Bids to solve the black hole firewall paradox are producing a free-for-all in theoretical physics – cue time reversal, walls of ice and bouncing stars
Take what you know about black holes and throw it out.
Last week famed physicist Stephen Hawking caused an uproar with his assertion that black holes do not exist – at least not as we've defined them for the past 40 years. Rather than letting nothing, not even light, escape their grasp, Hawking says that this "point of no return" is a fallacy, and black holes will sometimes let trapped light back out.
His idea is a proposed solution to the firewall paradox. The paradox has it that if black holes are as we think, they must be surrounded by rings of fire, though that would violate general relativity.
Hawking's notion does away with a key part of a black hole – its event horizon. It sounds far out, and has been met with heated debate, but even if Hawking is wrong, his high-profile claim highlights the fact that efforts to solve the paradox have turned cosmology on its ear. Firewalls have turned this corner of physics into a cosmic Wild West: a strange frontier where black holes can take on the form of exotic stars, fuzzballs, time machines or iced wormholes (see "The firewall paradox").
The pillars of theoretical physics – quantum mechanics and general relativity – are in a stand-off. One of them will have to blink if this paradox is to be undone.
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