---
title: "Linearised Gravity and Gravitational Waves"
description: "Small departures from flat spacetime obey a wave equation, so a changing mass distribution sends ripples of geometry that stretch one transverse direction while squeezing the other. Detectors measure "
canonical: https://lightmysky.com/learn/science/linearised-gravity-and-gravitational-waves-mt_z0tMZDGMHi
source: https://lightmysky.com/learn/science/linearised-gravity-and-gravitational-waves-mt_z0tMZDGMHi.md
retrieved: 2026-09-12
---

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# Linearised Gravity and Gravitational Waves

Small departures from flat spacetime obey a wave equation, so a changing mass distribution sends ripples of geometry that stretch one transverse direction while squeezing the other. Detectors measure that strain, and it is tiny because geometry couples weakly to matter.

Subject: Science · Area: Astronomy & Astrophysics · Ages 23 to 24
Page: https://lightmysky.com/learn/science/linearised-gravity-and-gravitational-waves-mt_z0tMZDGMHi

## Ready when they can

- Explains why the linearised equations resemble electromagnetic radiation with a different source
- Describes what a passing wave does to a ring of freely floating test masses
- Says why there is no monopole or dipole gravitational radiation

## Lesson: Ripples in the shape of space

Mass and energy warp the spacetime around them, and objects follow the straightest paths through that warped geometry. Small departures from flatness obey a wave equation, so accelerating lumps send ripples outward at the speed of light. The derivation reads against electromagnetic radiation, with a different source doing the shaking.

**Example.** Not every motion radiates. A smooth steady change stays silent, while lumpy unsymmetric acceleration sings: two black holes or neutron stars spiraling together, or a massive star collapsing. Steady currents broadcast nothing, and the same holds for gravity.

A passing wave never pushes you forward; it stretches one sideways direction while squeezing the other, then swaps, over and over. A ring of freely floating marbles becomes an oval, first tall and thin, then wide and flat. Even balanced pulsing and wobbling stay silent as monopole and dipole: only uneven quadrupole lumps qualify.

The effect is tiny because geometry couples to matter very weakly. Even waves from colliding black holes shift a several kilometer detector arm by far less than a proton width. LIGO bounces laser light between mirrors to measure that strain, and in 2015 it caught merging black holes, opening listening alongside looking.

**Recap.** Moving lumps shake spacetime at light speed, squeeze rings sideways, and whisper so faintly that lasers must strain to hear.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [Curvature and the Einstein Field Equation](https://lightmysky.com/learn/science/curvature-and-the-einstein-field-equation-mt_1CxE6kRDt_)
- [The Schwarzschild Solution: Orbits, Redshift and the Horizon](https://lightmysky.com/learn/science/the-schwarzschild-solution-orbits-redshift-and-the-horizon-mt_dNZpRDeZUE)
- [Radiation from an Accelerating Charge](https://lightmysky.com/learn/science/radiation-from-an-accelerating-charge-mt_Qr0rMEOYAg)