The cosmologist and pop-science icon Stephen Hawking, who died last March on Einstein’s birthday, spoke out from the grave recently in the form of his last scientific paper. Appropriately for a man on the Other Side, the paper is about how to escape from a black hole.
Cleansed of its abstract mathematics, the paper is an ode to memory, loss and the oldest of human yearnings, the desire for transcendence. As the doomed figure in Bruce Springsteen’s “Atlantic City” sings, “Everything dies, baby, that’s a fact, but maybe everything that dies someday comes back.”
Dr. Hawking was the manifestation of perseverance; stricken by Lou Gehrig’s disease, he managed to conquer the universe from a wheelchair. The fate of matter or information caught in a black hole is one that defined his career, and it has become one of the deepest issues in physics.
Black holes are objects so dense that, according to Einstein’s law of general relativity, not even light can escape. In 1974, Dr. Hawking turned these objects, and the rest of physics, inside-out. He discovered, to his surprise, that the random quantum effects that rule the microscopic world would cause black holes to leak and, eventually, explode and disappear.
In the fullness of time (which in many cases would be longer than the current age of the universe), all the mass and energy that had fallen into the hole would come back out. But, according to the classical Einstein equations, black holes are disturbingly simple; their only properties are mass, electrical charge and angular momentum. Every other detail about what falls into a black hole disappears from the universe’s memory banks. A black hole has no complications — no hair — the saying went.
So the fountain of matter and energy exiting a black hole would be random, Dr. Hawking emphasized in a paper in 1975. If you fell into one and came back out, you would lack all the details that had made you: male or female, blue eyes or brown, Yankee fan or Red Sox fan. The equation describing that fate is inscribed on Dr. Hawking’s tombstone, in Westminster Abbey, where it presumably will endure the ages.
That’s some kind of reincarnation. If nature can forget you, it could forget anything — a deathblow to the ability of science to reconstruct the past or predict the future. “It’s the past that tells us who we are,” Dr. Hawking told a conference at Harvard a couple years ago. “Without it, we lose our identity.”
In effect, Dr. Hawking maintained in his 1975 paper, the paradoxical quantum effects that Einstein had once dismissed, saying that God doesn’t play dice, were adding an extra forgetfulness to nature. “God not only plays dice,” Dr. Hawking wrote, “but he often throws them where they can’t be seen.”
Those were fighting words to other physicists; it was a basic tenet that the proverbial film of history can be run backward, to reconstruct what happened in, say, the collision of a pair of subatomic particles in a high-energy collider.
Recent years have brought a glimmer of hope. Andrew Strominger of Harvard discovered that, when viewed from the right mathematical perspective — that of a light ray headed toward the infinite future — black holes are more complicated than we thought. They have what Dr. Strominger has called “soft hair,” in the form of those imaginary light rays, which can be ruffled, stroked, twisted and otherwise arranged by material coming into the black hole. In principle, this hair could encode information on the surface of the black hole, recording all those details that Einstein’s equations supposedly leave out.
Whether this is enough to save physics, let alone a person falling into a black hole, is what Dr. Hawking was working on in the years before he died.
“When I wrote my paper 40 years ago, I thought the information would pass into another universe,” he told me at the Harvard conference. Now, he said, it’s on the surface of the black hole. “The information will be re-emitted when the black hole evaporates.”
Other experts, including Juan Maldacena of the Institute for Advanced Study in Princeton, have been more measured, saying that if soft hair does not solve the information paradox, it might at least help.
In his recent, posthumous report, which drew a flurry of press, Dr. Hawking and his colleagues endeavored to show how this optimistic idea could work. Besides Dr. Hawking, the paper’s authors were Dr. Strominger as well as Malcolm Perry and Sasha Haco of Cambridge University.
Dr. Strominger is hopeful that physicists one day will be able to understand black holes just by reading what is written in this soft hair.
“We didn’t prove it,” he said in an email. But, he added, they did succeed in showing how all the pieces could fit together: “If our guess is right, this paper will be of central importance. If not, it will be a technical footnote.”
Few of us, including Dr. Hawking, ever harbored the hope that solving the information paradox would bring back our parents, the dinosaurs or Joe DiMaggio from whatever was waiting in Atlantic City. Somewhere along the way we’ve all made some sort of accommodation with the idea that our personal timelines will come to an end, but we take some comfort in knowing that we will be remembered, and that our genes and books and names will carry on.
Last year’s Pixar/Disney movie “Coco,” which I happened to watch with my daughter recently, tells the story of a young Mexican boy who visits the Land of the Dead to find an ancestor who can help him in his quest to become a musician. The Land of the Dead is a lively place, but its denizens can only stay there, it turns out, as long as someone remembers them. When the memories vanish, so even do the animated skeletons
Some astronomers now say that even this pale version of salvation might be in jeopardy. A mysterious force called dark energy is speeding up the expansion of the universe. Someday, these experts say, if the expansion continues, making the galaxies fly away faster and faster, the rest of the universe will be permanently out of sight to us, and we will be forever out of sight of it. It would as if we were surrounded by a black hole, into which all our information and memory were disappearing.
Our little bubble of the Milky Way might always remember Aretha and Cleopatra and Shakespeare and Hawking. But will the rest of the universe might remember us?