He was a gentleman who helped create the world's most deadly weapon; a humble man who collected as many honors as almost any man of his time. Before he died of a heart attack last week at 77, Danish Physicist Niels Bohr left an unmistakable imprint on the 20th century.
For a boy who always wanted to be a physicist, Niels Henrik David Bohr could have chosen no better age in which to live. By the time he was in college, physics was in fascinating chaos. Blow after blow had shattered its foundations: Albert Einstein proved that matter is energy, Max Planck proved that energy comes in indivisible packets he called quanta. Lord Rutherford proved that though the very name atom means "indivisible" in Greek, atoms are not indivisible. Nothing seemed certain. One physicist declared that all students should be warned: "Caution! Dangerous structure! Closed for reconstruction!"
Chewing on Stubs. In this chaos Bohr found his own future. In 1912, he went to Rutherford's laboratory at Manchester, England, just after Rutherford had advanced the theory that atoms are miniature solar systems with electrons revolving like planets around a sunlike nucleus. The idea had serious faults, which Bohr, then 27, spotted promptly; he corrected them by applying the unfamiliar principles of Planck's new quantum theory.
Bohr's atomic model answered dozens of questions that had the physicists of the time chewing their pencil stubs. It won him a Nobel Prize, but it, too, had faults which were gradually corrected by mathematical abstractions that seemed to grow more and more bizarre. Bohr himself did much of the correcting, and even the most recent concepts of atomic structure reflect his genius for inventive analysis.
Golden Age. In 1920, Bohr organized the University of Copenhagen Institute for Theoretical Physics, which quickly became a kind of scientific shrine, attracting students from all over the world. "The unique and exciting feature of Copenhagen," wrote Professor John A. Wheeler of Princeton, "lies in the stimulus that Bohr gives. I know of nothing with which to compare it except the school of Plato." J. Robert Oppenheimer, who was later to head the atom-bomb-making Los Alamos Scientific Laboratory, said about physics in the 19205: "It was a heroic time. It was not the doing of any one man; it involved the collaboration of scores of scientists from many different lands. But from first to last, the deeply creative, subtle and critical spirit of Niels Bohr guided, restrained, deepened, and finally transmuted the enterprise." He never dogmatized. "Every sentence I utter," Bohr liked to tell his students, "must be understood not as an affirmation but as a question." Once he defined truth as "something that we can attempt to doubt, and then perhaps, after much exertion, discover that part of the doubt is unjustified."
Niels Bohr deeply resented any restrictions that hindered the search for scientific truth. When the Nazis began to harass the great German universities, he wrote to physicists who he thought might be in danger of persecution and invited them to Copenhagen. Many came, and whenever any of them arrived, Bohr always made certain that he or one of his colleagues was at the railroad station to welcome them to his pleasant refuge.