Discovery
| Parameter | Value |
|---|---|
| Detection date | September 14, 2015, 09:50:45 UTC |
| Signal | GW150914 |
| Source | Merger of two black holes (36 + 29 M☉ → 62 M☉) |
| Distance | 1.3 billion light-years (z ≈ 0.09) |
| Energy radiated | 3 M☉c² ≈ 5.4 × 10⁴⁷ J in 0.2 s |
| Nobel Prize in Physics | 2017 (Weiss, Thorne, Barish) |
Technical Explanation
1. Laser interferometry — LIGO is a Michelson interferometer with 4 km arms. A Nd:YAG laser (1,064 nm, 200 W, amplified to 750 kW in Fabry-Pérot cavities) is split into two beams traveling the perpendicular arms and recombining on a photodetector. With no signal, the beams interfere destructively ("dark port"). A gravitational wave stretches one arm and compresses the other, creating a phase shift proportional to the strain h = ΔL/L.
2. Extreme sensitivity — The strain measured for GW150914 is h ≈ 10⁻²¹, corresponding to ΔL ≈ 4 × 10⁻¹⁸ m over 4 km. To achieve this sensitivity: 40 kg mirrors of ultra-pure fused silica, quadruple pendulum suspension (seismic isolation >10¹⁰ above 10 Hz), ultra-high vacuum (10⁻⁷ Pa in the beam tubes), and quantum squeezing (reducing shot noise below the standard quantum limit).
3. Noise filtering — Noise sources include: seismic (<10 Hz), mirror thermal (10–100 Hz), photon shot noise (>100 Hz), and environmental disturbances (wind, trains, ocean waves). The GW150914 signal sweeps 35–250 Hz in 0.2 seconds (chirp) — a characteristic inspiral → merger → ringdown pattern predicted by numerical relativity.
4. Inter-detector coincidence — LIGO operates two sites (Hanford, Washington and Livingston, Louisiana) separated by 3,002 km. The signal must appear in both detectors within ≤10 ms (light travel time). GW150914 arrived at Livingston 6.9 ms before Hanford — consistent with the source direction.
Why It Worked
The key is vibrational isolation: the mirrors are suspended by four stages of pendulums with a natural frequency of ~0.5 Hz, passively filtering vibrations above a few Hz by a factor of >10¹⁰. Power recycling (Fabry-Pérot cavity) boosts the effective laser power from 200 W to 750 kW, reducing shot noise. The coincidence between two sites 3,002 km apart eliminates local false positives.
The detection came exactly 100 years after Einstein published general relativity. The signal was so strong it was visible to the naked eye in the raw data (no filtering needed). The LIGO team initially suspected a blind injection — a test procedure used to verify the analysis pipeline. It took confirmation that no injection had been scheduled that day.
Causal Chain
General relativity (Einstein, 1915) → Gravitational wave prediction (1916) → Weber bars (1960s, unconfirmed) → Interferometric concept (Weiss, 1972) → LIGO funded (NSF, 1992) → Initial LIGO (2002–2010, no detections) → Advanced LIGO (upgrade 2010–2015) → GW150914 (September 14, 2015) → Nobel 2017 → Neutron star merger GW170817 (first multi-messenger event, 2017) → Gravitational-wave astronomy
Anecdote
GW150914 was detected on September 14, 2015 — exactly 100 years after Einstein published general relativity. The signal was so strong that it was visible to the naked eye in the raw data (without filtering). The LIGO team initially suspected a blind injection test — a procedure used to verify the analysis chain. It took several checks to confirm that no injection had been scheduled that day.
Sources
References verified during the August 2026 fact-checking audit: these are the pages
against which this bulletin's claims were checked.