Astronomy
Gravitational Waves from Binary Black Hole Mergers
Quick fact
The first detected gravitational wave signal, GW150914, was produced by the merger of two black holes with masses about 36 and 29 times the Sun's mass, which released more energy in half a second than all the stars in the observable universe combined.
Why this is interesting
You've probably seen ripples when you toss a stone into a pond. But what if the universe itself could ripple? In 2015, scientists detected ripples in space and time from two black holes crashing into each other a billion light-years away.
Read the full explanation
Understanding Gravitational Waves from Binary Black Hole Mergers
Imagine a heavy ball resting on a stretched rubber sheet. The ball makes a depression. If you roll another ball nearby, both balls will spiral around and eventually meet, creating waves in the sheet. This is like two black holes orbiting each other. They are so massive that they make the fabric of spacetime itself curve. As they orbit, they lose energy by emitting gravitational waves, which are ripples in that fabric. These waves travel across the universe at the speed of light, stretching and squeezing space as they pass. When the two black holes finally merge, they create a single, larger black hole and emit a final burst of ripples. This process has three phases: inspiral (spiraling closer), merger (when they collide), and ringdown (the resulting black hole settling into a stable shape). The signal detected by LIGO looks like a 'chirp'—the frequency and amplitude increase over time, matching the inspiral and merger.
A deeper explanation
Gravitational waves are a direct consequence of Einstein's general relativity, which describes gravity as the curvature of spacetime. When massive objects accelerate, they generate ripples that propagate outward. In a binary black hole system, the two black holes orbit each other, radiating gravitational waves. This radiation carries away orbital energy and angular momentum, causing the orbit to shrink and the frequency to increase over time. The emitted waveform depends on the masses and spins of the black holes, allowing us to infer their properties. The merger of two black holes proceeds through three stages: inspiral, merger, and ringdown. The inspiral phase is well modeled by post-Newtonian approximations; the merger requires numerical relativity simulations; and ringdown involves the quasinormal modes of the resulting black hole. The detection of these waves is extremely challenging because the stretching of space is minuscule—about 10^-21, meaning a 4-km detector changes by less than a proton width. Advanced LIGO uses laser interferometry to measure this. The detection of GW150914 marked the first direct evidence of black holes as predicted, confirmed that black holes exist in binary pairs, and demonstrated that black hole mergers are a new, powerful source of gravitational waves. This opens a new window for observing the universe, allowing us to study phenomena that emit no electromagnetic radiation, such as black holes and dark matter candidates. It also provides a way to test general relativity in the strong-field regime and to measure the expansion of the universe via standard sirens.