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A simulation of a binary blackhole merger in action

What is LIGO?

LIGO project was conceptualised in late 80’s and started in the early 90’s. It began as a first ever attempt – in form of a huge and ultra-sensitive interferometric observatory –  to measure Einstein’s elusive gravitational waves, predicted first in 1916. It underwent a long trial with no successes but rapid upgrades. The primitive detectors took data in several cycles/runs between 2002 and 2010. No gravitational waves were detected during those runs. In 2010, LIGO undertook a long sabbatical from these data-taking cycles (called ‘science-runs’), and began major upgrade operations. In parallel, VIRGO project in Italy and GEO600 project in Germany caught up to LIGO. The LIGO Scientific Collaboration (LSC) became a synonym for LIGO-Virgo Collaboration (LVC). By 2010, GEO600 was assigned the task of a R&D facility where future technology for LIGO was to be tested. VIRGO also stopped its science-runs and joined LIGO in major upgrade operations. After 5-odd years of upgrades, the first two detectors in USA (in Hanford-H1 and Livingston-L1) were intended to go online (for the so-called ‘observation-runs’) in September 2015, while VIRGO would continue with upgrades until mid-late 2016 and then join LIGO. In preparation for the first observation-run O1 in late September last year, LIGO began its last 2-week test run, called ‘engineering-run 8 (ER8)’. At this point, we knew that LIGO detectors had been through enough upgrades and their sensitivity to gravitational waves had begun to be at par with the requirements. We were very optimistic. However, we didn’t account for the possibility of measuring a signal in the test run itself; the signal was found in ER8 data, before the observation-run O1 even began. This was very exciting.

LIGOH1
LIGO Hanford Observatory (H1)
LIGOL1
LIGO Livingston Observatory (L1)

The Human Story

I joined the LSC (a.k.a LVC) in late 2014 when I started my PhD at the Max-Planck-Institute (in Observational Relativity and Cosmology & Data-analysis). I was in Italy on holidays (meeting a couple of classmates from IIT Guwahati), when one of my colleagues (Marco Drago) at the Max-Planck-Institute (in Hannover, Germany) sent a LSC-wide email citing a disturbance in the data, on 14 September 2015. The disturbance looked frighteningly similar to what one would expect from a cosmic event, in this case a binary blackhole merger. It was initially suspected that it was a fake test-signal injected into the data; fake test-signal injections are done to verify, test and calibrate our data-analysis search methods. Marco was wondering in his email if there was a fake injection planned at that moment in time. He couldn’t find any record of one and he wondered if an unauthorised injection may have missed the catalogues. I returned to Germany 2 days later while enquiries were made across LSC and it was checked if Marco’s suspicion was correct. Within 3 days, all channels were exhausted and it became highly likely – yet not completely sure – that no such injection was made. As soon as I returned, my supervisor (one of the leaders of data-analysis within LIGO) briefed me with a little more detail about ‘the event’. It was to be kept a secret within the collaboration (while deeper detective work took place in parallel to disqualify malicious intent from our list of suspects) and our data-analysis team got down to the task of digging deeper into the data. With each passing day, more suspects were ruled out and it became clear that it was most likely the first gravitational wave signal ever measured. This was officially confirmed within the collaboration by the end of October. It is pointless to say that the entire community got really excited at the prospects that this discovery represented. There were many long threads of emails circulating within the community domain; data-analysis collaborators from around the world started on their caffeine-riddled quest to define the source behind the signal. It was concluded that the signal represented a pair of black holes circling and gravitating toward each other in a binary system. At their merger, an immense amount of energy was released in the cosmic explosion in form of gravitational waves (very very very tiny distortions in the fabric of space-time), which LIGO succeeded in measuring. It was determined from the properties of the signal that the black holes were roughly equal to 30 suns in one – each, totalling roughly 60 solar-masses. Their distance was determined to be roughly 1.3 billion light-years i.e. the event occurred 1.3 billion years ago! In terms of equivalent ‘visible’ luminosity, for roughly 1/10th to 1/5th of second, the explosion shone brighter than all the stars in the Universe combined! This represented the first instance in history when gravitational waves were detected by mankind while (binary) black holes, i.e. the strangest of all objects in the universe, were confirmed to exist. 

Signal
Signal waveforms recorded at LIGO Observatories
Sky
Location of signal in the sky

In terms of a life of an individual, besides the extra attention, nothing much has changed (for most people). We are obviously very happy that the efforts of past 25 years have finally paid off. It feels like a truly great achievement to be part of such a gang-of-science-people. At the same time, we also realise the possibility of measuring many more such phenomenal (theorised) events in the cosmos. LIGO finished its O1-run on January 12, 2016. We have already gotten down to the task of analysing the rest of the data from O1 (for similar kind of sources, as well as from other exotic objects such as ‘neutron stars’), and prepare our techniques for the second 6-month observation-run O2 starting in June 2016.

What does the discovery represent?

This discovery of the gravitational waves also serves as a direct proof of Einstein’s theory of General Relativity, published in 1915. Precisely after 100 years of the advent of General Relativity, its smoking gun(s), i.e. gravitational waves, were found. With more precise measurements and more such events, we will soon be able to find further corrections/modifications to the theory of General Relativity, and be able to explore it in great depth. This starts a new era in astronomy, of Gravitational Wave Astronomy. Gravitational waves are also predicted to be emitted at the Big Bang; those ancient signals (called ‘primordial gravitational waves’) are extremely extremely weak. In roughly two or more decades, we hope to be able to be sensitive enough (with incredibly more sensitive future detectors, including the upcoming duo in Japan and India, besides LIGO’s 2 and VIRGO’s 1) to capture the holy grail of the Universe.

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— Avneet Singh

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