The Physics World 2016 Breakthrough of the Year goes to “the LIGO Scientific Collaboration for its revolutionary, first-ever direct observations of gravitational waves”. Nine other achievements are highly commended and cover topics ranging from nuclear physics to material science and more.

Almost exactly 100 years after they were first postulated by Albert Einstein in his general theory of relativity, gravitational waves hit the headlines in 2016 as the US-based LIGO collaboration detected two separate gravitational-wave events using the Advanced Laser Interferometer Gravitational-wave Observatory (aLIGO). The first observation was made on 14 September 2015 and was announced in February 2016. A second set of gravitational waves rolled through LIGO’s detectors on 26 December 2015, and this so-called “Boxing Day event” was announced in June 2016. Gravitational waves are ripples in the fabric of space–time, and these observations mark the end of a decades-long hunt for these interstellar undulations.

Among the almost 1000 scientists involved, there are many Greek researchers: Erotokritos Katsavounidis, a Senior Research Scientist at MIT’s Kavli Institute, Vicky Kalogera from Northwestern University, Antonis Kontos from MIT, Michael Agathos, Andrew Melatos from the University of Melbourne, Mary Sakkelariadou from King’s College, Nicholas Demos from California State University, Leon Deligiannidis  and Antonis Mytidis from the University of Florida.

The measurements also herald the start of the era of gravitational-wave astronomy and multi-messenger astronomy, whereby gravitational-wave observations are combined with those made by optical and radio telescopes and other detectors observing the cosmos. Indeed, LIGO’s twin detectors will soon be joined by a global network of gravitational-wave detectors.

LIGO detected three events in its four months of observation, and this was no mean feat. The instruments are sensitive enough to detect a change in length less than 1000th the size of a single proton between its interferometer’s arms – which is an incredible feat of engineering.

LIGO has already changed our view of the universe – its observations are the first direct evidence for the existence of black holes. Also, the stellar-mass black holes that merged in both events do not fit our current understanding of black holes. Astronomers had thought that such binaries would either not form at all or, if they did, they would be too far apart to merge within the age of the universe. Also, the LIGO collaboration had expected that its first detections would come from binary neutron-star mergers rather than coupling black holes, which were thought to be rare. But the data from the recent discoveries suggest that the rate of binary-black-hole mergers is higher than expected.

The Greek scientists in the team that made first direct detection of gravitational waves