Photo by NASA/JPL-Caltech
Don Page has a unit of time named after him: Page time. It’s the time it would take for a black hole to emit half its radiation. “It’s longer than the age of the universe,” says the University of Alberta physicist, whose area of expertise is cosmology and theoretical gravitational physics. “I like to say they named it for me because I’m such an old guy.”
He mentions Page time in an effort to explain the significance of a recent article by his former supervisor, Stephen Hawking, in which the renowned physicist pronounced, “There are no black holes.” Sci-fi fans everywhere can relax, though. Hawking doesn’t want to do away with the idea of black holes, just with the current definition, explains Page.
Traditionally, it was thought a black hole’s gravitational force was so strong that nothing — not even light — could escape its boundary, known as the event horizon. But in the 1970s, physicists began to realize that this idea doesn’t jibe with a central principle of quantum mechanics: information is never lost.
Since that time, the physics community has determined that both energy and information do escape from black holes.
“There’s stuff inside the black hole, stuff just outside the black hole and stuff that’s been radiated and is farther away from the black hole,” Page says. The “stuff” (information and energy) in each of these areas constitutes a “system.”
But this creates a further conundrum for physicists. Why? You’ll need to follow this closely.
According to quantum mechanics, one system can be correlated to another. This is called entanglement. “The stuff that’s just outside the black hole must be entangled with what’s been radiated previously, which means that it’s no longer entangled with what’s inside the black hole,” says Page. This is because of the assumption that information is not lost, he says.
“So across this threshold — between the inside and the outside of the black hole — you get these huge energy densities.” These energy densities, which wrap around the inside of black holes and wouldn’t be visible from the outside, are known as “firewalls.”
And herein lies the conundrum: very few people in the physics community believe in a firewall because it “conflicts with what most of us expect based on our past observations,” says Page. Of course, physicists are reliant on past observations because, well, they’re not exactly going to find any eyewitness accounts. “We have no reports from anyone who has fallen into an old black hole, so we don’t have actual observations that directly conflict with firewalls. … Obviously, we can’t rule it out entirely. But we don’t see events of this magnitude in the universe.”
Don Page (second from left) meets with fellow physicists (from left) James Hartle, Stephen Hawking and Thomas Hertog in April 2014.
In other words, expecting the existence of firewalls would be pretty much like expecting the Earth to suddenly halt in its orbit of the sun.
Hawking’s paper was an attempt to deal with this apparent contradiction. He proposed that instead of an event horizon, from which nothing can escape, black holes are bounded by an “apparent horizon,” where the effects of entanglement are perhaps not as great. In doing away with the traditional definition of black holes, he could claim “there are no black holes.”
But has Hawking solved the conundrum of the firewall? “I don’t think he has the full answer yet,” says his former student.
In April, Page arrived in England, where he met with Hawking and others. They continued work on the problem. “I saw very little in his opinions to disagree with, but I don’t think he yet has an argument strong enough to convince the firewall advocates who disagree with Stephen and me and believe there are firewalls,” says Page, who is preparing for publication what he calls “a partial argument” against firewalls.
As physicists debate the question, the rest of us will have to wait to find out the answer. But, hopefully, not a Page time.