Ronald W P Drever

Ronald W P Drever was born in 1931 in Bishopton, Glasgow, UK and is Professor of Physics, Emeritus, at the California Institute of Technology (Caltech), USA. He received his Bachelor of Science in 1953 and his PhD in Natural Philosophy in 1958 from the University of Glasgow, UK. He served as a Visiting Associate in 1977 and was successively Professor (1979-2002) and Professor Emeritus (2002- ) at Caltech. He is a Fellow of the American Academy of Arts and Sciences and the Royal Society of Edinburgh.

Kip S Thorne

Kip S Thorne was born in 1940 in Logan, Utah, USA and is the Feynman Professor of Theoretical Physics, Emeritus, at the California Institute of Technology (Caltech), USA. He received his Bachelor of Science in Physics from Caltech in 1962 and his PhD in Physics from Princeton University, USA in 1965. He returned to Caltech, first as a Research Fellow (1966-1967), and then successively as Associate Professor (1967-1970), Professor in Theoretical Physics in 1970, the William R Kenan, Jr. Professor (1981-1991), the Feynman Professor of Theoretical Physics (1991-2009), and the Feynman Professor of Theoretical Physics, Emeritus (2009- ). He is a member of the US National Academy of Sciences and the American Academy of Arts and Sciences.

Rainer Weiss

Rainer Weiss was born in 1932 in Berlin, Germany and is currently Professor Emeritus in Physics at the Massachusetts Institute of Technology (MIT), USA. He received his Bachelor of Science in Physics in 1955 and his PhD in Physics in 1962 from MIT. He was an Instructor and Assistant Professor of Physics at Tufts University, USA from 1960 to 1961 and from 1961 to 1962 respectively. He served as Research Associate in Physics at Princeton University, USA from 1962 to 1964. He then joined MIT, where in the Department of Physics he was successively Assistant Professor (1964-1967), Associate Professor (1967-1973), Professor (1973-2001) and Professor Emeritus (2001- ). He is a member of the US National Academy of Sciences and the American Academy of Arts and Sciences.

On 14 September 2015, the two detectors of the Laser Interferometer Gravitational-Wave Observatory (LIGO) simultaneously observed a gravitational-wave signal. The signal matches the waveform predicted by general relativity for the merger of a pair of black holes. Scientists estimate that the black holes for this event were about 29 and 36 times the mass of the Sun, and the event took place 1.3 billion years ago. About three times the mass of the Sun was converted into gravitational waves in a fraction of a second. These observations demonstrate the existence of binary stellar-mass black hole systems. This is the first direct detection of gravitational waves and the first observation of a binary black hole merger.

Ronald Drever, Kip S Thorne and Rainer Weiss are the primary figures responsible for the conception and design of LIGO. LIGO's recent direct detection of gravitational waves represents the first probe of physics in the limit of strong gravity, where massive objects moving at velocities close to that of light drive nonlinear waves in spacetime.

LIGO's discovery ranks among the most significant ever made in astronomy, and its importance can be viewed from a number of distinct perspectives. Most simply, LIGO has provided a third strand to the means by which we can observe the universe, in addition to 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 the subsequent discovery of pulsars and quasars.

The direct observation of gravitational radiation validates a basic prediction of general relativity, showing such fundamental expectations based on causality to be correct. However, LIGO's results go much beyond the weak spacetime fluctuations already inferred from the orbital decay of pulsars in binary systems. By probing the region of strong and time-dependent gravitational fields from which the waves originate, they reveal remarkable properties of black holes. 

Ripples in spacetime created by merging black holes (numerical simulation). 
Credit: R. Hurt/Caltech-JPL

The L-shaped detector of the Laser Interferometer Gravitational-Wave Observatory
Credit: Caltech/MIT/LIGO Lab

In 1916, Albert Einstein in his general theory of relativity first predicted the existence of gravitational waves — ripples in the fabric of spacetime. Gravitational waves travel at the speed of light. When massive objects move around or collide with one another, they disturb the surrounding spacetime like a pebble tossed into a still pond. By studying these waves, astronomers hope to learn about the cosmic phenomena that create them, which are often "invisible" objects such as black holes. The gravitational waves are extremely weak when they reach us, so are very difficult to detect.

Gravitational waves cause incredibly small but detectable distortions in the positions of objects in the spacetime through which they pass. LIGO's design takes advantage of this fact: Its two L-shaped detectors have three mirrors, one at the end of each arm and one at the point where the arms meet. The two arms are the same length. Laser beams bounce off the mirrors back to a photo detector. LIGO is set up so that normally, when both laser beams travel the same distance, their light waves are exactly out of phase and cancel out each other when they meet at the centre and no light hits the photo detector. If a gravitational wave passes through the detector, however, one arm will become slightly longer whereas the other arm will get slightly shorter, causing the light travelling through the two arms to have a phase difference. Such a mismatch creates an interference pattern when the two laser beams meet, thus revealing the gravitational wave's presence. 

The longer the arms, the farther the laser travels, and the more sensitive to length changes the instrument becomes. LIGO attempts to measure a change in arm length 10,000 times smaller than a proton. Its arms are 4 km long. By bouncing the laser beam about 280 times before it finally merges with the beam from the other arm, the distance travelled by each laser beam is increased to 1,120 km and the sensitivity of the detector is dramatically increased. Twin facilities, one in Hanford, Washington, and the other in Livingston, Louisiana, USA, wait for gravitational waves caused by binary systems or supernovae. The two sites are located more than 3,000 km apart to ensure that local vibrations, such as that caused by a micro-earthquake, are not mistaken for signals from gravitational waves.

LIGO technician inspects one of the interferometer's mirrors.
Credit: Caltech/MIT/LIGO Lab

The interferometer of LIGO 

During normal operations, when the beams from the two arms of LIGO recombine (yellow), the waves should cancel each other out, rendering the resultant beam dark. If a gravitational wave changes the relative length of the arms (blue), the waves will not match up, and the combined beams will reveal signals.