Albert Einstein had predicted the existence of gravitational waves 100 years ago. The American-European scientific collaboration LIGO-Virgo has now detected them.
The history of physics will remember 2016 as a most extraordinary year, one which inaugurated a new path towards cosmic observation with the birth of gravitational astronomy. This unprecedented field of investigation was initiated by the simultaneously acquired scientific data collected by two twin Laser Interferometer Gravitational-Wave Observatory
(LIGO) facilities, specially designed for the detection of gravitational waves. One is located in Hanford, Washington and the other in Livingston, Louisiana in the United States..
On 15 September and 26 December 2015, the detectors vibrated with the passage of two distinct gravitational signals,
the first to be intercepted by humans. They were both emitted by binary black hole systems in the final stages of their lives as dual entities. Two dark bodies, dozens of times more massive than the sun but much smaller, that spiral towards each other at speeds comparable to that of light until they coalesce into a single black hole. These astronomical systems do not emit electromagnetic radiation, the primary source of our current information about the cosmos.
Therefore, the two gravitational signals detected represent the first direct proof of the existence of such systems.
After a feverish bout of joint data analysis, the European-American scientific collaboration LIGO-Virgo informed the world about the two events, detected just a few months apart on 16 February and 15 June 2016. Nearly 100 years earlier, in 1916, Albert Einstein had predicted the existence of gravitational waves as part of his geometric theory of gravitation, the general theory of relativity. But gravitational radiation eluded physicists’ measurement tools for over half a century until the beginning of the ‘60s when the first machine was built to detect them, Joseph Weber’s resonant bars. Modern interferometric etectors stretch over many kilometres. There are three in the world: the twin LIGO
facilities and the Italian/French joint venture Virgo in Italy.
Unfortunately, our local twin was not available for reception at the time of the two detected signals as it had been closed down to carry out technical upgrades and increase its sensitivity. Based on a collaboration agreement stipulated between the LIGO and Virgo scientific communities in 2007, the analysis of the data of the three facilities is shared and scientific articles are signed by the nearly 1,000 scientists involved. In the months following the September detection, LIGO-Virgo embarked on a sophisticated data analysis process that enabled the extremely weak signal to be identified and separated from background noise. Thus in spite of the fact that the first signal was detected by LIGO, the announcement of it was made in two parallel press conferences, one in Washington and the other in Pisa (where Virgo is located).
The study of gravitational radiation requires measurements that must be made by many operational detectors at the same time. One single operational facility is not sufficient to indicate with any high level of confidence that a gravitational signal has been detected. Two facilities can certify the nature of the signal but not identify the location
in space from which it came by means of triangulation. The search for gravitational waves is therefore by its very nature an area that requires international collaboration.
Why doesn’t Europe have two detectors of gravitational waves like the US? And moreover, why is Virgo an Italian/French joint venture and not a European one? In the ‘80s, the idea of a European interferometer was discussed by various teams operating in France, Italy, Germany and Great Britain, but no agreement was reached on a scientific or even a political level. The British and German groups subsequently decided to join forces with the US’ project, and became part of the LIGO scientific collaboration in 1997. The French and Italian national scientific research authorities, CNRS and INFN respectively, managed to raise their own funds to start a joint venture: Virgo. The unification of European efforts for the detection of gravitational waves was achieved later by the European Space Agency (ESA), with its eLISA space interferometry project, which involves launching three orbiting satellites by 2028, arranged at the apexes of a giant equilateral triangle rotating around the sun.
The LISA Pathfinder, a test satellite, was launched in December 2015 from the ESA spaceport in Kourou (French Guiana) and has successfully tested the operating principles and technologies that will be launched into space on eLISA. The failure to create a land-based European interferometer in the past will be remedied by a future project: the Einstein Telescope, a third generation interferometric detector, ultra cryogenic and underground, funded by the EU commission as part of its 7th Framework Programme. Excitement is mounting over the next data collections by the detectors. When the two LIGO facilities are once again operational in autumn (after a maintenance period) and Virgo joins them in the first months of 2017, the scientific community will finally be able to rely on three detectors, and the era of gravitational astronomy will begin in earnest.
Albert Einstein had predicted the existence of gravitational waves 100 years ago. The American-European scientific collaboration LIGO-Virgo has now detected them.
