LIGO

Interferometer Lab

Space-time / light / measurement

Model & sources

DIFFERENTIAL LENGTHam

OPTICAL SIGNAL PHASEnrad

ARM CIRCULATING POWERkW

Click the scene to capture input · drag to orbit · scroll or pinch to zoom · right-drag to pan

STRAIN → CALIBRATED SIGNAL

— injected- - reconstructed·· residual ·

RECOMBINATION AT THE BSδphase ×10⁸

Signal phase magnified · DC bias schematic

The measurement chain

Effective differential length

↓ phase response + local oscillator

Output power modulation

↓ photodiode · calibration

Reconstructed strain

Assembling the interferometer

Embedded WebGL · no network required

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2015 · Physics

LIGO Gravitational-Wave Detector

Two four-kilometer laser arms, recycling mirrors, a dark port and a synthetic compact-binary waveform make differential strain measurable.

A passing gravitational wave stretches one of LIGO’s four-kilometer arms while squeezing the perpendicular one. The instrument is a modified Michelson interferometer operated near a dark fringe: the returning laser fields are adjusted to produce little light at the readout photodetector, and differential arm motion changes that output.

This laboratory is a reduced-order model of the published Advanced LIGO topology. It projects a synthetic plus, cross, circular or leading-order 10 + 10 solar-mass inspiral waveform onto both arms, solves stored power in the recycled Fabry–Pérot cavities, forms a DC readout with a small offset and reconstructs strain through an idealized 15 Hz feedback loop. Break an arm, detune the cavities or remove the offset and the linear calibration is marked unavailable.

Everything shown is a teaching model, not observatory data: the synthetic compact-binary waveform has no merger or ringdown, detector noise is a deterministic illustrative mixture, arms are compressed and mirror motion is magnified for visibility, while every plotted value stays in physical units.

The 2015 detection of GW150914 helped earn the 2017 Nobel Prize in Physics and opened gravitational-wave astronomy. LIGO is one of the most sensitive displacement-measurement systems ever constructed.

Specifications

Instrument: Modified Michelson interferometer with 4 km arm cavities
First detection: GW150914 — 14 September 2015
Representative strain: h ≈ 10⁻²¹ (a few attometers over 4 km)
Modeled: Recycled cavity storage, DC readout, idealized feedback and calibration validity
Synthetic: Injected waves, illustrative noise and magnified motion; not measured LIGO data

All exhibits

LIGO

Interferometer Lab

Numerical checks of the reduced-order model

In the current issue