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.
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.
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 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.
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.
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.