Measure a passing change in geometry

Gravitational waves are propagating disturbances in spacetime — open definition geometry. Systems such as orbiting compact binaries can radiate energy in them. As a binary loses orbital energy, its motion can evolve toward a merger. Gravitational waves are distinct from electromagnetic waves and do not require air or another sound-carrying medium to travel through space.

A detected signal may be converted into an audible representation for people to hear. That rendering is not proof that ordinary sound traveled across the vacuum from the source. A graph, a sonification — open definition and the underlying calibrated time series are different presentations of evidence. Each is useful when its relationship to the measurement is explicit.

Laser interferometers compare signals traveling along different arms. A passing wave can produce a tiny differential response related to changes in the arms’ effective lengths. Strain — open definition is a dimensionless fractional change, often introduced through h ≈ ΔL/L in a simplified aligned example. A complete detector response also depends on direction, polarization, frequency and the optical configuration.

Ground motion, instrumental disturbances and other noise can also change a detector signal. Comparing separated instruments, checking calibration and environmental channels, and evaluating the waveform against background events are central to a detection claim. A matching-looking pair of curves is useful evidence, but it does not remove the need to quantify whether noise could plausibly account for them.

For an invented aligned example, take a 4,000 m reference length and fractional change 3 × 10⁻²¹. The corresponding change is 1.2 × 10⁻¹⁷ m. The strain itself has no metre unit: it is a ratio. This multiplication is a scale illustration, not a full interferometer — open definition simulation or a quoted observation of a particular merger.

Consider a second check: two hypothetical detectors are separated by a straight-line distance of 6,000 km. Using 300,000 km/s, a plane wave traveling at c cannot have an arrival-time difference larger in magnitude than 0.020 s between them. A candidate separated by 0.030 s fails that simple common-wave timing test under the stated assumptions, even if its displayed shapes look similar.

Test a common-wave timing claim

The LIGO collaboration’s GW150914 account connects calibrated detector strain with a compact-binary merger interpretation. The waveform and its arrival at separate sites provide evidence beyond a dramatic illustration of two black holes. Parameters such as masses and distance are inferred through modeling, and their uncertainties belong with the reported result.

The course ends with a shared method rather than a single exotic object. State the frame, separate received signals from events, keep track of proper time — open definition and coordinate time, identify the measurement and test alternatives. Relativity becomes useful when it connects those definitions to predictions that clocks, images and interferometers can challenge.