Black holes are regions of spacetime where gravity is so intense that nothing, not even light, can escape. The boundary beyond which escape is impossible is known as the event horizon. Given their immense gravitational power, a natural question arises about what happens when two of these cosmic behemoths encounter one another. The reality involves a complex physical process where two distinct objects combine.
The Fundamental Answer: Black Hole Mergers
The question of whether a black hole can swallow another is answered with a definitive yes, though scientists prefer the term “merger” or “coalescence.” The true event is a mutual process where the two objects combine to form a single, more massive black hole. This process is distinctly different from accretion, which is when a black hole gradually pulls in and consumes surrounding matter like gas, dust, or a star.
In a merger, the two black holes eventually spiral toward each other until their event horizons touch and then coalesce, a highly energetic event that fundamentally reshapes the local spacetime. The resulting single black hole retains the combined mass, minus the energy radiated away during the collision. Even if one black hole is substantially larger than its companion, the process is still classified as a merger because both event horizons participate in the final unification.
The Inspiraling Dance: Conditions for Collision
A black hole merger begins long before the final collision, often starting with a binary black hole system—two black holes orbiting a common center of mass. These systems can be formed either from the collapse of a binary star system or through dynamic interactions in dense stellar environments like globular clusters. For the two black holes to ultimately collide, they must bleed off orbital energy and angular momentum, a process that shrinks the distance between them.
This orbital decay occurs primarily through the emission of gravitational waves, which are ripples in the fabric of spacetime predicted by Einstein’s theory of General Relativity. As the black holes orbit, they generate these waves, carrying energy away from the system and causing the orbit to shrink gradually. This slow, tightening spiral is known as the inspiral phase, and it can take billions of years when the black holes are far apart. As the distance decreases, the gravitational wave emission becomes dramatically stronger, causing the orbital decay to accelerate rapidly.
The Moment of Merger: Gravitational Wave Emission
The inspiral culminates in the moment of merger, a brief, violent event where the two separate event horizons finally touch and combine into a single, highly distorted horizon. This is the Universe’s most powerful known event, where mass is converted directly into energy in the form of gravitational waves. The energy released during this fraction of a second can briefly outshine the total light output of all the stars in the observable universe.
The characteristic gravitational wave signal produced during the final moments is known as the “chirp” because of the way its frequency and amplitude increase over time. As the black holes spiral faster and closer, the frequency of the emitted waves increases, which translates into a rising pitch if the signal is converted into an audible sound wave. The peak of the chirp signifies the moment the two black holes become one, followed by a brief “ringdown” phase as the newly formed, irregular black hole settles into a stable spherical shape. The detection of this chirp signal by observatories like LIGO and Virgo provides the direct evidence that these mergers occur.
The Resulting Black Hole
The consequence of a black hole merger is a single, larger black hole, but its properties are not a simple addition of the two initial objects. The mass of the final black hole is slightly less than the arithmetic sum of the two merging black holes’ masses. This difference represents the enormous amount of energy radiated away as gravitational waves during the collision and subsequent ringdown, with the mass-energy loss often amounting to around five percent of the total mass.
The resulting black hole also inherits a significant spin, or angular momentum, from the orbital motion and the intrinsic spins of its predecessors. The final spin is dictated by the initial spin directions and the mass ratio of the two merging objects. For black holes of roughly equal mass, the merger typically produces a remnant with a spin parameter around 0.7, on a scale where 1 represents the maximum possible spin. The rapid rotation causes the final object to be a Kerr black hole, a rotating, stable configuration.