After a session of overviews, six sessions were dedicated to the detectors (status, advanced techniques, fundamental noise sources, and future detectors), one to theory and sources, and one to data analysis.
Since the last conference two years ago in Australia, the general progress is remarkable: the LIGO large interferometric detectors (2 in Hanford, Wa, and one in Livingston, La) have been all completed and are getting close to the expected sensitivity at high frequency, the construction of Virgo, in Tuscany, has included a successful pre-commissioning period and is now completed, the TAMA and GEO prototype/detectors are also running. If the sensitivity of the interferometers still remains to be improved, particularly in the low frequency range, the good news are that they all demonstrate a good duty cycle: when the site activity stops for an "engineering run" or a "science run", they do run unattended for hours to weeks. This is quite remarkable, given the complexity and the delicacy of these instruments. The analysis of the first data from LIGO, TAMA, and GEO detectors allows the determination of upper limits on the amplitude and event rate for various possible GW sources. Although these limits are not yet quantitatively interesting, they demonstrate the ability to acquire and store the data, and to filter them with appropriate templates. These studies have also the merit to show that the final sensitivity (after data filtering) is strongly affected by the lack of stationarity of the noise. This has a consequence on the detailed design of the interferometers, that the Tama group has started to take into account: it is important to improve the noise stationarity, as well as the noise spectral density.
GEO plays the double role of a detector, and of an advanced prototype: after a "science run" in coincidence with LIGO, it will implemented as a double recycling interferometer, using advanced technologies in the last suspension stages: monolithic fiber suspensions, controlled damping of the violin modes, thermal control of the curvature radius, The Australian AIGO group also develops advanced instrumentation in collaboration with LIGO and with Virgo.
"Advanced techniques" and "Noise sources" were mainly focused on two points: thermal noise and quantum noise. Thermal noise issues are the most immediate concern: since the previous Amaldi meeting in Perth, it was found that the mirror's thermal noise, due to the mechanical losses of the reflective coatings, could become relevant, particularly in advanced uncooled detectors like the Advanced LIGO. It may limit the sensitivity if the planned sapphire substrates are not yet available and have to be replaced by silica. If the material of the coating layers cannot be sufficiently improved, it remains the possibility of using non-Gaussian beams, which should provide a lower phase noise level, for the same mirror fluctuations. This thermal noise would anyway become negligible in a cryogenic detector, were quantum noise should be the main limitation. Experimental and theoretical studies were presented on the quantum noise problems. "Quantum locking", a new idea, which consists in measuring the radiation pressure noise on each mirror and providing active feedback, does not require the use of squeezing or QND techniques. The first large cryogenic prototype, CLIO, is being built, underground, in the Kamioka mountain.
Except for Nautilus and Explorer the acoustic detectors have not been running much in the last years, but good progress has been made in the noise and the bandwidth of the transducers, which will keep them competitive for some more time with the wideband interferometers. A promise for the future is the development of spherical cryogenic detectors: three of them may be built soon, in Leiden (mini-Grail), in Sao Paulo (Mario Schenberg) and in Rome (Sfera), involving two new groups and new countries in the field.
One full session was dedicated to LISA, and to the SMART-2 test flight, foreseen in 2006-2007. The spacecraft will carry two similar but different payloads, one built by NASA and one by ESA. They will both test the functionality and the sensitivity of the interferometers, of the "gravity reference sensors", and of micro-thrusters to be used later in LISA. The progress on these critical issues is impressive. The collaboration-competition compromise is not yet totally clear.
The session on theory and GW sources was mainly focused on the description of new possible sources to be detected by LISA and by LIGO-Virgo and the corresponding advanced detectors, on the possibility to use LISA signals and their redundancy for testing alternative theories, and on the progress in numerical relativity. Recent achievements, and new investments in computing facilities, should allow soon for an interesting comparison between ''exact" models and real coalescence signals.
The data analysis session became very large, with (too) many (too)
short talks, and more than 20 posters. At the next issue of the Amaldi
conference ( Summer 2005, in Japan), it may be wise to focus on the
results of the working detectors (upper limits , noise studies,)
and to push other papers towards the GWDAW (Gravitational Wave Data
Analysis Workshop) that the community organizes each year in December.
The session on future detectors proposed a full description of the
Advanced LIGO, and considerations about Japanese and European projects
for cryogenic detectors, improved SQUID's for acoustic detectors, and
even a "post-LISA" proposal. Funds and manpower may be limited, but
the research field is young, and there is no lack of new ideas!