It has been announced that scientists have detected gravitational waves for the first time, after a long and arduous search that rivaled the search for the Higgs boson in the difficulty involved even though it did not match it in costs. The announcement came in the form of a paper published yesterday in the journal Physical Review Letters, based on results obtained on September 14, 2015. You can read the paper here.
Like with the Higgs, the results were not a surprise since they confirm a long-predicted expectation rather than something novel. The achievement lies in the skill of the experimenters in designing and carrying out a search for something so elusive. I described the LIGO experiment three years ago and you can read that post for further details.
Emanuele Berti describes the history of the search for gravitational waves, the design of LIGO, what the researchers found, and its implications.
On September 14, 2015, within the first two days of Advanced LIGO’s operation, the researchers detected a signal so strong that it could be seen by eye (Fig. 2). The most intense portion of the signal lasted for about 0.2 s and was observed in both detectors, with a combined signal-to-noise ratio of 24. Fittingly, this first gravitational wave signal, dubbed GW150914, arrived less than two months before the 100-year anniversary of the publication of Einstein’s general relativity theory.
In physics, we live and breathe for discoveries like the one reported by LIGO, but the best is yet to come. As Kip Thorne recently said in a BBC interview, recording a gravitational wave for the first time was never LIGO’s main goal. The motivation was always to open a new window onto the Universe.
Gravitational wave detection will allow new and more precise measurements of astrophysical sources. For example, the spins of two merging black holes hold clues to their formation mechanism. Although Advanced LIGO wasn’t able to measure the magnitude of these spins very accurately, better measurements might be possible with improved models of the signal, better data analysis techniques, or more sensitive detectors. Once Advanced LIGO reaches design sensitivity, it should be capable of detecting binaries like the one that produced GW150914 with 3 times its current signal-to-noise ratio, allowing more accurate determinations of source parameters such as mass and spin.
2015 marked the centenary of the publication of Einstein’s General Theory of Relativity that first predicted the existence of such waves.