2016

Gravitational Waves

In the view of the universe elegantly advocated by Albert Einstein in his early twentieth-century theory of general relativity, our three dimensions of space and one dimension of time are intimately linked into a continuum called “spacetime.” Furthermore, Einstein and others realized that spacetime can become warped or curved in the presence of mass or energy, and thus ripples or waves could theoretically propagate through spacetime like the waves on a pond.

At least, that is theoretically the case. The problem scientists encountered throughout the rest of the twentieth century, however, was that the magnitude of these predicted gravitational waves in the spacetime continuum was extremely small and impossible to detect with existing technology. In addition, the kinds of events or disturbances that could produce detectable gravitational waves—like the supernova explosion of an enormously massive star, or the merger of two black holes—are rare and/or extremely distant. Thus, detecting gravitational waves had to wait for technological advances.

Those advances finally arrived with the advent of two enormous detectors specially designed to search for gravitational waves: the US Laser Interferometer Gravitational-wave Observatory (LIGO) and the European Virgo Interferometer. Both facilities use lasers to search for the tiny changes in the distances between reference targets that would be caused by a passing gravitational wave. The instruments can achieve exquisite sensitivity, comparable to being able to measure the distance to the nearest stars to within an accuracy of a human hair. LIGO began operations in 2002 and Virgo in 2003, and since 2007 both facilities have jointly shared their data and analyses, helping to refute or confirm each other’s potential detections.

After more than a decade of searching, and after careful data processing and peer review of their results, LIGO and Virgo finally announced the first detection of gravitational waves (from the merger of two supermassive black holes) in February 2016, confirming the last major unproven prediction of Einstein’s theory of general relativity. More detections have been made since, and astronomers are now excited to use gravitational waves as new tools to study extremely violent and high-energy phenomena across the universe.

Computer simulation of ripples in the spacetime continuum—gravitational waves—caused by the merger of two supermassive, co-orbiting black holes.