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Deep Space

Gravitational Waves: Read a LIGO Chirp

Understand what a laser interferometer measures and how scientists check that a brief signal came from space.

By AstroboatPublished 2 min read

Prepared with AI assistance using the primary sources linked below. How Astroboat works

A ripple in spacetime is not sound

Accelerating massive systems such as orbiting compact objects can generate gravitational waves. A passing wave changes the relative distances between freely suspended test masses in a characteristic pattern. The wave travels at light speed, but it is not light and does not need air. An audio chirp made from the signal is a representation of the measured waveform, not sound recorded by a microphone in space.

Reference: LIGO Lab: What Are Gravitational Waves?; LIGO Collaboration: The GW150914 Detection

The instrument measures a fractional change

LIGO compares laser light that travels along two perpendicular arms. A wave can alter their relative lengths, changing how the returning light interferes. Strain is a fractional measure of the distortion. A strain of one part in 10 to the 21st corresponds to a characteristic length scale of about 4 times 10 to the minus 18 metres over a 4-kilometre arm. The exact response depends on wave direction and polarization.

Reference: LIGO Lab: What Are Gravitational Waves?; LIGO Collaboration: The GW150914 Detection

Read frequency and time, not volume alone

In a compact binary inspiral, the orbit tightens and the wave frequency rises before merger. That upward sweep is the chirp. A plot may show filtered data and a model waveform together, so read the legend before treating every small wiggle as a feature of the source. Researchers compare the measured pattern with predictions to estimate properties such as the objects' masses.

Reference: LIGO Collaboration: The GW150914 Detection

A second detector helps distinguish a signal from noise

The first direct detection, GW150914, was seen in both LIGO detectors. Researchers checked whether the patterns and arrival times were consistent and whether local disturbances or instrumental effects could explain them. When reading about another event, look for which detectors contributed and whether the report is an initial candidate or a vetted result. A waveform image without that context is not the full evidence.

Reference: LIGO Collaboration: The GW150914 Detection; LIGO Collaboration: Vetting GW150914

Sources and further reading