"Ringing" black hole proves Einstein right yet again
A quirk of Albert Einstein’s theory of general relativity says that after two black holes collide and merge, the newly-created black hole should “ring” like a bell, sending gravitational waves rippling through spacetime. What’s more, the pitch and decay of these waves should directly tell us about the object’s mass and spin. Now, astronomers have managed to observe exactly this for the first time, proving the preeminent scientist right once again.
As mysterious as black holes seem to be, Einstein’s theories suggest that they only really have three observable properties – mass, spin and electric charge. This is often called the No-Hair Theorem, where “hair” is a metaphor for other characteristics, which are thought to be swallowed up by the black hole itself.
Whether or not black holes have “hair” has been debated for decades, but Einstein also proposed a way to test the theory. When two black holes collide and form a new one, they distort the very fabric of reality, sending ripples through spacetime. Details about their mass and spin should be encoded in some of the “tones” of those waves.
Gravitational waves themselves have been detected regularly over the last four years, but decoding the properties of the object from the tones has so far not been done. So researchers from MIT, Caltech, Stony Brook University and Cornell University set out to do just that.
The team focused on an event called GW150914 – the very first detection of gravitational waves in September 2015. In this signal, the researchers identified the ringing pattern of the black hole and used Einstein’s proposed equations to calculate what the leftover black hole’s mass and spin should be, if the theory is correct. These can then be compared to actual measurements of the black hole’s mass and spin taken through other means.
If the team’s findings are very different from the known measurements, it would suggest that black holes do have other properties, hinting at exotic physics beyond general relativity. But sure enough, the team found that the ringing pattern lined up directly with its mass and spin.
“We all expect general relativity to be correct, but this is the first time we have confirmed it in this way,” says Maximiliano Isi, lead author of the study. “This is the first experimental measurement that succeeds in directly testing the no-hair theorem. 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.”
But that picture might not live forever – and the researchers say they’re excited for the day when this technique returns an unexpected result. That could indicate an even stranger object than a black hole.
“In the future, we’ll have better detectors on Earth and in space, and will be able to see not just two, but tens of modes, and pin down their properties precisely,” says Isi. “If these are not black holes as Einstein predicts, if they are more exotic objects like wormholes or boson stars, they may not ring in the same way, and we’ll have a chance of seeing them.”
The research was published in the journal Physical Review Letters.