Gravitational waves detected from two merging black holes

Physicists have detected the existence of gravitational waves, the type of waves predicted by Albert Einstein in his theory of relativity 100 years ago, in two black holes.

A simulated image of two black holes merging, creating ripples 1.3 billion years ago. Photo: SXS

At a press conference yesterday in Washington, USA, Mr. David Reitze, executive director of the Laser Interferometer Gravitational-Wave Observatory (LIGO), announced the discovery before other researchers.

According to Mr. Reitze, gravitational waves, ripples in space-time, were created by the merger of two black holes. One black hole had a mass equivalent to 29 suns while the other was equivalent to 36 suns. Each black hole had a diameter of about 50 km.

Scientists at LIGO estimate that about 1.3 billion years ago, two black holes collided at half the speed of light. Gravitational waves passed through all matter and traveled across the universe before reaching Earth. Gravitational waves stretched and compressed the space around Earth like jelly.

However, the waves are so small that only LIGO, which can measure distortions of 0.001 the size of a proton, can observe them. The gravitational waves were detected on September 14, 2015.

Scientists heard the sound of two black holes colliding, lasting 0.2 seconds. While gravitational waves are not sound waves, the frequencies emitted by the collision in the final 0.001 seconds when the two black holes were a few kilometers apart are audible to humans, according to Deirdre Shoemaker, a physicist at LIGO.

LIGO is a system of two identical transmitters, located in Livingston, Louisiana, and Hanford, Washington. They were carefully built to detect the tiny vibrations from propagating gravitational waves. The project was founded by scientists from the California Institute of Technology (Caltech) and the Massachusetts Institute of Technology (MIT) and is funded by the American Academy of Sciences.

The LIGO team detected gravitational waves with a confidence level of more than 5 sigma. "It took us six months to be sure that this result was correct," Shoemaker said.

According to VnExpress