Phenomena · Space Codex
Gravitational waves
Merging black holes and neutron stars shake spacetime itself.
Ripples in spacetime
At a glance
Key points
Since the first detection in 2015, LIGO, Virgo and KAGRA have confidently recorded 390 events.
- 390 confident detections
- First heard in 2015
- Confirmed Hawking's area theorem in 2025
Hearing collisions
As two black holes spiral together, the wave's frequency and strength rise in a 'chirp' that ends at merger, followed by a ringdown as the new black hole settles. LIGO measures stretches of spacetime smaller than a thousandth of a proton's width.
Light and waves together
Black-hole mergers emit no light. The neutron-star merger GW170817 did, letting telescopes watch heavy elements such as gold forming.
A century-long hunt
Einstein's general relativity predicted gravitational waves in 1916, but the effect seemed far too small ever to measure. The first indirect evidence came from the Hulse–Taylor binary pulsar, whose orbit shrinks exactly as wave emission predicts. LIGO's upgraded detectors caught GW150914 in September 2015, days before their first formal observing run began, and the detection earned Rainer Weiss, Barry Barish and Kip Thorne the 2017 Nobel Prize in Physics.
How the detectors work
Each LIGO detector splits a laser beam down two perpendicular 4 km arms and recombines it. A passing wave stretches one arm and squeezes the other by a tiny fraction of a proton's width, shifting how the beams interfere. Two sites, in Washington and Louisiana, catch the same signal milliseconds apart, which rules out local noise and helps locate the source; Virgo in Italy and KAGRA in Japan sharpen the aim.
What comes next
The next observing run, IR1, is planned to begin in early to mid-November 2026 with both LIGO detectors, Virgo and KAGRA joining as they can. Lower frequencies need other tools: pulsar timing arrays reported evidence of a nanohertz background in 2023, and ESA's LISA, three spacecraft 2.5 million km apart, is planned to launch in 2035.
Missions
- 2015–
LIGO–Virgo–KAGRA
NSF / EGO / NAOJActiveO4 ran 24 May 2023 – 18 Nov 2025; a six-month run (IR1) is due to start in late 2026.
Related entries
- Black holeEvent horizon · gravity light cannot escape
- Neutron starsCity-sized stellar cores
Sources
- LIGO/Virgo 2016, PRL 116:061102 (arXiv) (opens in a new tab)
- ESA: LISA gets go-ahead (opens in a new tab)
- Nobel Prize in Physics 2017: press release (opens in a new tab)
- LIGO/Virgo 2016, PRL (arXiv) (opens in a new tab)
- LIGO: O4b catalog (GWTC-5.0) (opens in a new tab)
- LIGO: O4 wraps up (opens in a new tab)
- LIGO observing plans (opens in a new tab)
- LVK 2025 (arXiv) (opens in a new tab)
- LVK 2025, PRL 135:111403 (opens in a new tab)
Last reviewed against these sources. Built from public NASA, ESA and JPL data; not affiliated with or endorsed by any agency.