Gravitational Waves and Interferometric Antennas

The VIRGO project is a physics experiment; its aim is the detection of gravitational waves.

Among all the forces of Nature, Gravity is the one that has been known to man for the longest time. One of its basic properties - that all bodies fall to ground with the same acceleration- was recognized by Galileo at the beginning of the seventeenth century; the static law of Gravity was established by Newton at the end of the same century. Finally, Albert Einstein connected the perturbations of the gravitational field to the structure of the space-time.

Einstein's theory predicts the existence of gravitational waves, that is perturbations of the gravitational field, which, as it is for electromagnetic field, spread out through space at the speed of light. Right from their source, these waves radiate, like ripples on the surface of a pond. Spreading out, the amplitude of the waves decreases very slightly when interacting with matter, thus, unlike electromagnetic radiation, gravitational waves are not stopped by interstellar matter.

The detection and the study of gravitational waves are of the greatest importance in order to determine the basic characteristics of one of the fundamental interactions that occour in Nature.

The weakness of Gravity makes it extremely hard to detect gravitational waves: to date, after 30 years of experiments and studies, we only have an indirect proof of their existence deduced from very accurate and careful observations of a binary system of pulsars. It has not yet been possible to detect directly the gravitational waves: it remains one of the major challenges of experimental physics.
  Gravitational Waves as probes to the Universe Gravitational waves are not only relevant to the fundamental problems of Physics; they are also important for the information they carry.

We are urged to decode all the messages that we receive from outer space in order to study the Universe and its evolution: gravitational waves will significantly complete the cosmic signals explored so far, that is, electromagnetic waves and cosmic rays. Gravitational waves are generated by quite different processes as electromagnetic waves and cosmic rays, and they will add insight to the physics of the celestial phenomena where they are produced.

Gravitational waves strong enough to be detected are expected to be generated in astrophysical events not yet fully understood, as supernova explosions, as the catastrophic collisions of inspiralling binary systems, as the interaction of black holes with companion stars, as the Big-Bang itself. The detection and the measurement of gravitational waves from these events will give us the tools that we are still missing to their full knowledge. Antennas for gravitational waves make possible to study those events mentioned also in regions heavily obscured by dust, that absorbs the electromagnetic radiation, and are therefore so far hidden to our observations, as, for example, in the center of our Galaxy.

Finally, let us remark that every new instrument used to observe nature has generated unespected discoveries that have enriched our knowledge and, often, have deeply modified our understanding of the world.
  Interferometric antennas for gravitational waves Gravitational waves distort space-time: along two perpendicular directions, the distances between fixed points will increase (resp. decrease), at the arrival of a gravitational wave. The variation is very small, and it is proportional to the distance: it would be the size of an atom, on the distance between Earth and Sun, and it is a billion times smaller in a detector several kilometres long. Tiny variation of distance as that can be detected using the phenomenon of interference.

An interferometer for gravitational waves is made of two optical resonant cavities, each consisting of two mirrors set at a distance of a few kilometers: a laser beam is split and injected in the two orthogonal cavities, it goes back and forth a number of times, reflected each time by the mirrors at the end of each cavity, and it is finally recombined. The variation of the optical path's lenght, caused by the variation of distance of the mirrors due to the arrival of the gravitational wave, produces a shift of the relative phase of the the beams, and thus a variation in the intensity of the beam after the recombination. This variation is proportional to the amplitude of the gravitational wave.

  • The VIRGO Project
  • The VIRGO project is setting up a laser interferometer made of two orthogonal cavities 3 kilometers long: actually, multiple reflexions extend the optical lenght of each arm to 120 km. The site chosen is near Cascina, a village not far from Pisa. VIRGO will be sensitive to gravitational waves in a wide spectrum of frequencies, from 10 Hz to 1000 Hz: that will include gravitational radiation produced by various sources, like supernovae, the coalescence of binary systems, and fast rotating neutron stars in our Galaxy.

    The Italian and French scientists involved in the project are developing the most advanced techniques in the field of high power ultrastable lasers and high reflectivity mirrors; in the field of highly efficient seismic damping, and of position and alignement control. The entire interferometer needs very carefully designed optics, made according to status-of-the-art technology, in order to reach the sensitivity required by the goal, and will be isolated very accurately from the environment, so that it is only sensitive to gravitational waves.

    Optics: the laser for VIRGO is the first of a new generation of ultrastable lasers, and the most stable oscillator built to date. The mirrors for VIRGO must combine the highest reflectivity (better than 99.999%) with the best surface quality (better than one hundreth of a micron). It took about ten years to develop the mirrors, and a dedicated workshop has been set up.

    Damping: extreme care has been taken to avoid spurious displacements of the optical components, due to seismic noise; insulation is obtained with a system of 5 compound pendula, to which each mirror is suspended.

    Moreover, the light path needs to be evacuated to a very high level, because the presence of residual gas would slightly modify the phase of the light beam, and thus perturb the measurement; the 6 km. long, 1.2m diameter evacuated beam pipe will be one of the largest vacuum vessels in the world. Finally, the environment of the interferometer will be much quieter than what achievable on a spacecraft.

    VIRGO will run all day long, and all around the year, waiting for signals arriving at any time and from any direction in the Universe. These signals will be detected, recorded and analyzed in a computer center; the final data will be at the disposal of the international scientific Community, for further studies.

    VIRGO will join in the research of gravitational waves the LIGO interferometers, presently being built in the USA. The analysis of data provided simoultaneously by VIRGO, LIGO, and by other interferometric antennas, or by cryogenic resonant detectors already operating in Italy, Australia, and the USA, will provide insights in the gravitational theory.