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Try as we might, we just can’t prove Einstein wrong. One prediction of his theory of general relativity is that black holes are simple objects – and listening to them “ring” indicates this is true.

According to general relativity, any black hole can be described in full by three properties: its mass, its spin, and its electrical charge. In practice, this boils down to the first two, because we do not expect black holes to accumulate charge.

All other information about a black hole, like the properties of the objects that have fallen into it, is not observable from beyond the event horizon. This extra information is called “hair” and so the theorem is known as the no-hair theorem.

Our observations of black holes have all been consistent with this idea. But Maximiliano Isi at the Massachusetts Institute of Technology and his colleagues wanted to test it in a different way: using gravitational waves, ripples in spacetime caused by massive moving objects.

We know that when a pair of black holes merge, the leftover black hole should “ring” like a bell, emitting gravitational waves in several frequencies. Just as the note of a bell is determined by its shape, the frequencies of waves produced by the black hole are determined by its mass and rotation. “These frequencies and their lifetimes are inextricably tied to the shape of the bell, so you can listen to the ringing and learn about its structure,” says Isi.

The frequency which lasts the longest is called the fundamental, but there are also shorter-lived notes called overtones. “The fundamental rings like a high-quality wine glass, and the overtones are more like a thud,” says Leo Stein at the University of Mississippi.

Isi and his team managed to find one of these overtones in a gravitational wave signal detected by LIGO in 2015. This is the first time anyone has found more than one tone in a gravitational wave.
The mass and spin of the black hole had already been calculated by the LIGO team based on all the information in the signal. Isi and his colleagues have now used just the overtone frequencies they found to estimate the mass and spin.

They reckon the black hole is about 68 times the mass of the sun and spinning about 100 times a second. That is a good match with the previously calculated value. Since Isi’s estimate is based on the no hair theoem, this suggests it is correct. In other words, Einstein is still right.

The result is not particularly precise. The black hole could still deviate from relativity by up to 20 per cent. But there is a good chance that it can be repeated. “The result was from the loudest binary black hole signal we’ve had so far, but there are more signals that haven’t been analysed yet,” says Katerina Chatziioannou at the Flatiron Institute in New York. Those measurements should allow physicists to nail down whether black holes have hair.