Nothing escapes a black hole’s immense gravity, but it may still be possible to detect what is going on in one
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Measuring “entanglement islands” that contain copies of information that black holes have lost could help us find an answer to Stephen Hawking’s black hole paradox
We may be able to find out what happens to matter that falls into a black hole, something previously thought impossible. This is because some parts of a black hole’s interior, called “islands”, may actually poke far enough outside the hole for us to measure them. If we can do this, then Stephen Hawking’s long-standing black hole paradox might finally be resolved.
In the 1970s, Stephen Hawking realised that when the laws of quantum mechanics are applied to the sphere around a black hole where light can no longer escape, called its event horizon, then radiation should be emitted by the black hole. This means the black hole slowly “evaporates”.
This Hawking radiation, which contains only some of the information of whatever is sucked in by a black hole’s all-consuming gravity, will eventually cause the black hole to totally evaporate, which creates a paradox. In physics, information can’t just disappear, but if something falls into a black hole and the Hawking radiation only contains some of the information about it, then when the black hole evaporates some information would be lost forever.
Competing theories
Physicists have come up with many ideas to resolve the paradox, but the most popular ones focus on what happens to the Hawking radiation after it is created. An approach called complementarity says that the lost information is actually stored at the black hole’s boundary while also passing through to its interior, appearing in different ways to an observer depending on whether they have fallen into the black hole or remain outside.
Although this appears to create two “copies” of the information and violate a fundamental principle of quantum mechanics, called the no-cloning theorem, these copies are actually just different viewpoints, described in Albert Einstein’s theory of relativity, so that no single observer could see both copies at the same time.
However, in 2012, a group of researchers found black hole scenarios in which this complementarity would, in fact, lead to an observer actually being able to access both copies at the same time, again violating the no-cloning theorem, so these researchers proposed that there was actually an incredibly hot “firewall” around the black hole’s horizon that would destroy everything on contact, again separating the two copies.
Physicists have argued over which interpretation is correct for more than a decade, and settling the matter using an experiment seemed impossible because it would require sending a person or instrument past the black hole’s horizon, which is by definition inescapable.
But, in 2019, researchers found that for some specific simplified hypothetical black holes, some information about the interior might be detectable on the surface of the event horizon, in an entanglement island.
At the event horizon of a black hole, you have virtual particle pairs popping in and out of existence. If one of these quantum entangled pairs is split up by the event horizon, then one will be held in the black hole and the other will fly out as Hawking radiation.
The entanglement islands, if we could measure them, would reveal what happens to the prisoner particles and the seemingly duplicated information that would exist if the complementarity approach is correct.
Tantalisingly, researchers found these surfaces could even extend slightly beyond the event horizon, but the distance they poked out appeared to be smaller than the smallest possible length we can measure physical effects on, which made islands seem like more of a mathematical curiosity than a physical reality.
Now, Raphael Bousso and Geoff Penington at the University of California, Berkeley, have calculated that for more complicated black holes, like the ones that actually exist in our universe, the islands can extend even further outside the black hole’s horizon. For a supermassive black hole, this could be by as much as an atom’s length, which raises the prospect of measuring one.
“We show these islands actually protrude beyond the horizon of the black hole far enough that, in principle, there is no obstruction to probing them and coming back out,” says Bousso. “That’s actually pretty dramatic because it means that there’s some extremely surprising and radical new physics that is no longer hidden behind black hole horizons or hits you when you try to jump into a black hole, but which is, in principle, accessible to us.”
Measuring a black hole
Getting a scientific instrument within an atom’s width of a black hole horizon would require far more advanced technology than our current spaceships, because the gravitational pull is so strong. The closest black hole that is massive enough to have a potentially detectable island is thousands of light years away. But, says Bousso, “with futuristic technology, the laws of physics don’t prohibit you from probing it”.
“You need to get within about the size of an atom from its horizon and then get back out again. That’s pretty hard to do. Like really, really hard to do,” says Pennington. One way this could be made easier is if the black hole itself was electrically charged, which is theoretically possible but unlikely given that the universe is electrically neutral. You could then make your spaceship the opposite charge to the black hole which, assuming you could get it close enough in the first place, could then propel your ship away again.
If the probe is vaporised before it gets to the event horizon from something other than an obvious physical effect resulting from radiation or gravitational tidal forces, then this would show the firewall interpretation is correct, says Pennington. If the probe returns unscathed, then it would show complementarity is probably correct.
Being able to differentiate between complementarity and a firewall would be an important development, says Juan Maldacena at the Institute for Advanced Study in Princeton, New Jersey, even though the measurement remains tricky and he isn’t convinced we would be able to do this for a physical black hole.
There could be an alternative, though, he says, which doesn’t involve getting dangerously close. “You can do this observation without dying, in some sense.”
“You are not going to be able to do it for a realistic black hole, but one might be able to do something similar for toy black holes that you can make in a laboratory,” says Maldacena. “Of course, they are not the same as the big black holes you have in nature, they will be tiny microscopic black holes. For those, maybe you could try to actually measure this effect.”
One way would be to simulate one on a quantum computer, which Maldacena in a separate work calculated would require around a million qubits, some way beyond the 1000 qubits of today’s best quantum computers. However, it may be possible to test for these islands in simpler models that require fewer qubits, he says.
Reference: arXiv DOI: 10.48550/arXiv.2312.03078