What Happens When Two Black Holes Collide?
QUICK ANSWER
When two black holes collide, they merge into a single larger black hole and release enormous energy as gravitational waves, ripples in spacetime that propagate at the speed of light. LIGO detected the first such merger in September 2015, confirming Einstein's century-old prediction of gravitational waves.
When two black holes collide, the result is one of the most violent events in the universe, releasing more energy in fractions of a second than the rest of the observable universe combined. The energy comes out as gravitational waves, ripples in the fabric of spacetime itself. LIGO first detected these waves directly in 2015, opening a new way to observe the universe.
What happens during the merger?
A dramatic, fast collision. According to NASA, when two black holes get close, they spiral around each other, losing energy as gravitational waves until they finally merge. The actual merger takes a tiny fraction of a second. The two event horizons combine into one larger horizon, and the resulting black hole's mass is slightly less than the sum of the two original masses (the difference being radiated away as gravitational waves). The new black hole is initially highly distorted but quickly settles into a stable shape.
What are gravitational waves?
Ripples in spacetime. Einstein predicted in 1915 that accelerating masses should create waves in the fabric of spacetime itself, stretching and compressing space as they pass. The waves travel at the speed of light. For typical events on Earth, the waves are far too weak to detect. But the collision of two black holes generates enormous gravitational waves, briefly making them detectable from billions of light-years away. The waves stretch space by tiny amounts, less than the diameter of a proton, requiring extraordinarily sensitive detectors to measure.
How were they first detected?
By LIGO in 2015. The Laser Interferometer Gravitational-Wave Observatory (LIGO) consists of two facilities (in Louisiana and Washington state) that use lasers to measure tiny changes in distance over miles. On September 14, 2015, both detectors recorded a brief signal matching exactly what theorists predicted for a black hole merger. The event, called GW150914, came from two black holes (29 and 36 solar masses) merging 1.3 billion light-years away. The discovery was announced in February 2016 and won the 2017 Nobel Prize in Physics.
How often do black holes collide?
More often than expected. Since the first detection in 2015, LIGO and the European Virgo detector have detected dozens of black hole mergers and a few neutron star mergers. The rate works out to perhaps one black hole merger somewhere in the observable universe every few minutes. Most occur too far away or are too weak for us to detect. As detectors improve, the count keeps growing. Each detection provides new information about the population of black holes and the physics of strong gravity.
When two black holes collide, they spiral together and merge into a single larger black hole, releasing enormous energy as gravitational waves. LIGO first detected such waves in September 2015, confirming Einstein's century-old prediction and opening a new window on the universe. Dozens of mergers have been detected since, with the rate growing as instruments improve. Black hole collisions are now one of the most exciting frontiers in astrophysics.
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