On the morning of the twenty-fifth of September 1933, the Austrian physicist Paul Ehrenfest walked into Professor Jan Waterink''s Pedagogical Institute for Afflicted Children in Amsterdam, shot his fifteen-year-old son, Vassily, in the head, then turned the gun on himself. Paul died instantly, while Vassily, who suffered from Down syndrome, was in agony for hours before being pronounced dead by the same doctors who had cared for him since his arrival at the institute, in January of that same year. He had come to Amsterdam because his father had decided that the clinic where the boy had spent the better part of a decade, located in Jena, in the heartland of Germany, was no longer safe for him with the Nazis in power. Vassily-or rather Wassik, as almost everyone called him-had to endure severe mental and physical disabilities during his short life; Albert Einstein, who loved the boy''s father as if they were brothers and was a regular houseguest at the Ehrenfests'' home in Leiden, nicknamed Wassik "patient little crawlikins," for he had such trouble getting around, and would sometimes feel so much pain in his knees that he could not stand. And yet, even then, the child did not lose his seemingly boundless enthusiasm, dragging himself along the carpet, with his useless legs trailing behind, to meet his favorite "uncle" at the door. Wassik spent most of his life institutionalized, but he was a cheerful child nonetheless, often sending postcards to his parents in Leiden featuring quaint German landscapes, or letters with accounts of his daily life, written in his unsure hand, telling them what new things he had learned, how his best friend had fallen ill, how hard he was trying to be a good boy, just as they had taught him, and how in love he was with not one but two girls in his class, as well as his teacher, Mrs. Gottlieb, who was the most caring and wonderful person he had ever met, a thought that would bring tears to his father''s eyes, as Paul Ehrenfest was, first and foremost, a teacher. Paul had suffered from extreme melancholy and bouts of crippling depression all his life.

Like his son, he had been a weak boy, often ill. When he was not nursing nosebleeds, coughing due to his asthma, or dizzy and wheezing from lack of breath after escaping the bullies who teased and taunted him at school- Pig''s ear, donkey''s ear, give ''em to the Jew that''s here! -he would feign some other ailment, a fever perhaps, a cold, or an unbearable stomachache, just to stay at home in his mother''s arms, hidden from the outside, safely wrapped in her embrace, as if deep down, in some way, little Paul, the youngest of five brothers, could foretell that she would die when he was ten, and all his prior suffering was nothing but a premonition, an anticipation of loss that he dare not speak of, to himself or to others, afraid that if he said it out loud, found the courage to put it into words, her death would somehow hasten forward to meet him; so he remained silent, fearful, and sad, shouldering a weight that no child should bear, a dark foreknowledge that haunted him past her death, past his father''s demise six years after hers, and that would follow behind him like the toll of a bell up to the day of his undoing, by his own hand, at fifty-three years of age. However much he was at odds with himself and the world, Paul was the most gifted member of his household and the best student of any class he ever took part in. He was well liked by his friends, highly esteemed by his classmates, and appreciated by his teachers, but nothing could convince him of his self-worth. Yet he was by no means an introvert; on the contrary, he would pour out everything he took in, delighting those around him with fabulous displays of knowledge and his uncanny ability to translate the most complicated ideas into images and metaphors that anyone could understand, threading together concepts from disparate fields that he drew from the ever-growing number of books he fed on with ravenous, spongelike intelligence. Paul was capable of absorbing everything around him while making no differentiations. His mind was fully porous, lacking, perhaps, some essential membrane; he was not so much interested in the world as he was invaded by its many forms. With nothing to keep him safe, he felt raw and exposed to the information flowing constantly back and forth across his blood-brain barrier.

Even when he obtained his PhD and became firmly established as a distinguished professor, after succeeding the great Hendrik Lorentz in the theoretical physics chair at the University of Leiden, the only thing that really sparked joy in Paul was giving himself to others, to the point that, as one of his many beloved students remarked, "Ehrenfest distributed all that was living and active in him," such that sometimes it looked like "he gave away everything he had found or observed, without building up a reserve, a kind of stronghold, within himself." As a physicist, he made no earth-shattering discoveries, but he enjoyed the full respect of such towering figures as Niels Bohr, Paul Dirac, and Wolfgang Pauli. Albert Einstein wrote that, no more than a few hours after having met Paul, he felt "as though our dreams and aspirations were meant for each other." These friends of Paul''s admired not only his critical and intellectual capacities but something rather different, a virtue that is usually lacking among giants: ethics, character, as well as a deep, some would say overwhelming, desire to understand, to grasp the core of things. Ehrenfest sought relentlessly what he called der springende Punkt, the leaping point, the heart of the matter, as for him deriving a result by logical means was never enough: "That is like dancing on one leg," he would say, "when the essence lies in recognizing connections, meanings and associations in every direction." For Ehrenfest, true understanding was a full-body experience, something that involved your entire being, not just your mind or reason. He was an atheist, a doubter and a skeptic with such a rigid standard for truth that he sometimes became a figure of fun among his peers: in 1932, at the end of a meeting of three dozen of Europe''s best physicists at the Niels Bohr Institute in Copenhagen, a parody of Faust was staged to celebrate the centennial of Goethe, and Paul was cast as the great scholar Heinrich Faust himself, unwilling to be convinced by the demon Mephistopheles, portrayed by Wolfgang Pauli, of the existence of the neutrino, a newly postulated fundamental particle. They called him the Conscience of Physics, and while there was a hidden barb in that nickname due to Ehrenfest''s undeterred opposition to the road that not only physics but the whole of the exact sciences seemed to be taking during the first decades of the twentieth century, many of his colleagues would visit him regularly at his home in Leiden, just across the river from the university, to try their ideas on him, and on his wife, since Tatyana Alexeyevna Afanassjewa was an accomplished mathematician in her own right.

She coauthored some of Ehrenfest''s most important scientific papers, including the one that first made his name (though it did almost nothing for her career) and eventually led to his appointment as successor to the much-revered Lorentz: it was a summary article on statistical mechanics, a favored subject of his mentor, the ill-fated Ludwig Boltzmann. Boltzmann was one of the strongest advocates of the atomic hypothesis, a veritable trailblazer who first discovered the role that probability plays in the behavior and properties of atoms. Like Ehrenfest, Boltzmann also suffered greatly during his restless and unhappy life, crippled as he was by severe bouts of uncontrollable mania and abysmal depression, the effects of which were compounded by the vicious antagonism that his revolutionary ideas bred among his peers. Ernst Mach, a staunch positivist who contended that physicists should not speak of atoms as anything but theoretical constructs-since, at the time, there was no direct evidence for their existence-hounded and mocked Boltzmann without end, interrupting one of his lectures on atoms with the mean-spirited question: "Have you ever seen one?" The Bull, as his friends called Boltzmann due to his corpulence and his stubborn tenacity, despaired at the ferocity of his critics, and even though he laid down one of the fundamental equations of modern physics, his statistical interpretation of the second law of thermodynamics, in his personal life he could not escape the slow and constant advance of his mental disorder, which seemed, like the entropy of the universe he had so wonderfully captured in his equation, to be constantly and irreversibly increasing, leading to inevitable randomness and decay. He admitted to his colleagues that he lived in perpetual fear that he might suddenly lose his mind during a lecture. Toward the end of his life he could hardly breathe from his asthma, his eyesight dimmed to the point where he could no longer read, and his headaches and migraines became so utterly unbearable that his physician ordered him to completely abstain from any scientific activity. In September 1906, Boltzmann hanged himself with a short rope from the crossbars of the window frame in a room at the Hotel Ples, during a summer holiday in Duino, near Trieste, while his wife and young daughter were out swimming in the calm, turquoise waters of the Adriatic. Speak the truth, write with clarity, and defend it to your very end was Boltzmann''s personal motto, and Paul, his disciple, took it to heart.

The weight of the respect that Ehrenfest carried among so many outstanding physicists was due to his capacity to bring other people''s ideas into sharp focus and capture their fundamental essence, transmitting this knowledge with such pa.