≈10-12

(SEP 1.3x10-3)Earthlike/ Moonlike

SunRotating torsionbalance. 1hr to 36’modulation

Eöt-Wash, PRL 1999

(-1.9 ± 2.5)x10-12Be − Cu

Earth

Rotating torsionbalance. ≈ 1hr modulation

Eöt-Wash, PRD 1994

(-0.3 ± 0.9)x10-12Al − PtSunTorsion balance. 8TMs. Not rotating. 24hr modulation byEarth rotation

Braginsky & PanovJETP 1972

(1.3±1)x10-11

10-8 ÷10-9

Al − Au

Manycombinations

Sun

Earth

Torsion balance. Notrotating. 24hr modulation by Earthrotation

Torsion balance. Notrotating. No signalmodulation

Roll, Krotkov & DickeAnn. Phys. 1964

Eötvös et al. ≈1900collected in Ann. Phys. 1922

a a≡ ∆η

36 yr

16 yr

MaterialsSource massApparatusAuthors

10-785Rb & 87Rb

EarthCold atoms droppingFray et al with T. Hänsch., PRL 2004

10-9g measurement with cold atoms (Cs)Peters, Chung & Chu, Nature 1999

a a≡ ∆η

GGG current sensitivity: 2.3x10-7 (macroscopic, fast rotating differential accelerometer in the field of the Sun)

Target: 10-15

10-16 (10-17)

85Rb & 87Rb

EarthCold atoms droppingDimopoulos, Graham, Hogan, Kasevich, PRL 2007

State of the art (II)State of the art (II)

Ongoing

Differ by 2 neutrons only!!!

Satellite, orbit and the VEGA launcher Satellite, orbit and the VEGA launcher

To fly in near circular near equatorial orbit

GG satellite in the bay of VEGA (Kourou launch site)

2.2 m~600 km altitude

Passive attitude stabilization by 1-axis rotation at 1 Hz

550 kg total mass (with 20% margin) of which ; 100kg launch adapter, 80 kg payload)

Drag Free Control around orbit frequency

To be operated from Italian station in Malindi (Kenya)

1 yr nominal mission duration (up to 3 yr)

1.6 m

GG differential accelerometer (I)GG differential accelerometer (I)

NOTE: We do not fly a vacuum chamber (use venting to space instead..)

GG differential accelerometer (II)GG differential accelerometer (II)

A second accelerometer has been designed could be accommodated:, same composition test cylinders (for zero check) CONCENTRIC with the EP violation one... There is only one center of mass of the spacecraft!!Only the EP accelerometer will fly:• Once you reach the target sensitivity (TMs relative displacements of 0.5 pm), the signature of an EP violation

signal in the field of the Earth is well known and so far we have found no perturbation with the same signature competing with it to the level of GG target

• TMs material choice to maximize physical chance of violation…

Test masses material choice in GG (I)Test masses material choice in GG (I)

Co-rotation makes many disturbing effects DC. Test masses do not need to be manufactured to very high precision => More freedom in the choice of materials to maximize chance of EP violation and significance of test

EP violation not expected to depend on macroscopic properties of matter (density, chemical, mechanical, electric or magnetic characteristics)

/ / /zB L Iµ µ µ

Barion number, Lepton number and z component of Isospin (normalized to mass in unit of the mass of H atom) have been identified (Fischbach & Talmadge, 1998)

=> choose test masses materials so as to maximize difference in all 3 these properties!

Test masses material choice in GG (II)Test masses material choice in GG (II)

Figure adapted (CH2 added) from : E. Fischbach, C. L. Talmadge: “The Search for Non-Newtonian Gravity; Springer- Verlag, New York, 1998.

Test masses material choice in GG (III)Test masses material choice in GG (III)

HDPE (High Density Polyethylene) identified – to be tested in GGG (has interesting side consequences.. Not conductive, capacitance read-out possible without capacitance plates in between test cylinders … differential by definition .. )

GG lock/unlock mechanismsGG lock/unlock mechanisms

Lock/unlock of inner test cylinder

1. Mechanical stops

2. Launch safe lock/unlock (non magnetic actuators)

3. Fine lock/unlock (inch-worm actuators)

“bunny ear” lock/unlock of coupling arm

Designed by DTM Technologies (Ferrari)

GG EP violation signal recoveryGG EP violation signal recovery

EP violation signal after demodulation (from 1Hz rotation)

Drivers and requirements: some numbers (I)Drivers and requirements: some numbers (I)

Drivers and requirements: some Drivers and requirements: some numbers (II)numbers (II)

Drag Free Control:

• FEEP thrusters developed for LISA-PathFinder + adapted to GG (=> GG spin frequency reduced to 1 Hz; surface of solar panels increased)

• Small ad-hoc optical spin sensor built in Torinocapable of measuring spin frequency to 10-5

Drivers and requirements: some numbers (III)Drivers and requirements: some numbers (III)

Drivers and requirements: some numbers (IV)Drivers and requirements: some numbers (IV)

Drivers and requirements: some numbers (V)Drivers and requirements: some numbers (V)

Drivers and requirements: some numbers (VI)Drivers and requirements: some numbers (VI)

…passive MLI sufficient

Drivers and requirements: some numbers (VII)Drivers and requirements: some numbers (VII)

Electric charging & patch effects:

• passive electric grounding of the test masses• co-rotation of the test masses and the capacitance transdusers (make patch effects

DC or slowly varying if they do slowly vary.. .as they do…)• gold coating• direct measurement of the effects of any patches of charges on test masses, as we

have done in GGG (see later..)

Error budget (I)Error budget (I)

How it is built

Establish requirements

Implement Drag Free ControlHeritage from GOCE!

Run GG Simulator

Analyze time history of test masses relative displacements

Single out systematic effects and check their magnitude and signature

Error budget (II)Error budget (II)

A simulator in the lab: A simulator in the lab: ““GG on the Ground (GGG)GG on the Ground (GGG)””

GGG lab at INFN Pisa-San Piero a Grado

Same number of degrees of freedom; same dynamical properties; position of relative equilibrium of the test masses in the horizontal plane is NOT stabilized by local gravity (as it should be as a test of experiment in space…)

GGG sensitivity: major improvements (I)GGG sensitivity: major improvements (I)

FFT of relative displacements of GGG test cylinders in the horizontal, not rotating, plane of lab

GGG sensitivity: major improvements (II)GGG sensitivity: major improvements (II)

PSD of relative displacements of GGG test cylinders in the horizontal, not rotating, plane of lab

GGG current sensitivity to EP violation in the field of the SunGGG current sensitivity to EP violation in the field of the Sun

GGG has measured 6x10-9 m at diurnal frequency with coupling period of 13 s =>

ηsun~2.3x10-7

Limited by terrain tilts: apparatus not suspended, active tilt control only.Main issue: tilt sensors dependence on temperature

sGGGsGGG (suspended GGG) (suspended GGG) -- ASI funds (I)ASI funds (I)

New chamber has the right symmetry and has been designed to minimize disturbances on GGG

sGGG will be suspended inside chamber by cardanic joint (not rotating) to reduce low frequency terrain tilts passively, in addition to active tilt control now in use (Note: active tilt control is limited by thermal effects on tilt sensor and requires good thermal stabilization to be effective)

An Experiment Simulator will be built by Thales Alenia Space-Italy for the new GGG, similarly to the Simulator built for the space experiment, to be compared with experimental measurements …

New chamber + new rotor (under completion)

sGGGsGGG (suspended GGG) (suspended GGG) –– ASI funds (II)ASI funds (II)

• With the cardanic suspension already manufactured we expect a terrain tilt reduction at low frequencies by about 2000 (based on measured intrinsic stiffness k ~2x 980 N/m + exploitation of lever effect )

• With active terrain tilt control (+thermal stabilization) plus passive attenuation we expect to detect 1 pm displacements (GG target is 0.5 pm) i.e., with current natural test masses period of 13 s: => ηsun~4x10-11

Longer natural period possible (sensitivity would increase quadratically..) but shall we encounter the motor ball bearings noise???

Thermal stability in new chamberThermal stability in new chamber

Thermal stability of tiltmeter inside chamber (multi stage thermal control): to a few tenths of mdeg down at diurnal frequency (requires 20 mW only)

A better A better tiltmetertiltmeter built and under test to improve active tilt controlbuilt and under test to improve active tilt control

Double pendulum (one simple + one inverted, aligned, coupled by tiny cantilever), based on knife edge suspensions.

Capacitance transducer with ad hoc electronic board developed in the lab based on the AD7745 24 bit capacitance to digital converter capable to measure up to 4 picoFarad to a few tenths of femtoFarad. No additional electronics is needed outside the vacuum chamber; data are transferred to the computer outside via USB port.

Designed to reach 100 s period (equivalent to a simple pendulum 2500 m long!!!) .. Extremely sensitive…

Preliminary tests with coarse balancing give 40 s period (equivalent to a 400 m simple pendulum..). Fine balancing also possible

MeasurementMeasurement of of electricelectric patchpatch effectseffects (I)(I)

Apply a force to the external test cylinder with a capacitance plate (both grounded)

Since outer and inner test cylinders are coupled, they will move relative to each other

Their differential motion is measured by the capacitance bridges (main sensors) located in between the test cylinders

MeasurementMeasurement of of electricelectric patchpatch effectseffects (II)(II)2

2=V

o

QFSε

Charge Q changes sign with applied potential, patch charge q does not!

First, apply unipolar potential as square wave and measure effect on TMs at frequency of square wave; then switch to bipolar potential (square wave with same frequency) and measure effect on TMs: the effect of patch charge q (if any) will be upconverted to the frequency of square wave and amplified by factor 4Q/q (q=0 would give no signal at square wave frequency)

0.750.18

+

+

∆∆

V

V

x my m

µµ

MeasurementMeasurement of of electricelectric patchpatch effectseffects (III)(III)

2± ≡ =V patch

o

QqF F qSε

is the charge of patch we want to measure

0.02750.006

±

±

∆∆

V

V

x my m

µµ

4 4±

+ + += =patch patchV

V V V

q Vdisplacementdisplacement Q V

by measuring the displacements in the two cases we measure Vpatch …

(no need to know how system responds to

From these measurements:

0.35 VpatchV

(4 cm2 Al plate, no gold coating: smaller patches expected for larger surfaces with gold coating)

Values measured for Vpatch are 0.3 to 0.5 V

MeasurementMeasurement of of electricelectric patchpatch effectseffects (III)(III)• No modeling needed, very neat measurement

4 / 4 /= applied patchQ q V V

Remember: the effect of the patch, in addition to being upconverted to the frequency of the applied potential is also amplified by the factor:

• You measure directly the effect of patch charges on the tests masses upconverted to known frequency (can be done also in space for GG test masses)

• We have made this measurement with GGG spinning for 10.6 d, in order to measure time variation of patch effect amplitude over such long time….

Here it is about 300

⇒ At diurnal frequency the effect of patch charges (small surface, no coating…will be reduced) on GGG test masses is 7 pm, corresponding to 8 µV(GG target requires to measure 0.5 pm)

““Galileo Galileo GalileiGalilei (GG)(GG)””

GG undergoing Phase A-2 Study by ASI (Agenzia Spaziale Italiana) Preliminary (April 2009) Report available on the Web:

http://eotvos.dm.unipi.it/PA2