MINNEAPOLIS. Don’t try to conduct a quantum experiment near a black hole — its mere presence destroys all quantum states in its vicinity, researchers say.
Physicists announced this on April 17 at a meeting of the American Physical Society . Researchers have found that any quantum experiment conducted near a black hole can cause a paradox in which the black hole reveals information about its interior — something physicists say is forbidden. The paradox, the team reports, is if a black hole simply destroys any quantum states that approach.
This disruption may have implications for future theories of quantum gravity. These sought-after theories aim to unify quantum mechanics, the set of rules governing subatomic particles, and general relativity, which describes how mass moves on cosmic scales.
“The idea is to use properties [теорій]which you understand how [є] quantum mechanics and gravity to explore aspects of the fundamental theory that is quantum gravity,” says theoretical physicist Gautam Satishchandran of Princeton University.
Here’s how Satishchandran, along with theoretical physicists Dane Danielson and Robert Wald, both of the University of Chicago, did just that.
A quantum experiment near a black hole creates a paradox
First, the team imagined a person they called Alice conducting the famous double-slit experiment in a lab orbiting a black hole. In this classic example of quantum physics, a scientist sends a particle, such as an electron or photon, into a pair of slits in a solid barrier. If no one observes the progress of the particle, an interference pattern characteristic of waves appears on the screen on the other side of the barrier, as if the particle passed through both slits at the same time. But if someone or some device measures the path of the particle, it will be registered as having passed through this or that slit. The quantum state of the particle, apparently, being in two places at the same time, is destroyed.
The team then imagined another person, Bob, sitting just inside the black hole’s event horizon—the limit beyond which even light cannot escape the black hole’s gravity. Even though Bob is doomed, he can still make measurements. The laws of physics apply equally both inside the horizon and outside it. “On the horizon, you won’t even notice you’ve fallen,” says Satishchandran.
When Bob observes which slit Alice’s particle passed through, the particle’s quantum state is destroyed. It would also tell Alice that Bob is present and mess up her experiment. But this is a paradox—nothing done inside a black hole should have any effect on the outside. According to the laws of physics, Bob should not be able to communicate with Alice at all.
“The paradox is that black holes are a one-way street,” says Satishchandran. “Nothing done inside the black hole can affect my experiment outside. But we simply created a scenario in which the experiment will definitely have an impact.”
The paradox is resolved if the black hole acts as an “observer”
The team then guessed at a possible solution to this paradox: the black hole itself causes the quantum state of the Alice particle to collapse, regardless of whether Bob is there or not. “There seems to be an effect that no one has accounted for in these theories that comes to the rescue,” Danielson says.
Salvation came in the fact that charged particles emit or emit light when shaken. No matter how carefully Alice sets up her experiment, her particle will always emit a small amount of radiation as it moves, physicists have shown. This radiation will have a different electromagnetic field depending on where the Alice particle went.
When the radiation crosses the black hole’s event horizon, the black hole registers this difference, effectively observing the original particle to destroy its quantum state.
“The horizon actually ‘knows’ where the particle went,” Mathematically speaking, Satishchandran says. Alice blames the black hole for ruining her experiment, not Bob, and the paradox is resolved.
The team went further with this idea. If Alice’s particle is a graviton, a particle of gravity, the same thing happens to an electron. And if the horizon in question is not a black hole, but a cosmic horizon that marks the edge of the visible universe, then the Alice particle will still collapse the team announced at the same meeting.
Theories of quantum gravity must take all this into account
According to the researchers, the ultimate goal of this experiment is not to create a complete theory of quantum gravity, but to sketch a diagram into which a possible future theory should fit.
“We are not in the business of creating theories of quantum gravity,” says Satishchandran. “But we would like to … provide benchmarks that will hopefully tell us something more fundamental about what such theories look like.”
It’s not clear how to get to a full theory from here, agrees physicist Alex Lupsaska of Vanderbilt University in Nashville, who was not involved in the study. But the idea that black holes can act as quantum observers is interesting in itself.
“I think this is a true fact that should be part of the final theory of quantum gravity,” he says. “But whether this is an important clue we’re gathering towards a final theory of quantum gravity, or just an interesting detour on the way to unraveling that theory, is unknown.”