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How do gravitational waves work?

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Quick answer

Gravitational waves are ripples in the fabric of spacetime, generated when massive objects accelerate or merge. They propagate outward at the speed of light, carrying energy. Detectors on Earth measure the minuscule stretching of space they cause, opening a new window on violent cosmic events.

Gravitational waves are ripples in spacetime produced when massive objects accelerate, such as two black holes spiraling together. They stretch and squeeze space itself as they travel outward at the speed of light.

Step by step

  1. Massive objects like black holes or neutron stars orbit each other and gradually spiral inward.
  2. As they accelerate, they disturb the surrounding spacetime, emitting gravitational waves.
  3. These waves travel outward at the speed of light, carrying energy away from the system.
  4. When a wave reaches Earth, it alternately stretches and squeezes space in perpendicular directions.
  5. Laser interferometers measure the resulting tiny changes in arm lengths to detect the wave.

Key things to understand

  1. Any accelerating mass distorts spacetime, but only extreme events like merging black holes create detectable waves.
  2. The waves travel outward at the speed of light, carrying energy away from the source.
  3. As a wave passes, it alternately stretches and squeezes space in perpendicular directions.
  4. Detectors like LIGO use laser interferometers to measure these tiny length changes.
  5. The first direct detection in 2015 confirmed a prediction made by Einstein a century earlier.

Labelled diagram

  1. Source

    Two massive objects spiral together, emitting gravitational waves.

  2. Wave travels

    The ripples move outward at the speed of light.

  3. Wave reaches Earth

    Space stretches and squeezes as the wave passes.

  4. Detector

    Laser beams in perpendicular arms change length slightly.

  5. Signal

    Interference pattern shifts, revealing the wave.

How a gravitational wave is detected

Real-world examples

  • The first detection, GW150914, came from two black holes merging about 1.3 billion light-years away.
  • LIGO's two detectors in the United States, in Hanford and Livingston, confirmed the signal almost simultaneously.
  • The Virgo detector in Italy and KAGRA in Japan joined the network to pinpoint sources more accurately.
  • Future space-based detector LISA will observe lower-frequency waves from supermassive black hole mergers.

Common mistake

✗ Myth: Gravitational waves are sound waves or require a medium like air to travel.

✓ Fact: Gravitational waves are ripples in spacetime itself and do not need a medium; they can travel through the vacuum of space.

People also ask

What causes gravitational waves?
Gravitational waves are caused by accelerating masses, especially when massive objects like black holes or neutron stars orbit and merge. The acceleration distorts spacetime, sending out ripples that propagate at the speed of light.
How fast do gravitational waves travel?
Gravitational waves travel at the speed of light in a vacuum, about 300,000 kilometres per second. This speed was confirmed by the simultaneous detection of gravitational waves and gamma rays from a neutron star merger in 2017.
Who discovered gravitational waves?
Albert Einstein predicted gravitational waves in 1916 from his general theory of relativity. They were first directly detected in 2015 by the LIGO collaboration, a discovery that earned the 2017 Nobel Prize in Physics.
How do we detect gravitational waves?
Detectors such as LIGO split a laser beam along two perpendicular 4-kilometre arms and recombine it. A passing gravitational wave changes the arm lengths by a tiny fraction of a proton's width, shifting the interference pattern.
Why are gravitational waves important?
They carry information about violent events, like black hole mergers, that emit no light. This gives astronomers a new way to observe the universe, complementing electromagnetic telescopes.
Do gravitational waves lose energy as they travel?
Yes, the wave's amplitude decreases with distance because its energy spreads over a larger area. However, the waves themselves are not absorbed by matter and can cross the universe almost unimpeded.
Can gravitational waves affect everyday objects?
The stretching they cause is incredibly small—far less than the size of an atom—so everyday objects are unaffected. Only sensitive instruments like LIGO can detect them.

Part of the guide: Space & Astronomy →