The random disorder of subatomic particles – their entropy – might be the key to solving a long-standing puzzle about how black holes lose information. This proposal could help unite gravity and quantum mechanics.
“Quantum gravity nowadays is really far beyond our limits,” says at the Max Planck Institute for Gravitational Physics in Golm, Germany. “But I think black holes are a good arena to test it.”
Black holes are both a blessing and a curse to researchers because their huge mass is compressed down to a tiny point, meaning they are simultaneously governed by the two pillars of physics – general relativity, which deals with the very large, and quantum mechanics, which focuses on the very small. Yet trying to combine these frameworks has often led to trouble.
Advertisement
According to quantum mechanics, information about a particle can’t be destroyed. But material that falls into a black hole can theoretically never be recovered, leading to a loss of information. This incompatibility is known as the information paradox.
Firewall trouble
More than 40 years ago, Stephen Hawking proved that certain entities could escape a black hole through what’s now called Hawking radiation. Subatomic particles at a black hole’s edge constantly wink into and out of existence. Every once in a while, the universe generates an entangled particle pair, one of which falls into the black hole while the other is flung away at high speed. This saps the black hole’s energy, causing it to slowly evaporate and shrink.
In 2012, researchers showed that this process would sever entanglement, an extremely violent act that would create a burning firewall at the black hole’s event horizon. But this resolution to the information paradox has left many physicists dissatisfied and seeking another answer.
Alonso-Serrano and her colleagues offer a new solution. In previous work, they showed that for burning objects, like a coal lump in a fire, a certain amount of information was always concealed in each photon due to its inherent random noise or entropy. By analogy, Hawking radiation also contains hidden information, allowing some of the information to escape without producing a firewall.
If a black hole shrinks enough, it could start to be governed by the laws of quantum gravity. To see how that would affect the entropy, Alonso-Serrano and her team turned to a model known as the Generalized Uncertainty Principle (GUP) found in many quantum gravity proposals.
Quantum remnant
Ordinary quantum mechanics says that we can’t simultaneously know with high precision certain properties about a particle, such as its position and speed. GUP extends this feature to the fabric of space-time, stating that the more you know about the curvature of a particular part of space, the less you can know about the energy it carries.
Because GUP relates space-time and energy, which is in turn related to entropy, the researchers showed a black hole would give off more and more Hawking radiation as it evaporated but that each emitted particle would carry less information in its entropy. This eliminates the firewall, but means that a black hole would not evaporate entirely.
At a certain point, the black hole would be unable to shrink further because in GUP models there is a fundamental limit to how small objects can be – a property known as the Planck length, which is roughly 20 orders of magnitude smaller than a proton.
Once the black hole reached this size, it would leave behind “a tiny quantum remnant black hole thing – I’m not sure that black hole is the right word for it anymore – that still contains some information about how it was formed and what fell into it,” says at Los Alamos National Laboratory near Santa Fe, New Mexico, who was not involved in the work.
“Quantum gravity is really, really hard,” Miller adds. The findings are “interesting in the sense that people are pushing what we know or don’t know about quantum gravity. But this is still just one of many proposals for how to solve the information paradox.”
Reference: Arxiv,
Read more: Gravity may be created by strange flashes in the quantum realm
