Expressed in this way, I think the process sounds fairly straightforward. However, the main issue for the designers was to manage the amount of ‘noise’ in the system, given that the strength of the ‘signal’ was expected to be so small. Although every effort was made to isolate the detector’s elements from the general drone of planet Earth (e.g. the force of the wind, the vibration of a passing lorry, seismic events, and so on), nevertheless the signal was still buried in a sea of noise. So techniques were required to aid in the uncovering of any tiny gravity wave signal present. The main means of doing this was to trawl the noise looking for specific patterns representative of what we expect gravitational wave signals to look like. Many years of research involving supercomputer solutions of Einstein’s gravity theory equations have allowed us to predict the expected LIGO signal for a variety of astrophysical events. In other words, the wave signal has specific tell-tale frequencies and characteristics.
The signal confirming the existence of gravity waves was received at 10.51 UT (GMT) on 14 September 2015. In the subsequent 5 months, prior to the announcement of the discovery in February 2016, an army of researchers from 80 institutions in 15 countries exercised their expertise to work out what it was that LIGO had actually detected. And the details of what it ‘saw’ are completely staggering! For me this was definitely a ‘wow!’ moment. There are so many occasions in recent times when new discoveries have been made on the frontiers of science, and what we have found in nature is way beyond anything we imagined. For me, this is one of those occasions. Quoting the Executive Director of the LIGO, David Reitze - "Take something about 150 km in diameter, and pack 30 times the mass of the Sun into that, and then accelerate it to half the speed of light. Now, take another thing that's 30 times the mass of the Sun, and accelerate that to half the speed of light. And then collide them together. That's what we saw here. It's mind boggling." Basically he’s describing two monster black holes spiralling around each other, getting closer and closer to each other due to the huge amount of orbital energy being lost in the form of gravitational waves. One has a mass of 30 solar masses (1 solar mass = the mass of our Sun) and the other about 35 solar masses. In the moments just before they collide and coalesce they are orbiting each other several tens of times per second. At the moment when their event horizons merge, and they become one, the event produces a pulse of pure radiate energy in the form of gravitational waves equivalent to 3 solar masses (E equals m c squared!). And it is the huge energy of this pulse that allowed the LIGO systems to detect the event, even though the black holes were roughly 1.3 billion light years away. To put this pulse of gravity wave energy into perspective, the mass equivalent of the Sun’s radiant energy released over its lifetime of about 4.6 billion years is a mere 0.00031 solar masses, and the mass equivalent of the energy released in the brightest supernova explosion yet observed is roughly 0.006 solar masses. Extraordinary!
In the diagram below, the output from the Washington State detector is shown in orange, and that of the Louisiana detector in blue. The detector output predicted by Einstein’s gravity theory is also shown by the finer line. The correspondence between the signal predicted by theory, and that which was actually observed is remarkable. Also the signal was seen in both LIGO systems at virtually the same time, the small delay in detection between them being explained by their 3000 km separation. In the past, we have detected gravity waves by indirect methods – for example, by measuring the loss of orbit energy of compact binary systems and comparing it with the expected energy of gravity wave emission (see blog post on September 1, 2012), but this is the first time they have been detected directly.