THE starting point is familiar enough. “A black hole is a part of space-time so incredibly warped, and where gravity becomes so incredibly intense, that nothing can escape, not even light,” says physicist at the University of Oxford.
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That is also where the problems begin. Black holes are cast-iron predictions of general relativity, Einstein’s peerless theory of gravity, and yet they stretch it to breaking point. Its equations fail catastrophically at a black hole’s centre, known as its singularity, where the warping of space-time simply goes off the scale. “Everything you calculate goes to infinity,” says Ferreira. “It has no meaning.”
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Even Einstein thought that black holes were too absurd to be real. They emit no light, so we cannot see them. Yet we infer their presence from their influence on nearby matter as they suck in gas and dust and stars, the contortions of which produce awesome light shows. In 2015, when we detected gravitational waves for the first time, the observed ripples in space-time matched the predicted signal from two black holes spiralling into one another and merging.
Actually, black holes are rather common. Space is pockmarked with ones formed when over-massive stars collapse and die: our galaxy alone holds perhaps 100 million of these smaller black holes. Most galaxies also have a humongous one at their centre. The one at the heart of the Milky Way packs over 4 million solar masses into a region that would fit inside the orbit of Mercury.
So how can you think about something so extreme that even abstract mathematics cannot depict it? With difficulty, it turns out. “I have a mental picture of a classical black hole in terms of deformed space-time, like a trampoline with a very heavy weight on it,” says at the University of California, Santa Barbara. “The problem is that this classical picture ultimately fails at a fundamental level.”
To get down to the nitty-gritty of what happens at a black hole’s point of no return, its event horizon, you need to bring quantum theory into the picture – but quantum theory and general relativity famously don’t agree. “When we think about what happens to information that falls into a black hole, we run into a conundrum so severe it suggests there is something deeply wrong with our classical description of space-time,” says Giddings.
“Black holes might be fuzzballs of tangled space-time string”
General relativity says that when matter falls into a black hole, information is destroyed; quantum mechanics says it can’t be. A unified theory requires us to somehow reconcile the two, probably by reimagining space-time as only an approximate thing, says Giddings. String theory offers one way, and might turn what we think are black holes into “fuzzballs” with no singularity and no event horizon – dense, star-like objects that essentially amount to a tangled ball of space-time string.
Or perhaps not – and that is precisely why black holes are so tempting, says , an astronomer at Yale University. “I actually describe them as seductive,” she says. “They lie at the margin of what is knowable and what is not.”
This article appeared in print under the headline “How to think about… Black holes”
