What the detector measures

A laser is split into perpendicular paths, reflected along the arms, and recombined. A change in optical path difference changes the interference signal. Strain h=ΔL/L is the differential arm-length change divided by a reference length; it is not a direct reading of a mirror’s absolute motion. The detector’s optical-mechanical response and control system shape the output, so calibration is part of the measurement. LIGO’s calibration study describes validation with a photon radiation-pressure actuator and optical/radio-frequency references, and reports uncertainty for GW150914.

Read the GW150914 numbers

The LIGO–Virgo discovery paper reports coincident signals in 2015, a frequency sweep from about 35 to 250 Hz, peak strain near 1.0×10⁻²¹, and matched-filter signal-to-noise ratio 24. In a plot, frequency describes the changing waveform, strain the fractional arm response, and SNR how prominent the template-matched signal was against noise under that analysis. No one number confirms an event by itself: researchers compare sites, investigate instrumental and environmental disturbances, and test analysis consistency. The LIGO event summary explains this context.

A useful reading order is to check the axis and whether the plot is calibrated strain, then ask whether the signal is consistent across sites, and only then interpret the noise and waveform fit. Do not infer source mass or distance directly from a waveform’s height.

The two arms can also respond to ordinary disturbances. Scientists therefore examine differential readout together with environmental and instrument records; a simple interferometer diagram does not show the full noise-rejection and calibration work.

Limits

Strain is reconstructed through a calibration model. Estimating source mass or distance requires waveform models and further statistical analysis. These are GW150914 results, not universal detector thresholds.

Sources