Tests of the Equivalence Principle via differential accelerometry on-ground, in a balloon and in space
Abstract
Einstein’s theory of general relativity -- currently the best known description of gravitational phenomena -- relies on the validity of the so-called Einstein Equivalence Principle, which in turn is based on the Weak Equivalence Principle (WEP). The WEP states that the ratio of the gravitational mass to the inertial one is the same for all bodies, hence all fall with the same acceleration independently of their mass and composition. This is the Universality of Free Fall (UFF), a property which makes gravitation a universal phenomenon. The experimental tests which investigate the validity of UFF are among the most significant to verify Einstein's theory and among the most promising for the discovery of new physics beyond general relativity. The best tests have reached an accuracy of parts in (10^{13}), using rotating torsion balances (in the field of Earth and Sun) and Lunar Laser Ranging measurements (in the field of the Sun). We present here a concept for a new experimental test of WEP carried out in the gravity field of the Sun. Two test masses of different materials are the central elements of a differential accelerometer with zero baseline. A possible WEP violation would show up with a non-zero differential acceleration signal for the two elements falling in the given gravitational field. This basic concept of the measurement lends itself to three stages of development, with increasing projected accuracy levels. In the first stage, which is foreseen to be performed on ground, the differential accelerometer is placed on a pendulum, in such a way as to make the common center of mass coincident with the center of mass of the pendulum. Ensuring a very precise centering, such a system should provide a high degree of attenuation of the local seismic noise, which, together with an integration time of the order of tens of days, would allow verification of the WEP with an accuracy improved by at least an order of magnitude with respect to the state of the art. One of the strengths of this experiment is the know-how acquired from a previous study and technology development (GREAT: General Relativity Accuracy Test) that involved a test of the WEP in the gravity field of the Earth, in free fall inside a co-moving capsule released from a stratospheric balloon. This is the second stage, with a projected accuracy of a few parts in (10^{15}). The capsule would be carried at high altitude and released. During the fall, inside the capsule the detector would be in turn released, freely falling in vacuum; it would be initially spun about a horizontal axis in order to modulate a possible violating signal at the spin frequency (therefore enabling to distinguish the violation from other perturbations). The high accuracy of the experiment requires resolving a very small signal out of components like the instrument intrinsic noise and the noise components associated with its motion and gravity gradients. The detector would operate in cryogenic conditions and would be capable of rejecting linear and angular noise while retaining the signal of a possible violation. The third stage would imply accommodating the differential accelerometer, core of the measurement, in a very-low-noise environment as that offered by a space platform. This platform should have to employ a drag-free technology. A sensible advantage of space is the possibility of very long integration times, thereby lowering the random components of noise acting on the detector. The differential accelerometer concept will be discussed, along with design and prototyping performed along the years. Then an outline of the three subsequent stages of the experiment will be given.
- Publication:
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40th COSPAR Scientific Assembly
- Pub Date:
- 2014
- Bibcode:
- 2014cosp...40E1253I