The discovery uncovered

Almost exactly one hundred years after the publication of Einstein’s paper on General Relativity, the LIGO and Virgo collaborations have published a paper in which they show a gravitational signal emitted by the merger of two black holes. The signal has been observed with 5-sigma accuracy and is the first direct observation of gravitational waves.


On Thursday, 11 February, Barry Clark Barish, one of the fathers of the LIGO experiment, presented the latest results in a packed Auditorium.

Ripples in space-time, the fabric of the Universe: this is how we can picture gravitational waves. In his visionary paper published in June 1916, Einstein predicted that masses deform space-time and, therefore, any change in their position causes a distortion that propagates at the speed of light, resulting in gravitational waves.  

It wasn’t until 1975, almost 60 years later, that Russell Hulse and Joseph Taylor, who were awarded the Nobel Prize in 1993, inferred the existence of gravitational waves by observing the neutron star binary system PSR1913+16 in which the orbital period of the pulsar has decreased over the years: the measurement was perfectly in line with the loss of energy through gravitational waves predicted by General Relativity. However, we have had to wait another 40 years for the first direct observation: a beautiful, perfectly shaped signal coming from the unimaginable collision of two black holes with masses of around 36 and 29 times that of the Sun.

The signal was recorded by LIGO’s interferometers on 14 September 2015, at the beginning of the new run following a long upgrade campaign from 2010 to 2015. The first ever gravitational signal shows a distortion that becomes more intense and reaches a higher frequency as the black holes spiral towards the collision point and fades out again after the collision, when a considerable part of the initial energy is dissipated in gravitational waves.

But don’t imagine a huge explosion! There is no air out there so no sound (sound is a vibration or mechanical wave that needs a medium to propagate) and nothing but gravitational waves can escape from a black hole, not even light, so from our vantage point, everything happened in darkness and silence. Indeed, such an event can only be “seen” through the gravitational perturbation it causes. In other words, we now have a very powerful instrument to study previously invisible events in the Universe. And you don’t have to worry about this catastrophic event happening anywhere near you: the paper reports that it occurred at about 1000 million light-years from the Earth. Nothing to worry about, then, but a huge discovery for humankind. 

LIGO, Virgo, and others 

The teams behind the LIGO interferometer in the US, the Virgo interferometer in Italy and the GEO600 interferometer in Germany have been collaborating since 2004, and in 2007 they signed a Memorandum of Understanding to analyse their data together and exchange technologies. The current paper is jointly signed by the three collaborations.

Other interferometers are currently under development around the world: KAGRA under construction in Japan; the Indigo project submitted to the Indian Government; and, taking a longer term perspective, the Einstein Telescope to be located on the Earth's surface, and the LISA observatory, which will be orbiting in space.

The basic principle of any laser interferometer for the direct observation of gravitational waves is based on an L-shaped design: the gravitational wave produces a distortion of the local metric such that one axis of the interferometer is stretched while the orthogonal direction shrinks. This distortion between the two arms oscillates with the frequency of the gravitational wave.


by Antonella Del Rosso