In a packed lecture hall at Columbia University in 1958—or so the story goes—eminent physicist Wolfgang Pauli was presenting a radical new theory. In the audience was Niels Bohr, another eminent physicist, who, at lecture's end, stood up and announced:
"We are all agreed that your theory is crazy. The question that divides us is whether it is crazy enough to have a chance of being correct."
Stanford University theoretical physicist Leonard Susskind when he came up with his holographic principle—an idea that has recently gained traction in the physics community. The principle, which states that our universe is a three-dimensional projection of information stored in two dimensions at the boundary of space, certainly ranks as crazy. But is it crazy enough?
After reading Susskind's entertaining new book, "The Black Hole War," you may decide that, yes, the holographic principle may well be on the good side of crazy. But before he gets to the holographic principle, Susskind gives an explanation, both lucid and enjoyable, of why black holes are so crucial to the future of physics and to any eventual reconciliation of relativity and quantum mechanics.
Einstein's general theory of relativity describes the world of the very large: planets, stars, galaxies, black holes, the warped curvature of space. Quantum mechanics describes the world of the very small, the bizarre precincts of subatomic particles, where gravity is trivial. General relativity is classical, in that it can be used to make definite predictions about reality. Quantum mechanics is not: One can predict outcomes only in terms of their probability. The dream of many physicists is to find a way to unify these two seemingly antagonistic conceptions of reality.
It turns out, though, that black holes may have just the right ingredients for a unification recipe. A black hole is a region of space at whose center is the remnant of a collapsed star. But "remnant" isn't really the right word: Because of intense gravitational pressure, that star has become what physicists call a singularity—an infinitesimal point of infinite density. Black holes suck up everything in their vicinity; gravity in a black hole's interior is so strong that nothing can escape, not even light.
In the 1970s, Cambridge University theoretical physicist Stephen Hawking made the brilliant deduction that black holes dissipate their energy the way everything else in the universe does, through the radiation of heat—a process known now as Hawking radiation. Gradually, that is, black holes evaporate. In 1983, Hawking also claimed that, along with everything else a black hole has gobbled up, "information is lost in black hole evaporation." ("Information" means the same thing as it does in computer science: data that can be measured in bits—0's or 1's. The "information" is what the bits encode.)
For Susskind, this was a declaration of war. Susskind is a quantum theorist, and a central principle of quantum mechanics is that information is conserved; it can never be annihilated. If Hawking was right, "the foundations of [quantum mechanics] were destroyed."
"War" may seem an overblown word to describe the debate between Susskind and Hawking, especially since the two are friends and Susskind's admiration for Hawking is boundless—he considers him a "truly heroic figure" and "the first to enter a remote country and bring back gold." At Hawking's 60th birthday celebration, Susskind declared that "of all the physicists I have known he has had the strongest influence on me and my thinking." But perhaps "war" has the right emotional tone to convey his near-panic about Hawking's conjecture.
So how did Susskind, and several others, save quantum mechanics? It has to do with the peculiar nature of singularities at the heart of black holes. Infinitesimally small, infinitely dense, they are macroworld objects that behave with the quantum weirdness of the microworld.
In the quantum world, particles behave like particles or like waves, depending on what experiment is being performed; it's as though there were two separate realities. Within black holes, there's a similar duality. Susskind asks us to imagine two space travelers: Alice, who drifts toward a black hole, and Bob, who stays behind on the space station. "To Bob . . . it takes an eternity for Alice to reach the point of no return, but to Alice it may take no more than the blink of an eye." As Alice reaches that point—the black hole's invisible, spherical horizon—she appears to Bob to be frozen in time. From Alice's point of view, though, she passes through the horizon "without any sense of slowing down or speeding up." Her reality is completely different from Bob's.
Alice's fate isn't kind. The tidal forces of the black hole will tear her apart. According to the Hawking version, that's the end of her and her bits—the information of which she's made up.
But as Susskind and his colleagues discovered, that's not the case. The black hole's heat is a shredded and scrambled version of the information Hawking thought was lost, and "[i]nformation leaks out in the Hawking radiation in the same way that it escapes from an evaporating pot of water."
It gets weirder. That heat—comprising the information that fell into the black hole—exists as a thin layer coating the black hole's horizon. It's as though the black hole were a three-dimensional projection of that two-dimensional layer of information—in short, a hologram.
As black holes go, so goes the universe:
"The most notable inhabitants of the universe—the galaxies—are built around giant black holes that are continually gobbling up stars and planets. Out of every 10,000,000,000 bits of information in the universe, 9,999,999,999 are associated with the horizons of black holes."
And consider this: As the universe's expansion accelerates, the light from faraway galaxies, the most distant of which are receding at light speed, will cease to reach us. Yet, to our eyes, nothing will look any different; this "cosmic horizon" will simply seem frozen, much as Alice appears to Bob when she enters a black hole.
"It is as if we all live in our own private inside-out black hole," Susskind says. Could that ultimate, visible layer of the universe contain the information we experience as our three-dimensional reality?
This necessarily skeletal account may suggest the flavor of Susskind's bold thinking but does little justice to his book.