In 2016, the LIGO collaboration made history by detecting gravitational waves, first predicted by Einstein a century earlier in general relativity. These waves arose from two black holes merging, forming a final black hole lighter than the sum of the originals. Yet general relativity states nothing escapes a black hole's event horizon. So how does this energy escape?
For a black hole, all mass-energy concentrates at a central singularity, shielded by the event horizon. Anything crossing it—particle or light—is doomed to spiral into the singularity.
Einstein's general relativity shows accelerating massive bodies emit gravitational waves: spacetime ripples traveling at light speed, stretching and squeezing space as they carry energy. Per E=mc², this energy equates to mass.
On September 14, 2015, LIGO captured waves from a 36- and 29-solar-mass black hole merger, yielding a 62-solar-mass remnant. The missing 3 solar masses? Converted to gravitational wave energy. Similar-mass mergers can radiate up to 5% of total mass this way.
Each black hole retains its event horizon throughout—no naked singularities. So how do waves escape?
Related: How are gravitational waves detected?
Consider stellar fusion: four protons fuse into helium-4, lighter than the inputs due to nuclear binding energy. Bound states are stabler, less massive.
Likewise, merging black holes grow more tightly bound gravitationally. Mass loss stems from gravitational binding energy, not horizon leakage.
A Newtonian analogy: Two 1-kg masses at infinite separation hold 1.8×1017 J energy. Bringing them closer:
At black hole scales, general relativity governs, but the principle holds: energy shifts from unbound masses to bound system plus waves.
Orbital dynamics and spins dictate conversion: up to 5% for equals, 11% with aligned high spins, dropping to 0.0001% for extreme mass ratios (1 vs. 1 million solar masses).
In essence, no energy escapes horizons during mergers. LIGO detects spacetime's reconfiguration as gravitational binding energy radiates via waves.
