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Black Holes & Spacetime

Gravitational Waves

Gravitational waves are ripples in spacetime itself, produced when massive objects such as black holes or neutron stars merge.

Unlike light, they pass through matter almost unimpeded. They were first detected directly in 2015.

  1. Observe

    A tiny, rhythmic stretching and squeezing of space that speeds up and then stops.

  2. Measure

    Laser interferometers compare two perpendicular arms to detect changes smaller than a proton's width.

  3. Model

    General-relativity simulations predict the signal for different masses and spins.

  4. Explain

    Massive objects merged, and spacetime rang like a bell.

What is it?

Einstein's general relativity predicts that accelerating masses disturb spacetime and send out waves at the speed of light. Almost every motion produces them, but only extreme events, like two black holes or neutron stars spiraling together, make waves strong enough to detect.

The first direct detection, GW150914, was recorded by the LIGO detectors on 14 September 2015 and announced in 2016. It came from two black holes, each roughly thirty times the Sun's mass, merging about 1.3 billion light-years away. Since then detectors have recorded on the order of a hundred or more merger candidates.

How does it happen?

  1. Stage 1
    Compact objects orbit each other
  2. Stage 2
    They spiral inward, losing energy
  3. Stage 3
    They merge
    Peak wave emission
  4. Stage 4
    Detectors measure the signal
    Ripples pass through Earth

What do we actually observe?

A chirp
The signal rises in frequency and amplitude as the objects spiral in, then ends after the merger and a brief ringdown.
Strain
The effect is a fractional change in length of order one part in a billion trillion (10⁻²¹).
Sky position
A network of several detectors is needed to localize the source, so telescopes can look for light.

How do scientists detect it?

  • LIGO (Hanford and Livingston). Two 4-kilometer laser interferometers in the United States.
  • Virgo and KAGRA. Detectors in Italy and Japan that improve sky localization.
  • Pulsar timing arrays. Sensitive to much lower-frequency waves from supermassive black-hole systems; evidence for a background was reported in 2023.
  • LISA (planned). A space-based detector under development by ESA with NASA participation.

Why does it matter?

  • A new sense: gravitational waves reveal events that emit little or no light, such as black-hole mergers.
  • Tests of general relativity in the strongest gravity we can observe.
  • Multi-messenger astronomy: in 2017 a neutron-star merger was seen in gravitational waves and across the electromagnetic spectrum.

The SpaceWeatherTracker connection

Unlike ordinary space weather, gravitational waves are disturbances in spacetime itself, not streams of particles or changes in magnetic fields. They do not affect satellites or power grids, but they extend the same observe, measure, model and explain approach to the most extreme events.

This is an explainer. SpaceWeatherTracker does not show live data for this phenomenon; see the “Space Phenomena Now” section on the hub for what is currently monitored.

Related phenomena

Primary family: Black Holes & Spacetime

Gravitational Waves: common questions

What are gravitational waves?

Ripples in the fabric of spacetime, predicted by Einstein and produced by accelerating masses, strongest when compact objects such as black holes merge.

What creates gravitational waves?

Any accelerating mass, but detectable waves come from extreme events: merging black holes, merging neutron stars, and possibly other violent phenomena.

How are gravitational waves detected?

Laser interferometers such as LIGO split a laser into two long perpendicular arms and measure minuscule changes in their relative length when a wave passes.

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