Billions of years ago, a collision between two black holes sent gravitational waves rippling through the universe. In 2019, signals from these waves were detected at the gravitational wave observatory LIGO (United States) and the detector Virgo (Italy).
What has excited scientists, however, is the mass of one of the parent black holes, which defies traditional knowledge of how black holes are formed.
What exactly was detected?
- It was a signal from a gravitational wave, a relatively new field of discovery. Gravitational waves are invisible ripples that form when a star explodes in a supernova; when two big stars orbit each other; and when two black holes merge. Travelling at the speed of light, gravitational waves squeeze and stretch anything in their path.
- Gravitational waves were proposed by Albert Einstein in his General Theory of Relativity over a century ago. It was only in 2015, however, that the first gravitational wave was actually detected — by LIGO. Since then, there have been a number of subsequent detections of gravitational waves.
- The signal detected at LIGO and Virgo, as described by the LIGO Collaboration, resembled “about four short wiggles” and lasted less than one-tenth of a second.
What is a ‘black hole’?
- A black hole is a place in space where gravity pulls so much that even light can not get out. The gravity is so strong because matter has been squeezed into a tiny space. This can happen when a star is dying. Because no light can get out, people can’t see black holes. They are invisible.
- Black holes can be big or small. Scientists think the smallest black holes are as small as just one atom. These black holes are very tiny but have the mass of a large mountain. Mass is the amount of matter, or “stuff,” in an object.
- Another kind of black hole is called “stellar.” Its mass can be up to 20 times more than the mass of the sun. There may be many, many stellar mass black holes in Earth’s galaxy. Earth’s galaxy is called the Milky Way.
- The largest black holes are called “supermassive.” These black holes have masses that are more than 1 million suns together. Scientists have found proof that every large galaxy contains a supermassive black hole at its centre. The supermassive black hole at the centre of the Milky Way galaxy is called Sagittarius A. It has a mass equal to about 4 million suns and would fit inside a very large ball that could hold a few million Earths.