Will an astronaut who falls into a black hole be crushed or burned to a crisp?
In March 2012, Joseph Polchinski began to contemplate suicide — at least in mathematical form. A string theorist at the Kavli Institute for Theoretical Physics in Santa Barbara, California, Polchinski was pondering what would happen to an astronaut who dived into a black hole. Obviously, he would die. But how?
According to the then-accepted account, he wouldn’t feel anything special at first, even when his fall took him through the black hole’s event horizon: the invisible boundary beyond which nothing can escape. But eventually — after hours, days or even weeks if the black hole was big enough — he would begin to notice that gravity was tugging at his feet more strongly than at his head. As his plunge carried him inexorably downwards, the difference in forces would quickly increase and rip him apart, before finally crushing his remnants into the black hole’s infinitely dense core.
But Polchinski’s calculations, carried out with two of his students — Ahmed Almheiri and James Sully — and fellow string theorist Donald Marolf at the University of California, Santa Barbara (UCSB), were telling a different story1. In their account, quantum effects would turn the event horizon into a seething maelstrom of particles. Anyone who fell into it would hit a wall of fire and be burned to a crisp in an instant.
The team’s verdict, published in July 2012, shocked the physics community. Such firewalls would violate a foundational tenet of physics that was first articulated almost a century ago by Albert Einstein, who used it as the basis of general relativity, his theory of gravity. Known as the equivalence principle, it states in part that an observer falling in a gravitational field — even the powerful one inside a black hole — will see exactly the same phenomena as an observer floating in empty space.
Without this principle, Einstein’s framework crumbles.
Well aware of the implications of their claim, Polchinski and his co-authors offered an alternative plot ending in which a firewall does not form. But this solution came with a huge price. Physicists would have to sacrifice the other great pillar of their science: quantum mechanics, the theory governing the interactions between subatomic particles.
The result has been a flurry of research papers about firewalls, all struggling to resolve the impasse, none succeeding to everyone’s satisfaction. Steve Giddings, a quantum physicist at the UCSB, describes the situation as “a crisis in the foundations of physics that may need a revolution to resolve”.
With that thought in mind, black-hole experts came together last month at CERN, Europe’s particle-physics laboratory near Geneva, Switzerland, to grapple with the issue face to face. They hoped to reveal the path towards a unified theory of ‘quantum gravity’ that brings all the fundamental forces of nature under one umbrella — a prize that has eluded physicists for decades.
The firewall idea “shakes the foundations of what most of us believed about black holes”, said Raphael Bousso, a string theorist at the University of California, Berkeley, as he opened his talk at the meeting. “It essentially pits quantum mechanics against general relativity, without giving us any clues as to which direction to go next.”
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