[First Section]


New Astronomy, Vol. 8 (5) (2003) pp. 371-390
2002 Elsevier Science B.V. All rights reserved.
PII: S1384-1076(02)00233-6

A rotating differential accelerometer for testing the equivalence principle in space: results from laboratory tests of a ground prototype

A.M. Nobili a * nobili@dm.unipi.it , D. Bramanti a bramanti@mail.dm.unipi.it , G.L. Comandi a comandi@mail.dm.unipi.it , R. Toncelli a toncelli@mail.dm.unipi.it and E. Polacco b polacco@df.unipi.it

a Space Mechanics Group, Department of Mathematics, University of Pisa, I-56127 Pisa, Italy
b Department of Physics, University of Pisa, and INFN, I-56127 Pisa, Italy

Received 19 August 2002; accepted 1 November 2002

Communicated by F. Melchiorri

Abstract

We have proposed to test the equivalence principle (EP) in low Earth orbit with a rapidly rotating differential accelerometer (made of weakly coupled concentric test cylinders) whose rotation provides high frequency signal modulation and avoids severe limitations otherwise due to operation at room temperature [PhRvD 63 (2001) 101101]. Although the accelerometer has been conceived for best performance in absence of weight, we have designed, built and tested a variant of it at 1-g. Here we report the results of measurements performed so far. Losses measured with the full system in operation yield a quality factor only four times smaller than the value required for the proposed high accuracy EP test in space. Unstable whirl motions, which are known to arise in the system and might be a matter of concern, are found to grow as slowly as predicted and can be stabilized. The capacitance differential read-out (the mechanical parts, electronics and software for data analysis) is in all similar to what is needed in the space experiment. In the instrument described here the coupling of the test masses is 24,000 times stiffer than in the one proposed for flight, which makes it 24,000 times less sensitive to differential displacements. With this stiffness it should detect test masses separations of 1.510-2 m, while so far we have achieved only 1.5 m, because of large perturbations--due to the motor, the ball bearings, the non-perfect verticality of the system--all of which, however, are absent in space. The effects of these perturbations should be reduced by 100 times in order to perform a better demonstration. Further instrument improvements are underway to fill this gap and also to reduce its stiffness, thus increasing its significance as a prototype of the space experiment.

PACS: 04.80.Cc; 07.10.-h; 06.30.Bp; 07.87.+v

Keywords: Gravitation; Relativity; Instrumentation: detectors; Methods: laboratory; Methods: data analysis

*Corresponding author.


Table of contents

  1. Introduction
  2. Design of the apparatus
  3. Adjustments and settings of the apparatus
  4. Read-out and data acquisition
  5. Results from measurement data
  6. Concluding remarks
  7. Note added in proof


Acknowledgements
References

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