John Mester

Critical states in 2D disk-shaped type-II superconductors in periodic external magnetic field

Abstract Following the procedure of Mikheenko and Kuzovlev, we present analytical solutions of field and current patterns in thin film disk-shaped type-II superconductors in perpendicular time-varying periodic external magnetic fields. We also calculate the magnetic moment and effective susceptibility. The analysis is carried out within the framework of the critical state model assuming a constant critical current. Our results are compared to that of Mikheenko and Kuzovlev; and we discuss the discrepancies.

The STEP mission: principles and baseline design

The Satellite Test of the Equivalence Principle (STEP) will test the equality of fall of objects in Earth orbit to an accuracy approaching one part in 108 by measuring the difference in rate of fall of test cylinders in cryogenic differential accelerometers in a drag-free satellite. This paper describes the current baseline design and principles used in the design of the STEP mission.

The Science Case for STEP

Abstract The Satellite Test of the Equivalence Principle (STEP) will advance experimental limits on violations of Einstein’s Equivalence Principle (EP) from their present sensitivity of 2 parts in 10 13 to 1 part in 10 18 through multiple comparison of the motions of four pairs of test masses of different compositions in an earth-orbiting drag-free satellite. Dimensional arguments suggest that violations, if they exist, should be found in this range, and they are also suggested by leading attempts at unified theories of fundamental interactions (e.g., string theory) and cosmological theories involving dynamical dark energy. Discovery of a violation would constitute the discovery of a new force of nature and provide a critical signpost toward unification. A null result would be just as profound, because it would close off any possibility of a natural-strength coupling between standard-model fields and the new light degrees of freedom that such theories generically predict (e.g., dilatons, moduli, quintessence). STEP should thus be seen as the intermediate-scale component of an integrated strategy for fundamental physics experiments that already includes particle accelerators (at the smallest scales) and supernova probes (at the largest). The former may find indirect evidence for new fields via their missing-energy signatures, and the latter may produce direct evidence through changes in cosmological equation of state—but only a gravitational experiment like STEP can go further and reveal how or whether such a field couples to the rest of the standard model. It is at once complementary to the other two kinds of tests, and a uniquely powerful probe of fundamental physics in its own right.

The Gravity Probe B test of general relativity

The Gravity Probe B mission provided two new quantitative tests of Einsteins theory of gravity, general relativity (GR), by cryogenic gyroscopes in Earths orbit. Data from four gyroscopes gave a geodetic drift-rate of −6601.8 ± 18.3 marc-s yr−1 and a frame-dragging of −37.2 ± 7.2 marc-s yr−1, to be compared with GR predictions of −6606.1 and −39.2 marc-s yr−1 (1 marc-s = 4.848 × 10−9 radians). The present paper introduces the science, engineering, data analysis, and heritage of Gravity Probe B, detailed in the accompanying 20 CQG papers.

STEP: A Status Report

This paper presents an overview of the current technical status of STEP, the Satellite Test of the Equivalence Principle. STEP was originally presented as a candidate for ESA’s M2 mission as a joint mission with NASA, and has since been studied as an M3 candidate, and under NASA as QuickSTEP and MiniSTEP. Studies especially during the last two years have resolved some long standing issues such as control of helium tide, improved the mission definition and error analysis, and have resulted in an improved baseline design which should be capable of comparing rates of fall to an accuracy approaching 10-18.

STEP and fundamental physics

The Satellite Test of the Equivalence Principle (STEP) will advance experimental limits on violations of Einsteins equivalence principle from their present sensitivity of two parts in 1013 to one part in 1018 through multiple comparison of the motions of four pairs of test masses of different compositions in a drag-free earth-orbiting satellite. We describe the experiment, its current status and its potential implications for fundamental physics. Equivalence is at the heart of general relativity, our governing theory of gravity and violations are expected in most attempts to unify this theory with the other fundamental interactions of physics, as well as in many theoretical explanations for the phenomenon of dark energy in cosmology. Detection of such a violation would be equivalent to the discovery of a new force of nature. A null result would be almost as profound, pushing upper limits on any coupling between standard-model fields and the new light degrees of freedom generically predicted by these theories down to unnaturally small levels.

STEP error model development

We describe the ongoing development of a comprehensive error model for the satellite test of the equivalence principle, STEP. The goal is to employ a model of the experiment and apparatus as a self-consistent whole. The model uses a set of input parameters based on experiment design and the measured characteristics of STEP sensor systems. The output of the model evaluates specific disturbances to the test masses in the general categories of thermal noise, gas pressure forces, electrical forces, magnetic forces, gravitational forces, radiation pressure and vibration. Use of the model to set experiment requirements and to evaluate design trade-offs are briefly discussed. PACS number: 0480C (Some figures in this article are in colour only in the electronic version; see www.iop.org)

Development of the Gravity Probe B flight mission

Abstract Gravity Probe B is an experiment to measure the geodetic and frame-dragging precessions, relative to the “fixed” “stars”, of a gyroscope placed in a 650 km altitude polar orbit about the earth. For Einsteins general relativity, the precessions are calculated to be 6.6 arcsec/yr for the geodetic precession and 0.042 arcsec/yr for the frame-dragging precession. The goal of the experiment is to measure these precessions to better than 0.01% and 1%, respectively. This paper gives an overview of the experiment and a discussion of the flight hardware development and its status. This paper also includes an estimate of the geodetic and frame-dragging errors expected for the experiment.

The step payload and experiment

Abstract The foundation of modern gravitational theory is the Equivalence Principle. General Relativity is incompatible with theories of other fundamental forces such as QED, suggesting that it is incomplete. For example, there may be additional forces coupled to baryon number or spin. In this case the Equivalence Principle may be violated below the experimentally verified level of one part in 10 12 . A violation could provide crucial information for new theories. A team of US and European scientists has assembled to do the Satellite Test of the Equivalence Principle (STEP) with the goal of improving this measurement to 1 part in 10 18 . In STEP two or more test masses “fall” around the earth in a drag free satellite. A difference in the rate of fall appears as a periodic difference in their acceleration. The test masses are cooled to less than 2K and are supported by frictionless superconducting bearings. Ultra-sensitive SQUID position sensors measure their relative motion and their common motion is removed by adjustments during acceleration maneuvers. Any Equivalence Principle signal is separated from major disturbances by rotation of the spacecraft. STEP is planned to be launched by 2004, with nominal mission lifetime of 6 months.

Ultralow magnetic fields and Gravity Probe B gyroscope readout

Abstract We describe the generation of an ultralow magnetic field of −11 Tesla in the flight dewar of the Gravity Probe B Relativity Mission. The field was achieved using expanded-superconducting-shield techniques and is maintained with the aid of a magnetic materials control program. A high performance magnetic shield system is required for the proper function of gyroscope readout. The readout system employs a dc SQUID to measure the London moment generated by the superconducting gyro rotor in order to resolve sub-milliarcsecond changes in the gyro spin direction. In addition to a low residual dc magnetic field, attenuation of external field variation is required to be 10 12 at the gyro positions. We discuss the measurement of the dc magnetic field and ac attenuation factor and the performance of the readout system.