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Niels Bohr.

Born: 1885, Copenhagen, Denmark

Died: 1962, Copenhagen, Denmark

Major Works: Atomic Theory and the Description of Nature (1934), Atomic Physics and Human Knowledge (1958)

Major Ideas

The electrons that surround the nucleus of an atom can only occupy certain discrete states--those states being defined by Planck's constant, h. "In-between" states are not permissible.

The configuration of electrons in an atom is the primary factor that determines the chemical properties of an element.

Quantum mechanical descriptions of the states of atoms should be considered the most complete possible descriptions.

The classical ideal of complete causal explanation of any phenomenon is impossible to reach and should be abandoned in favor of statistical explanation.

Niels Henrik David Bohr was the child of an auspicious Danish family. His father, Christian Bohr, was an internationally known physiologist who raised two famous children, Harald, the mathematician, and Niels, the physicist. Niels shared his father's love for natural science and passed this love along to his own children, as he continually referred them to everyday experiences as examples of intriguing physical phenomena. Illustrations abound of the warm and familial atmosphere of the Bohr household, an atmosphere that encouraged personal growth and intellectual achievement.

Bohr was awarded his Ph.D. in 1911 by the University of Copenhagen. His thesis dealt quite ably with the electron theory of metals but never gained wide recognition, partly because there were few speakers of Danish who knew enough of the topic of promote its broader publication.

The years immediately following the completion of his degree were very important in Bohr's life. In 1911, he studied with J. J. Thomson; in 1912, he began his fruitful and lengthy relationship with Ernest Rutherford, and in that same year he married Margrethe Norlund, who was to be his faithful companion for more than fifty years. These events set the stage for the most significant single year of Bohr's professional life, 1913. In this year Bohr solved some of physical science's most nagging problems in a trilogy of papers. His initial break-through came as an insight that Planck's quantum of action should be incorporated into the explanation of the mysterious stability of Rutherford's atom. This insight served to entrench Planck's constant in physical reality to a degree unapproached up to that time. Such a move also guaranteed the further erosion, if not collapse, of many strongly held beliefs from classical Newtonian physics. Bohr's fame increased due to his unrelenting quest for a thorough-going and consistent interpretation of these new ideas.

Bohr quickly gained an international reputation, which brought him as much stress as it did comfort. He was in demand from many corners and spent many of his mature years writing and delivering lectures on new themes and old. Bohr worked tirelessly from 1921 until the end of his life administrating what would eventually be named the Niels Bohr Institute for Theoretical Physics in Copenhagen. He played a role in the early stages of the development of atomic weapons, whose use he abhorred and whose strict control he tenaciously sought. All his life, Bohr was exceptionally meticulous about his publications (even personal letters often went through several drafts), and hence did not publish a single book-length work. Three non-technical, philosophically oriented collections of his essays, however, have been published. In the 1934 publication of Atomic Theory and the Description of Nature, Bohr first develops his ideas about complementarity as the core of his philosophy of atomic physics. The two other collections, Atomic physics and Human Knowledge (1958) and Essays 1958-1962 on Atomic Physics and Human Knowledge (1963), represent refinements of the basic philosophical positions of the earlier volume and a further branching out into other disciplines as they related to physics. Bohr died at his home on November 18, 1962, shortly after apparently having recovered from an illness.

The Structure of the Atom

The young Niels Bohr, newly graduated with his doctorate, had the privilege of studying with two luminaries in atomic physics, J. J. Thomson and Ernest Rutherford. Thomson is generally regarded as the discoverer of the electron, and Rutherford first proposed the nuclear model of the atom that would provide the foundation for Bohr's own model. Bohr benefited greatly from his contact with these two men, though he clearly sees his collaboration with Rutherford as the most valuable of his scientific life. In 1911, Bohr spent several months with Thomson in Cambridge, where he quickly became aware that Rutherford would be the better mentor in his particular field of interest. So in March of 1912 Bohr moved to Manchester to work with Rutherford, beginning a lengthy friendship and collaboration.

When Bohr encountered Rutherford, the latter had recently proposed that the nucleus of the atom was extremely dense and widely separated from the electrons. Rutherford suggested that the electrons were in orbit around the nucleus, modeled on the shape of the solar system. Compared to the size of the nucleus, the distance between the nucleus and the electrons was great. Also, the electrons had been shown to be exceptionally light in comparison to the weighty nucleus. This model explained many things, and Bohr was highly attracted to it. The simplicity of the model persuaded many other physicists of its value as well.

However, most physicists, including Rutherford himself, were reluctant to believe that the model provided a literal picture of the atom, because of problems in explaining how it could hold together. According to classical electrodynamic theory, a system such as Rutherford's atom, in which orbiting particles were held in position by electromagnetic attraction to the oppositely charged central nucleus, could not hold itself together. The negatively charged electrons should, according to the accepted physics of the day, collapse into the positively charged nucleus, and with all dispatch. This turn of events would make the existence of stable matter impossible. But stable matter did exist; hence it was thought that Rutherford's model had some serious shortcomings. This state of affairs did not deter the young Bohr, who, as a matter of character, could not rest as long as such quandaries persisted.

Another such quandary befuddled the world of the natural sciences at the same time. It was well known that the absorption lines in the spectrum of the element hydrogen displayed numerical regularities, but physicists and spectroscopists were at a loss to explain the physical origin of these regularities. Bohr, through his own work on the electron theory of metals and through his close contact with important chemists of the day, was well aware of this puzzling situation.

Bohr's great insight began with the notion that these two apparently unrelated problems might share a common solution. He thought that by taking Planck's quantum of action more seriously than did its discoverer, one might find the key to unlock the dual mysteries of atomic stability and spectroscopic regularity. As Bohr himself explained in a 1955 essay, "In the hitherto entirely incomprehensible empirical laws for the line spectra of the elements was found a hint as to the decisive importance of the quantum of action for the stability and radiative reactions of the atom." This "hint" led Bohr to propose that the electrons did not collide with the nucleus, because the very nature of energy prohibited it. Bohr suggested that the orbits of electrons were quantized in much the same way as were Planck's resonators in the original quantum postulate of 1900. Electrons could not, as the planets could, assume just any position relative to the focus of the orbit. They could only occupy states, called "shells" by Bohr, that were proportional to the angular corollary of Planck's constant. The allowed orbits were calculated by Bohr from the mountain of data accumulated from years of spectroscopic investigations.

Not only were electrons prohibited from assuming orbits in between the discrete ones allowed by Planck's constant, they could not even momentarily be in these states. This meant that an electron could not move continuously from one state to the other, because to do so it would have to traverse the forbidden territory. But experiment showed that atoms did in fact go from one state to another under certain conditions. According to Bohr's theory, which has been thoroughly corroborated, these movements were discontinuous, just as radiation was discontinuous (as had been shown by Planck and Einstein). Electrons were said to jump from one stationary state to another (hence the origin of the phrase "quantum leap"). This transition resulted in the emission or absorption of radiation in discrete quantities, a process that explained the hitherto anomalous distribution of spectral lines.

Bohr's well-founded conjectures met with a mixture of praise and skepticism when they were published in the 1913 series of papers. The agreement of his ideas with the experimental data was undisputed, but the radical departure from classical physics was cause for grave concern on the part of many of Bohr's contemporaries. The debate about the proper physical interpretation of Bohr's notions continues, but the practical fruitfulness of the same theories is undebated.

The influence of Bohr's ideas on subsequent physics and chemistry is difficult to overestimate. The quantum revolution that has changed the civilized world of the twentieth century relies heavily on Bohr's atomic model. The shape of the periodic table of the elements depends largely on the conclusions of Bohr concerning the nature of the periodicity of the elements. He was able to predict successfully the properties of hitherto undiscovered elements and the nature of spectral data from hitherto unobserved regions of X-radiation. His observations explained why certain chemicals acted as they did and gave the first comprehensive tool that accounted for the mechanism of chemical bonding. Though Bohr's model was quickly shown to be inaccurate as a literal picture value of the shape of the atom, the practical value of the model continues. Bohr's ideas were exceptionally useful in the description of simple system, such as the hydrogen atom, but the world had to wait for those who would elaborate on Bohr's foundations before adequate explanations of more complicated systems were offered.

The philosophical repercussions have been no less profound, even though the conclusions reached have been fewer and more tentative. Much of Bohr's contribution to the changing of the thought patterns of contemporary scientists and philosophers can be traced to his uncompromising pursuit of the full implications of Planck's quantum of action. Planck was the first to suggest that radiation was available only in particular size units; Einstein suggested that radiation, including light, existed only in such units (through his work on the photoelectric effect); Bohr showed how this feature of energy had universal consequences for nature and our understanding of it. In a strong sense, Bohr tried to bring to completion the break with classical physics inaugurated by Planck's 1900 discovery. This effort was met with notable success.

Classical physics, epitomized by Newton and carried on with tremendous success by countless others, had no room for energy that had to be quantized, or for noncontinuous entities, or for spontaneous jumps of particles. These new ideas, along with the increasing experimental evidence in their favor, compelled an extensive rethinking of accepted doctrine in the physical sciences. Cherished notions such as objectivity, causality, and determinism simply could not be salvaged in the new system, at least not in the form in which they had been held previously.

The "Copenhagen Interpretation" of Quantum Physics

Nowhere is there written an official doctrine named "the Copenhagen interpretation" of quantum physics, but the writings of Niels Bohr hinge around a few important concepts that are at the heart of the understanding of physical reality that goes by that name. Much of Bohr's perception of the broader meaning of the fundamental physical discontinuity that characterizes quantum physics can be gleaned from his treatment of the notions of complementarity and correspondence.

Ideas that would later coalesce into the principle of correspondence were present as early as 1913 in Bohr's writings. Bohr's electron shells were radically different from anything encountered in physics before the quantum era, yet the world at the macroscopic, everyday level behaved just as the classical world-view predicted that it would. Thus Bohr knew that any very new theory must include within it the expected behavior at the visible level. The new theory met this requirement by proposing, and then demonstrating, that when very large numbers of atoms are involved in a system, the random jumps of the electrons "average out," resulting in the large-scale predictability of classical systems. The discontinuous behavior of matter does not become evident until one examines systems small enough that Planck's quantum of action cannot be ignored. The visible world is much too large for anything as small as Planck's quantum to make a noticeable difference, under most circumstances. But when experimental techniques are sophisticated enough to allow the observation of effects of sub-atomic events, then Planck's constant can no longer be ignored. Hence, when large numbers of atoms are being investigated, even according to quantum rules, the results correspond to classical results. Classical mechanics is then seen as a special case of quantum mechanics.

With the notion of complementarity, Bohr describes the way in which everyday concepts are inadequate to the task of depicting microscopic reality. He has been quoted as saying, "Anyone who is not shocked by quantum theory has not understood it." Complementarity allows the shocking aspects to be brought to light. Early in his career, when he realized the fundamental significance of Planck's quantum of action, Bohr could see, perhaps only hazily at first, that a wholesale revision of classical concepts was going to be needed.

The very notion of cause and effect, the cornerstone not only of classical physics but also of classical thought, assumed that systems varied continuously, moving regularly and predictably from one state to another. The quantum postulate suggested that matter and energy are not continuous but corpuscular, behaving not smoothly but in fits and starts. In such a scenario, our accustomed language and conceptual framework have only limited applicability. The pictures used to explain physical events, pictures drawn from our apparently continuous experience of our everyday world, could not explain atomic reality except in a very limited way. For example, electrons and light cannot accurately be said to be waves or particles, but something very different--we do not possess any picture at all to describe it. Hence our various pictures, the only means we have of linguistic description, should be expected to bump into contradictions, such as the wave/particle dilemma.

The debate over whether electrons, light, and other subatomic particles are ultimately wavelike or particlelike is a premier example of a complementary relationship between mutually exclusive descriptions. Both electrons and electromagnetic radiation (including light) stubbornly refuse to be classified as either wave or particle. Numerous experiments apparently confirm both models. To our minds, full of concepts based on the everyday world of classical physics, this makes no sense. Bohr realized this and concluded that our everyday concepts simply do not apply to the subatomic realm. Bohr proposed instead that these mutually exclusive yet jointly necessary pictures provide complementary interpretations of the phenomena. Neither is complete, yet neither is wholly mistaken. This rather vague and apparently inadequate means of description is simply the best we can do. The nature of physical reality precludes our being able, ever, to gain a more precise physical description of these subatomic entities.

In order to describe subatomic phenomena, then, we must resort to descriptions less precise than classical mechanics would tolerate. In quantum mechanics, this means that the most complete description possible is a statistical one. We cannot tell what a given subatomic system will do in the future, but since we know that on the average many atoms together approach classical limits, we can assign a probability to one outcome as opposed to another. Heisenberg's uncertainty principle (which post-dated Bohr's early thoughts on complementarity) sets an unsurpassable limit to the accuracy with which any given subatomic particle can be measured. This limit, which gives mathematical rigor to Bohr's notion of complementarity, requires that statistical approximations be the most accurate possible descriptions of individual atomic states. We will never be able to penetrate the atom with an application of our classical pictures to the behavior of what is found there.

Attempts to describe the microscopic run into further trouble when one considers the act of observation. There are dual difficulties here. First, the experimental apparatus is necessarily an object in the classical world, so the information it yields must be interpreted in light of complementarity. Second, observation connects a classical, macroscopic observer to a nonclassical, quantum reality. The classical observer, armed with classical concepts, assumes the existence of a continuous causal chain from the event to the observation of the event. For quantum-level systems, this assumption is grossly inaccurate. In fact, the very act of observation produces unpredicatable behavior on the part of the observed system, making an objective description of the event impossible. The most that can be done is to describe the overall observation-event, which involves the inseparable observer-observed system. Again, this causes classical pictures of events to lose their relevance.

Considerations such as these persuaded Bohr to promote the abandonment of the traditional notions of objectivity and causation. Since we would never be able to observe any alleged causal connection between discontinuous atomic states, and since we would never be able to conduct experiments without a crucial influence by the observer, there was no reason to hang on to these concepts at all.

Bohr's endeavor to eliminate the traditional concepts of causality met with stiff opposition from some of the most celebrated physicists of his day. Planck, Einstein, and Schrodinger all continued to believe that there is an underlying continuity in subatomic events and that causation is universal, despite the apparent obstacles to demonstrating this belief experimentally.

One of the most famous debates in the history of science is the one between Bohr and Einstein. These two brilliant men engaged in sometimes heated exchanges over the proper interpretation of quantum theory. Bohr insisted that random jumps and statistical approximations were permanent parts of physical theory, while Einstein maintained that quantum physics was inherently incomplete and that "God does not play dice." Einstein and other created numerous thought experiments designed to envision situations in which quantum mechanics, and the limits represented by Heisenberg's principle and complementarity, could be shown to be incomplete. Einstein seemed to think that eventually experimental technique would surmount the problems that prohibited a classical space-time description of all phenomena. In each case, Bohr illustrated how the situation envisioned by Einstein would not in fact supersede the limits imposed by uncertainty and complementarity.

Einstein and Bohr never reached agreement on these issues, and both types of interpretations persist. Experiment and consensus since the early days of the theory, however, clearly favor Bohr's interpretation over Einstein's. More and more sophisticated experiments have served to strengthen the idea that the statistical and counterintuitive reading of quantum mechanics defended by Bohr (and refined by many others) is the most adequate.

Further Reading

Folse, Henry J. The Philosophy of Niels Bohr: The Framework of Complementarity. Amsterdam: North-Holland, 1985. A thorough discussion of the notion of complementarity, with useful reference to popular misunderstandings of the notion.

Honner, John. The Description of Nature: Niels Bohr and the Philosophy of Quantum Physics. Oxford: Clarendon Press, 1987. An interesting analysis of Bohr's philosophy in comparison with other world-views, including mysticism.

Jammer, Max. The Conceptual Development of Quantum Mechanics. New York: McGraw-Hill, 1966. An unparalleled historical source that recounts the theoretical development of quantum theory from its inception through the various debates about its proper interpretation.

Mehra, Jagdish, and Helmut Rechenberg. The Historical Development of Quantum Theory: 5 vols. New York: Springer-Verlag, 1982, 1987. An exhaustive narration of the events leading the development of quantum theory. The authors begin with an analysis of nineteenth-century physics and the difficulties that caused the twentieth century to begin on such a radical note.

Murdoch, Dugald. Niels Bohr's Philosophy of Physics. Cambridge: Cambridge University Press, 1987. An exhaustive account of Bohr's interpretation of physical reality. The book is on providing philosophical and cultural context for the various ideas promoted by Bohr. It also includes an engaging account of the debate between Bohr and Einstein.

Rozental, Stefan, ed. Niels Bohr: His Life and Work as Seen by His Friends and Colleagues. Amsterdam: North-Holland, 1967. A series of essays by the family and friends of Bohr, including many revealing anecdotes and detailed biographical information. Austin, D. Brain
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Author:Austin, D. Brian
Publication:Great Thinkers of the Western World
Date:Jan 1, 1992
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