Table of contents

A review of the main features of resonant mass gravitational wave (GW) detectors is given. After a brief survey of their main characteristics and news about each of them, recent experimental results are presented.

The objective of the TAMA 300 interferometer was to develop advanced technologies for kilometre scale interferometers and to observe gravitational wave events in nearby galaxies. It was designed as a power-recycled Fabry–Perot–Michelson interferometer and was intended as a step towards a final interferometer in Japan.

The present successful status of TAMA is presented. TAMA forms a basis for LCGT (large-scale cryogenic gravitational wave telescope), a 3 km scale cryogenic interferometer to be built in the Kamioka mine in Japan, implementing cryogenic mirror techniques. The plan of LCGT is schematically described along with its associated R&D.

The instability in the r-modes of rotating neutron stars can (in principle) emit substantial amounts of gravitational radiation (GR) which might be detectable by LIGO and similar detectors. Estimates are given here of the detectability of this GR based on the non-linear simulations of the r-mode instability by Lindblom, Tohline and Vallisneri. The burst of GR produced by the instability in the rapidly rotating 1.4M_⊙ neutron star in this simulation is fairly monochromatic with frequency near 960 Hz and duration about 100 s. A simple analytical expression is derived here for the optimal signal-to-noise ratio S/N for detecting the GR from this type of source. For an object located at a distance of 20 Mpc we estimate the optimal S/N to be in the range 1.2–12.0 depending on the LIGO II configuration.

In this review we examine the dynamics and gravitational wave detectability of rotating strained neutron stars. The discussion is divided into two halves: triaxial stars and precessing stars. We summarize recent studies on how crustal strains and magnetic fields can sustain triaxiality, and suggest that Magnus forces connected with pinned superfluid vortices might contribute to deformation also. The conclusions that could be drawn following the successful gravitational wave detection of a triaxial star are discussed, and areas requiring further study identified. The latest ideas regarding free precession are then outlined, and the recent suggestion of Middleditch et al (Middleditch et al 2000 New Astronomy5 243; 2000 Preprint astro-ph/0010044) that the remnant of SN1987A contains a freely precessing star, spinning down by gravitational wave energy loss, is examined critically. We describe what we would learn about neutron stars should the gravitational wave detectors prove this hypothesis to be correct.

Binary black holes are the most promising candidate sources for the first generation of earth-based interferometric gravitational-wave detectors. We summarize and discuss the state-of-the-art analytical techniques developed during the last few years to better describe the late dynamical evolution of binary black holes of comparable masses.

The laser interferometer space antenna (LISA) can be used to map the gravitational wave background by measuring the orbital modulation of the detector response. Using the known antenna pattern and orbit, and the measured modulation, it is possible to solve for the sky luminosity distribution.

The late stage of the inspiral of two black holes may have important non-Newtonian effects that are unrelated to radiation reaction. To understand these effects we approximate a slowly inspiralling binary by a stationary solution to Einstein's equations in which the holes orbit eternally. Radiation reaction is nullified by specifying a boundary condition at infinity containing equal amounts of ingoing and outgoing radiation. The computational problem is then converted from an evolution problem with initial data to a boundary value problem. In addition to providing an approximate inspiral waveform via extraction of the outgoing modes, our approximation can give alternative initial data for numerical relativity evolution. We report results on simplified models and on progress in building 3D numerical solutions.

We present results from simulations of axisymmetric relativistic rotational core collapse. The main objective of our investigation is to compute the waveforms of gravitational radiation emitted in such events, extending previous Newtonian simulations to relativity. The general relativistic hydrodynamic equations are formulated in flux-conservative form and solved using a high-resolution shock-capturing scheme. The Einstein equations are solved assuming a conformally flat 3-metric and the quadrupole formula is used to extract waveforms of the gravitational radiation emitted during the collapse. A comparison of our results with those of Newtonian simulations shows that gravitational wave amplitudes agree within 30%. Surprisingly, in some cases, relativistic effects actually diminish the amplitude of the gravitational wave signal. We further find that the parameter range of models suffering multiple coherent bounces due to centrifugal forces is considerably smaller than in Newtonian simulations.

A population of stellar mass black hole binaries may exist in globular clusters. The dynamics of globular cluster evolution imply that there may be at most one black hole binary in a globular cluster. The populations of binaries are expected to have orbital periods greater than a few hours and a thermal distribution of eccentricities. In the LISA band, the gravitational wave signal from these binaries will consist of several of the higher harmonics of the orbital frequency. A Monte Carlo simulation of the galactic globular cluster system indicates that LISA will detect binaries in 10% of the clusters with an angular resolution sufficient to identify the host cluster of the binary.

By using a recent model for the evolving star formation rate, we investigate the statistical distribution of gravitational wave amplitudes due to supernovae that result in neutron star formation in the Einstein–de Sitter cosmology. To account for the uncertainty in gravitational wave emission for this source type, we use a random mixture of three simulated waveform types computed by Zwerger and Müller. We investigate statistical parameters of the resulting gravitational wave amplitude distribution in our frame.

Black hole–torus systems may represent astrophysical transient sources powered by the spin energy of a Kerr black hole. Equivalence in poloidal topology to pulsar magnetospheres suggests a long-lived state in suspended accretion when the black hole spins rapidly. The rotational energy is released in several channels: baryon-poor jets along open flux tubes extending from the horizon to infinity, gravitational radiation from the torus, winds and, conceivably, neutrino emissions. Gravitational radiation is expected to be representative of the rotational energy of the black hole, exceeding the output along the open flux tube by some two orders of magnitude. LIGO/VIRGO detections can hereby provide us with a calorimetric test of Kerr black holes.

The gravitational radiation signals to be anticipated from events involving black-hole galactic nuclei depend on the spin of the underlying object. To obtain evidence about the spin of Seyfert AGN black holes, we can rely on future ultra-high resolution spectral/spatial x-ray studies of iron K line fluorescence from the innermost regions of accreting matter. Normal galaxies present more of a challenge. To account for the highest energy cosmic rays, we propose that ultra-relativistic particle acceleration can occur near the event horizons of spun-up supermassive black-holes at the non-active nuclei of giant elliptical galaxies. This conjecture about the black hole spin associated with such nuclei is subject to verification via the characteristic TeV curvature radiation expected to be detected with upcoming gamma-ray observatories.

X-ray observations suggest that neutron stars in low mass x-ray binaries (LMXB) are rotating with frequencies in the range 300–600 Hz. These spin rates are significantly less than the break-up rates for essentially all realistic neutron star equations of state, suggesting that some process may limit the spin frequencies of accreting neutron stars to this range. If the accretion-induced spin up torque is in equilibrium with gravitational radiation losses, these objects could be interesting sources of gravitational waves. I present a brief summary of current measurements of neutron star spins in LMXBs based on the observations of high-Q oscillations during thermonuclear bursts (so-called 'burst oscillations'). Further measurements of neutron star spins will be important in exploring the gravitational radiation hypothesis in more detail. To this end, I also present a study of fast chirp transform (FCT) techniques as described by Jenet and Prince (Prince T A and Jenet F A 2000 Phys. Rev. D 62 122001) in the context of searching for the chirping signals observed during x-ray bursts.

We present analytical and numerical studies of the Fourier transform (FT) of the gravitational wave (GW) signal from a pulsar, taking into account the rotation and orbital motion of the Earth. We also briefly discuss the Zak–Gelfand integral transform and a special class of the generalized hypergeometric function of potential relevance. The Zak–Gelfand integral transform that arises in our analytic approach has also been useful for Schrödinger operators in periodic potentials in condensed matter physics (Bloch wavefunctions) and holds promise for the study of periodic GW signals for long integration times.

We study the generation of a gravitational wave (GW) background produced from a population of core-collapse supernovae, which form black holes in scenarios of structure formation of the Universe. We obtain, for example, that a pre-galactic population of black holes, formed at redshifts z ≃ 30–10, could generate a stochastic GW background with a maximum amplitude of h_BG ≃ 10⁻²⁴ in the frequency band ν_obs ≃ 30–470 Hz (considering a maximum efficiency of generation of GWs, namely, ε_GW = 7 × 10⁻⁴). In particular, we discuss what astrophysical information could be obtained from a positive, or even a negative, detection of such a GW background produced in scenarios such as those studied here. One of them is the possibility of obtaining the initial and final redshifts of the emission period from the observed spectrum of GWs.

The sole influence of plane gravitational waves (PGW) on the magnitude of the gradient of a scalar field (MGSF) defined inside a medium, is calculated. The conditions on the relations between the fluctuations in the gradient of the scalar field (GSF) and PGW leading to increase or decrease of the GSF due to the sole influence of PGW, are given. Two special cases of laser interferometers and cryogenic bar detectors are presented as an application of the proposed approach.

Possible effects, which could be caused by a hypothetical super-strong interaction of photons or massive bodies with single gravitons of the graviton background, are considered. If full cosmological redshift magnitudes are caused by the interaction, then the luminosity distance in a flat non-expanding universe as a function of redshift is very similar to the specific function which fits supernova cosmology data by Riess et al (1998 Astron. J.116 1009). From another side, in this case every massive body, moving slowly relative to the background, would experience a constant acceleration, proportional to the Hubble constant, of the same order as a small additional acceleration of Pioneer 10, 11.

By comparing the observed orbital decay of the binary pulsars PSR B1913+16 and PSR B1534+12 to that predicted by general relativity due to gravitational-wave emission, we are able to bound the mass of the graviton to be less than 7.6 × 10⁻²⁰ eV/c² at 90% confidence. This is the first such bound to be derived from dynamic gravitational fields. It is approximately two orders of magnitude weaker than the static-field bound from solar system observations, and will improve with further observations.

In a recent paper by Wang et al (Wang Y, Stebbins A, and Turner E L 1996 Phys. Rev. Lett.77 2875) the influence of gravitational lensing on increasing the estimated rate of gravitational radiation sources was considered. We show that the authors used the incorrect model for this case and thus they gave an overestimated rate of possible events for possible sources of gravitational radiation for the advanced LIGO detector. We also show that if we use a more correct model of gravitational lensing, one could conclude that stronger influence on increasing rate of estimated events of gravitational radiation for the advanced LIGO detector could give gravitational lenses of galactic masses but not gravitational lenses of stellar masses as Wang et al concluded. Moreover, binary gravitational lenses could give essential distortion of gravitational wave form templates, especially the gravitational wave template of periodic sources, and the effect could be significant for templates of quasi-periodic sources which could be detected by a future gravitational wave space detector such as LISA.

The groups operating cryogenic bar detectors of gravitational waves are performing a coordinated search for short signals within the International Gravitational Event Collaboration (IGEC). We review the most relevant aspects of the data analysis, based on a time-coincidence search among triggers from different detectors, and the properties of the data exchanged by each detector under a recently-upgraded agreement. The IGEC is currently analysing the observations from 1997 to 2000, when up to four detectors were operating simultaneously. 10% and 50% of this time period were covered by simultaneous observations, respectively, of at least three or at least two detectors. Typical signal search thresholds were in the range 2–6 10⁻²¹/Hz. The coincidences found are within the estimated background, hence improved upper limits on incoming GW (gravitational wave) bursts have been set.

The GEO 600 laser interferometer with 600 m armlength is part of a worldwide network of gravitational wave detectors. Due to the use of advanced technologies like multiple pendulum suspensions with a monolithic last stage and signal recycling, the anticipated sensitivity of GEO 600 is close to the initial sensitivity of detectors with several kilometres armlength. This paper describes the subsystems of GEO 600, the status of the detector by September 2001 and the plans towards the first science run.

Using one arm of the Michelson interferometer and the power recycling mirror of the interferometric gravitational wave detector GEO 600, we created a Fabry–Perot cavity with a length of 1200 m. The main purpose of this experiment was to gather first experience with the main optics, its suspensions and the corresponding control systems. The residual displacement of a main mirror is about 150 nm rms. By stabilizing the length of the 1200 m long cavity to the pre-stabilized laser beam, we achieved an error point frequency noise of 100 μHz Hz^−1/2 at 100 Hz Fourier frequency. In addition we demonstrated the reliable performance of all included subsystems by several 10-hour-periods of continuous stable operation. Thus the full frequency stabilization scheme for GEO 600 was successfully tested.

The data acquisition system of the gravitational wave detector GEO600 is recording the first data now. Data from detector subsystems and environmental channels are being acquired. The data acquisition system is described and first results from the detector characterization work are being presented. We analysed environmental influences on the detector to determine noise propagation through the detector. Long-term monitoring allowed us to see long-timescale drifts in subsystems.

TAMA300, an interferometric gravitational-wave detector with 300 m baseline length, has been developed and operated with sufficient sensitivity to detect gravitational-wave events within our galaxy and sufficient stability for observations. The interferometer was operated for over 24 h stably and continuously. With a strain-equivalent noise level of h ∼ 5 × 10⁻²¹ Hz^−1/2, a signal-to-noise ratio (SNR) of 30 is expected for gravitational waves generated by a coalescence of 1.4 M_⊙−1.4 M_⊙ binary neutron stars at 10 kpc distance. In the summer of 2000, we carried out a two-week data-taking run, called data taking 4 (DT4), collecting 160 h of data to be analysed in the search for gravitational waves. In this paper, we review the design of the TAMA300 interferometer and the results of DT4. In addition, improvements after DT4 and recent results are also reported.

The VIRGO Central Interferometer (CITF) is a short suspended interferometer operated with the central area elements of the VIRGO detector. The main motivation behind the CITF is to allow the integration and debugging of a large part of the subsystems of VIRGO while the construction of the long arms of the antenna is being completed. This will permit a faster commissioning of the full-size antenna. In fact, almost all the main components of the CITF, with the exception of the large mirrors and a few other details, are the same as those to be used for the full-size detector. In this paper the present status of the VIRGO CITF is reported.

The goal of the laser interferometer gravitational-wave observatory (LIGO) project is to detect and study gravitational waves from astrophysical sources. Currently, three interferometers with arm lengths of several kilometres and a design strain sensitivity of order 2 × 10⁻²³ Hz^−1/2 are being commissioned at two independent sites in Hanford (WA) and Livingston (LA). This paper describes the current work towards achieving LIGO's final sensitivity.

The robust statistic proposed by Creighton (Creighton J D E 1999 Phys. Rev. D 60 021101) and Allen et al (Allen et al 2001 Preprint gr-gc/010500) for the detection of stationary non-Gaussian noise is briefly reviewed. We compute the robust statistic for generic weak gravitational-wave signals in the mixture-Gaussian noise model to an accuracy higher than in those analyses, and reinterpret its role. Specifically, we obtain the coherent statistic for detecting gravitational-wave signals from inspiralling compact binaries with an arbitrary network of earth-based interferometers. Finally, we show that excess computational costs incurred owing to non-Gaussianity is negligible compared to the cost of detection in Gaussian noise.

In the search for gravitational wave burst signals, the coincidence of two or more detectors is necessary. We outline here some of the problems that arise when performing a coincidence analysis, given the fact that the detectors have, in general, different sensitivities.

A variety of gravitational-wave stochastic backgrounds populate the sensitivity window of LISA: signals produced by galactic and extra-galactic binary systems, and relic gravitons generated in the early Universe. We review our present astrophysical understanding of the main sources, address the prospects of detection with LISA and discuss possible follow-on missions.

We report on our experience gained in the signal processing of the resonant GW detector AURIGA. Signal amplitude and arrival time are estimated by means of a matched-adaptive Wiener filter. The detector noise, entering in the filter set-up, is modelled as a parametric ARMA process; to account for slow non-stationarity of the noise, the ARMA parameters are estimated on an hourly basis. A requirement of the set-up of an unbiased Wiener filter is the separation of time spans with 'almost Gaussian' noise from non-Gaussian and/or strongly non-stationary time spans. The separation algorithm consists basically of a variance estimate with the Chauvenet convergence method and a threshold on the Curtosis index. The subsequent validation of data is strictly connected with the separation procedure: in fact, by injecting a large number of artificial GW signals into the 'almost Gaussian' part of the AURIGA data stream, we have demonstrated that the effective probability distributions of the signal-to-noise ratio χ² and the time of arrival are those that are expected.

Gravitational wave astronomy will require the coordinated analysis of data from the global network of gravitational wave observatories. Questions of how to optimally configure the global network arise in this context. We have elsewhere proposed a formalism which is employed here to compare different configurations of the network, using both the coincident network analysis method and the coherent network analysis method. We have constructed a network model to compute a figure-of-merit based on the detection rate for a population of standard-candle binary inspirals. We find that this measure of network quality is very sensitive to the geographic location of component detectors under a coincident network analysis, but comparatively insensitive under a coherent network analysis.

A change of non-astrophysical origin in the detector state or in the statistical nature of data while an interferometer is in lock reflects an abnormality. The change can manifest itself in many forms: transients, drifts in noise power spectral density, change in cross correlation between channels, etc. We advance the idea of an algorithm for detecting such change points whose design goal is reliable performance, i.e. a known false alarm rate, even when statistically unmodelled data such as those from the physical environmental monitors are included. Reliability is important since following up on such change points could be fairly labour intensive. Such an algorithm need not be simply a collection of isolated independent monitors running in parallel. We present the first design steps towards building this detector characterization robot along with some preliminary results and outline some possibilities for the future.

We extend a coherent network data-analysis strategy developed earlier for detecting Newtonian waveforms to the case of post-Newtonian (PN) waveforms. Since the PN waveform depends on the individual masses of the inspiralling binary, the parameter-space dimension increases by one from that of the Newtonian case. We obtain the number of templates and estimate the computational costs for PN waveforms: for a lower mass limit of 1M_⊙, for LIGO-I noise and with 3% maximum mismatch, the online computational speed requirement for single detector is a few Gflops; for a two-detector network it is hundreds of Gflops and for a three-detector network it is tens of Tflops. Apart from idealistic networks, we obtain results for realistic networks comprising of LIGO and VIRGO. Finally, we compare costs incurred in a coincidence detection strategy with those incurred in the coherent strategy detailed above.

Detecting a stationary, stochastic gravitational wave signal is complicated by the impossibility of observing detector noise independently of the signal. Here we describe a method of identifying this source of systematic error by varying the orientation of one of the detectors, leading to separate and independent modulations of the signal and noise contribution to the cross-correlation. The method can be applied to measurements of a stochastic gravitational wave background by the ALLEGRO/LIGO Livingston Observatory detector pair. We explore—in the context of this detector pair—how this new measurement technique is insensitive to a cross-correlated detector noise component that can confound a conventional measurement.

The fast chirp transform (FCT) provides a powerful formalism for the detection of signals with variable frequency, in much the same way that the Fourier transform provides a formalism for the detection of constant frequency signals. The FCT algorithm has significant applications in the detection of gravitational waves, particularly the gravitational wave signal produced by an inspiralling compact binary. We report on progress towards a parallelized search code to identify inspiral events in interferometer data and estimate the mass parameters.

The basic problem for gravitational wave detectors is to detect very small signals in the presence of noise which is often not Gaussian and not stationary. We cope with this problem by applying data filters, matched to short bursts, based on power spectra obtained online. We describe the new procedure adopted by the Rome group, where the signal-to-noise ratios obtained with various algorithms are compared and the best one is selected. The selected algorithm depends on the noise characteristics at that particular time.

Pattern matching techniques such as matched filtering will be used for online extraction of gravitational wave signals buried inside detector noise. This involves cross correlating the detector output with hundreds of thousands of templates spanning a multi-dimensional parameter space, which is very expensive computationally. A faster implementation algorithm was devised by Mohanty and Dhurandhar using a hierarchy of templates over the mass parameters, which speeded up the procedure by about 25–30 times. We show that a further reduction in computational cost is possible if we extend the hierarchy paradigm to an extra parameter, namely, the time of arrival of the signal. In the first stage, the chirp waveform is cut-off at a relatively low frequency allowing the data to be coarsely sampled leading to cost saving in performing the FFTs. This is possible because most of the signal power is at low frequencies, and therefore the advantage due to hierarchy over masses is not compromised. Results are obtained for spinless templates up to the second post-Newtonian (2PN) order for a single detector with LIGO I noise power spectral density. We estimate that the gain in computational cost over a flat search is about 100, which is about a factor of four over the previous less sophisticated hierarchical scheme.

Removal of narrowband noise features (also called lines) of known instrumental origin from a time series is important for improving the performance of algorithms, such as those for the detection of transients. We present a new method for removing lines which (i) is not based on any model for the features to be removed, (ii) is designed so as not to affect transients substantially and (iii) works in the time domain. Property (i) allows lines to be removed irrespective of their physical origin, (ii) ensures that transients remain detectable in the residual after line removal and (iii) means that, unlike Fourier domain methods, line power is not redistributed in the entire frequency band.

One of the types of signals for which the LIGO interferometric gravitational wave detectors will search is a stochastic background of gravitational radiation. We review the technique of searching for a background using the optimally filtered cross-correlation statistic, and describe the state of plans to perform such cross-correlations between the two LIGO interferometers as well as between LIGO and other gravitational-wave detectors, in particular the preparation of software to perform such data analysis.

A numerical procedure for noise recognition and uncoupling is described. The procedure is applied to a Michelson interferometer and is effective in seismic and acoustic noise uncoupling from the output signal of the interferometer. Due to the low data flow coming from the instrumentation this uncoupling can be performed in real time and it is useful as a data quality procedure for interferometer data output.

We have developed a simple and robust system based on standard UNIX tools and frame library code to transfer and merge data from multiple gravitational wave detectors distributed worldwide. The transfer and merger take place with less than 20 minute delay and the output frames are available for all participants. Presently VIRGO and LIGO participate in the exchange and only environmental data are shared. The system is modular to allow future improvements and the use of new tools like Grid.

The standard IGEC approach to detection of gravitational waves with many detectors is a simple time coincidence search. We discuss the problems of false alarm and false dismissal assessment, in the case of both stationary and non-stationary noise. The significance of any cumulative excess of found coincidences over the background is determined by maximum likelihood methods.

Dual recycling is the combination of power recycling and signal recycling. GEO 600, a German-British interferometric gravitational wave detector being built in Germany, has no arm cavities and needs to use dual recycling from the beginning, while the other interferometric detectors may use it at a later stage to enhance their sensitivity. The control scheme to be used in GEO 600 has been demonstrated at the Garching 30 m prototype. This paper summarizes those results and the adaptions necessary in order to apply the scheme for GEO 600.

The most important point of a resonant sideband extraction (RSE) experiment is the signal extraction for control of the interferometer. We proposed a new signal-sensing method for the single modulation scheme. This method uses the third harmonic demodulation (THD) with a particular asymmetry in the interferometer which makes the third-order sidebands vanish at the detecting port. We have successfully locked a suspended-mass RSE interferometer for the first time by the THD method. The transfer function of the interferometer was measured to confirm the RSE effect.

Future gravitational wave detectors will include some form of signal mirror in order to alter the signal response of the device. We introduce interferometer configurations which utilize a variable reflectivity signal mirror allowing a tunable peak frequency and variable signal bandwidth. A detector configured with a Fabry–Perot cavity as the signal mirror is compared theoretically with one using a Michelson interferometer for a signal mirror. A system for the measurement of the interferometer signal responses is introduced. This technique is applied to a power-recycled Michelson interferometer with resonant sideband extraction. We present broadband measurements of the benchtop prototype's signal response for a range of signal cavity detunings. This technique is also applicable to most other gravitational wave detector configurations.

Using a quantum mechanical approach, we show that in a gravitational-wave interferometer composed of arm cavities and a signal recycling cavity, e.g., the LIGO-II configuration, the radiation-pressure force acting on the mirrors not only disturbs the motion of the free masses randomly due to quantum fluctuations, but also and more fundamentally, makes them respond to forces as though they were connected to an (optical) spring with a specific rigidity. This oscillatory response gives rise to a much richer dynamics than previously known, which enhances the possibilities for reshaping the LIGO-II's noise curves. However, the optical–mechanical system is dynamically unstable and an appropriate control system must be introduced to quench the instability.

The current plan for the Advanced LIGO dual-recycled optical configuration and control scheme is described, with emphasis on those elements which differ from Initial LIGO. Plans for prototyping this system are discussed, and the prototyping effort at the LIGO Caltech 40 Meter Interferometer Laboratory is described in some detail. Finally, some of the modelling efforts at Caltech are reviewed.

The design of a 10 m all-reflective prototype Sagnac interferometer with suspended optics is described and the experimental results are presented. It uses a polarization scheme to allow detection of the dark fringe on the symmetric port of the beamsplitter for optimal interference contrast. The necessary low-frequency response of the interferometer requires delay lines in the arms. To deal with the noise introduced by scattered light in the delay lines, a laser frequency sweep frequency shifts the scattered light so that it does not produce noise near zero frequency. This results in a shot-noise-limited phase sensitivity of Δϕ = 1.6 × 10⁻⁹ rad Hz^−1/2 at frequencies as low as 200 Hz. Scaling this prototype to several kilometres with kilowatts of circulating power requires several technical improvements in high-power solid-state lasers, second harmonic generation and the fabrication of large mirrors, which are likely to be made in the next 10 years.

The baseline design concept for a seismic isolation component of the proposed 'Advanced LIGO' detector upgrade has been developed with proof-of-principle experiments and computer models. It consists of a two-stage in-vacuum active isolation platform that is supported by an external hydraulic actuation stage. Construction is underway for prototype testing of a full-scale preliminary design.

The vibration isolation system for TAMA300 has a vibration isolation ratio large enough to achieve the requirement in the observation band around 300 Hz. At a lower frequency range, it is necessary to reduce the large fluctuation of mirrors for stable operation of the interferometer. With this aim, the mirror suspension systems were modified and an active vibration isolation system using pneumatic actuators was installed. These improvements contributed to the realization of a continuous interferometer lock for more than 24 h.

The TAMA SAS seismic attenuation system was developed to provide the extremely high level of seismic isolation required by the next generation of interferometric gravitational wave detectors to achieve the desired sensitivity at low frequencies. Our aim was to provide good performance at frequencies above ∼10 Hz, while utilizing only passive subsystems in the sensitive frequency band of the TAMA interferometric gravitational wave detectors. The only active feedback is relegated below 6 Hz and it is used to damp the rigid body resonances of the attenuation chain. Simulations, based on subsystem performance characterizations, indicate that the system can achieve rms mirror residual motion measured in a few tens of nanometres. We will give a brief overview of the subsystems and point out some of the characterization results, supporting our claims of achieved performance. SAS is a passive, UHV compatible and low cost system. It is likely that extremely sensitive experiments in other fields will also profit from our study.

Several R&D programmes are ongoing to develop the next generation of interferometric gravitational wave detectors providing the superior sensitivity desired for refined astronomical observations. In order to obtain a wide observation band at low frequencies, the optics need to be isolated from the seismic noise. The TAMA SAS (seismic attenuation system) has been developed within an international collaboration between TAMA, LIGO, and some European institutes, with the main objective of achieving sufficient low-frequency seismic attenuation (−180 dB at 10 HZ). The system suppresses seismic noise well below the other noise levels starting at very low frequencies above 10 Hz. It also includes an active inertial damping system to decrease the residual motion of the optics enough to allow a stable operation of the interferometer. The TAMA SAS also comprises a sophisticated mirror suspension subsystem (SUS). The SUS provides support for the optics and vibration isolation complementing the SAS performance. The SUS is equipped with a totally passive magnetic damper to suppress internal resonances without degrading the thermal noise performance. In this paper we discuss the SUS details and present prototype results.

The VIRGO suspensions are chains of passive mechanical filters designed to isolate the interferometer mirrors from seismic noise starting from a few Hz. In order to reduce the low-frequency swing of the mirror along the beam, an active control system, acting at the level of the suspension point, damps the main resonant modes of the system (all below 2.5 Hz). Another control loop, at the level of the optical payload, makes use of a digital camera monitoring the mirror position in all six degrees of freedom. Its main goal is to decrease the rms angular displacements of the mirror, on a time scale of several minutes, down to less than 1 μrad. All the seven suspensions of the VIRGO central interferometer are presently in operation, while the assembly of the last two, for the terminal mirrors, is in progress. The design and performance of the system are described in this paper.

The VIRGO superattenuator (SA) is effective in suppressing seismic noise below the expected thermal noise level above 4 Hz. However, the residual mirror motion associated with the SA normal modes can saturate the interferometer control system. This motion is reduced by implementing a wideband (DC–5 Hz) multidimensional active control (the so-called inertial damping) which makes use of both accelerometers and position sensors and of a digital signal processing (DSP) system. Feedback forces are exerted by coil–magnet actuators on the top of an inverted pendulum pre-isolator stage. The residual root mean square motion of the mirror in 10 s is less than 0.1 μm.

Difficulties in obtaining ideal vertical vibration isolation with mechanical springs are identified as being due to the mass of the elastic element which is in turn due to its energy storage requirement. A new technique to minimize this energy is presented—being an Euler column undergoing elastic buckling. The design of a high performance vertical vibration isolation stage based on this technique is presented together with its measured performance.

We have investigated applications of a high-performance damping metal, called M2052, which is a manganese-based alloy containing copper, nickel and iron. Using an all-metal prototype of a vibration isolation system, we have tested the property of M2052. As its actual application to a gravitational wave detector, we have used M2052 in the damping system of a suspended optics in TAMA300, which is a 300 m long interferometric gravitational wave detector built at the Mitaka campus of the National Astronomical Observatory in Japan. The results of the experiments are reported.

The Glasgow group is involved in the construction of the GEO600 interferometer as well as in R&D activity on technology for advanced gravitational wave detectors. GEO600 will be the first GW detector using quasi-monolithic silica suspensions in order to decrease thermal noise significantly with respect to steel wire suspensions. The results concerning GEO600 suspension mounting and performance will be shown in the first section. Section 2 is devoted to the present results from the direct measurement of thermal noise in mirrors mounted in the 10 m interferometer in Glasgow which has a sensitivity limit of 4 × 10⁻¹⁹ m Hz^−1/2 above 1 kHz. Section 3 presents results on the measurements of coating losses. R&D activity has been carried out to understand better how thermal noise in the suspensions affects the detector sensitivity, and in section 4 a discussion on the non-linear thermoelastic effect is presented.

Thermal noise in the mirror substrates is expected to be the main limit to the VIRGO sensitivity in the 50–500 Hz frequency range. The mechanical quality of the mirror substrates and the geometry of their suspension are expected to affect the noise level of the detector output. High mechanical Q have been obtained for different large fused silica substrates under VIRGO suspension conditions. Moreover, calcium fluoride substrates are shown to provide a more promising option for the design of future cryogenic, low thermal noise interferometers.

Thermal noise in mirror suspension wires is the main limitation of low-frequency sensitivity of interferometric gravitational wave detectors. In order to minimize the pendulum thermal noise, a monolithic design, using a low dissipation material, is proposed for VIRGO. High mechanical Qs and high breaking strengths have been obtained for monolithic fused silica fibres. A low-dissipation and high-strength bonding technique using potassium silicate bonding is proposed.

The low frequency facility is a VIRGO R&D experiment having the goal of performing a direct measurement of the thermal noise of the VIRGO suspensions by means of a two-mirror Fabry–Perot cavity suspended to the last stage of the attenuating chain. The present status of advancement of this experiment is reported: the apparatus, including mechanical and optical parts, has been completely built and put into operation. Vacuum facilities and the first control loops are active. First measurements on the suspended cavity are in progress.

We report the measurement of the transfer function of a flexure suspension system which is used in the ANU thermal noise experiment. This suspension hinges over four thin machined flexure membranes ensuring uniaxial motion. We have successfully measured the transfer function of the suspension using a novel high dynamic range measuring technique.

The normal modal expansion is the most frequently used method to estimate the thermal noise of interferometric gravitational wave detectors. However, the method does not agree with new estimation methods, direct approaches, when the loss is distributed inhomogeneously. We have checked the modal expansion and direct approaches experimentally using a mechanical oscillator, such as a mirror. The experiments showed that the modal expansion is invalid. On the other hand, the measured spectra are consistent with the direct approaches. We calculated the thermal noise of a real mirror with inhomogeneous loss using the direct approaches. This calculation showed that the thermal motions caused by loss in the reflective coating and at coil–magnet actuators are comparable with the sensitivity goals of future gravitational wave detector projects. In addition, according to our calculation, a mechanical loss may cause much larger or much smaller thermal motion than is expected in modal expansion, depending on the loss distribution. The thermal fluctuation caused by coating loss is about three times larger than the estimation of the modal expansion. The thermal motion of the coil–magnet actuators is 15 times smaller. This implies that, in order to discuss the research strategy for the thermal noise, in addition to the loss quantity represented by Q-values, one should identify the distribution of all the major loss origins and use direct approaches instead of depending on the conventional modal expansion method.

We systematically measured the intrinsic mechanical quality factors of 13 kinds of bulk fused silica from four companies using a nodal support system. Some of them have actually been adopted as mirror substrates in interferometric gravitational wave detectors. The measured quality factors widely ranged from 7 × 10⁵ to 4 × 10⁷. They turned out to be independent of one specific property, such as the amount of OH content, suggesting that the loss mechanism has several origins. We found that many of the samples showed smaller losses at lower frequency. From the viewpoint of the mirror thermal noise, it would be fortunate if the loss continued to decrease with decreasing frequency. We also found that an annealing process increases their quality factors.

Niobium membrane flexure suspensions have been proved to be able to achieve high pendulum Q-factors, whereas niobium cantilever suspensions have been proved to give high internal Q-factors in sapphire test masses. Here we present the proposed sapphire test mass suspension systems based on the use of niobium flexures. This suspension system has the advantage of being robust while maintaining the high Q of the sapphire test mass. We show that it is also advantageous to use Nb flexures in future cryogenic detectors.

Ocean waves interacting in shallow water at the shore generate land waves propagating inland. To study these waves vertical, horizontal and tilt seismic noise were measured simultaneously at one location. Vibration isolators designed for gravitational wave research were used for detection. Cross-correlation between the above components was calculated. We found correlations between all of them. However, only the correlation between horizontal and vertical motions could be addressed to land waves, and other correlations are thought to be due to local rigid body motion of the large building in which the experiments were located.

Thermal noise is one of the dominant noise sources in interferometric length measurements and can limit the sensitivity of gravitational wave detectors. Our goal is to analyse the off-resonant thermal noise of a high Q pendulum. Therefore we interferometrically detect the length changes of a 2.3 cm long optical resonator, which for good seismic isolation consists of two multiple stage pendulums. We are able to lock the length of this optical resonator to a frequency-stabilized laser beam and as a result get the spectral density of the differential mirror movement.

This paper describes the design of a novel double-flexure two-axis tilt sensor with a tilt readout based on an optical walk-off sensor. The performance of the device has been investigated theoretically and experimentally. The walk-off sensor has demonstrated a sensitivity of 10⁻¹¹ rad Hz^−1/2 at 1 Hz. The tilt sensor has measured seismic noise ∼10⁻⁹–10⁻¹⁰ rad Hz^−1/2 for frequency in the 2–10 Hz range.

A breadboard optical bench for SMART-2 has been designed and set up using preliminary optics and mounting techniques. A feature of the design is its relative insensitivity to laser frequency noise, achieved by closely matching the arm lengths of the main measurement interferometer. The design also introduces the use of an auxiliary unequal arm length interferometer to measure laser frequency fluctuations.

Hydroxy-catalysis bonding has been investigated as a possible method for attaching optics of different materials to a low thermal expansion base plate. Bonds involving combinations of ULE, Zerodur and fused silica have been successfully tested for its reliability under vibration and thermal cycling.

A phasemeter based on a programmable logic device has been developed. The phasemeter communicates through a standard PC bus and is software-controlled. The general principle of operation and its implementation are described. At a signal frequency of 10 kHz the phasemeter has been demonstrated to have a noise level of less than 10⁻⁶ cycles Hz^−1/2 for frequencies down to 1 mHz.

The acceleration perturbations of a drag-free satellite with a free-floating spherical proof mass for the LISA space mission to detect gravity waves are presented. These calculations show that the RSS of all disturbances is less than 3.4 × 10⁻¹⁶ m s⁻² Hz^−1/2. This is about a factor of 10 less than the calculated value for cubical single-axis proof masses and could potentially decrease the low-frequency response correspondingly. The performance calculations presented here have been confirmed by the flight of the DISCOS drag-free satellite.

The success of the LISA project depends on the ability of the disturbance reduction system to shield the proof masses from all external forces and to maintain tight pointing requirements relative to the other two spacecrafts. μN-thrusters are required to compensate for the solar radiation pressure acting on the spacecraft. The force noise from these thrusters must be low enough not to disturb the freely floating proof masses. To date, these noise requirements have not been demonstrated, mostly because no thrust-stand exists with sufficient sensitivity. We present the status of our μNewton thrust-stand that will verify that the thrusters proposed for LISA will meet the noise requirements.

The extreme level of isolation from stray forces required for LISA makes the development of 'drag-free control' technologies essential to the mission. We report here on the progress in the development of a capacitive, six degree-of-freedom, position sensor designed to meet the required low levels of position read-out noise (1 nm Hz^−1/2) and stray force noise (3 × 10⁻¹⁵ N Hz^−1/2) across the LISA bandwidth of 0.1 mHz to 0.1 Hz. In this paper we briefly discuss sensor design and expected performance before presenting preliminary noise measurements made with a prototype sensor.

We report here on a torsion pendulum facility for ground-based testing of the Laser Interferometer Space Antenna (LISA) gravitational sensors. We aim to measure weak forces exerted by a capacitive position sensor on a lightweight version of the LISA test mass, suspended from a thin torsion fibre. This facility will permit measurement of the residual, springlike coupling between the test mass and the sensor and characterization of other stray forces relevant to LISA drag-free control. The expected force sensitivity of the proposed torsion pendulum is limited by the intrinsic thermal noise at ≈3 × 10⁻¹³ N Hz^−1/2 at 1 mHz. We briefly describe the design and implementation of the apparatus, its expected performance and preliminary experimental data.

Cassini, a joint American/European interplanetary scientific mission to Saturn, will be continuously and coherently tracked for 40 days during its solar oppositions in the next three years, starting on 26 November 2001. Doppler tracking searches for gravitational waves in the millihertz frequency band will be performed by using newly implemented Ka-band (≈32 GHz) microwave capabilities on the ground and onboard the spacecraft. Use of the Ka-band coherent microwave link will suppress solar plasma scintillations to levels below those identified by remaining instrumental noise sources, making the Cassini Doppler tracking experiments the most sensitive searches for gravitational waves ever attempted in the millihertz frequency band. This paper provides a short review of the Doppler response to gravitational radiation, the noise sources and their transfer functions into the Doppler observable and estimates of the anticipated Cassini Doppler tracking sensitivities to gravitational radiation.

Interferometric gravitational wave detectors require high optical power, single frequency lasers with very good beam quality and high amplitude and frequency stability as well as high long-term reliability as input light source. For GEO 600 a laser system with these properties is realized by a stable planar, longitudinally pumped 12 W Nd:YAG rod laser which is injection-locked to a monolithic 800 mW Nd:YAG non-planar ring oscillator. Frequency control signals from the mode cleaners are fed to the actuators of the non-planar ring oscillator which determines the frequency stability of the system. The system power stabilization acts on the slave laser pump diodes which have the largest influence on the output power. In order to gain more output power, a combined Nd:YAG–Nd:YVO₄ system is scaled to more than 22 W.

The development of a power-scalable diode-laser-pumped continuous-wave Nd:YAG laser for advanced long-baseline interferometric detectors of gravitational waves is described. The laser employs a chain of injection-locked slave lasers to yield an efficient, frequency-stable, diffraction-limited laser beam.

The next generation of interferometric gravitational wave detectors will employ laser powers approaching 200 W to increase shot-noise limited sensitivity. Optical components that transmit the laser light will exhibit increased thermal lensing induced by bulk absorption and concomitant changes in the material refractive index, resulting in significant changes in the modal characteristics of the beam. Key interferometer components such as electro-optic modulators and Faraday isolators are particularly at risk, since they possess relatively large absorption coefficients. We present a method for passive correction of thermally induced optical path length (ΔΛ) changes induced by absorption in transmissive optical components. Our method relies on introducing material in the optical path that possesses a negative index temperature derivative, thereby inducing a compensating opposite ΔΛ. We experimentally demonstrate a factor of 10 reduction in higher order spatial mode generation for terbium gallium garnet, a Faraday isolator material.

As the first generation of laser interferometric gravitational wave detectors nears operation, research and development has begun on increasing the sensitivity of the instrument while utilizing the existing infrastructure. In the laser interferometer gravitational wave observatory (LIGO), significant improvements are being planned for installation around 2007, increasing strain sensitivity through improved suspensions and test mass substrates, active seismic isolation and higher input laser power. Even with the highest quality optics available today, however, finite absorption of laser power within transmissive optics, coupled with the tremendous amount of optical power circulating in various parts of the interferometer, results in critical wavefront deformations which would cripple the performance of the instrument. A method of active wavefront correction via direct thermal actuation on optical elements of the interferometer is discussed. A simple nichrome heating element suspended off the face of an affected optic will, through radiative heating, remove the gross axisymmetric part of the original thermal distortion. A scanning heating laser will then be used to remove any remaining non-axisymmetric wavefront distortion, generated by inhomogeneities in absorption of the substrate, thermal conductivity, etc. A proof-of-principle experiment has been constructed at MIT, selected data of which are presented.

We describe a study and the preliminary experimental results on the possibility of using adaptive optics systems for the reduction of geometrical fluctuations of input laser beams in long baseline interferometric detectors of gravitational waves. The experimental tests aimed to test the efficiency of Hermite–Gauss versus Shack–Hartmann wavefront reconstruction and feedback diagonalization. These preliminary results seem to indicate that the adaptive optics systems may be integrated in the near future as stabilization stages before a passive mode cleaner cavity, provided that the operational band of the mirror is increased together with the efficiency of the control system.

We present novel techniques for overcoming problems relating to the use of high-power lasers in mode cleaner cavities for second generation laser interferometric gravitational wave detectors. Rearranging the optical components into a double pass locking regime can help to protect locking detectors from damage. Modulator thermal lensing can be avoided by using a modulation-free technique such as tilt locking, or its recently developed cousin, flip locking.

The optical sensing subsystem of a LIGO interferometer is described. The system includes two complex interferometric sensing schemes to control test masses in length and alignment. The length sensing system is currently employed on all LIGO interferometers to lock coupled cavities on resonance. Auto-alignment is to be accomplished by a wavefront-sensing scheme which automatically corrects for angular fluctuations of the test masses. Improvements in lock stability and duration are noted when the wavefront auto-alignment system is employed. Preliminary results from the commissioning of the 2 km detector in Washington are shown.

The VIRGO injection system, designed to provide a stable single-frequency 20 W laser to the VIRGO interferometer, is under commissioning. All functions have been demonstrated to work close to requirements, and the integration of all of them is in progress.

GEO 600 uses two 8 m triangular ring cavities as a modecleaner system for the stabilization of the laser. To isolate the cavities with respect to the seismic noise the optical components are suspended as double pendulums. The resonances of these pendulums are damped by a local-control loop via magnet–coil actuators acting on the intermediate masses. The suspension scheme and the measured key data (i.e. finesse, linewidth, visibility, throughput and in-lock durations of the cavities, as well as the isolation performance and the resulting frequency stability) of the modecleaner system will be given in this paper. Furthermore an overview of the GEO 600 interferometer suspension will be given.

Power recycling will soon be implemented on TAMA300. This paper gives motivation for the TAMA recycling experiment, discusses the planned length sensing/control system and considerations for the lock acquisition process.

This paper gives an overview of the automatic mirror alignment system of the modecleaner and main interferometer of the GEO 600 gravitational wave detector. In order to achieve the required sensitivity of the detector, the eigenmodes of all optical cavities have to be aligned with respect to the incoming beams (or vice versa) and kept aligned for long measuring periods. Moreover the beam spots have to be centred on the mirrors to minimize coupling of residual angular mirror motion into changes of the optical path length. An overview of the principles and setup for the automatic alignment is given, and first results of the modecleaner and 1200 m cavity alignment system are presented, including the error-point spectra of mirror angular motions, which are smaller than 10⁻⁸ rad Hz^−1/2 below 10 Hz.

VIRGO is a laser interferometer aiming at the first detection of gravitational waves emitted by astrophysical sources. The signal detection system consists of all the output optics and the electronics necessary for the measurement of the interferometer output signal. An output mode cleaner has been developed in this context. The system has been installed at the detector site and is now being used for the central interferometer, the first step in the construction of VIRGO. The first results obtained so far are presented.

In this paper, we report on the concept and performance of a highly effective broadband Faraday isolator. This device is based on Nd:Fe:B permanent magnets and can be used in interferometric gravitational wave detectors in which lasers oscillating in the visible or near infrared region are used. The degree of optical isolation of 30 dB, provided by the device, is achieved when operating with laser beams of up to 2 mm, and up to 25 dB when the aperture of the magneto-optic element (10 mm) is completely filled.

We describe recent modifications to the resonant mass gravitational wave detector Niobé, which have resulted in improved sensitivity and more efficient cryogenics. Data are presented indicating a noise temperature of 400 μK for a bandwidth of 2 Hz. A preliminary search for anomalous burst events correlated with cosmic ray showers does not indicate statistically significant correlations, but is of insufficient duration to obtain a definitive conclusion.

To measure the standard quantum limit (SQL) a high quality transducer must be coupled to a high quality mechanical system. Due to its monolithic nature, the monolithic sapphire transducer (MST) has high quality factors for both types of resonances. Single loop suspension is shown to yield a mechanical quality factor of 6 × 10⁸ at 4 K and Whispering Gallery electromagnetic modes display a quality factor of 2 × 10⁶ at 4 K. From standard analysis we show that the MST has the potential to measure the noise fluctuations of the mechanical oscillator at the SQL. Also, we point out a new way to determine if the transducer back-action is quantum limited. We show that if the fluctuations are at the quantum limit, then the amplitude of the oscillations will be amplified by the ratio of the ringdown time to the measurement time, which is an inherently easier measurement.

The noise performance of Allegro since 1993 is summarized. We show that the noise level of Allegro is, in general, stationary. Non-Gaussian impulse excitations persist despite efforts to isolate the detector from environmental disturbances. Some excitations are caused by seismic activity and flux jumps in the SQUID. Algorithms to identify and automatically veto these events are presented. Also, the contribution of Allegro to collaborations with other resonant-mass detectors via the International Gravitational Event Collaboration and with LIGO is reviewed.

Cosmic ray showers interacting with the resonant mass gravitational wave antenna NAUTILUS, cooled down to 0.1 K, have been detected. The experimental results show large signals at a rate much greater than expected. Since August 2000 NAUTILUS has been running at T = 1.1 K (non-superconductor aluminium). There is evidence at the 3 standard deviation level that the rate of the large signals is dependent on the bar temperature. The rate is compatible with zero at a bar temperature of 1.1 K.

Since the beginning of 2000 the EXPLORER gravity wave (GW) detector has been operated continuously after a stop devoted to improve the apparatus. The antenna has been equipped with a new read-out. The use of a new transducer, characterized by a very small gap, and a dc-SQUID with a high coupling, led to a better sensitivity and a larger bandwidth. The EXPLORER sensitivity in terms of spectral noise amplitude, at present (June 2001), is 10⁻²⁰ Hz^−1/2 over a bandwidth of 35 Hz and 3 × 10⁻²¹ Hz^−1/2 with a bandwidth of about 6 Hz, corresponding to a sensitivity to short conventional GW bursts of h = 4 × 10⁻¹⁹. The performance is stable and the apparatus is taking data with a duty cycle in excess of 80%.

The ultra-cryogenic gravitational wave detector NAUTILUS is gathering data in Frascati (Rome), in its second science run since June 1998. The measured strain sensitivity at the two resonances is 4 × 10⁻²² Hz^−1/2 over a bandwidth of 1 Hz and better than 3 × 10⁻²⁰ Hz^−1/2 over a band of about 25 Hz, with a duty cycle of about 80%, mainly limited by cryogenic operations. At the beginning of 2002, the detector will be upgraded with a new Al bar, transducer and SQUID, and will be tuned to the 935 Hz frequency of the recently discovered pulsar in SN 1987A. The future sensitivity of the detector is presented and discussed.

In the frame of the AURIGA collaboration, a readout scheme based on an optical resonant cavity has been implemented on a room temperature resonant bar detector of gravitational waves. The bar equipped with the optical readout has been operating for a few weeks and we report here the first results.

We describe the experimental efforts to set up the second AURIGA run. Thanks to the upgraded capacitive readout, fully characterized and optimized in a dedicated facility, we predict an improvement in the detector sensitivity and bandwidth by at least one order of magnitude. In the second run, AURIGA will also benefit from newly designed cryogenic mechanical suspensions and the upgraded data acquisition and data analysis.

The results of the first cooldown of MiniGRAIL are presented. MiniGRAIL is a 65 cm diameter spherical gravitational wave detector made of CuAl6%, having a mass of 1150 kg. The sphere is suspended from a seven-stage vibration isolation system with a total weight of about 950 kg. The sphere should be cooled to below 20 mK. During the first run the sphere was cooled down to 1.8 K. A forced helium flow was used to cool the sphere down to 4 K, which took only two days. Further cooling to 1.8 K was done using a 1 K pot. We measured the temperature dependence of the mechanical quality factor and made an evaluation of the heat leaks (for more details see www.minigrail.nl).

We are developing a two-mode inductive resonant transducer for MiniGrail. We report several quality factor measurements, down to 4.2 K, performed on a scaled size resonator in different conditions: when suspended from a wire and when clamped, by thermal contraction techniques, into a hole of a sphere of 150 mm diameter and 14 kg mass. Q-factor measurements of a first resonator prototype at 4.2 K for MiniGrail are also presented. Finally, a fabrication process for a Nb film pick-up coil is described.

The first phase of the Brazilian Graviton Project is the construction and operation of the gravitational wave detector Mario Schenberg at the Physics Institute of the University of São Paulo. This gravitational wave spherical antenna is planned to feature a sensitivity better than h = 10⁻²¹ Hz^−1/2 at the 3.0–3.4 kHz bandwidth, and to work not only as a detector, but also as a testbed for the development of new technologies. Here we present the status of this detector.

High-energy cosmic ray particles are expected to be a significant source of noise in resonant mass gravitational wave detectors close to the quantum limit. The spherical, fourth generation antennas have been designed to attain such a limit. In this work we will show how the energy of a cosmic ray particle interacting with such an antenna is distributed over its eigenmodes. We will then make some comments on the relevant consequences of such a distribution for gravitational wave detection.

'Mario Schenberg' is a spherical resonant-mass gravitational wave (GW) detector that will be part of a GW detection array of three detectors. The other two will be built in Italy and the Netherlands. Their resonant frequencies will be around 3.2 kHz with a bandwidth of about 200 Hz. This range of frequencies is new in a field where the typical frequencies lie below 1 kHz, making the transducer development much more complex. In this paper, the design of the mechanical part of the transducer will be shown, as well as the attachment method to the sphere and the expected sensitivity.

The gravity wave detector at the University of Western Australia is based on a bending flap of 0.45 kg tuned near the fundamental resonant frequency of a 1.5 tonne resonant bar of 710 Hz. The displacement of the bending flap is monitored with a 9.5 GHz superconducting re-entrant cavity transducer. The performance of the transducer is related to the development of a low-noise microwave pump oscillator to drive the transducer. In this study we describe how to improve the quality of the existing microwave pump oscillator using a second servo frequency control system.

The resonant-mass technique for the detection of gravitational waves may involve, in the near future, the cooling of very large masses (about 100 tons) from room temperature (300 K) to extreme cryogenic temperatures (20 mK). To cool these detectors to cryogenic temperatures an exchange gas (helium) is used, and the heat is removed from the antenna to the cold reservoir by thermal conduction and natural convection. With the current technique, cooling times of about 1 month can be obtained for cylindrical bar antennas of 2.5 tons. Should this same technique be used to cool a 100 ton spherical antenna the cooling time would be about 10 months, making the operation of these antennas impracticable.

In this paper, we study the above-mentioned cooling technique and others, such as thermal switching and forced convection from room temperature to liquid nitrogen temperature (77 K) using an aluminium truncated icosahedron of 19 kg weight and 25 cm diameter.

Starting from commercial chips, a two-stage dc superconducting quantum interference device (SQUID) was developed in order to use it as a low-noise amplifier on the resonant gravitational wave detector AURIGA. The SQUID was coupled to a high-Q electrical resonator, operating in the kilohertz frequency range, which was employed to simulate the real detector. The resonator was successfully stabilized by means of a capacitive damping network. SQUID additive noise and back-action noise were measured as functions of temperature. The best noise temperature of the SQUID amplifier, measured at 1.5 K, was better than 16 μK, and corresponds to a minimum detectable energy of 200 resonator quanta.

We designed a mechanical isolation system for a spherical resonant gravitational wave detector we are building in Brazil. We have used the finite element method to perform the dynamical analysis. The system is a multiple stage passive pendulum formed by cylinders joined by C springs and rods. Our results showed that the designed system could allow a 280 dB attenuation factor in the bandwidth, from 3.1 to 3.2 kHz, where the SCHENBERG detector will be sensitive.

Tests of the new AURIGA readout have recently started in the AURIGA ultracryogenic test facility. The most important modifications, with respect to the previous version of the readout, are a new, heavier, resonant capacitive transducer, the tuning of the electrical mode to the two mechanical ones and the use of a low noise two-stage SQUID amplifier. Results regarding the quality factor and the noise of the transducer mechanical and electrical modes are presented.

The cross-coupled interferometer is a new design for interferometric gravitational wave detectors. Similar to the baseline gravitational wave detectors proposed for Advanced LIGO, it uses long-arm cavities in which the signal is generated. The signal fields are then extracted from the arm cavities with an additional cavity behind the long-arm cavities. The tuning of this signal extraction cavity and the parallel tuning of the signal recycling mirror can be used to optimize the peak frequency and the bandwidth of the detector independently. If we replace the signal recycling mirror by a small cavity, it is possible to amplify signals in two different frequency bands.

Experiments relating to concepts for extending interferometer operation, particularly at low frequencies, are discussed. This includes work with suspensions connected by a suspension-point interferometer. A new concept for achieving similar frequency extension without requiring an additional interferometer between suspensions is outlined, as well as a technique for improving positioning of laser beams relative to centres of gravity of test masses in gravity-wave interferometers and other instruments.

We discuss the new concept of a sensitive and wide-band detector, consisting of a solid sphere nested inside a hollow one: the dual sphere. The advantage is that it would be possible to keep both the omni-directionality and high sensitivity of the spherical geometry without giving up the wide band. In the few kHz range the dual sphere would be complementary to 'advanced' interferometers. We also discuss the main technological and scientific challenges that the construction of such a system poses, particularly regarding material choice, fabrication, cooling, suspension and readout.

LIGO is exploring cryogenics as a technique of last resort to reduce the thermal noise of mirrors and suspensions in gravitational wave interferometric detectors. Some of the cryogenic, or cryogenic-related, R&D done at LIGO is reported here. Some ideas being considered to make cryogenics possible may be useful even for room temperature detectors.

Hollow spheres have the same theoretical capabilities as the usual solid ones, since they share identical symmetries. The hollow sphere is, however, more flexible, as thickness is an additional parameter one can vary to approach given specifications. I will briefly discuss the more relevant properties of the hollow sphere as a GW detector (frequencies, cross-sections), and suggest some scenarios where it can generate significant astrophysical information.

We present a progress report on the measurement of mechanical noise in mirror suspension prototypes. Excess noise in the metal wires has been detected. An advanced technique for the fused silica fibres test has been developed.

The main sources of damping of the test mass oscillation associated with the electric field of a multistrip actuator are described. The possibility of cool electrical damping with the small increase of the test mass thermal noise is discussed.

We report the reduction of thermal lensing in cryogenic sapphire mirrors, which are planned to be used in the large scale cryogenic gravitational wave telescope project. We measured three key parameters of sapphire substrate for thermal lensing at cryogenic temperature. They are the optical absorption coefficient, thermal conductivity and temperature coefficient of refractive index at cryogenic temperature. On the basis of these measurements, we estimated the shot noise sensitivity of the interferometer with thermal lensing by using a wavefront tracing simulation. We found that thermal lensing in cryogenic sapphire mirrors is negligible.

This special issue of Classical and Quantum Gravity constitutes the proceedings of the 4th Edoardo Amaldi Conference on Gravitational Waves held in Perth, Western Australia in July 2001. The field of gravitational wave detection has been slowly emerging since the pioneering experiments of Joseph Weber more than 30 years ago alerted the world to the possibility of actual detection of gravitational waves. The funding of kilometre scale interferometers and the planning of million kilometre scale space interferometers has grown the field of gravitational wave detection to a size where major review meetings are essential to obtain an overview of the field.

The early days of gravitational wave detection were characterized by hope: the hope that nature would surprise us with unexpected strong sources of gravitational waves. In fact nature did have a surprise for us, but it was not the enormous population of colliding black holes for which we had hoped. Instead it was the Hulse-Taylor binary pulsar. This proved the existence of gravitational waves and defined a class of source which has become a major target for detection. More than this, it gave us the knowledge that a broad spectrum of gravitational waves definitely exists. Today we know that this spectrum awaits us like an unexplored continent, like Australia before European settlement. We know that the Great South Land exists, but its rivers and mountains, deserts and plains wait to be explored. We create and perfect our detectors with the knowledge and anticipation that we are privileged to be the first human beings to explore this unknown spectrum.

Edoardo Amaldi led the development of gravitational wave research in Italy. He forged the first links between the gravitational wave community and the high-energy physics community which led to the long term hosting of the Explorer gravitational wave detector at CERN. His links also stretched across the Atlantic, leading to close collaboration between the first cryogenic resonant mass gravitational wave detectors at Stanford, Louisiana and Rome, and then later to the Niobe detector at Perth. Today, high-energy physics and gravitational wave physics are understood to be deeply entwined, providing alternative and complementary means of addressing fundamental questions in cosmology. The international research links and collaborations which Amaldi supported have broadened and today are represented by GWIC, the Gravitational Wave International Committee, which is now the official sponsor of the Amaldi Conferences. The Amaldi Conferences (with emphasis on terrestrial detectors) along with the LISA Symposia (which emphasize space interferometers, and which have also been published by this journal) have allowed the breadth of progress in the field to be presented while specialized workshops (such as the data analysis workshops) cater for specialized subgroups.

Like radio in its pioneering days, the field of gravitational wave detection has been extraordinarily innovative. Detectors are complex instruments which combine mechanical, optical, photonic, digital, laser and vacuum systems, all of which are critical components and all of which must exceed the prior state of the art by an enormous margin. Every subsystem has required the development or discovery of major innovative solutions. The innovations that have arisen in this field are broad and extensive. While some of them can be found in these proceedings, their applications are unlikely to be discussed. I believe that it is worth listing a few of them to give ammunition to those who need to answer the perennial question 'what is its use to the average pedestrian?'. Useful and economically valuable technologies that have arisen directly or indirectly from gravitational wave research are

• Improved SQUID magnetometers: brain scanning and many other applications • Low noise microwave oscillators: improved radars • Sapphire clocks: flywheel oscillators for atomic clocks • Low outgassing vacuum materials: high performance vacuum systems • Ultra-precision mirrors and coatings: improved optical systems • High efficiency lasers: numerous present and future applications from materials cutting to medicine and metrology • Improved vibration isolation: electron microscopy, atomic force microscopy and semiconductor manufacture etc • Low acoustic loss materials: metrology systems • Gravity gradiometers: mineral exploration • High performance position sensors: accelerometers, tilt sensors, seismometers • Laser stabilization: metrology systems, time standards • Data analysis: weak signal detection • Very low optical absorption materials: high laser power applications

Amaldi 4 was the second major international conference hosted by the UWA Gravitational Wave Research Group. The first was the 5th Marcel Grossmann Meeting in 1988, when gravitational wave research was still a subfield within the context of relativistic astrophysics. Both in 1988 and in 2001, the organization was a major effort and a substantial distraction from research. However, in both cases, I believe the benefits outweighed the costs. In the case of Amaldi 4, I believe that it is no coincidence that in the same year we achieved the first substantial level of funding for our gravitational wave research in Australia. After struggling for many years with inadequate funding, the Australian Consortium is now able to play a substantial role in the development of gravitational wave technology through its high optical power test facility at Gingin. Funding announced after the conference allows us to properly staff this facility and to become a partner in Advanced LIGO.

Amaldi 4 brought additional benefits to Australia. Besides ACIGA's Gingin facility, the Gravity Discovery Centre Foundation (a public charitable foundation) is constructing a public education facility. The site, in beautiful and unusual Australian bushland, has been set aside for the future long baseline gravitational wave detector AIGO. It combines clear skies, extremely high botanical biodiversity, Aboriginal cultural significance and, of course, the Australian link to the worldwide effort to detect gravitational waves. This set of ingredients is ideal for the promotion of a scientific understanding of our place in the universe, in the broad context of astronomy, our diverse cultures and our fragile planet in an awesome universe. It is also an ideal opportunity for demonstrating how fundamental science and the search for knowledge inevitably lead to discoveries of benefit to humanity. During the excursion, the conference participants saw the first stage of the Gravity Discovery Centre under construction. The Amaldi conference, which included two public lectures and a reception in which potential donors could meet with conference participants, brought wider public recognition of the Discovery Centre project which itself led to sufficient significant donations for the next stage of the discovery centre to be able to begin construction in 2002.

Australian gravitational wave research for many years consisted only of the development of the Niobe resonant mass detector at UWA, and it was pleasing that at Amaldi 4 we were able to present the lowest noise temperature ever observed in a resonant mass detector. However, of greater significance was the opportunity for the Australian Consortium to present its broad research effort. The vision of AIGO has allowed many universities across Australia to pool their diverse expertise to create a truly national research effort. Amaldi 4 allowed the entire Australian programme to be fully presented to an international audience. For example, the Adelaide laser group presented exciting results on high power lasers, the ANU group presented definitive results on configurations, the UWA group presented important new vibration isolation concepts, and the ANU data analysis group presented interesting results demonstrating the need for a Southern Hemisphere detector. Concurrently with Amaldi 4, the Australian Conference on General Relativity and Gravitation was held and much Australian theory work was presented.

Acknowledgments

The success of Amaldi 4 was due to many people. The organizers thank in particular Barry Barish for his consistent support and Seiji Kawamura for so graciously giving precedence to the Australian bid for the conference. Thanks to the International Union of Pure and Applied Physics, the West Australian Government and the University of WA for sponsorship. Thanks to Alan Robson, Deputy Vice Chancellor of UWA for hosting a reception, and John deLaeter and Rob Jewkes for their support. Thanks to Debbie Greenwood for her brilliant and dedicated organization, and to Ron Burman for his efficient treasurership. Thanks to Yarra Korczynskyj for organizing the excursion and David McClelland for his chairmanship of the Scientific Organizing Committee. Finally, thanks to the session convenors and the participants who were responsible for the excellent science presented at the conference, and to Andrew Wray of IOP for his efficient management of the refereeing process for the articles in this volume.

D G BlairUniversity of Western Australia

Proceedings of the 1st Edoardo Amaldi Conference Villa Tuscolana, Frascati, Rome, Italy (14-17 June 1994) Published in 1995 Gravitational Wave Experiments ed E Coccia, G Pizzella and F Ronga (Singapore: World Scientific)

Proceedings of the 2nd Edoardo Amaldi Conference CERN, Switzerland (1-4 July 1997) Published in 1999 Gravitational Waves ed E Coccia, G Pizzella and G Veneziano (Singapore: World Scientific)

Proceedings of the 3rd Edoardo Amaldi Conference California Institute of Technology, Pasadena, California (12-16 July 1999) Published in 2000 Gravitational Waves (AIP Conference Proceedings volume 523) ed S Meshkov (New York: AIP)

Proceedings of the 4th Edoardo Amaldi Conference University of Western Australia, Perth, Western Australia (8-13 July 2001) Published in 2002 Class. Quantum Grav.19 1227-2050

Proceedings of the 1st International LISA Symposium Rutherford Appleton Laboratory, Chilton, UK (9-12 July 1996) Published in 1997 Class. Quantum Grav.14 1397-1585

Proceedings of the 2nd International LISA Symposium California Institute of Technology, Pasadena, California (6-9 July 1998) Published in 1998 Laser Interferometer Space Antenna (AIP Conference Proceedings volume 456) ed W M Folkner (New York: AIP)

Proceedings of the 3rd International LISA Symposium Max-Planck-Institut für Gravitationsphysik, Golm, Germany (11-14 July 2000) Published in 2001 Class. Quantum Grav.18 3965-4164