With a pull so strong not even light escapes, a black hole is defined by its gravity. But now a model that ignores gravity is proving surprisingly useful for pinning down how these cosmic giants work.
Black holes are where big ideas in cosmology, such as gravity and quantum mechanics, collide. That makes them great for testing new theories. 鈥淎 black hole is a bit like the hydrogen atom of quantum gravity,鈥� says of the University of York, UK. 鈥淚t鈥檚 a place to test ideas and test theories, and see what may or may not happen.鈥�
His team modelled a minimal black hole, defined only by having an inside and an outside, using quantum theory. To their surprise, they found that this object reproduces a lot of the features of real black holes that are thought to rely on gravity, including Hawking radiation, which could occur via a process called quantum tunnelling.
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This chimes with suggestions that gravity is not a fundamental component of the universe but an emergent property of quantum mechanics, just as waves are an emergent property of water molecules.
Firewall paradox
, a theoretical physicist at the University of Amsterdam, the Netherlands, who came up with this idea, agrees. 鈥淭his indeed sheds some light on my ideas on the emergence of gravity from entropy,鈥� he says. 鈥淚n particular, he makes the point that quantum information is a key concept that is relevant.鈥�
Braunstein thinks the toy black hole also dodges the so-called black hole firewall paradox, a grisly thought experiment involving someone falling into a black hole that reveals an inconsistency between quantum mechanics and general relativity.
Last year, of the University of California, Santa Barbara, and colleagues showed how the quantum entanglement between photons emitted by a black hole through Hawking radiation and those still on the inside should at some point cause . As a result, someone falling into the black hole would be burned to a crisp. But this contradicts general relativity, which says that someone falling into a black hole shouldn鈥檛 notice a difference when they cross the event horizon.
Using their gravity-free black hole model, Braunstein and colleagues showed that it was possible to create a black hole in which the firewall doesn鈥檛 appear until the last instants of the black hole鈥檚 life, when it is too small for someone to fall into anyway. 鈥淚nstead of being a cutesy picture, [the toy black hole] is a fantastic contender for the real physics,鈥� says Braunstein.
Polchinski isn鈥檛 so sure. 鈥淏raunstein鈥檚 work is based on a somewhat non-standard model of the Hawking process,鈥� he says. 鈥淚t is interesting that this actually makes it possible to delay or evade the problem, but I do not think that this is how things will turn out in detail.鈥�
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