STE-QUEST (Space-Time Explorer and Quantum Test of the Equivalence Principle): the mission concept test of gravitational time dilation

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1 13th ICATPP Conference on Astroparticle, Particle, Space Physics and Detectors for Physics Applications Como, STE-QUEST (Space-Time Explorer and Quantum Test of the Equivalence Principle): the mission concept test of gravitational time dilation S. Schiller Heinrich-Heine-Universität Düsseldorf ESA Mission Concept Science Study Team: K. Bongs (UK), P. Bouyer (F), L. Iess (I), A. Landragin (F), E.M. Rasel (D), S. Schiller (D), U. Sterr (D), G.M. Tino (I), P. Wolf (F) ESA study scientist: L. Cacciapuoti

2 Mission Overview Medium-class mission (cost to ESA < 470 M ) Under consideration for slot M3 in ESA s Cosmic Vision Program Target take-off date: 2021 Science goals: Establish more firmly the metric nature of the theory of gravitation, search for Physics beyond the Standard Model plus General Relativity Test the Weak Equivalence Principle Test time dilation in the terrestrial and in the solar gravitational potential Application to other fields: master clock in space for precision experiments world-wide mapping of the gravitational potential of the Earth with ultra-high spatial resolution

3 The conceptual basis for the STE-QUEST experiments General Relativity Metric theory of gravity Gravitational redshift Lense-Thirring effect Shapiro delay Perihelion shift Schiff effect Earth & moon free fall Binaries dynamics Universality of Free Fall (Weak equivalence princip.) Einstein Equivalence Principle Local Position Invariance (Universality of grav. Redshift constancy of constants) Local Lorentz Invariance (Special Relativity) The trajectory of a point mass in a gravitational field depends only on its initial position and velocity, and is independent of its composition. The result of any local nongravitational experiment is independent of its location in space and time.

4 Weak Equivalence Principle Test Comparison of the free-fall trajectories of two different quantum waves: 85 Rb isotope and 87 Rb isotope Searches for anomalous coupling of gravity to matter Measurement is most sensitive at perigee Atom interferometry is used; goal: achieve 1x10-15 sensitivity in the violation parameter after 5 years mission duration

5 Gravity modifies time: the gravitational time dilation g t 2 t 2 t 1 t 1 START STOP time J. S. Bell, in Fundamental Symmetries, edited by P. Bloch et al. (Plenum, New York, 1987) 1911

6 Gravity modifies time: the gravitational frequency shift (redshift) The comparison of the frequencies stemming from two identical clocks located at different positions yields the nonzero result (if at rest): () r U ( r ) U ( r ) 1 clock clock 2() r c Near Earth surface: (height difference in m) Effect verified to 7x10-5 in 1976 clock 2 at r 2 clock 1 at r observation point r

7 Testing the gravitational frequency shift U Reference Clock Clock on satellite i 0

8 Phase I: Earth s gravitational frequency shift - Goal: High-precision measurement of the gravitational redshift - Use best available space clock - Use largest useful height difference Two measurement approaches Repeated measurements of difference of space clock frequency between apogee and perigee - gain in sensitivity of N 1/2, N = number of orbits - uses the stability properties of ground and satellite clocks, rather than their accuracy - for 4 years of measurement: measurement inaccuracy 0.17 ppm (MWL), 0.04 ppm (LCT) Absolute comparison between ground clock and satellite clock - U/c 2 ~ 6.2 x space clock inaccuracy (PHARAO with Rb atoms) 1 x ground clock accuracy: better (gravitational potential uncertainty at apogee is not relevant) - measurement at 0.16 ppm Gravity Probe A (1976): 70 ppm ACES (2014): ~ 2 ppm

9 Phase II Sun s gravitational redshift - First precise test of Sun gravitational time dilation - New approach towards determination of geopotential Unequal clock frequencies Frequency comparison links to STE-QUEST allow terrestrial clock comparisons in common-view Solar clock redshift: daily amplitude of 4x10-13 Compensated by Doppler shift due to Earth motion Assume: ground clocks and link combined instability at 1x10-18 (@ 1 h 1 d) and 1000 comparisons (N 1/2 improvement): Null test of solar gravitational redshift at 0.1 ppm. Measurement does not require operation of atomic clock or frequency comb on satellite Equal clock frequencies To sun

10 The satellite atomic clock PHARAO Eng. model PHARAO (for ACES mission on ISS, 2014): cold Caesium atoms, interrogated at 9.1 GHz STE-QUEST: PHARAO upgrade to Rubidium atoms, interrogated at 6.8 GHz Goal: higher accuracy due to more advantageous atomic properties for Rb Frequency instability: ( ) < 3x10-14 for = s (with MOLO) Frequency inaccuracy: <1x10-16

11 The microwave-optical local oscillator (MOLO) f n n* f rep = f n* = f L = f cav f n = n f rep f L MENLO 6.8 GHz 100 MHz fs laser repetition rate frequency generation 1064 nm laser optical lock reference cavity reference cavity To Laser Link Terminal TESAT PTB ACES f cav Hz Will provide a 6.8 GHz signal with frequency instability ( ) < 1x10-15 for = s (after removal of linear drift; (drift)=2x10-16 /s over 1000 s) Ca. 40 times higher than best quartzes For interrogation of the hyperfine transition in Rubidium atoms

12 The satellite - to - ground links Links transmit frequency information; two-way principle: - From satellite to ground - From ground to satellite Two types of links: - laser (pro: higher performance) - microwave (pros: not weather-sensitive, simultaneous contact to more ground stations) Two-way links allow measurement and cancellation of 1 st order Doppler shift Microwave link uses multiple frequencies in order to cancel atmospheric and ionospheric effects Heritage: ACES-MWL, LCT on TerraSAR-X

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15 Solar arrays: 1.6 kw Total wet mass: 1.25 t Launch mass: 1.36 t Instruments: 373 kg

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17 Ground stations 3 MWL ground stations (weather is not an issue), ACES heritage 3 LCT ground stations (not necessarily colocated with MWL, need cloudfree sky) Ground atomic clocks need not be at same location, but can be connected by fiber-optic link Current baseline locations: Matera (I), Boulder (USA), Tokyo (J) LCT ground terminal MWL ground terminal (2 or 3 m dish)

18 Summary Science goals: Establish more firmly the metric nature of the theory of gravitation, search for Physics beyond the Standard Model plus General Relativity Test the Weak Equivalence Principle with matter waves, goal accuracy : 1 x Test time dilation in the terrestrial and in the solar gravitational potential with goal accuracy 4 x 10-8 and 1.6 x 10-7, resp. Application to other fields: master clock in space for precision experiments world-wide mapping of the gravitational potential of the Earth with ultra-high spatial resolution Status: Instrument consortia are being formed 2012: Assessment studies (2 by industry on spacecraft/mission aspects and several by agencies/institutions on payload components) Mid-2013: downselection of M3 missions by ESA

19 Differential measurements of gravity potential With clocks of accuracy at 1x10-18, can in prinicple measure potential differences with equivalent height resolution of 1 cm Extremely high spatial resolution Good time resolution (~ 1 day) Can be measured anywhere U STE-QUEST Frequency comparison by: - free-space link (~ 10 km) - optical fiber (~ 100 km) - transponder (any distance, intercontinental) Link can be at optical or microwave frequency Two-way link permits Doppler cancellation U(r 1 ) U(r 2 ) Altitude of clocks must be independently determined with < 1 cm error () r U( r) U( r ) 1 clock clock 2() r c The onboard atomic clock is not required (only the laser coherent terminal)