Remembering John Wheeler, physicist and teacher

Professor John Wheeler was a giant among physicists and a perfect gentleman among scholars. He had a profound impact not only on all those who were associated with him, but also on many generations of physicists who follow his paths. He always will be in our collective memories.

It was only a few years ago when many of his associates celebrated his 90th birthday with a symposium on "Science and Ultimate Reality" at Princeton. About a year before the celebration, Wheeler, in his typical way, wrote an article in The New York Times about the glory of quantum physics and the mystery of its physical origin. In an interview conducted by Dennis Overbye, Professor Wheeler said gleefully that he had suffered a minor heart attack just a year earlier and that was a good thing, because the mishap reminded him that time was running short and he had better hurry up to solve the problem of tomorrow. His youthful enthusiasm for solving fundamental problems held no reservations even at his "tender" age of 90.

I first met Professor Wheeler in the fall of 1960 when I was an undergraduate at Princeton. He guided me to use Regge Calculus to exhibit the structure of a wormhole as my senior thesis. Later, when I was in graduate school, he guided me to study the stability of superheavy nuclei and introduced me to his idea of toroidal nuclei.

Wheeler's class teaching was in the style of the old masters. He often would come to the class without notes, and would begin his gentle and deliberate narration step by step from memory, writing down the concepts and formulas in detail and explaining the thinking that was needed at each step, as if he was thinking aloud. In the process, we students could observe how a great mind was at work, as if pulling out the thread of the silk continuously from a cocoon. On a few occasions, he might get stuck in the middle of the narration, and would stop in front of the blackboard. We students would wait eagerly

to see how he would resolve the difficult points in question. A short pause later, he would resume and march on to get the key concepts across. He was well known for drawing pictorial representations of objects that were informative, complex, and often with philosophical implications. His pictorial depiction of the "participatory universe" and its participating observers by the letter U (or perhaps it is the first letter of his initial J), with an eye at the end, is legendary!

Wheeler believed in the simplicity of nature. He reminded us often that once the solution of a physics problem became known, one would not but marvel at the simplicity of the solution and would hit oneself regretfully over why such a solution did not present itself earlier in the first place. Wheeler's high regard for the simplicity of nature led him to explore the possibility that the whole universe consisted of a single electron. Indeed, no picture would be simpler if the whole universe consisted of a single electron, with all other particles as composite entities of electrons and positrons. Wheeler examined the consequences of such a picture as hard as he could until the late 1940s, with the development of the model of poly-electrons, bound states of composite objects of many electrons and positrons. He reluctantly gave up this simple single-electron description of the universe when many elementary particles were discovered. However, as we now know, it was this picture of a single-electron-world line, winding itself up and down in time to give rise to electrons and positrons, that led to Richard Feynman *42's space-time picture of particles and antiparticles. Feynman won the Nobel Prize in physics in 1965.

Wheeler's belief in the simplicity of nature led him to search for a description of mass and charge that was free of singularities. In the 1950s, he constructed the singularity-free description of a mass that was a gravitational-electromagnetic entity (geon). He also

constructed the singularity-free wormhole solution of a mass and a charge to represent a "mass without mass" and a "charge without charge." Wheeler initially was perturbed by the solution of an inevitable gravitational collapse, and he examined the hydrodynamics

of gravitational collapse with many of his students, only to come to the conclusion that there was no escape from gravitational collapse. Hence, he adopted the term "black hole" to represent a gravitationally collapsed object.

Wheeler's belief in the simplicity of nature extended also to his search for answers to fundamental questions such as "Why the quantum?", "How come existence?", and "How come physical laws?" Nothing would be simpler if the physical laws arose by themselves from "nothing." Hence he proposed the simplest of these possibilities, "it from bit," in which the laws of nature could emerge from the primordial chaos of binary bits perhaps as a result of quantum uncertainty. Out of an ensemble of universes, a universe with a

successful evolution would leave behind participating observers to record the evolution. In this way, the physical laws arose from no law and out of nothing. Were Taoist masters Lao Tsu and Chuang Tsu alive today, they would look approvingly at Wheeler's participatory universe with great favor!

As reported by Kip Thorne *65, Feynman said of Wheeler: "Some people think Wheeler's gotten crazy in his later years, but he's always been crazy." Wheeler had an inventive mind that accepted no boundaries. He was always curious about the world around him, from the vast expanse of the universe down to the most basic and inner question of man's existence. He always told his students that "no question is stupid enough not to be an interesting question." Wheeler liked to tell us the story of a battleship (in a dead-silent environment) in enemy waters in which a stupid seaman's answer of flushing off an enemy mine by blowing from the mouth turned out to lead to the solution of blowing by the water hose – a tentative solution led to other better and better solutions. Wheeler's daring spirit to make new and unconventional proposals left a very deep impression on his students. He argued that it was only through daring supposition and the ability to abstract the most complicated subject to come up with a few words, a few lines, that may eventually bring out the promising lines of approach. No supposition is silly enough to be dismissed right away. Just by trying out one way, another promising method may follow. Examples are best illustrated by his pursuit of the one-electron universe that led to development of the space-time concept, and the search for singularity-free solutions that led to the inevitability of the gravitational collapse and black holes.

Among other things, Wheeler's "black hole," "wormhole," "geons," "quantum foams," "mass without mass," "charge without charge," "law without law," "it from bit," "Planck length," "superheavy nuclei," and "neutron drip line" have become parts of the physics vocabulary.

In nuclear physics, Professor Wheeler made many important contributions. Wheeler was the first to propose the collective motion description for the nuclear structure of beryllium and carbon nuclei. It was indeed a pity that when the Nobel Prize in physics was awarded in 1975 for the discovery of the nuclear collective motion, Wheeler was not included as one of the honorees. Soon after the discovery of nuclear fission, the classic paper of Bohr and Wheeler in 1939 laid the foundation for the dynamics of the nuclear-fission process. His papers with Hill, with Ford, and with Griffin in the 1950s provided the groundwork for the dynamics and scattering of nuclear systems. Wheeler was the first to propose superheavy nuclei and toroidal nuclei. He introduced the S matrix, which was later taken up by Heisenberg. He was the first to study the solution of the Schroedinger equation by the phase-amplitude method. His contributions to gravitation physics are well known. In his later years, he turned to examine the foundation of quantum mechanics and the question of existence, which also had a significant impact.

Professor Wheeler's love for science was profound and contagious. He inspired many generations of physicists. Wheeler declared categorically that the greatest discovery is yet to come. It is such a sweeping pronouncement that has driven many bright minds working through thick and thin to search for new discoveries to reveal nature as it is. In the process, we all benefit from the fruits of their continuous search. At the end, whether the discovery will be the greatest or not will not matter. It is the spirit of pursuit that is the essence of the quest.

Wheeler's First Moral Principle says, "Never do a calculation without knowing the answer." He urged his students to make first an estimate before every evaluation and to try a simple physical argument before every derivation. A right guess will reinforce intuition, while a wrong guess will bring new insights. This way of developing intuition should indeed be a guiding principle for all serious students of physics.

Among his many admirable personal qualities, Wheeler held his students in high regard and encouraged them to reach great heights of achievement. He had an uncanny ability to get the best out of his students. Many were the times when he encouraged his students to take up assignments that later would be pivotal in the development of their careers. In 1962, Wheeler was lecturing on general relativity. On many occasions when Wheeler was away, he would ask Thorne, a second-year graduate student, to substitute for him to teach the general relativity class. We all now know what an outstanding physicist Thorne has turned out to be. When John Toll *52 was a graduate student, Wheeler asked him to take an important administrative assignment in Project Matterhorn. Toll later held many important positions in many educational and scientific organizations. Wheeler's ability to nourish the development of his students gave an extra dimension to his impact as a mentor.

Wheeler had genuine concern for the welfare of his students. From time to time, he would write letters to his students to provide a note of encouragement. In the early 1970s, he wanted to get in touch with one of his students, Bei-Lok Hu *72 (now at Maryland), but he did not know his whereabouts. He wrote five letters and sent them out simultaneously to five locations in order that his message would reach him. Few other mentors would try as hard and would be as concerned about their students.

Wheeler had warm friendships with his students, both when they were working under him and afterward. In my student days, he often invited his students to tea in his Princeton home near the Institute for Advanced Study. In earlier years, when Einstein was still living, sometimes his Sunday tea party might include Einstein as a special guest. During the summer, Wheeler would stay at his cottage in Maine and often would invite his students to visit him. I visited him once in early 1970s at his cottage, which was on a small island by the sea, connected to the shore by a causeway. Around noon, he looked at his watch and stopped working on our physics problem. He changed to a swimming suit, opened the screen door, and took a dip to swim just outside his cottage. His warmth and friendship provide a role model for his students – to treat others with respect and sincerity. Later, when my three children, Janet '89, Albert '91, and Lisa '99, went to study at Princeton and came to see him, he would receive them with warmth and kind words of encouragement.

Wheeler was productive and kept on publishing even in his 90s. His inquisitive spirit was an inspiration for all his students. He sat and listened to the scientific talks in the symposium at Princeton to honor his 90th birthday all the way through – for many hours on end, without showing any sign of being bored or tired. It is not surprising that his students have a good example to follow and continue to be productive in making scientific contributions. We all hope that we can be scientifically productive, even in our old age.

We remember Professor Wheeler's inquisitive spirit. We remember his kindness, his encouragement, and his warm friendship. We remember the many wonderful pieces of physics treasure that he left us. By his enthusiastic participation, he greatly enriched the universe. He was one of the most honorable members of the participatory universe. He will always be in our collective memories.

Cheuk-Yin Wong '61 *66 has been working at Oak Ridge National Laboratory since obtaining his Ph. D degree at Princeton, with sabbatical leaves at Niels Bohr Institute, Copenhagen, and at M.I.T. Wong was elected a Fellow of the American Physical Society in 1978 and served as chairman of the Overseas Chinese Physics Organization in 1999-2000. He authored the textbook Introduction to High-Energy Heavy-Ion Collisions, edited six books, and is a former editor of the International Journal of Modern Physics E and Chinese Physics.