Gravitational-Wave Astronomy LARGE SCIENTIFIC PROJECT in 1916 the existence of compact objects like black holes and neutron stars was not yet known, and the idea of massive bodies like the Sun rotating near the speed of light with the density of atomic nuclei would have seemed like fantasy. In 1936, Einstein denied the existence of gravitational waves, unable to identify them as exact solutions to his equations. Fortunately, the article, written with Nathan Rosen, known for both the EPR paradox and the Einstein – Rosen bridges, was never published, since a reviewer identified an inconsistency. Shortly thereafter, Einstein and Rosen corrected the error and published a different conclusion [ 2 ]. Yet, a fundamental question remained: even if such waves exist mathematically, can they be detected? Do they possess a physical reality? Can they deposit energy in a detector? This dilemma persisted until 1957, at the famous Chapel Hill conference, where some of the leading researchers in relativity concluded that the passage of gravitational waves would indeed modify the distance between free-falling test masses, and that if these masses were coupled to a dissipative element, the passage of the wave would generate heat [ 3 ].
From Weber’ s bars to interferometry
At the beginning of the 1960s, the experimental quest began. The first attempt to detect gravitational waves was carried out by the American physicist Joseph Weber, who introduced resonant bars: metallic cylinders designed to oscillate at their resonant frequency when traversed by a gravitational wave. In 1969, Weber went as far as to claim the detection of several signals, which might have originated from galactic supernovae [ 4 ]. However, it soon became clear that the recorded signals were too intense to be attributed to gravitational waves. Despite this, Weber had the merit of launching
Figure 1. a system of freely falling masses when a gravitational-waves passes through the plan of the page.
the experimental efforts and introducing a fundamental idea: that of coincident detection using instruments separated by large distances, in order to exclude local noise sources, such as seismic activity, electrical glitches, or transient mechanical or electronic disturbances specific to individual instruments. During the same period, two Russian physicists, M. E. Gertsenshtein and V. I. Pustovoit, understood that the Michelson interferometer, the same instrument for which Albert Abraham Michelson received the Nobel Prize in 1907, had the right characteristics for detecting gravitational waves. In 1972, Rainer Weiss, a young physics professor at MIT in Boston, conducted a detailed analysis
Figure 2. Evidence of orbital decay of PSR B1913 + 16 Source: https:// en. wikipedia. org / wiki / Hulse – Taylor _ pulsar of the noise sources limiting the sensitivity of such an instrument, and proposed a conceptual scheme for a gravitational-wave detector. This would become one of the foundational ideas that led to LIGO and Virgo. The document, about thirty pages long, was not formally published at the time, but remained an internal MIT report [ 5 ]. Also for this work, Weiss would later be awarded the Nobel Prize in Physics, together with Kip Thorne and Barry Barish, for their decisive contributions to LIGO. In parallel, neutron stars were being discovered in the form of pulsars, by Jocelyn Bell Burnell. At the same time, the first black hole candidates began to emerge from X-ray astronomy. The invention of the laser in 1960 opened the way for the first prototypes of laser interferometers. One of the earliest was developed by Robert Forward in Malibu: although its sensitivity was ten orders of magnitude worse than that of today’ s LIGO and Virgo, it represented a crucial first step toward interferometric detection of gravitational waves [ 6 ].
Binary pulsars and the proof of the gravitational-wave existence
In 1974, two American radio astronomers, Russell Hulse and Joseph Taylor, discovered the binary pulsar PSR B1913 + 16, a system composed of two compact objects, one of which is a pulsar. The presence of the pulsar- a highly precise cosmic clock- allowed for extremely accurate orbital measurements and made it possible to test general relativity in strong-field conditions. This discovery earned the two scientists the 1993 Nobel Prize in Physics. Observations of the system over several years showed that the orbital period was decreasing over time, in spectacular agreement with
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