BOHR, NIELS HENRIK DAVID

(b. Copenhagen, Denmark, 7 October 1885; d. Copenhagen, 18 November 1962)

atomic and nuclear physics, chemistry, epistemology, philosophy of physics. For the original article on Bohr see DSB, vol. 2.

Bohr’s legacy to physics and its interpretation is a controversial one. He contributed decisively to the development of atomic and nuclear physics in many ways (especially to quantum theory from 1913 to 1925), and he is widely recognized as possessing remarkable insight into the nature of physical problems. Yet his theoretical approach and interpretive outlook sometimes have been questioned as vague, unclear, or inconsistent. Scholarship from 1970 to 2007 emphasized both the radical successes and some of the interpretive challenges of Bohr’s work.

Influences on Bohr’s Work. Much attention has been paid to possible philosophical influences on Bohr’s thought, especially, it seems, because his work is both groundbreaking and strangely conservative and is not easily made sense of via traditional analyses of the trajectory of the physics of his time. Thus some have looked to influences such as Immanuel Kant, Søren Kierkegaard, and William James (especially as conveyed to Bohr by his teacher, Harald Høffding) to account for his willingness to use incomplete and non-causal models or for his apparent views on the role of observation in quantum phenomena. However, although the influence of various well-known philosophical views of the time should be acknowledged, at least on the way Bohr described his ideas and probably on his methods, Bohr’s work is best explained by focusing on the close attention he paid to specific issues in physics, the particulars of his methods, and his interactions with other physicists.

Correspondence Principle. It has become increasingly clear that Bohr’s much-misunderstood correspondence principle, a critical component of his work from at least 1918 to 1925, was internally motivated and more robust than the loose heuristic it has often been made out to be. Bohr relied on it for epistemological reasons concerning our ability to infer atomic properties from empirical phenomena such as spectra. His emphasis on this empirical approach over simple hypothesizing can be seen in his justification for his 1913 model via the application of the frequency relation E=h? to the Balmer formula for the hydrogen spectrum. But that method justified claims only about stationary states and transitions between them. To allow investigation of other atomic properties, Bohr looked to mathematical limiting relations in order to set up some sort of correspondence between those properties and the properties of observable emitted radiation. He then used the classical limit as a guide to indicate how observable emitted radiation corresponded to presumed electron motion.

This sense of correspondence is precisely the idea that led to both the unusual Bohr-Kramers-Slater theory and Werner Heisenberg’s quantum mechanics, and it influenced much of Bohr’s later work. The success of classical electrodynamics, especially in spectroscopy, convinced Bohr that something must be vibrating long enough to emit (and therefore correspond to)

electromagnetic radiation at precise frequencies. But that meant that the oscillators and radiation were not independently detectable (thus dubbed “virtual”), and that they conserved energy only statistically. When energy conservation was shown to hold strictly, Bohr questioned the reality of the oscillators, which helped lay the foundation for Heisenberg’s purely formal use of the classical mechanics of oscillatory motion.

Copenhagen Interpretation. Previous orthodoxy accepted the idea that Bohr and Heisenberg shared a Copenhagen interpretation (often conflated with John von Neumann’s collapse interpretation) of Heisenberg’s new quantum mechanics. But in fact there were significant differences. It has become well known that Heisenberg wrote his 1927 paper on the indeterminacy relations independently of Bohr and waited to show it to him until he had it completed. Upon reading the paper, Bohr corrected a fault with Heisenberg’s microscope thought experiment and pushed Heisenberg to explain indeterminacy in a wider context, especially in terms of Bohr’s developing conception of complementarity, which he thought was a better way to conceive of indeterminacy than Heisenberg’s formal derivation and disturbance interpretation. Heisenberg relented only by conceding Bohr’s point in an endnote.

Albert Einstein was one of the more visible critics of the Copenhagen approach, and the discussions between Bohr and Einstein regarding the status and interpretation of the newly developed quantum mechanics looms large in legend. For some time, conventional wisdom had been that Bohr won the friendly debates. That view changed somewhat in the late twentieth century, during which time scholars began to question the clarity of Bohr’s work, especially his response to the 1935 EinsteinPodolsky-Rosen

paper regarding whether quantum mechanics is complete. Bohr’s response contained such confusing but crucial language as that expressing the claim that there exists a non-mechanical, at-a-distance “influence on the very conditions which define the possible types of predictions” of remote particles (1935, p. 700). Bohr’s language was always chosen with almost obsessive attention to the nuances of the way things were expressed, yet the reasoning behind the language often got lost in the end result. In particular, in that response Bohr apparently did have specific formal and philosophical arguments in mind. Bohr’s interpretive views of quantum mechanics never really invoked disturbance by observation, but were more focused on the recognition of the apparent holism of quantum phenomena of entangled systems, which he recognized in an early way even in 1925.

Interactions with Colleagues. Some scholarship has suggested that Bohr won his converts not through the cogency and successes of his theories but from the forcefulness of his attempts to forge forge consensus. It is true that some of Bohr’s work, especially his methods and interpretation of the developing physics, was not convincing to those who were looking for more rigorous foundations or were concerned primarily with mathematical formalism. However, Bohr’s intuition and ability to see through problems were widely appreciated, and his influence spread in virtue of his decisive and well-recognized successes. Those were achieved through his collaborative methods, which involved much discussion, reworking of solutions, and repeated dictation and feedback from those around him.

Another legendary aspect of Bohr’s life concerns his meeting with Heisenberg in 1941, during which they apparently discussed prospects for a German atomic bomb. Heisenberg later indicated that he had told Bohr he had been trying to undermine those prospects. While what was said at that meeting is a matter of much speculation (and became the subject of a play by Michael Frayn), in 2002 the Niels Bohr Archive released drafts of letters Bohr had written, but never sent, to Heisenberg. In those letters, Bohr gently expressed his dismay that Heisenberg would try, perhaps not entirely consciously, to make it seem as if he (Heisenberg) had expressed anything other than support for the German cause at the time. The strain that issue placed on the relationship between Heisenberg and the man who from 1943 on had pushed for the peaceful, open sharing of all science and technology was clear.

SUPPLEMENTARY BIBLIOGRAPHY

WORKS BY BOHR

“Can Quantum-Mechanical Description of Physical Reality Be Considered Complete?” Physical Review 48 (1935): 696– 702. Bohr’s response to EPR.

Niels Bohr Collected Works, 12 vols. Edited by Léon Rosenfeld and Erik Rüdinger. Amsterdam: North-Holland, 1972–2006. Comprehensive, with excellent introductions.

The Philosophical Writings of Niels Bohr, 4 vols. Woodbridge, CT: Ox Bow Press, 1987–1998. New editions of the popular collections of Bohr’s essays.

“Documents relating to 1941 Bohr-Heisenberg meeting.” Niels Bohr Archive. Available from http://www.nba.nbi.dk.

OTHER SOURCES

Beller, Mara. Quantum Dialogue: The Making of a Revolution. Chicago: University of Chicago Press, 1999. Critical of Bohr and his influence.

Blædel, Niels. Harmony and Unity: The Life of Niels Bohr. Madison, WI: Science Tech, 1988. Contains insights into Bohr’s personal history.

Einstein, Albert, Boris Podolsky, and Nathan Rosen. “Can Quantum-Mechanical Description of Physical Reality Be Considered Complete?” Physical Review 47 (1935): 777– 780. Written mostly by Podolsky and not entirely representative of Einstein’s views.

Favrholdt, David. Niels Bohr’s Philosophical Background, Vol. 63, Historisk-Filosofiske Meddelelser. Copenhagen: Munksgaard, 1992.

Faye, Jan, and Henry J. Folse, eds. Niels Bohr and Contemporary Philosophy, Vol. 153, Boston Studies in the Philosophy of Science. Dordrecht: Kluwer Academic, 1994.

Frayn, Michael. Copenhagen. New York: Anchor Books, 2000. Play about the Bohr-Heisenberg meeting.

French, Anthony P., and P. J. Kennedy, eds. Niels Bohr: A Centenary Volume. Cambridge, MA: Harvard University Press, 1985.

Jammer, Max. The Conceptual Development of Quantum Mechanics. New York: McGraw-Hill, 1966. Claims influence on Bohr from Kierkegaard and James.

Mehra, Jagdish, and Helmut Rechenberg. The Historical Development of Quantum Theory, 6 vols. New York: Springer-Verlag, 1982–2001.

Pais, Abraham. Niels Bohr’s Times: In Physics, Philosophy, and Polity. Oxford, U.K.: Clarendon Press, 1991.

Rozental, Stefan, ed. Niels Bohr: His Life and Work as Seen by His Friends and Colleagues. Amsterdam: North-Holland, 1985.

Scott Tanona