No account of Bohr's achievements should neglect his role as a mentor. As head of the Copenhagen Institute, Bohr was a mentor for many young physicists. Many of these scientists went on to make discoveries whose significance proved no less than Bohr's own work, and Bohr was proud to have contributed.

One of Bohr's favorite protégées was Wolfgang Pauli, fifteen years younger than Bohr but very willing to criticize him both jovially with his sharp wit and seriously with his brilliant scientific reasoning. Like many other theoretical physicists, Pauli was inept in the laboratory. Before he became famous for his exclusion principle, he was well known throughout Europe for the "Pauli effect"âit was joked that equipment broke down the moment he entered the lab. Once, when an apparatus in Goettingen blew up for no particular reason, it was determined that the explosion had coincided exactly with the stopping of a train bearing Pauli as he passed through Goettingen station on the way to Copenhagen.

Pauli had an easy manner that delighted all, but Bohr could not help taking his scientific criticism into serious consideration. Bohr had established the basic framework for electrons and their arrangement, but he had no answer as to why each orbit had a set number of electrons. In 1925 Pauli demonstrated his famous exclusion principle, which held that no two electrons could simultaneously occupy the same quantum state. As a consequence, he was able to determine the rules governing electrons and the orbits they fit into. Bohr was delighted with Pauli's contribution, which he had helped to flesh out in their correspondence.

Not long after, Werner Heisenberg arrived from Germany and began working with Kramers. Suddenly Heisenberg perceived a way that the vibrations of electrons could be expressed mathematically, and soon, with the help of colleagues Max Born and Pascual Jordan, he wrote the paper that formulated the basic working of the atom in terms of matrices, laying the foundation for what would be called quantum mechanics. Bohr expressed great admiration for Heisenberg, recognizing that a quantitative understanding of atomic structure was no longer so far out of reach.

Disagreements soon began to brew between Pauli and Heisenberg, who suggested throwing out the idea of orbits completely. Before this debate could be resolved, a new paper brought havoc and excitement to Copenhagen. This paper came from Ernest Schrödinger, who began work based on an insight given in a two-year-old paper by Louis de Broglie that suggested that matter might behave like a wave. Schrödinger soon worked out his theory to show that electron behavior could be expressed in wave theory. Immediately a colloquium was called at Copenhagen, and a period of intense examination began by the world's brightest minds. The fundamental question was whether Schrödinger's work contradicted or supported Heisenberg's. But soon it was proved that wave theory yielded the same mathematical results as Heisenberg's quantum mechanics. Far from ending the debate, this convergence merely led to more questions that continued to be debated at Copenhagen and around the world.

During this time, Bohr took a secondary role. He had been working on a path that might have led him to the uncertainty principle, but he did not have the mathematical virtuosity of Heisenberg. While new generations of physicists were making names for themselves, Bohr guided them. The work that led to the phenomenal achievements of Pauli, Heisenberg, Schrödinger, and others could not have come about independently, without the progress being made by scientists in laboratories all over the world. Both as an individual mentor for such scientists, and as the leader of discussions at Copenhagen, Bohr pushed ideas to their limit while directing their ultimate critical tests. It was not uncommon for a scientist to hold off on publication until he or she had a chance to receive Bohr's evaluation and criticism.

Moreover, Bohr gave these scientific pursuits a philosophical backbone. In 1927, at the famous Volta Commemoration Conference held in Como, Italy, Bohr delivered a speech that addressed the problems raised by quantum mechanics. Bohr proposed what he called the principle of complementarityâwhat he considered his greatest contribution to science and understanding in general, and what others have acclaimed as the most important scientific concept of the century. Bohr attempted to resolve the questions raised by the uncertainty principle and the wave/particle duality of matter and light. He said that subatomic reality could not be understood in exact numbers because the very act of measuring atomic behavior changed its conditions. Thus he argued that our goal should be to view our knowledge of a particle's position as complementary to knowledge of its velocity. In the same manner, we should view our knowledge of the electron as a particle as complementary to knowledge of it as a wave. The goal would then be to exhaust all possible information and combine it to obtain the most complete understanding possible. This understanding would never allow us to predict certain atomic behavior, but it would improve estimates of probability.

Bohr believed that his doctrine was the best synthesis possible of the conflicting phenomena that had been observed. But not everyone was willing to make the concession that reality could never be completely understood. The most famous critic was Einstein. Just as he had taken issue with Bohr's argument about spectral theory, Einstein disliked formulations that left anything unknowable. Despite Bohr's repeated demonstrations of the inevitable limitations of experiments, Einstein insisted that this was a human limitation to be overcome, and that Bohr was accepting an incomplete answer as the final one. Physicists have gradually come to Bohr's side, but debates continue, and many are unwilling to give up on classical ideas of the certain observability of reality.