The question of whether one black hole can consume another is often posed. A black hole is a region of spacetime where gravity is so intense that nothing, not even light, can escape its boundary, known as the event horizon. This extreme gravitational pull results from a massive star collapsing in on itself. When two black holes meet, the outcome is not consumption but a powerful combination, or coalescence, which ranks among the most energetic events in the universe.
The Orbital Dance of Black Hole Pairs
The precursor to a black hole merger is a binary system, where two black holes orbit a common center of mass. These systems form either when two orbiting massive stars collapse into black holes, or through dynamical capture in a dense stellar environment like a globular cluster. In the latter case, two independently formed black holes drift close enough for mutual gravity to lock them into orbit.
Once established, the binary system begins a long process of orbital decay. As the objects circle one another, their acceleration causes them to emit gravitational waves, which are ripples in the fabric of spacetime. This energy loss acts as a constant brake, causing the orbital separation to shrink gradually. This phase, known as the inspiral, involves the black holes slowly spiraling closer together over vast stretches of cosmic time.
The rate of orbital decay accelerates dramatically as the distance between them closes. The continuous emission of gravitational radiation tightens the orbit, moving the pair from a slow, wide dance to a final, rapid plunge.
The Violent Act of Coalescence
The final stage of the inspiral accelerates from millions of years to mere seconds when the black holes are separated by only a few times the diameter of their event horizons. In this chaotic, relativistic environment, the black holes hurtle toward each other at speeds that are a significant fraction of the speed of light, drastically warping the surrounding spacetime.
The actual merger occurs when the two separate event horizons touch and combine, forming a single, larger, and highly unstable event horizon. This is a mutual combination where their spacetime boundaries dissolve into one. The newly formed black hole is initially distorted and immediately begins to shed its excess energy to stabilize.
This stabilization process is called the ringdown, where the distorted object oscillates like a struck bell, radiating away excess gravitational energy. The final mass of the remnant black hole is measurably less than the sum of the two original masses. This missing mass is converted entirely into a burst of gravitational wave energy, following Einstein’s mass-energy equivalence principle.
The energy released in these final moments is immense, briefly shining with more power than all the stars in the observable universe combined. The resulting object is a single, more massive black hole that quickly settles into a stable configuration defined only by its mass, spin, and electric charge.
Detecting the Aftermath Through Ripples in Spacetime
The only way to observe these mergers is by detecting the massive burst of energy released as gravitational waves. These waves, predicted by Albert Einstein, are ripples in the fabric of spacetime that travel outward from the collision at the speed of light. They cause a tiny, temporary stretching and squeezing of space as they pass.
The Laser Interferometer Gravitational-Wave Observatory (LIGO) was designed to detect these faint distortions. It operates by using laser beams to measure the distance between mirrors separated by several kilometers. When a gravitational wave passes, it minutely alters these distances, providing a signature of the event.
The most recognizable signal of a black hole merger is the “chirp,” a characteristic rise in the frequency and amplitude of the gravitational waves as the black holes spiral faster together. This rising tone culminates in a final, loud burst at the moment of coalescence. The first direct detection of this chirp, named GW150914, confirmed that two black holes can combine to form a single, larger entity.