MICROSCOPE

MICROSCOPE and the Equivalence
Principle / MICROSCOPE et le
principe d’équivalence
Joel Bergé (ONERA)
For the Microscope team
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Joel Bergé, Gravitational Waves Fiesta, March 1st, 2016
Tuesday, March 8, 16
MICROSCOPE
Micro-satellite à traînée compensée pour l’Observation du Principe d’Equivalence
Drag-free microsatellite for the observation of the Equivalence Principle
CNES mission to test Equivalence Principle with 10-15 precision / Mission du CNES
pour mesurer le Principe d’Equivalence avec une précision de 10-15
PI: Pierre Touboul (ONERA)
Co-PI: Gilles Métris (Observatoire de la Côte d’Azur)
Partners: ESA, ZARM, PTB, DLR
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Joel Bergé, Gravitational Waves Fiesta, March 1st, 2016
Tuesday, March 8, 16
Weak Equivalence Principle: the universality of free fall
Galileo observed that all bodies fall at the same rate: the traveled distance is proportional to the time of fall
=> x ~ t2
This means that their acceleration is constant and the same for all bodies: a = g (g = 10 m/s2 on Earth)
All test bodies follow the same universal trajectory
in a gravitational field, independently of their mass,
detailed internal structure and composition.
Newton’s laws: F = mi a, Fg = mg g
F = Fg => mi = mg
mi = inertial mass: “opposes” changes in motion (universal)
mg = gravitational mass: feels gravity (specific to the
gravitational force)
mg/mi = “gravitational charge”
For all test bodies, the inertial mass and the gravitational mass are equal: mi = mg
Precision measured in terms of the Eötvös ratio
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Joel Bergé, Gravitational Waves Fiesta, March 1st, 2016
Tuesday, March 8, 16
Equivalence Principle and General Relativity
Universality of gravitation: locally, an
observer cannot tell if he/she is at
rest in a gravity field or in an
accelerated frame far from any
gravitational field source; gravity is
inescapable
Weak Equivalence Principle
The motion of freely-falling particles are the same in a gravitational field and a uniformly
accelerated frame, in small enough regions of spacetime.
As mass is a form of energy (E=mc2), Einstein generalized the WEP to a principle
encompassing other interactions: Einstein Equivalence Principle
“In small enough regions of spacetime, the (nongravitational) laws of physics reduce to
those of special relativity; it is impossible to detect the existence of a gravitational field
by means of local experiments.”
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Joel Bergé, Gravitational Waves Fiesta, March 1st, 2016
Tuesday, March 8, 16
Equivalence Principle and General Relativity
Direct consequence of the EEP: we cannot define the acceleration due to gravity, but
instead define “unaccelerated” as “freely falling”
In other words: gravity is not a force; “zero acceleration” = “moving freely in the presence
of whatever gravitational field happens to be around”
Very profound implication: it is not possible to define inertial frames, but
only locally inertial frames
Gravity = Curvature
Other direct implications (with no need of full GR): gravitational redshift and time dilation
+ Implications of GR: perihelion precession of Mercury, gravitational lensing, gravitational waves
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Joel Bergé, Gravitational Waves Fiesta, March 1st, 2016
Tuesday, March 8, 16
The Equivalence Principle
Weak Equivalence Principle
The motion of freely-falling particles are the same in a gravitational field
and a uniformly accelerated frame, in small enough regions of spacetime.
Einstein Equivalence Principle
In small enough regions of spacetime, the (nongravitational) laws of
physics reduce to those of special relativity; it is impossible to detect the
existence of a gravitational field by means of local experiments.
Strong Equivalence Principle
In small enough regions of spacetime, all laws of physics reduce to those
of special relativity; it is impossible to detect the existence of a gravitational
field by means of local experiments.
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Joel Bergé, Gravitational Waves Fiesta, March 1st, 2016
Tuesday, March 8, 16
Should we doubt the Equivalence Principle?
• Towards a “Theory of Everything”
General Relativity: gravity, large scale, low energy
Quantum Physics: electroweak / nuclear forces, small scale, high energy
=> how to unify them?
• Dark energy / Cosmological constant
The expansion of the Universe is accelerating
=> concordance problem
• Solutions to those problem
string theory, quantum gravity, modified gravity
=> all predict a weak violation of the WEP
• Underlying question: is General Relativity the right theory of gravitation
on all scales?
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Joel Bergé, Gravitational Waves Fiesta, March 1st, 2016
Tuesday, March 8, 16
WEP tests up to now
Will 2006
Wagner+ 2012
Eötvös, Eöt-Wash
Torsion balance
Lunar Laser Range
Williams+ 1996
Probes fall of Earth and Moon towards the Sun
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Joel Bergé, Gravitational Waves Fiesta, March 1st, 2016
Tuesday, March 8, 16
The near future, what to expect?
Will 2006
Wagner+ 2012
Eötvös, Eöt-Wash
Torsion balance
Lunar Laser Range
Williams+ 1996
Probes fall of Earth and Moon towards the Sun
Expected violations
(string theory, new
scalar fields...)
Microscope
10-15
20
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Joel Bergé, Gravitational Waves Fiesta, March 1st, 2016
Tuesday, March 8, 16
MICROSCOPE measurement principle
Aim: test mi = mg at the 10-15 level
10-15: difference of weight of a 500,000 ton-tanker with or without a 0.5 mg
drosophilia on board
Differential electrostatic accelerometer:
2 coaxial cylindrical inertial sensors
Sensors forced to follow the same orbit (permanent pico-meter control)
=> we measure the electrostatic forces needed to keep the sensors centered.
Signal measured along an ultra-sensitive axis.
Drag-free control on 6 degrees-of-freedom to achieve highest possible stability
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Joel Bergé, Gravitational Waves Fiesta, March 1st, 2016
Tuesday, March 8, 16
MICROSCOPE measurement principle
Signal: gravity field modulated by satellite’s motion
around the Earth => sine of known frequency fEP
fEP can be varied by either:
- Keeping the satellite in inertial motion
- Or spinning it
How to extract the signal? Easy! We must look for a noise-dominated sine in measured
time series.
.
.
. Time domain
Frequency domain
Equivalence Principle
Violation (3x10-15)
Baghi+ in prep
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Joel Bergé, Gravitational Waves Fiesta, March 1st, 2016
Tuesday, March 8, 16
MICROSCOPE instrument: T-SAGE (Twin Space Accelerometer for
Gravitation Experiment)
2 differential accelerometers
• SUREF: reference, test masses of the same material (Pt/Rh)
• SUEP: used for Equivalence Principal test, test masses of
different materials (Pt/Rh, Ti)
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Joel Bergé, Gravitational Waves Fiesta, March 1st, 2016
Tuesday, March 8, 16
Integration in the satellite
© CNES/Emmanuel GRIMAULT, 2015
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Joel Bergé, Gravitational Waves Fiesta, March 1st, 2016
Tuesday, March 8, 16
MICROSCOPE satellite
© CNES/Emmanuel GRIMAULT, 2015
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Joel Bergé, Gravitational Waves Fiesta, March 1st, 2016
Tuesday, March 8, 16
MICROSCOPE: Ready for the launch and the space experiment
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Shipped next week to Kourou
Launch scheduled April 22, 2016 from the Kourou spaceport, on a
Soyuz rocket
Commissioning phase until August 2016
First science measurements in September 2016
Mission will last 2 years
• Main goal
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Perform the most precise test of the Weak Equivalence Principle ever (10-15), and
the first one in space
• Additional goals
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Specific search for new physics
Geodesy
Very high atmosphere physics
Demonstrate that the 6 degree-of-freedom drag-free technology is ready
(complement to LISA Pathfinder)
Joel Bergé, Gravitational Waves Fiesta, March 1st, 2016
Tuesday, March 8, 16
To know more about MICROSCOPE
On the web
• http://microscope.onera.fr
• https://microscope.cnes.fr
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Joel Bergé, Gravitational Waves Fiesta, March 1st, 2016
Tuesday, March 8, 16