Richard Rhodes - The Making of the Atomic Bomb

One reason Rutherford was unaware of Nagaoka’s Saturnian model of the atom is that it had been criticized and abandoned soon after Nagaoka introduced it because it suffered from a severe defect, the same theoretical defect that marred the atom Rutherford was now proposing. 188The rings of Saturn are stable because the force operating between the particles of debris that make them up—gravity—is attractive. The force operating between the electrons of Nagaoka’s Saturnian electron rings, however—negative electric charge—was repulsive. It followed mathematically that whenever two or more electrons equally spaced on an orbit rotated around the nucleus, they would drift into modes of oscillation—instabilities—that would quickly tear the atom apart.

What was true for Nagaoka’s Saturnian atom was also true, theoretically, for the atom Rutherford had found by experiment. It the atom operated by the mechanical laws of classical physics, the Newtonian laws that govern relationships within planetary systems, then Rutherford’s model should not work. But his was not a merely theoretical construct. It was the result of real physical experiment. And work it clearly did. It was as stable as the ages and it bounced back alpha particles like cannon shells.

Someone would have to resolve the contradiction between classical physics and Rutherford’s experimentally tested atom. It would need to be someone with qualities different from Rutherford’s: not an experimentalist but a theoretician, yet a theoretician rooted deeply in the real. He would need at least as much courage as Rutherford had and equal self-confidence. He would need to be willing to step through the mechanical looking glass into a strange, nonmechanical world where what happened on the atomic scale could not be modeled with planets or pendulums.

As if he had been called to the cause, such a person abruptly appeared in Manchester. Writing to an American friend on March 18, 1912, Rutherford announced the arrival: “Bohr, a Dane, has pulled out of Cambridge and turned up here to get some experience in radioactive work.” “Bohr” was Niels Henrick David Bohr, the Danish theoretical physicist. 189He was then twenty-seven years old.

“There came into the room a slight-looking boy,” Ernest Rutherford’s McGill colleague and biographer A. S. Eve recalls of Manchester days, “whom Rutherford at once took into his study. 190, 191Mrs. Rutherford explained to me that the visitor was a young Dane, and that her husband thought very highly indeed of his work. No wonder, it was Niels Bohr!” The memory is odd. Bohr was an exceptional athlete. The Danes cheered his university soccer exploits. He skied, bicycled and sailed; he chopped wood; he was unbeatable at Ping-Pong; he routinely took stairs two at a time. He was also physically imposing: tall for his generation, with “an enormous domed head,” says C. P. Snow, a long, heavy jaw and big hands. 192He was thinner as a young man than later and his shock of unruly, combed-back hair might have seemed boyish to a man of Eve’s age, twelve years older than Rutherford. But Niels Bohr was hardly “slight-looking.”

Something other than Bohr’s physical appearance triggered Eve’s dissonant memory: probably his presence, which could be hesitant. He was “much more muscular and athletic than his cautious manner suggested,” Snow confirms. “It didn’t help that he spoke with a soft voice, not much above a whisper.” All his life Bohr talked so quietly—and yet indefatigably—that people strained to hear him. Snow knew him as “a talker as hard to get to the point as Henry James in his later years,” but his speech differed dramatically between public and private and between initial exploration of a subject and eventual mastery. 193Publicly, according to Oskar Klein, a student of Bohr’s and then a colleague, “he took the greatest care to get the most accurately shaded formulation of the matter.” Albert Einstein admired Bohr for “uttering his opinions like one perpetually groping and never like one who [believed himself to be] in the possession of definite truth.” 194If Bohr groped through the exploratory phases of his deliberations, with mastery “his assurance grew and his speech became vigorous and full of vivid images,” Lise Meitner’s physicist nephew Otto Frisch noted. 195, 196And privately, among close friends, says Klein, “he would express himself with drastic imagery and strong expressions of admiration as well as criticism.” 197

Bohr’s manner was as binary as his speech. Einstein first met Bohr in Berlin in the spring of 1920. “Not often in life,” he wrote to Bohr afterward, “has a human being caused me such joy by his mere presence as you did,” and he reported to their mutual friend Paul Ehrenfest, an Austrian physicist at Leiden, “I am as much in love with him as you are.” 198Despite his enthusiasm Einstein did not fail to observe closely his new Danish friend; his verdict in Bohr’s thirty-fifth year is similar to Eve’s in his twenty-eighth: “He is like an extremely sensitive child who moves around the world in a sort of trance.” At first meeting—until Bohr began to speak—the theoretician Abraham Pais thought the long, heavy face “gloomy” in the extreme and puzzled at that momentary impression when everyone knew “its intense animation and its warm and sunny smile.” 199

Bohr’s contributions to twentieth-century physics would rank second only to Einstein’s. He would become a scientist-statesman of unmatched foresight. To a greater extent than is usually the case with scientists, his sense of personal identity—his hard-won selfhood and the emotional values he grounded there—was crucial to his work. For a time, when he was a young man, that identity was painfully divided.

Bohr’s father, Christian Bohr, was professor of physiology at the University of Copenhagen. In Christian Bohr’s case the Bohr jaw extended below a thick mustache and the face was rounded, the forehead not so high. He may have been athletic; he was certainly a sports enthusiast, who encouraged and helped finance the Akademisk Boldklub for which his sons would one day play champion soccer (Niels’ younger brother Harald at the 1908 Olympics). He was progressive in politics; he worked for the emancipation of women; he was skeptical of religion but nominally conforming, a solid bourgeois intellectual.

Christian Bohr published his first scientific paper at twenty-two, took a medical degree and then a Ph.D. in physiology, studied under the distinguished physiologist Carl Ludwig at Leipzig. Respiration was his special subject and he brought to that research the practice, still novel in the early 1880s, of careful physical and chemical experiment. Outside the laboratory, a friend of his explains, he was a “keen worshipper” of Goethe; larger issues of philosophy intrigued him. 200

One of the great arguments of the day was vitalism versus mechanism, a disguised form of the old and continuing debate between those, including the religious, who believe that the world has purpose and those who believe it operates automatically and by chance or in recurring unprogressive cycles. The German chemist who scoffed in 1895 at the “purely mechanical world” of “scientific materialism” that would allow a butterfly to turn back into a caterpillar was disputing the same issue, an issue as old as Aristotle.

In Christian Bohr’s field of expertise it emerged in the question whether organisms and their subsystems—their eyes, their lungs—were assembled to preexisting purpose or according to the blind and unbreathing laws of chemistry and of evolution. The extreme proponent of the mechanistic position in biology then was a German named Ernst Heinrich Haeckel, who insisted that organic and inorganic matter were one and the same. Life arose by spontaneous generation, Haeckel argued; psychology was properly a branch of physiology; the soul was not immortal nor the will free. Despite his commitment to scientific experiment Christian Bohr chose to side against Haeckel, possibly because of his worship of Goethe. He had then the difficult work of reconciling his practice with his views.