Observational Astronomy and Gravitational Waves

What a gravitational wave actually is

General relativity describes gravity not as a force pulling objects together in empty space, but as the curvature of space-time itself, caused by the presence of mass. A massive object warps the space-time around it, and a second massive object nearby follows that curved geometry, which is what we experience as gravitational attraction. This is worth stating precisely because a past question tested exactly this pairing: "light bends near a massive body due to curvature of space-time" and "massive objects warp the space-time around them" are both genuine, correct predictions of general relativity, not competing or contradictory claims.

A gravitational wave is a ripple in this space-time fabric, radiating outward at the speed of light, produced by extremely violent astronomical events, most notably the merger of two massive, compact objects such as black holes or neutron stars. Einstein predicted their existence in 1916 as a direct consequence of general relativity, but the waves are so extraordinarily faint by the time they reach Earth that he doubted they could ever actually be detected.

LIGO: how something that faint gets measured at all

A gravitational wave passing through space stretches and compresses space itself by an amount so small it is almost impossible to describe intuitively: a change in length far smaller than the width of a proton, over a detector arm several kilometres long.

The Laser Interferometer Gravitational-Wave Observatory (LIGO) measures exactly this. It splits a laser beam down two long, perpendicular arms, reflects each beam back, and recombines them. Under normal conditions the two beams cancel out in a precisely predictable way; a passing gravitational wave stretches one arm and compresses the other by a minuscule amount, throwing that cancellation very slightly out of balance in a detectable pattern. LIGO's 2016 announcement, confirming the direct detection of gravitational waves from the merger of two massive black holes billions of light years away, was the observational confirmation of Einstein's century-old prediction, and it is what a question referencing "the 2016 announcement" is asking you to identify as confirming general relativity, specifically the existence of gravitational waves.

eLISA: the same idea, moved into space

The evolved Laser Interferometer Space Antenna (eLISA) is a proposed space-based gravitational wave observatory, using the same core interferometer principle as LIGO but scaled up dramatically: three separate spacecraft, flying in a triangular formation with sides millions of kilometres long, exchanging laser beams between each other rather than reflecting them down an earthbound arm just a few kilometres in length. Moving the detector into space allows a vastly longer arm length than any earthbound facility could build, and avoids the ground-based seismic noise that limits detectors like LIGO, making it sensitive to different, typically lower-frequency, sources of gravitational waves than LIGO can detect.

LIGO-India: joining the global detector network

India is building its own facility, LIGO-India, sanctioned as the first mega science project undertaken on Indian soil, at an estimated cost of roughly Rs 2,600 crore, in collaboration with the LIGO Laboratory run by Caltech and MIT in the United States, at a site in Hingoli district, Maharashtra. The purpose of adding a geographically separate detector to the existing global network is specific: with detections from three or more widely separated observatories, scientists can triangulate the exact direction a gravitational wave came from, in the same way multiple listening stations can locate the source of a sound, which a single detector alone cannot do.

Quick revision points

• General relativity: gravity is the curvature of space-time caused by mass, not a simple attractive force. Light bending near massive objects and massive objects warping space-time are both genuine, compatible predictions of the theory, not competing claims.
• A gravitational wave is a ripple in space-time, radiating at the speed of light, from violent events like black hole or neutron star mergers, predicted by Einstein in 1916.
• LIGO detects them via laser interferometry: splitting a beam down two long perpendicular arms and looking for the minuscule imbalance a passing wave causes. 2016 announcement: first direct detection, confirming a black hole merger and confirming general relativity's prediction.
• eLISA: a proposed space-based version, three spacecraft in a triangular formation with arms millions of kilometres long, avoiding ground seismic noise and reaching much lower-frequency sources than LIGO.
• LIGO-India: India's own facility, ~Rs 2,600 crore, at Hingoli, Maharashtra, built with Caltech/MIT's LIGO Laboratory. A geographically separate detector allows triangulating a wave's source direction, which one detector alone cannot do.