Editor's Note: Yesterday morning, renowned physicist John Archibald Wheeler died of pneumonia. He was an iconic figure: a veteran of the Manhattan Project, a pioneer of the search for a quantum theory of gravity, and an originator of such evocative terms as "black hole." Most physics students know him as co-author of the standard textbook on Einstein's general theory of relativity—a tome that defies almost every stereotype of a textbook, much as Wheeler's own career defied almost every generalization. He was rigorous yet playful, and he always had a pithy, Zen-like phrase for profound ideas and questions: "it from bit," "mass without mass" and "Why the quantum?". An out-of-the-box thinker who wasn't afraid to speculate, he always carefully identified speculation as such. In so doing, he opened up space for his  colleagues to push the boundaries. Here is a profile by John Horgan, published by Scientific American in 1991:

Questioning the "It from Bit"

It's hard keeping up with John Archibald Wheeler. When we leave his third-floor office at Princeton University to get some lunch, he spurns the elevator—"Elevators are hazardous to your health," he declares and charges down the stairs. He hooks an arm inside the banister and pivots at each landing, letting centrifugal force whirl him around the hairpin and down the next flight. "We have contests to see who can take the stairs fastest," he says over a shoulder.

Outside, Wheeler marches rather than walks, swinging his fists smartly in rhythm with his stride. He pauses only when he reaches a door. Invariably, he gets there first and yanks it open for me. After passing through, I wait a moment in reflexive deference—after all, the man will be 80 years old in July—but a moment later he's past me, barreling toward the next doorway.

The metaphor seems so obvious I almost suspect it is intentional. Wheeler, a professor emeritus of physics at Princeton and the University of Texas at Austin, where he holds a joint appointment and spends a few weeks each year, has made a career of racing ahead of other scientists and throwing open doors for them. He has helped gain acceptance—or at least attention—for some of the most outlandish ideas of modern physics, from black holes to multiple-universe theories. "He has this great ability to see what is important before anyone else and persuade others that this is so," says David Deutsch, a physicist at the University of Oxford.

Wheeler is also renowned for his coinages, analogies and aphorisms, both self-made and co-opted. Among the one-liners he bestows on me are, "If I can't picture it, I can't understand it" (Einstein); "Unitarianism [Wheeler’s official religion] is a featherbed to catch falling Christians" (Darwin); "Never run after a bus or woman or cosmological theory, because there'll always be another one in a few minutes" (a professor of French history at Yale); and "If you haven’t found something strange during the day, it hasn't been much of a day" (Wheeler). Lately Wheeler has been drawing his colleagues' attention to something strange indeed. It is a worldview uniting information theory, which seeks to maximize the efficiency of data communications and processing, with quantum mechanics. As usual, Wheeler has packaged the concept in a catchy phrase: "it from bit." And as usual, he delights in being ahead of—or at least apart from—the pack. "I hope you don't think I'm too much like Daniel Boone," he says slyly. "Anytime someone moved to within a mile of him, he moved on."

Wheeler might have been dismissed as fun but flaky long ago if he did not have such unassailable credentials. The son of two librarians "who were interested in ideas, interested in the world, interested in adventures" (and who obviously endowed him with an omnivorous appetite for reading), he entered Johns Hopkins University at the age of 16 and emerged with a PhD in physics six years later.

He subsequently journeyed to Copenhagen to study with Niels Bohr, the great Danish physicist, "because he sees further ahead than any man alive," Wheeler wrote on his application for the fellowship. In 1939 Bohr and Wheeler published the first paper successfully explaining nuclear fission in terms of quantum physics. Wheeler's expertise in nuclear physics led to his involvement in the construction of the atomic bomb during World War II and, during the Cold War's early years, the hydrogen bomb.

I had heard that beneath Wheeler's puckish demeanor lay a core of steel. That is apparent when I ask if he has any second thoughts about helping to create nuclear weapons. His eyes narrowing, he acknowledges that "a lot of my friends have gone around giving what I call 'scare-the-dope speeches'" deploring such weapons. But he has no regrets. Nuclear weapons, he insists, saved lives by ending World War II quickly and by deterring Soviet aggression thereafter.

When his involvement in the H-bomb project ended, Wheeler immersed himself in studying relativity and gravity—which he calls his "lifelong love"—at Princeton. In 1966 he proposed that a brilliant cloud of gas known as the Crab nebula was illuminated from within by a whirling sphere of solid neutrons created by the implosion of a star. Astronomers later detected such spinning neutron stars, or pulsars, both in the Crab nebula and elsewhere in the Milky Way.

Wheeler also speculated that matter could collapse even beyond the solid­neutron state, becoming so dense that nothing—not even light—could escape its gravitational clutches. Such an object was first proposed by J. Robert Oppenheimer and Hartland S. Snyder in 1939, but it had been dismissed as a theoretical curiosity and not something that might actually exist.

Wheeler recalls discussing such "completely collapsed gravitational objects" at a conference in 1967, when someone in the audience casually dropped the phrase "black hole." Wheeler immediately adopted the phrase for its brevity and "advertising value," and it caught on. Largely because of Wheeler's proselytizing, black holes now play a crucial role in astrophysics and cosmology.

In the 1950s Wheeler grew increasingly intrigued by the philosophical implications of quantum physics. According to quantum theory, a particle such as an electron occupies numerous positions in space until we observe it, when it abruptly "collapses" into a single position. Wheeler was one of the first prominent physicists seriously to propose that reality might not be a wholly physical phenomenon. In some sense, Wheeler suggested, reality grows out of the act of observation, and thus consciousness itself; it is "participatory."

These ruminations helped to inspire two of the odder notions of modern physics. In 1957 Hugh Everett III of Princeton, in a Ph.D. thesis supervised by Wheeler, proposed the many worlds theory: Although we can observe a particle in only a single position, it actually occupies all the positions allowed it by quantum theory—in different universes. Four years later another Princeton physicist, Robert H. Dicke, introduced the anthropic principle. It asserts that the universe is the way it is because if it were not, we would not be here to observe it. Although many physicists recoiled from such ideas as untestable and therefore unscientific, Wheeler urged that they be taken seriously.

At the same time, Wheeler began to draw his colleagues' attention to some intriguing analogies between physics and information theory, which was first proposed by Claude E. Shannon of Bell Laboratories in 1948. Just as physics builds on an elementary, indivisible entity that depends on the act of observation—namely, the quantum—so does information theory. Its "quantum" is the binary unit, or bit, which is a message representing one of two choices: heads or tails, yes or no, 0 or 1.

In addition, information theory provided a new way of viewing entropy, one of the most important, and confusing, concepts in physics. Entropy is defined as the disorder, or randomness, or "shuffledness," as one physicist has put it, of a system. Shannon had proposed that the information in a given system—the sum total of all its possible messages—is a function of its entropy; as one increases, so does the other. Wheeler pointed out that entropy, like a quantum event, is thus tied to the state of mind of the observer. The potential information of a system is proportional to one's ignorance, and so, therefore, is the entropy of the system.

Wheeler was not the only scientist to recognize these links, "but he was probably the first to recognize the potential implications for fundamental physics," says physicist Wojciech H. Zurek of Los Alamos National Laboratory. In the early 1970s Wheeler's speculation bore some tangible fruit when yet another of his graduate students, Jacob Bekenstein, described a black hole in terms of information theory. The surface area of a black hole's "event horizon," Bekenstein showed, is equal to its thermodynamic entropy, which in turn is equal to the information that the black hole has consumed.

Spurred by this and other findings, an ever larger group of researchers—including computer scientists, astronomers, mathematicians and biologists as well as physicists—has passed through the doors flung open by Wheeler. In the spring of 1989 a number of them gathered at the Santa Fe Institute in New Mexico to update one another on their progress. The proceedings of the meeting were published in 1990 as Complexity, Entropy and the Physics of Information.

The lead chapter of the book is based on Wheeler's address to the meeting, and it is vintage Wheeler. Over the course of 16 pages, he cites 175 sources, including the Greek poet Parmenides, Shakespeare, Leibniz, Einstein and graffiti in the men's room of the Pecan Street Cafe in Austin, Tex., which states: "Time is nature's way to keep everything from happening all at once." Wheeler also spends some time establishing what reality is not: It is not a "giant machine, ruled by any preestablished continuum physical law"; at its most fundamental level, it even lacks dimension, such as space or time.

What is reality, then? Wheeler answers his own question with the koanlike phrase "it from bit." Wheeler explains the phrase as follows: "Every 'it'—every particle, every field of force, even the spacetime continuum itself—derives its function, its meaning, its very existence entirely—even if in some contexts indirectly—from the apparatus-elicited answers to yes-or-no questions, binary choices, bits."

Elaborating on this idea, Wheeler evokes what he calls the "surprise" version of the old game of 20 questions. In the normal version of the game, person A thinks of an object—animal, vegetable or mineral—and person B tries to guess it with a series of yes-or-no questions. In surprise 20 questions, A only decides what the object is after B asks the first question. A can then keep choosing a new object, as long as it is compatible with his previous answers. In the same way, Wheeler suggests, reality is defined by the questions we put to it.

How do other scientists react to such propositions? Zurek, who organized the Santa Fe meeting and edited the proceedings, calls Wheeler's style "prophetic, leading the way rather than relating what's already been done."

Wheeler acknowledges that the ideas of the entire field are still raw, not yet ready for rigorous testing. He and his fellow explorers are still "trying to get the lay of the land" and learning how to converse in the language of information theory. Wheeler says the effort may lead to a powerful new vision of "the whole show" or to a dead end. "I like that phrase of Bohr's: 'You must be prepared for a surprise, a very great surprise.'"

Another favorite Wheelerism is "one can only learn by teaching." Wheeler has been the supervisor for some 50 PhDs in physics during his career, an "enormous number," according to Jeremy Bernstein, a physicist and science writer. Wheeler's most famous student was the late Richard P. Feynman, who received a Nobel Prize in 1965 for his work in quantum electrodynamics. Technically, Wheeler can teach no longer. "If you know of a school that lets its professors teach after they reach 70," he says, "let me know."

But of course, Wheeler can neither stop teaching nor stop learning. During my visit, we run into a young physicist who briefs us on his new cosmological theory, which posits that the universe is riddled with knotlike spatial "defects." "I can't believe space is that crummy," Wheeler declares. Noting the physicist's somewhat crestfallen expression, Wheeler touches his arm and says: "To hate is to study, to study is to understand, to understand is to appreciate, to appreciate is to love. So maybe I'll end up loving your theory." The smile returns to the young man's face, and Wheeler marches off.