When it comes to black holes, we are caught between a rock and a hard place. A black hole, it seems, either destroys information in violation of quantum mechanics or it is enveloped by a blazing firewall, defying Einstein’s general relativity. But a new analysis using the “many worlds” interpretation, which says that each possible outcome of a quantum event exists in its own world, shows that black holes present no such paradoxes.
In the 1970s, Stephen Hawking showed that all black holes give off thermal radiation and eventually evaporate. In doing so, they seemed to be destroying information contained in the matter that fell into them and thus falling foul of a cardinal rule of quantum mechanics: information cannot be created or destroyed.
Some argued that the outgoing “Hawking” radiation preserved the information. But this led to other problems. In 2013, a from the University of California at Santa Barbara showed that if this were the case, then given certain assumptions the event horizon — the black hole’s boundary of no return — would become intensely energetic, forming a firewall.
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But such firewalls go against the tenets of general relativity, which says that space-time near the event horizon should be smooth and devoid of any high-energy flare-ups. The black hole firewall paradox was born.
Branching out
Now, at the California Institute of Technology and his colleagues have shown that the paradox disappears when the evolution of black holes is understood in the context of the many-worlds interpretation of quantum mechanics.
The quantum state of the universe is described by something called the global wave function. Whenever there is a multiplicity of possible outcomes for a physical process in one world, this wave function in traditional quantum mechanics “collapses” to represent one outcome. But in the many-worlds interpretation, the wave function doesn’t collapse – rather it branches, with one branch for each outcome. When the branches can no longer interact, they evolve independently of each other, as separate worlds.
In this way of thinking, the formation of a black hole and its evaporation due to Hawking radiation — both of which are quantum mechanical processes with different possible outcomes — leads to multiple branches of the wave function. An observer monitoring a black hole also splits into multiple observers, one in each branch.
No contradiction
The new work shows that from the perspective of an observer in a given branch, space-time behaves as ordained by general relativity and the black hole has no firewall. But does that imply loss of information?
Not so, says team member , also of Caltech.
That’s because the principle of preservation of information applies to the global wave function and not to its individual branches, he says. The way the wave function changes with time, or its evolution, is said to be unitary. This unitary evolution means there is no loss of information. But only the global wave function evolves unitarily. Each individual branch doesn’t necessarily have to satisfy this condition.
“If you yourself are always on a single branch of the wave function, then you are not confined to expect unitarity on your specific branch,” says Chatwin-Davies.
So, information is preserved across all branches of the global wave function, but not necessarily in any one branch of the wave function. Given this scenario, a black hole that doesn’t lose information and yet has a smooth, uneventful event horizon without a firewall isn’t a contradiction.
at the University of California at Berkeley has independently arrived at similar conclusions in his work. He agrees that the many-worlds approach to quantum mechanics resolves the paradox surrounding information loss from black holes. “If I take only one of the worlds, it’s clearly not unitary. Unitarity seems to be violated because you are ignoring all the possible other worlds,” says Nomura. “Many worlds should be taken seriously.”
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