Gravitational Waves: LIGO's GW150914, Chirp Signals, and LISA

GW150914 Lasted 0.2 Seconds and Contained More Power Than All the Stars in the Observable Universe

At 5:51 a.m. EDT on September 14, 2015, the LIGO detector in Livingston, Louisiana, registered a signal. Seven milliseconds later, the Hanford, Washington detector registered the same signal — the travel time for a gravitational wave traveling at the speed of light between the two sites 3,002 km apart. In 0.2 seconds, the signal swept from 35 Hz to 150 Hz, increasing in frequency and amplitude before suddenly ending. The peak gravitational wave luminosity was approximately 3.6 × 10^49 watts — roughly 50 times the combined luminosity of all stars in the observable universe, concentrated into a fifth of a second. Two black holes, each roughly 30 solar masses, had merged 1.3 billion light-years away. The merger radiated approximately 3 solar masses worth of energy as gravitational waves: E = mc² gives 5.4 × 10^47 joules. This signal — GW150914 — confirmed a prediction of general relativity made exactly 100 years earlier by Albert Einstein. Rainer Weiss, Barry Barish, and Kip Thorne received the 2017 Nobel Prize in Physics for the detection.

How LIGO Detects Spacetime Ripples

A gravitational wave is a propagating perturbation in the curvature of spacetime. It stretches space in one transverse direction while squeezing it in the perpendicular direction, then alternates — a quadrupolar oscillation. This strain h is defined as h = ΔL/L, where ΔL is the change in length and L is the detector arm length. For GW150914, the peak strain was approximately 10^−21 — one part in 10^21. LIGO's 4-kilometer arms changed length by approximately 4 × 10^−18 meters — about four-thousandths the diameter of a proton.

Reading the Chirp Signal

The characteristic chirp waveform — rising in frequency and amplitude — contains an enormous amount of physical information. Matched-filter analysis compares the observed signal against a bank of template waveforms calculated using general relativity for different combinations of mass, spin, and orbital parameters. The best-fitting template reveals:

• Chirp mass: A specific combination of the two component masses that dominates the waveform evolution during inspiral — determined to ~1% precision from GW150914
• Component masses: 35.6 M☉ and 30.6 M☉ for GW150914 (with measurement uncertainty)
• Final black hole mass: 63.1 M☉ (the remaining 3.1 M☉ was radiated as gravitational waves)
• Luminosity distance: ~410 Mpc (1.3 billion light-years), inferred from signal amplitude
• Spin parameters: Weakly constrained for GW150914; better measured for other events

GW170817: The Multimessenger Revolution

On August 17, 2017, LIGO and Virgo detected a signal fundamentally different from all previous events. The chirp lasted 100 seconds — not 0.2 seconds — sweeping from 24 Hz up before the signal ended. The masses were tiny: 1.17 and 1.36 solar masses. A neutron star merger. 1.7 seconds after the merger, NASA's Fermi satellite detected a short gamma-ray burst from the same sky location. Then, optical telescopes worldwide found an optical transient — a kilonova — in galaxy NGC 4993, 40 megaparsecs (130 million light-years) away. Over weeks, X-ray, radio, ultraviolet, and infrared observations tracked the aftermath.

GW170817's scientific yield was unprecedented:

• Confirmed that neutron star mergers produce short gamma-ray bursts (long-debated)
• Demonstrated that r-process nucleosynthesis — the origin of heavy elements including gold, platinum, and uranium — occurs in neutron star mergers
• Constrained the neutron star equation of state from the tidal deformability measurement in the gravitational wave signal
• Provided an independent Hubble constant measurement: H₀ = 70⁺¹²₋₈ km/s/Mpc
• Confirmed that gravitational waves travel at the speed of light to within 1 part in 10^15

LISA: Gravitational Wave Astronomy in Space

LIGO's frequency band — roughly 10 Hz to several kHz — is set by its arm length and seismic noise floor. Gravitational waves from supermassive black hole mergers (millions to billions of solar masses), extreme mass ratio inspirals (a stellar-mass object spiraling into a supermassive black hole), and cosmological sources occur at millihertz frequencies — inaccessible from Earth. The Laser Interferometer Space Antenna (LISA), approved by ESA in 2024 for launch in the late 2030s, will consist of three spacecraft in a triangular formation 2.5 million km apart, exchanging laser beams to measure gravitational wave strains at frequencies from 0.1 mHz to 1 Hz. LISA is expected to detect thousands of gravitational wave sources — including thousands of binary white dwarf systems in our own galaxy — and observe the mergers of supermassive black holes out to the edge of the observable universe.