In April 1932 seven physicists, six men and one woman, attended a small annual gathering in Copenhagen. To be honest, only six of them were actually there. The seventh, Wolfgang Pauli, had originally intended to go, as he had in earlier years and would do so again, but he decided that spring instead to take a vacation. He was there in spirit, as you will see.

Four of the seven—Niels Bohr, Paul Dirac, Werner Heisenberg, and Wolfgang Pauli—would be placed in most physicists' selection of the century's top ten physicists. Lise Meitner, the only woman in the group, ranks high on anyone's list of the century's most important experimentalists. Another of the seven, Max Delbrück, changed fields soon after the meeting, though he never stopped defining himself as a physicist. He went on to become one of the founding fathers of modern molecular biology and ranks as one of that discipline's top ten. All of them taught and mentored a generation of future scientists. The last of the seven, Paul Ehrenfest, was perhaps the greatest teacher of them all.

Physics was fortunate to have at one moment a remarkable number of individuals to help create and shape the great revolution in science called quantum mechanics. Indeed, one could say that the revolution occurred because of them. It developed very differently from relativity, the twentieth century's other major departure from physics' past. Relativity, in the special theory of 1905 and the general theory of 1916, was the work of a single individual, Albert Einstein. Both the special theory and the general theory were essentially complete in Einstein's initial formulations, requiring no revisions or subsequent interpretations. Quantum mechanics, on the other hand, emerged in 1925–26 only after a long buildup. Its details evolved over time, and its meaning continued to be debated for years. Unlike relativity, it was the work of many who struggled together, often arguing with one another as they hammered out the theory's conclusions. Its final version, the so-called Copenhagen interpretation, was contested even by some of the creators of the revolution. The questioning has not ceased.

Together Pauli, Heisenberg, Dirac, and the others created something remarkable, something that has changed all our lives in a practical sense more than any other twentieth-century scientific upheaval has. The inventions it led to, such as the transistor and the laser, are both implements that affect our daily activities and tools for future research.

Considering the human side of science, there's no single answer to the question of what these seven physicists were like, since physicists are as different from one another as any other group. Among them we find the gregarious and the withdrawn, the philanderers and the faithful, the rooted and the wanderers. Some were abstemious and others drank too much. There were perhaps a disproportionate number of music lovers and mountain climbers among them, but that may be because they had been told these are things physicists do. Their working habits differed: some preferred the early morning and others the late night. Some always worked alone and others required discussions with their peers. But the founders of quantum theory had one thing in common: they were geniuses at the particular calling known as theoretical physics.

They had a second common trait, perhaps not independent of theoretical-physics genius. Three of the scientists, all born between 1900 and 1902, stand out for their precocity: by their mid twenties Pauli, Heisenberg, and Dirac were leaders in the founding of the field. Several older theoretical physicists, notably Bohr, Einstein, Max Born, and Erwin Schrödinger, also played extremely important roles in the revolution, but the youthfulness of its major participants remains a striking feature. All of them had revealed their powers and were famous in the field by the time they were thirty. More accurately, all but one had achieved great prominence by then, and that one, Schrödinger, may have simply been delayed in his intellectual flowering because his years between ages twenty-five and thirty were taken up by military service during World War I.

Among all these physicists, one stands out for his personal impact on the field and on the others, not simply for his thought or achievements. In his obituary for Bohr, Heisenberg wrote, "Bohr's influence on the physics and the physicists of our century was stronger than that of anyone else, even than that of Albert Einstein." This seems, at first sight, to be the sort of generous testimonial one makes in obituaries, but Heisenberg was not given to exaggeration. I was surprised to read it, for my generation does not think of Bohr this way (I confess to being a theoretical physicist myself, though hardly in the range of geniuses). But the more I delved into the matter, the more I came to understand its truth. It is not based on relative intellectual contributions, for Einstein's were certainly far greater than Bohr's. It revolves around how Bohr's style affected the way physicists think and work, how they individually and collectively strive for answers, how they relate to their mentors, their peers, and their students. In the process of wielding this power, Bohr also became the most loved theoretical physicist of the twentieth century.

Yes, loved. Respect and admiration were feelings young physicists had for all of these greats, but love is something different. Yet it is a term that appears again and again in memoirs when physicists speak of Bohr. It is interesting to explain why this is so, what it was about Bohr's persona, his behavior, his way of thinking and working, that led others to regard him with such warmth.

One of the many factors that contributed to this affection was Bohr's lack of pretension or pomp. There wasn't a trace of personal ambition or aggrandizement in him, though few of these geniuses seem to have had much need of this. They were all secure in the knowledge of their own stature, but Bohr had an almost childlike innocence about such matters. He also worked tirelessly to improve others' work and lives. There are countless examples of people owing him their positions, their careers, and sometimes their very survival. He had a sense of who was in need, when and how to intervene, and how to make a difference.

But combined with all this was something else that seems to have played a role. Being connected to others, or as Bohr's biographer Abraham Pais calls it, being conjoined, was a need for Bohr, almost a necessity. His discussions were carried out as a kind of Socratic dialogue in which he slowly shaped and molded his thoughts, so much so that some said he was a philosopher, not a physicist. Bohr also loved paradoxes, believing that seeing the many sides of a problem was the way to reach resolution and clarity. His close friend Einstein described him as uttering "his opinions like one who perpetually casts about, and never like one who believes he holds the whole defining truth." This astute remark captures much of Bohr's essence—he constantly strove for that defining truth.

Bohr's need for others, for conjointness, was displayed in his relaxation as well, be that skiing, sailing, simply walking, playing a game, or going to the movies. He made others feel needed because he did need them. Bohr was certainly a great man and without guile, but his constant engaging of those he was involved with was a major factor in creating the love they felt for him.

Another of these theoretical physicists was also much loved, though his peers would use that word guardedly. Later generations find the love for him to be even more puzzling than that for Bohr. He forms an interesting contrast to Bohr because while Bohr was invariably polite, Wolfgang Pauli was invariably rude. His insults and his aphorisms became legend, but part of the legend lies in the realization that these insults were directed without regard for rank or age. Einstein, Bohr, or Heisenberg might just as easily be affronted as a student. In doing so, he never meant to hurt others. As the well-known physicist Victor Weisskopf said, "Pauli possessed an almost childlike honesty, and always expressed his true thought directly," adding that once you became used to his style, it was easy to live with him.

Pauli might insult you, but he never ignored you, and the biting remarks directed at you became a kind of badge of honor, remembered and told to friends. With a gift for the bon mot, Pauli was often very funny in his insults. Only he would describe someone as "so young and already so unknown." But his expressive language communicated true feeling and commitment. More than thirty years after a meeting they had at the height of the quantum mechanics revolution, Bohr recalled a characteristic exchange for a historian: "I met Pauli who expressed the strongest dissatisfaction with mytreason, and in his emotional way, which we all treasured so highly, deplored that a new heresy should be introduced into atomic physics."

The athletic Bohr loved to go for long hikes, ski, and chop wood, while portly Pauli preferred cabarets, nightlife, and good wine. Bohr had six sons to whom he was devoted, while Pauli had no children. Bohr, while still being deeply attached to his native Denmark, worked tirelessly after World War II for world peace and disarmament. Pauli had no interest in world affairs. Living in quintessentially neutral Switzerland, he became a symbol of physics research unsullied by worldly concerns. Pauli certainly had more warts and blemishes than Bohr, but as we know, love is not only directed toward the pure. Weisskopf, who knew all of the physics geniuses of that era well, kept a photo of Pauli on his desk, while acknowledging that Bohr was his "intellectual father."

The contrast between the two, the affection felt for both of them, and the affection they felt for each other, is manifest in a skit put on by the young physicists at the April 1932 Copenhagen meeting. That year was the hundredth anniversary of the death of Johann Wolfgang von Goethe, the passing of the man, both humanist and scientist, widely regarded as the last true universal genius. As commemorations marking the occasion took place all over Europe, this small band of physicists at the annual informal gathering decided to have a celebration of their own. It took the form of a sketch, a tongue-in-cheek adaptation to the world of physics of Faust, Goethe's great drama. In the script, written primarily by Delbrück, noble Bohr was identified as the Lord, sardonic Pauli as Mephistopheles, and troubled Ehrenfest as Faust. As in Goethe's version Mephistopheles has the wittiest lines, but that was of course true of Pauli's real-life speech as well.

The skit, meant as comic relief from the intensity of the week's discussions, remains a fascinating portrayal of the world of physics seen through the eyes of its very young practitioners. They were the writers and producers of the parody as well as its actors. Though affectionately mocking their distinguished elders, many only a few years older than they were, these young physicists knew all too well that Bohr, Dirac, Heisenberg, and Pauli had made lasting contributions to their field by the time they were little older than twenty-five. They also remembered the warning uttered by the student Baccalaureus in Goethe's Faust

When more than thirty years are told,
As good as dead one is indeed:

(Faust, Part II, act 2, 222–23)

and worried about their own immediate future.

The year of the meeting was a pivotal one for them. The midsummer detection of the positron, the electron's antimatter companion, marked, as we shall see, the joining of special relativity to quantum mechanics. This completed for the community, with a few remarkable exceptions, the experimental confirmation of what is still the century's most profound and far-reaching physics revolution.

On the other hand, the discovery just before the meeting of the neutron and, a few months later, the first experimentally induced nuclear disintegration ushered in another revolution in physics, introducing us to the era of nuclear physics. Its effects on our world view and on mankind's potential for destruction still hover over us.

The year also saw the beginning of research with the cyclotron, signaling the transition in physics research from small science to big science. Whereas a single individual, James Chadwick, had discovered the neutron, efforts at the cyclotron required a team of experts and considerable financial resources. Large-scale experiments now became common. Only seven years after the meeting, a skeptical Bohr would comment that the fissionable material for a nuclear weapon could only be obtained by, metaphorically speaking, turning "the United States into one huge factory." And of course that happened.

Those discoveries of 1932, sometimes called the Miracle Year of experimental physics, also shifted the emphasis in physics from theory to experiment, from research done with pencil and paper to research done with sophisticated tools in a laboratory. The two modes of working inevitably go hand in hand, but there are times when one takes center stage and times when the other does. While theory's advances in understanding quantum physics had dominated the decade before the meeting, advances achieved in the laboratory marked the immediate period after it.

By concluding with the neutron's discovery, the Copenhagen skit points to this shift. It also eerily prefigures many of the personal problems the physicists, young and old, would encounter in the years to come. With hindsight, we see what a watershed 1932 was for them. Prior to it, they were a small community, the only tension among them induced by who would be the first to reach commonly pursued goals. They worked, ate, and traveled together, swam, played music, climbed mountains. Above all the physicists talked endlessly to one another, occasionally as rivals, but only in an intellectual sense because, in the end, they were friends and comrades. That congeniality was shattered by the ascent to power in Germany of Adolf Hitler in January 1933.

Though none of the seven physicists who are this story's focus was religiously observant, four of them, including Bohr, were at least part Jewish. In 1933 they had to begin worrying about personal safety and emigration. By little more than a decade later many of that small physics community found themselves pitted against one another in a deadly battle, thrust into the making of Faustian bargains they could not have contemplated a few years earlier.

I won't deny that I am prejudiced by a lifetime in the physics profession, but these individuals were true titans. We often do not know what of recent science will stand the test of time, but the contributions of these people will last forever. Part of their success was due to their being young and enthusiastic at a crucial moment, the dawn of quantum mechanics and of nuclear physics, but they seized that moment and shaped the field. Their work was the product of an ensemble: one of them was more original, another more critical, and yet another more daring. Together they created a magical instant in history. Hundreds of years from now, their names may only be footnotes in science textbooks, but their work will continue to shape the way our descendants think.

This singular time was epitomized by the few individuals gathering for a weeklong meeting in a Copenhagen room in April 1932.

[Adapted from the Introduction of Faust in Copenhagen: A Struggle for the Soul of Physics By Gino Segre, Viking $24.95.]