It was hailed as an elegant confirmation of Einstein’s general theory of relativity — but ironically the discovery of gravitational waves earlier this year could herald the first evidence that the theory breaks down at the edge of black holes. Physicists have analysed the publicly released data from the Laser Interferometer Gravitational-Wave Observatory (LIGO), and claim to have found “echoes” of the waves that seem to contradict general relativity’s predictions1.
The echoes could yet disappear with more data. If they persist, the finding would be extraordinary. Physicists have predicted that Einstein’s hugely successful theory could break down in extreme scenarios, such as at the centre of black holes. The echoes would indicate the even more dramatic possibility that relativity fails at the black hole’s edge, far from its core.
If the echoes go away, then general relativity will have withstood a test of its power — previously, it wasn’t clear that physicists would be able to test their non-standard predictions.
“The LIGO detections, and the prospect of many more, offer an exciting opportunity to investigate a new physical regime,” says Steve Giddings, a black-hole researcher at the University of California, Santa Barbara. The LIGO team says that it is aware of the prediction and searching its data for echoes.
The edge of a black hole, known as its event horizon, was long thought to lie beyond experimental reach. According to general relativity, anything that crosses the barrier will be captured by the black hole and have no chance of escape. It will be drawn to the black hole’s core, where all of the hole’s matter is concentrated. “Black holes were thought to be like bottomless pits,” says cosmologist Niayesh Afshordi at the University of Waterloo in Canada.
In the standard picture, this leaves nothing at the event horizon, and someone unlucky enough to cross it wouldn’t notice any sudden change in the environment. But in 2012, physicists based in California realized that if quantum physics is correct, then the event horizon should be replaced by a firewall, a ring of high-energy particles that would burn any matter that passes through to a crisp — and that contradicts general relativity2. The alternative is that black holes are firewall-free, but this would imply that quantum theory is wrong.
Other exotic theories that contradict general relativity also predict some structure at the horizon; for instance, some versions of string theory say that black holes are really ‘fuzzballs’: tangled up threads of energy with a fuzzy surface in place of a sharply-defined event horizon3. However, there did not seem to be any way to peer at the event horizon to find out what, if anything, was there, says Afshordi.
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