Gravitational waves are direct testimony to disruptions in spacetime itself”, reads the press release for this year’s Nobel Prize in Physics, from only three weeks ago.
“A wealth of discoveries awaits those who succeed in capturing the waves and interpreting their message.”
This optimistic message is a hopeful prediction of the exciting physics of the near future. What perhaps wasn’t expected was quite how soon these discoveries would begin.
On Monday, 16 October, one such revelation was announced: the first discovery in a new field known as ‘multi-messenger astrophysics’. The collision of two neutron stars was detected both with traditional method, electromagnetic waves, and with the indicator in its infancy: gravitational waves.
Such a collision had never before been observed; more excitingly, no astronomical event has before been observed in both spectra.
The news of this extraordinary occurrence arrived at Earth on 17 August of this year, when the Virgo and LIGO gravitational wave detectors picked up a signal that lasted for a hundred seconds. Then, 1.7 seconds later, a gamma ray burst in the constellation Hydra was picked up by Nasa’s Fermi Space Telescope.
These two events in such close succession caused a great deal of excitement among astronomers, who have been waiting for these tell-tale signs of a neutron star collision. It is estimated that 15 per cent of the world’s astronomers were involved in searching for the source of the signal.
Out of the astrophysicists who were gathering data on a tiny area of sky from 70 telescopes all over the world and in space, it was Dr Charlie Kilpatrick of the University of California at Santa Cruz who identified the source. With remarkable composure for the first person to see the light of a gravitational wave event, he sent a message to his colleagues through the messaging service Slack, simply saying “found something”.
Identifying the source yielded an astounding wealth of data. Dozens of papers emerged, detailing the remarkable variety of discoveries and deductions.
Primarily, this confirmed the source of enigmatic gamma ray bursts: it has long been theorised that the shortest of these bursts could come from a neutron star collision, also known as a kilonova.
A further prediction of these cataclysmic collisions is that they could be the source of the majority of the universe’s heavy elements, such as gold, silver, and platinum. The optical light emitted after the gamma ray burst showed all the signs of the formation of these elements.
Perhaps most remarkably is that it provided an estimate for the age of the universe. Analysis of the gravitational wave signal gives an estimate for a quantity known as the Hubble constant. This is a fundamental constant in the field of cosmology, and any estimate for the age or rate of expansion of the universe depends on it.
The most exciting thing about this discovery is the promise it holds for the future: “The era of gravitational wave astrophysics had dawned, but now it’s come of age,” says Julie McEnery, astrophysicist at Nasa’s Goddard Space Flight Center: “We’re able to combine dramatically different ways of viewing the universe, and I think our level of understanding is going to leap forward as a result.”