Click Image to Zoom. With their first few detections, LIGO and VIRGO have made significant contributions to our understanding of black holes and neutron stars. This graphic shows the masses for black holes detected through electromagnetic observations (purple); the black holes measured by gravitational-wave observations (blue); neutron stars measured with electromagnetic observations (yellow); and the masses of the neutron stars that merged in an event called GW170817, which were detected in gravitational waves (orange). The remnant of GW170817 is unclassified, and labeled as a question mark. Details/Credit: LIGO-Virgo/Frank Elavsky/Northwestern University.
Opening the Gravitational Wave Window
On September 14, 2015, the Laser Interferometer Gravitational-wave Observatory (LIGO), a ground-based gravitational wave observatory, made history by detecting the first gravitational waves from the merger of two stellar mass black holes. Since then, LIGO and its European counterpart VIRGO, have announced the detection of several additional black hole systems as well as a neutron star merger which also produced light detected by dozens of telescopes on ground and in space. This represents nothing less than the birth of an entirely new field of astronomy.
Click Image to Zoom. The LIGO Laboratory operates two detector sites, one near Hanford in eastern Washington, and another near Livingston, Louisiana. This photo shows the Livingston detector site. Credit: LIGO/Caltech
Ground Based vs Space Based
As LIGO, VIRGO, and other ground-based detectors increase their sensitivity, the number and quality of black hole and neutron star merger events observed will increase. New kinds of events, such as nearby supernovae, may be detected as well. However, there are some gravitational wave sources that are not detectable by even the most advanced ground-based detectors. Gravitational Waves at very low frequencies have wavelengths larger than the Earth itself. Deploying an antenna large enough to efficiently detect them requires going to space. LISA's three spacecraft will create an equilateral triangle in space and the paths between each pair of spacecraft, referred to as LISA's arms, will extend millions of miles. By measuring distance changes in these arms caused by passing gravitational waves, LISA will be able to measure their amplitude, direction and polarization. Astronomers will use this information to learn about the sources in this previously-unexplored region of the gravitational wave spectrum.