Bald as a black hole

Illustration of the “ringing” of a newly formed black hole in the form of gravitational waves.

According to Albert Einstein’s theory of general relativity, a black hole, born from the collision of two massive black holes, should “ring” in the aftermath and produce gravitational waves, much like a bell reverberates sound waves when struck. The theory predicts that the particular pitch and decay of these gravitational waves will be a direct signature of the newly formed black hole’s mass and spin.

Recently, analysis of the ringing of an infant black hole by physicists from Massachusetts Institute of Technology has found that the ringing does, in fact, predict the black hole’s mass and spin, supporting Einstein. The study was published in a recent issue of “Physical Review Letters”.

The finding also favours the notion that black holes lack any sort of “hair”, a metaphor referring to the idea that black holes, according to Einstein’s theory, should exhibit just three observable properties: mass, spin and electric charge. All other characteristics, which the physicist John Wheeler termed “hair”, would be swallowed up by the black hole itself, and would therefore be unobservable.

The MIT team’s findings support the idea that black holes are, in fact, hairless. The researchers were able to identify the pattern of a black hole’s ringing, and, using Einstein’s equations, calculated the mass and spin that the black hole should have, given its ringing pattern. This calculation matched the earlier observational measurements of the black hole’s mass and spin.

If the team’s calculations had deviated significantly from the measurements, it would have suggested that the black hole’s ringing encodes properties other than mass, spin and electric charge, and its explanation would require going beyond Einstein’s theory.

“We all expect general relativity to be correct, but this is the first time we have confirmed it in this way,” said Maximiliano Isi, the study’s lead author and a NASA Einstein Fellow in MIT’s Kavli Institute for Astrophysics and Space Research. “This is the first experimental measurement that succeeds in directly testing the no-hair theorem....[So] the picture of black holes with no hair lives for one more day.”