paul_j_steinhardt's picture
Albert Einstein Professor in Science, Departments of Physics and Astrophysical Sciences, Princeton University; Coauthor, Endless Universe

One of the sacred principles of physics is that information is never lost. It can be scrambled, encrypted, dissipated and shredded, but never lost. This tenet underlies the second law of thermodynamics and a concept called "unitarity," an essential component of unified theories of particles and forces. Discovering a counterexample or new ways to preserve information could be a real game-changer: one that alters our understanding of the fundamental laws of nature, transforms our concept of space and time, triggers a reconstruction of the history of the universe and leads to new prognostications about its future.

There is a real chance of breakthrough in the foreseeable future as theorists converge on one of the greatest threats to information preservation: black holes. According to Einstein's general theory of relativity, a black hole forms when matter is so concentrated that nothing, not even light, can escape its gravitational field. Any information that passes through the event horizon surrounding the black hole—the "point of no return"—is lost forever to the outside world. Suppose, for example, that Bob pilots a spaceship into the black hole carrying along three books of his choice. It appears that the titles and contents of the three books vanish. Either that or Einstein's general theory of relativity is wrong.

There is nothing shocking about having to correct Einstein's general theory of relativity. It's known to be missing an essential element, quantum physics. Einstein, and generations of theorists since, have sought an improved theory of gravity that incorporates quantum physics in a way that is mathematically and physically consistent. String theory and loop quantum gravity are the most recent attempts.

There is no doubt that quantum physics alters the event horizon and the evolution of a black hole in a fundamental way, as first point out in the work of Jacob Bekenstein, Gary Gibbons and Stephen Hawking in the 1970s. According to quantum physics, matter and energy are composed of discrete chunks known as quanta (such as electrons, quarks and photons) whose position and velocities are undergoing constant random fluctuations. Even empty space—a pure vacuum—is seething with microscopic fluctuations that create and annihilate pairs of quanta and anti-quanta. The seething vacuum just outside the event horizon occasionally produces a pair of quanta, such as an electron-positron duo, in which one escapes and one falls into the black hole. From afar, it appears that the black hole radiates a particle. This phenomenon repeats continuously, producing a spectrum of particles known as "Hawking radiation," whose properties are similar to the "thermal radiation" emitted by a hot body. Very slowly, the black hole radiates away energy and shrinks in mass and size until—well, here is where the story really begins to get interesting.

Thermal radiation only depends on the temperature of the emitting body, providing no other details about the body itself. So, if Hawking radiation is truly thermal, then the information inside the black hole is truly lost . For the last decade, though, leading physicists including Gerard 't Hooft, Leonard Susskind, and Stephen Hawking fiercely debated (and even bet on) the outcome—Susskind refers to the debate as the "black hole war." Aided by new theoretical tools developed by Juan Maldacena and other string theorists, physicists discovered that Hawking radiation is not quite thermal after all! The radiation deviates by a tiny amount from a perfectly thermal signal, and the tiny deviation incorporates information about whatever was inside. The titles of Bob's three books, for example, are not lost forever, although the information dribbles out incredibly slowly and is unimaginably scrambled. Thus, victory was declared in the black hole war.

But it may be an uneasy peace, for there remains the question of what happens to information after it falls into the horizon. This is a reasonable question because, curiously enough, passage through the horizon can be unremarkable (if the black hole is very big). There are no sign posts indicating to Bob that he has passed the point of no return, and his books remain intact. Now suppose Bob scribbles some notes in the margins of his book. What happens to this information?

Here there is a diversity of views. Some suggest that this information, too, is radiated away through the Hawking process and the black hole simply disappears. Some suggest that quantum physics makes the event horizon penetrable so that some information is radiated by the Hawking process but some escapes directly. Yet others suggest that the information is copied; one copy is radiated away and the other strikes the singularity, entering a new section of space-time that is causally disconnected from observers outside the black hole, so the two copies never meet.

Theorists have recently developed a number of new theoretical tools to attack the problem and are hard at work. Although the subject lies in the domain of quantum gravity, the implications for other fields, including my own, cosmology, will be profound. The answer will shape any future formulation of the laws of thermodynamics, quantum gravity and unified field theory. Since scrambling information, a.k.a., the entropy, determines the arrow of time, the results may inform us how time may have first emerged at the cosmic singularity known as the big bang. Or, if it proves possible for copies to bounce from the black hole singularity to a separate piece of space time, the same may apply to an even more famous singularity, the big bang. This would lend support to recent ideas suggesting that the large scale properties of the universe were shaped by events before the big bang and these conditions (a form of information) were transmitted across the cosmic singularity into a new phase of expansion. In fact, if information is forever preserved across singularities, the universe may undergo regularly repeating cycles of big bangs, expansion, and big crunches, forever into the past and into the future. To me, a breakthrough with these kinds of implications would be the ultimate game-changer.