Charles Q. Choi
Black holes are shrouded in mystery, with recent research only deepening scientists' understanding of how strange they must be.
The challenge of nailing down the nature of black holes has returned to the news, with renowned physicist Stephen Hawking saying recently that "there are no black holes"—at least not how we've thought of them.
General Relativity
The conventional view of black holes is that they possess a gravitational pull so powerful that nothing is able to escape, not even light. The boundary past which there is supposedly no return is known as the event horizon.
Now Hawking says there are no event horizons after all. That means that stuff—including light—can indeed escape from black holes. More on that in a minute.
First, let's explain the conventional view of how black holes work. One way to illustrate this is to pretend that a person—say an astronaut—is falling in.
Einstein's theory of general relativity holds that the astronaut would feel nothing in particular while crossing the event horizon. But the extreme gravity inside the black hole would stretch the astronaut's body thinner and thinner, a process whimsically known as "spaghettification." Here's how it would look:
Jason Treat, NG Staff
Source: Avery Broderick, Perimeter Institute for Theoretical Physics, University of Waterloo, Canada
Eventually it would be crushed at the singularity, a point of infinite density at the black hole's center.
Quantum Mechanics
This original picture of black holes holds that they essentially destroy all information about anything that ventures past their event horizons—astronauts included. But quantum physics, the best description so far of how the universe behaves on a subatomic level, includes a principle known as unitarity, which maintains that information cannot be destroyed.
To resolve this conflict, some scientists have recently (and controversially) suggested that black holes have "firewalls" at their event horizons. These are zones of extraordinarily destructive radiation. In this scenario, our astronaut would be instantly incinerated when crossing the event horizon, as would anything else falling into a black hole. The radiation released by the firewall would preserve information about the destroyed objects, astronauts included.
Here's how that would look:
Hawking's Idea
The problem with firewalls is that while they obey quantum physics, they contradict Einstein's well-tested equivalence principle.
This principle basically says that people do not experience their weight while in free fall, so falling through a black hole's event horizon should be an unremarkable event—which is why an astronaut would not even be aware of the transit.
This is where Stephen Hawking comes in. His idea resolves the problem of black holes defying the equivalence principle by doing away with their event horizons.
Instead of an event horizon, he says, black holes have "apparent horizons" that only temporarily entrap matter and energy, which are eventually released as radiation that retains all the original information about what fell into the black hole, albeit in highly scrambled form.
It would look like this:
Not the Last Word
However, many researchers have expressed skepticism that Hawking's idea solves the riddles of black holes.
"I would caution against any belief that Hawking has come up with a dramatic new solution answering all questions regarding black holes," said theoretical physicist Sean Carroll of the California Institute of Technology, who did not participate in Hawking's study. "These problems are very far from being resolved."
Theoretical physicist Leonard Susskind of Stanford University in California, who also did not take part in Hawking's research, suggests there may be another solution to the conundrums that black holes pose.
For instance, work by Susskind and Juan Maldacena of the Institute for Advanced Study hint that a phenomenon known as quantum entanglement might link two black holes via a wormhole (a shortcut connecting points distant in space and time). This line of thought might serve as the foundation for research that could settle the firewall controversy, Susskind said.
The bottom line: The mysteries surrounding black holes are far from resolved.
Look for The Truth About Black Holes on the cover of the March issue of National Geographic magazine, on newsstands February 25.
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