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Review
How ‘white holes’ could explain the mystery of dark matter
Can matter 'bounce' in a black hole? What would you see inside one? Carlo Rovelli's brilliant new book White Holes asks surprising questions
At some point, theoretical physics shades into science fiction. This is a beautiful little book, by a celebrated physicist and writer, about a phenomenon that is permitted by equations but might not actually exist. Or perhaps white holes do exist, and are everywhere: we just haven’t noticed them yet.
No such controversy exists about black holes, which were first proposed by Karl Schwarzschild, a German physicist then fighting on the Eastern Front in the First World War. When a large star burns through all its fuel, there is no more countervailing pressure to the gravity caused by its mass: the star collapses in on itself, creating a zone beyond which gravity is so strong that not even light can escape its clutches. Einstein did not believe in black holes, but the British physicist Roger Penrose did (and he won the 2020 Nobel for showing in 1964 that they were mathematically robust). In 2019 we saw the first photograph of a black hole, a giant at the centre of the M87 galaxy. The halo of hot gas swirling around it is testament to the eldritch void in the centre.
Or is it a void? Information – in the form of structured matter – can fall into a black hole, never to be seen again. But mathematics says information cannot be destroyed. So does it lurk somehow within?
For Rovelli, such questions require us to think more carefully about what happens inside a black hole – beyond the “event horizon” at which nothing can escape (except, as Stephen Hawking showed, dribbles of quantum radiation). For a distant observer, the hole seems black and final, but for a hypothetical observer inside, things are both more normal (you can still see the rest of the universe) and more weird: time runs much more slowly.
Conventionally, the very centre of a black hole is a “singularity”, where the density of matter at a single mathematical point is infinite. Rovelli argues that this kind of singularity does not happen in nature; in his alternative picture, the inrushing fall of matter is followed by a “bounce”, and things go into reverse: presto, we now have a “white hole”. Stuff can come out of a white hole but nothing can ever get in. The information that fell into the black hole is eventually beamed out again.
If this is so, why don’t we see trillions of white holes shining down at us from the night sky? The theoretical prestidigitator responds: because the maths ensures that a white hole looks exactly like a black hole from the outside. Moreover, a black hole only becomes white once much of its horizon has “evaporated” away, so white holes will look much smaller. They could be a kind of snow or dust throughout the cosmos, “minuscule white holes in the heavens”.
So much for the theory, dreamed up by Rovelli and his collaborator Hal Haggard. (Other theories of white holes are available: “I am still far from convinced that we have the truth in our pocket,” Rovelli admits disarmingly.) What elevates this book is the author’s supernatural concision (he gives a pellucid explanation of astronomical measurement via simple geometry in a couple of pages), and his artistic and philosophical elegance: he is constantly quoting Dante’s Inferno, and insists on a highly cultured view of science in which the literary art of analogy is just as important as logic and mathematics.
Many questions remain unanswered. Was the Big Bang itself another “bounce” from a previous implosion? Is “dark matter” actually a blizzard of white holes? Rovelli’s gift is to make such questions feel humanly important. “When we seek to understand white holes,” he writes, “we do so not as pure reason, not as part of a world different from the objects that we are trying to understand. We are processes guided by the same stars.” And so the universe continues to come to know itself.
White Holes: Inside the Horizon by Carlo Rovelli, tr Simon Carnell, is published by Allen Lane at £14.99. To order your copy for £12.99, call 0844 871 1514 or visit Telegraph Books