A passing gravitational wave changes distance only by a tiny fraction h ≈ 10⁻²¹ of whatever length you measure — so the absolute displacement ΔL = h·L scales directly with arm length L. A 4 km ground arm moves by a few attometres; a 2.5-million-km space arm moves by picometres — thousands of times more, easily resolved by laser interferometry.
Ground interferometers also sit on a shaking planet: seismic motion and gravity-gradient noise swamp anything below a few Hz, no matter how good the lasers are. Free-flying spacecraft feel none of that, so LISA's noise floor stays low all the way down to micro-hertz — right where slowly-orbiting supermassive black holes live, a band ground detectors can never reach.
- Frequency — sets the simulated source's oscillation rate; sweep it from the LISA band down at µHz–mHz to the LIGO band up at Hz–kHz.
- Source strength — scales the gravitational-wave strain amplitude, shifting both signals up or down together.
- Watch the arms: when a detector's signal is buried under its own noise floor, its arms stay essentially still; once the signal clears the floor, a clear, measurable stretch-and-squeeze appears.