Gravitational Waves: How Are They Caused and Detected?

On September 14, 2015 at 09:50:45 UTC, two detectors of the LIGO observatory simultaneously recorded a fleeting signal lasting less than 0.2 seconds. This was the first direct detection of gravitational waves — a century after Einstein predicted them in his General Theory of Relativity and long after most physicists thought they might never be detected. The event, designated GW150914, came from two black holes — 29 and 36 solar masses — merging 1.3 billion light-years away.

The Strain: LIGO detected a change in arm length of about 1/1000th the diameter of a proton — a strain of h ≈ 10^-21. The 4-km arms shrank and stretched by less than a proton's width to reveal one of the most energetic events in the observable universe.

What Are Gravitational Waves?

In Einstein's General Relativity (1915), mass curves spacetime. Accelerating masses — especially very massive, compact objects in extreme motion — ripple that curvature outward at the speed of light. These ripples are gravitational waves: transverse waves that alternately stretch and compress distances in the plane perpendicular to their travel direction (the two polarizations are called plus + and cross ×). Energy is carried away from the source, causing orbital decay and eventual merger.

How LIGO Works

LIGO is a Michelson interferometer with two 4-km arms arranged at right angles. A laser beam is split and sent down each arm, reflecting off mirrors (test masses) suspended as pendulums. If a gravitational wave passes, it differentially changes the arm lengths. When the beams recombine, any path difference causes destructive interference to differ slightly from perfect cancellation — a detectable signal in the photodetector. The system must isolate the mirrors from virtually all external vibrations to reach sensitivity of 10^-21.

Sept 14, 2015First GW detection (GW150914)

1.3B lyDistance to GW150914 source

3 M☉ energyRadiated as GWs in ~0.2 sec

2017Nobel Prize: Weiss, Barish, Thorne

GW170817: The First Multimessenger Event

On August 17, 2017, LIGO and Virgo detected GW170817 — the merger of two neutron stars. Unlike black hole mergers (which produce no light), this event generated a kilonova: a bright optical transient from heavy element synthesis (gold, platinum, strontium) and a short gamma-ray burst observed 1.7 seconds later. Over 70 observatories worldwide measured it. This launched the era of multimessenger astronomy: combining gravitational waves with electromagnetic signals for a more complete picture of cosmic events.

«With this discovery, we humans are embarking on a marvelous new quest: the quest to explore the warped side of the universe.»

— Kip Thorne, Nobel Prize lecture, 2017

Future: LISA and Next Generation

Ground-based detectors are limited by seismic noise at low frequencies. The Laser Interferometer Space Antenna (LISA), a joint ESA/NASA mission planned for launch around 2037, will consist of three spacecraft forming a triangle with 2.5 million km arms in space. LISA will detect supermassive black hole mergers across the observable universe, extreme mass ratio inspirals (EMRIs), and possibly signals from the early universe. Next-generation ground detectors (Einstein Telescope in Europe, Cosmic Explorer in the US) will push sensitivity by a factor of 10, enabling detections to the edge of the observable universe.

gravitational waves LIGO black holes Einstein spacetime astrophysics neutron stars general relativity

Sources:

Abbott et al. (LIGO Scientific Collaboration) – PRL 116, 2016: Observation of GW150914
Abbott et al. – PRL 119, 2017: GW170817 Multimessenger Observations
Thorne, K. – Nobel Prize Lecture 2017: Nobel Prize Archive
ESA – LISA Mission Overview