What Are Gravitational Waves?
QUICK ANSWER
Gravitational waves are ripples in the fabric of spacetime caused by massive accelerating objects. Predicted by Einstein in 1916, they were first directly detected by LIGO in September 2015 from two merging black holes. The discovery opened a new way to observe the universe and won the 2017 Nobel Prize.
Gravitational waves are one of the most remarkable predictions of Einstein's general relativity. They're literal ripples in the fabric of space and time, produced when massive objects accelerate. After being predicted in 1916, gravitational waves remained theoretical for nearly a century. Their first direct detection in 2015 opened a whole new way of observing the universe.
What are gravitational waves?
Ripples in spacetime that travel at the speed of light. According to NASA, gravitational waves are produced when massive objects accelerate, particularly when they orbit each other or collide. Einstein predicted them in 1916 as a consequence of his general theory of relativity, which describes gravity as the curvature of spacetime. When mass moves through spacetime, it creates ripples that propagate outward, much like ripples on a pond. Unlike light, these waves can pass through anything; nothing blocks them.
How were they detected?
By the LIGO observatory in 2015. The Laser Interferometer Gravitational-Wave Observatory (LIGO), funded by the US National Science Foundation, made the first direct detection on September 14, 2015 (announced February 2016). The signal, called GW150914, came from two black holes (about 30 solar masses each) merging into one about 1.3 billion light years away. LIGO uses two L-shaped detectors with 4 km arms in Washington State and Louisiana. The detectors measure changes in arm length smaller than 1/10,000 the width of a proton, caused by passing waves.
What have we learned from them?
More than expected. Since 2015, LIGO and partner detectors (Virgo in Italy, KAGRA in Japan) have detected over 100 gravitational wave events, mostly from merging black holes and a few from merging neutron stars. The 2017 detection of a neutron star merger (GW170817) was particularly important because it was also observed in light, allowing astronomers to study the same event in two ways. Gravitational waves have confirmed key predictions of general relativity, revealed populations of black holes we didn't know existed, and provided new measurements of cosmic expansion.
Why are they important?
They reveal phenomena invisible to light. Many cosmic events (like black hole mergers) emit little or no light, so they were essentially undetectable before gravitational wave astronomy. Gravitational waves let us study these events directly. They've also confirmed predictions of general relativity to extraordinary precision. The 2017 Nobel Prize in Physics went to LIGO's founders (Rainer Weiss, Kip Thorne, and Barry Barish) for the achievement. Future detectors like LISA, a planned NASA/ESA space mission, will detect even more events at lower frequencies.
Gravitational waves are ripples in spacetime caused by massive accelerating objects, predicted by Einstein in 1916 and first directly detected by LIGO in September 2015. The detection won the 2017 Nobel Prize in Physics and opened a new way of observing the universe. Over 100 events have been detected since 2015, revealing black hole and neutron star mergers invisible at other wavelengths. Gravitational wave astronomy is one of the most rapidly developing fields in physics.
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