The Shaw Prize in Astronomy for 2016 is awarded in equal shares to Ronald W P Drever, Kip S Thorne and Rainer Weiss for conceiving and designing the Laser Interferometer Gravitational-Wave Observatory (LIGO), whose recent direct detection of gravitational waves opens a new window in astronomy, with the first remarkable discovery being the merger of a pair of stellar mass black holes.
The discovery reported in 2016 [1] ranks among the most significant ever made in astronomy, and its importance can be viewed from a number of distinct points of view. Most simply, LIGO has added a third strand to the means by which we can observe the universe, which could previously only be carried out via electromagnetic radiation or energetic particles. LIGO has thus established an entirely new branch of astronomy, allowing us to study phenomena where signals from existing astronomical messengers are entirely lacking. The impact of this new tool seems likely to be as revolutionary as, for example, the opening up of radio astronomy and its discovery of pulsars and quasars.
The direct proof of the existence of gravitational radiation validates a basic prediction of general relativity. The general arguments for the existence of such waves are based on causality, and it is deeply satisfying to be able to demonstrate that such fundamental expectations are correct. However, the significance of the LIGO detection lies not so much in the proof of the existence of weak spacetime fluctuations, since these had been demonstrated to exist indirectly through study of the orbital decay of pulsars in binary systems. The true importance of the detection lies in the fact that the waves were apparently generated in the regime of strong and time-dependent gravitational fields, probing the properties of black holes.

Black holes have been a feature of astronomical discussion at least since the detection of quasars in the 1960s, but almost all arguments for the existence of black holes were indirect. In X-ray binaries, or in the centers of galaxies, we see objects that are too compact and massive to correspond to any alternative astronomical structure compatible with standard relativistic gravity. But a different theory of gravity might change this conclusion, e.g., by raising the maximum mass of a neutron star, so the existence of black holes could not be demonstrated without probing directly the characteristic features of general relativity, such as the event horizon. Even to demonstrate that a horizon existed would have been a huge step forward (towards which projects such as the event horizon telescope have been dedicated). But LIGO has achieved this and much more: probing not only the structure of spacetime in strong gravity, but also how this evolves dynamically. The signal for the event is as expected for the merger of a pair of black holes, even to the fine details of the ringdown oscillations of the horizon as the resultant single black hole settles to its final state, exactly as computed both analytically and numerically. All this pushes the validation of relativistic gravity into a completely new regime.

Perhaps the most impressive feature of LIGO’s achievement is that it required the focused efforts of many scientists over more than four decades. But along the way, progress always hewed closely to the vision presented in the proposal that the LIGO team submitted to the US NSF in 1989. Drever and Weiss are experimental physicists with contrasting styles. The former works more intuitively and the latter more analytically. Thorne is a theoretical physicist who specializes in general relativity. LIGO required a large team. Fortunately, a series of capable directors were appointed and they led LIGO to succeed in probing gravity in the limit where massive objects moving at relativistic velocities drive nonlinear ripples in spacetime. Such a triumph of team science is perhaps only matched by the 2012 detection of the Higgs boson at CERN.

[1] B. P. Abbott et al. (LIGO Scientific Collaboration and Virgo Collaboration), “Observation of gravitational waves from a binary black hole merger”, Phys. Rev. Lett. 116, 061102 (2016).

Astronomy Selection Committee
The Shaw Prize

31 May 2016  Hong Kong