LIGO's groundbreaking detection of gravitational waves marked a new era in astrophysics, deepening our understanding of black holes and their dynamic mergers.
On September 14, 2015, shortly after LIGO's activation with enhanced sensitivity, a gravitational wave rippled through Earth. Generated by the collision and merger of two massive black holes over a billion light-years away—far beyond the Milky Way—this signal traveled at light speed to reach us. LIGO's detectors registered minuscule arm expansions and contractions, shifting the laser interference pattern and confirming humanity's first direct observation of gravitational waves. By 2018, 11 events were cataloged, with 10 stemming from black hole mergers.
LIGO conducted two observing runs: the first from September 12, 2015, to January 19, 2016, and the second—with refined sensitivity—from November 30, 2016, to August 25, 2017. Mid-run, Italy's VIRGO detector joined, enabling three-way triangulation for precise sky localization.
LIGO paused operations for upgrades ahead of its next campaign in spring 2019. On November 30, the LIGO Scientific Collaboration released refined analyses targeting mergers between 1 and 100 solar masses.
Of the 11 detections, 10 involved black hole mergers; only GW170817 was a neutron star merger, at 130-140 million light-years. The most distant, GW170729, originated from a site now 9 billion light-years away due to cosmic expansion.
Related: How do gravitational waves escape from black holes?
These events span the lightest (GW170817: 1.46 and 1.27 solar masses) to heaviest (GW170729: 50.6 and 34.3 solar masses) mergers detected.
1. The most massive black hole mergers produce the strongest, most detectable signals
Gravitational waves offer a key advantage over light: their amplitude diminishes linearly with distance, unlike light's inverse-square law. A merger 10 times farther yields 10% of the signal strength, enabling detection of colossal events across cosmic scales. Yet, no mergers exceed ~50 solar masses; 20-50 solar masses dominate observations so far.
2. VIRGO's addition tripled detectors, boosting rates and localization
LIGO's first run lasted 4 months, the second 9. Yet, nearly half the detections occurred in the final month after VIRGO joined. 2017 highlights:
3. Black hole mergers unleash more energy than all observable Universe stars combined—for milliseconds
The Sun outputs 4×1026 W, fusing 4 million tons of mass per second. With ~1023 stars universe-wide, total stellar luminosity exceeds 1049 W. Yet, each merger briefly surpassed this, radiating immense power in gravitational waves.
4. Mergers convert ~5% of black hole mass to gravitational wave energy
Via E=mc², ~5% of the progenitors' mass radiates away as waves—the most energetic events since the Big Bang, outshining neutron star mergers, GRBs, and supernovae. Examples:
5. Future detections will uncover frequent low-mass black hole mergers
Massive mergers yield loudest signals, but volume scales with distance cubed—doubling range accesses 8x volume. Enhanced sensitivity will reveal nearby low-mass (7-20 solar masses) events, abundant yet subtle, alongside diverse binaries.
