Linearize Einstein's equations around Minkowski: $g_{\mu\nu} = \eta_{\mu\nu} + h_{\mu\nu}$ with $|h| \ll 1$. Choosing transverse-traceless gauge, $h_{\mu\nu}$ propagates as a wave:

with two physical polarizations $h_+$ and $h_\times$. A ring of free-falling test masses oscillates:

• $h_+$: ring stretches along $x$, contracts along $y$, swapping in half-period.
• $h_\times$: same pattern, rotated 45°.

The strain $h \sim \Delta L / L$ is dimensionless. From an inspiraling compact binary, $h \sim G M / (c^2 r) \cdot (v/c)^2$. For GW150914 (Sept 14, 2015) LIGO measured $h \sim 10^{-21}$ — a fractional length change one ten-thousandth the width of a proton.

The first detection (binary black-hole merger) won the 2017 Nobel Prize; the field is now routinely yielding mergers of black holes and neutron stars.

Interactive: ring of particles under $h_+$ or $h_\times$