Scientists hear the 'chirp' of baby black hole

Einstein was right - again.

This time, his 104-year-old theory of general relativity has been used to hear the "ringing" of a newborn black hole.

Astronomers heard it in the gravitational waves emitted in the wake of a collision more than 1 billion light-years away, which were detected on Earth in 2015. 

Gravitational waves are slight disturbances in the curvature of spacetime which scientists only just recently have been able to measure. Einstein's calculations suggested after black holes collide, the pitch and decay of the waves should tell you exactly how big the new mass is, and how fast it's spinning.

"We all expect general relativity to be correct, but this is the first time we have confirmed it in this way," said NASA's Maximiliano Isi, who led the new study.

"We detect an overall gravitational wave signal that's made up of multiple frequencies, which fade away at different rates, like the different pitches that make up a sound. Each frequency or tone corresponds to a vibrational frequency of the new black hole."

They managed to extract a signal from the mess that was from the newly born black hole - not the collision itself. 

Translating the data into audio, it reportedly sounded like a "chirp". Sadly, the researchers don't appear to have released the recording.

The study has also all-but proven black holes are "hairless" - lacking any other characteristics but mass, spin, and an electric charge - just as Einstein predicted. Everything else the black hole might have is swallowed up.

"This is the first experimental measurement that succeeds in directly testing the no-hair theorem," said Isi. "It doesn't mean black holes couldn't have hair. It means the picture of black holes with no hair lives for one more day."

Finding a black hole with "hair" - a metaphor to describe anything else that you might see or detect in a black hole - would be "tantalising evidence of physics beyond what Einstein's theory can explain".

But for now - like hairless black holes - Einstein's century-old mathematics lives another day.

The research was published in journal Physical Review Letters.