With his uncertainty principle, Werner Heisenberg asserted that there are definite limits to precise knowledge of atomic processes, a theory that has become a cornerstone of modern physics.
At the beginning of the nineteenth century, Sir Isaac Newton’s laws of physics, first published in 1687, were enjoying such enormous success, particularly regarding their power to predict the movements of heavenly bodies, that thinkers such as the French mathematician and astronomer Pierre-Simon Laplace
Werner Heisenberg.
The notion of universal determinism
It is against this background of classical physics that the significance of Werner Heisenberg’s principle of uncertainty, or indeterminacy, is most easily seen. In attempting to predict the exact future state of any sort of physical system—from solar to subatomic—it is necessary that one be able to measure precisely the qualities and coordinates of the parts of the system at a given point in time. The belief in universal determinism and absolute predictability held by many nineteenth century physicists was based largely on the assumption that such precision of measurement is theoretically unlimited. Heisenberg’s uncertainty principle dealt a fatal blow to this assumption and shook the supremacy of strict causality, or determinism. Heisenberg stated simply that it is impossible to measure simultaneously both the exact position and the exact momentum of a subatomic particle.
Heisenberg used the example of the gamma-ray microscope to demonstrate the validity of the uncertainty principle, proposing that concepts such as position and momentum can have meaning only if one specifies how they are to be measured. In this hypothetical experiment, one measures the location and speed of a particle such as an electron by shining a tiny ray of light, as little as one photon, on the particle. This light will be scattered by the particle and will then enter the microscope and make a mark on a photographic plate; the particle’s position and momentum can be calculated from this mark. The accuracy of the calculations, however, depends on the distance between the crests in the light waves used to make the observation. To obtain an exact measurement of the particle’s position, one would need to use light of very short wavelength. Such high-frequency light, however, contains much energy, and when the light is directed toward the particle, it will alter the particle’s momentum. By using light of longer wavelength and lower energy, on the other hand, one could obtain a precise measure of the particle’s speed, but uncertainty would then creep in with regard to its location. Therefore, the more one closes in on the position, the less accurately one can know the momentum, and vice versa. In the realm of microphysics, as the exact state of an atomic system can apparently never be fully open to view, physical processes at this level cannot be precisely predicted.
Heisenberg formulated his uncertainty principle early in 1927, the year after he became an assistant to Niels Bohr at Bohr’s research institute in Copenhagen. The two scientists engaged in almost daily dialogue on the foundations of quantum theory and the nature of physical reality. Near the end of February, 1927, there was a brief, but rather deliberate break when Bohr left to take a skiing vacation in Norway. During this time, Heisenberg conceptualized the gamma-ray microscope experiment and decided that the indeterminacy evident in the measurement of subatomic particles had to be considered a fundamental principle of quantum theory. When Bohr returned to Copenhagen, he realized that Heisenberg’s thinking was at variance with the ideas he had been pursuing. Bohr, who was also searching for basic principles, had been trying to understand the fact, established in part by Albert Einstein’s study of the photoelectric effect, that light, as well as matter, displays wavelike properties under some conditions and particle-like properties under other conditions. Physicists had been trying to understand the nature of the wave-particle duality for years.
Bohr realized that, as the property that appears depends on the type of experiment or observing apparatus one is using, one simply cannot describe microphysical phenomena as either particle-like or wavelike without also referring to the method of observation. The observer does not merely observe these properties, they are evoked. Bohr formulated the principle of complementarity
Bohr was convinced that the principle of complementarity revealed a basic fact about the possibilities and limitations of the knowledge of microphysics. Heisenberg believed that the principle of uncertainty expressed a similarly fundamental fact. The apparent disparity between these two principles was the focus of long and sometimes heated discussions between Bohr and Heisenberg. In the end, however, they were able to agree that uncertainty and complementarity were compatible, with Heisenberg’s principle understood as a particular mathematical formulation of the more general principle of complementarity. These two principles, together with Max Born’s probabilistic interpretation of electron waves, combined to form what has become known as the Copenhagen interpretation of quantum theory.
The basic issue at stake in the interface between the different but related principles of Heisenberg and Bohr concerns the appropriateness of using concepts familiar to classical physics and everyday life in understanding the realm of the atom. Heisenberg believed that concepts such as position and momentum, or particle and wave, are of limited applicability in this domain because of the limitations involved in their measurement. He thought that a clear and consistent theory could be expressed only in abstract mathematical terms. Bohr, on the other hand, maintained his strong conviction that concepts rooted in the everyday world of objects and events can, and indeed must, be used to describe microphysical phenomena, but that only one aspect of a complementary pair of concepts will be appropriate in a given experimental situation. Heisenberg recognized the great philosophical importance of Bohr’s approach and added to his famous 1927 paper enunciating the uncertainty principle a postscript in which he said that Bohr would present a related principle that would deepen and extend the meaning of the uncertainty principle. Bohr introduced the principle of complementarity in September of 1927, likewise acknowledging Heisenberg’s groundbreaking work.
In the new brand of physics ushered in by Heisenberg, abstract mathematics played a much greater role than in any previous form of physics. Quantum physics thus became a very powerful and influential mathematical tool that has been used to forge new theoretical developments in other fields of science such as chemistry and biology and to fashion a variety of technological innovations such as transistors, lasers, and microchips. All of this scientific and technological activity can be carried out with little concern for the profound philosophical questions posed by the uncertainty principle. Many scientists who, like Einstein, have been deeply concerned with the meaning of science for human life as a whole have given much thought to these issues.
Soon after Heisenberg and Bohr presented their principles of uncertainty and complementarity in 1927, the Copenhagen interpretation became established as the generally accepted foundation for quantum theory. A number of major physicists, including Einstein, challenged the conceptual cornerstones on which this version of the theory was built. The debate centered on the questions of objectivity and indeterminism. If the principle of uncertainty is taken as truly fundamental, then the state of a particle when it is not being observed should be considered. One would have to conclude that an unobserved particle has no definite characteristics. Actually, it could not be called a particle, or a wave, nor does it have any real position or momentum. As Heisenberg stated, “What we observe is not nature in itself, but nature exposed to our method of questioning.”
Einstein believed that any theory of physics that does not include physical reality cannot be considered a complete theory of nature. From 1927 to 1935, Einstein formulated a number of hypothetical experiments designed to discredit the uncertainty principle, but Bohr was able to refute each of these arguments. The Copenhagen interpretation maintained its sovereignty in the theoretical and practical work of the majority of physicists.
The controversy sparked by the uncertainty principle did not diminish. During the 1980’s, it emerged again as a lively topic of discussion, partially as a result of new experimental findings. Certain physicists and philosophers, notably David Bohm, continued in the spirit of Einstein to explore the possibility of formulating an expanded interpretation of quantum theory.
It is important to understand that although the uncertainty principle revolutionized microphysics and led indirectly to numerous technological developments, it had little impact on the physics of familiar objects. In the realm of everyday, easily perceived and measured objects and events, the determinism of classical physics still provides quite satisfactory predictions and explanations. Heisenberg’s uncertainty principle has been quite effective in shaking the assumptions and assurances of universal determinism that had guided the thinking of many people—scientists and nonscientists alike—since Laplace proposed the idea in the early nineteenth century.
Something that both advocates and opponents of the Copenhagen interpretation would certainly agree on is that the uncertainty principle has helped to reveal the perhaps unsuspected richness of reality, a wealth of patterns and potentialities too great to be grasped in a single observation or to be exhausted by a given experimental or conceptual structure. Heisenberg helped to push the search for an understanding of nature to a new level, to the point where matter meets mind and physics meets philosophy.
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Bohm, David. Causality and Chance in Modern Physics. 1957. Reprint. New York: Routledge, 1997. Reprint edition features a preface that refers to developments in Bohm’s sustained efforts to formulate an interpretation of quantum theory capable of encompassing both classical determinism and the indeterminism of the Copenhagen interpretation in a broader understanding of the laws of nature. A clearly articulated, searching, and sophisticated philosophical inquiry by a contemporary physicist. -
Bohr, Niels. Atomic Physics and Human Knowledge. New York: John Wiley & Sons, 1958. A collection of mostly short essays exploring the implications of research in atomic physics for various other fields, such as biology, anthropology, and philosophy. -
Cline, Barbara Lovett. Men Who Made a New Physics. 1965. Reprint. Chicago: University of Chicago Press, 1987. An interesting narrative, written for a general audience, of the lives, theories, and interrelationships of the physicists, primarily Einstein, Bohr, and Heisenberg, but also the earlier Ernest Rutherford and Max Planck, who between 1900 and 1930 created quantum theory. Includes illustrations, bibliography, and index. -
Crease, Robert P., and Charles C. Mann. The Second Creation: Makers of the Revolution in Twentieth-Century Physics. Rev. ed. New Brunswick, N.J.: Rutgers University Press, 1996. Readable volume follows the development of physics from its nineteenth century roots to the mysteries of the late twentieth century. Examines characters and personalities as well as the issues of physics. Includes discussion of Heisenberg’s work. -
Cropper, William H. Great Physicists: The Life and Times of Leading Physicists from Galileo to Hawking. New York: Oxford University Press, 2001. Presents portraits of the lives and accomplishments of important physicists and shows how they influenced one another with their work. Chapter 18 is devoted to Heisenberg. Includes glossary and index. -
Guillemin, Victor. The Story of Quantum Mechanics. 1968. Reprint. Mineola, N.Y.: Dover, 2003. Places quantum theory in the context of the history of physics and discusses the philosophical and religious implications of the new physics. Includes glossary of scientific terms, annotated bibliography, and index. -
Heisenberg, Werner. Physics and Beyond. Translated by Arnold J. Pomerans. New York: Harper & Row, 1971. Writing for a wide audience, Heisenberg demonstrates his belief that “science is rooted in conversations.” Gives a firsthand account of some of the conversations that have shaped modern physics and, in important ways, the modern world. Chapter 6 details the events surrounding the creation of the uncertainty principle. -
_______. Physics and Philosophy: The Revolution in Modern Science. 1958. Reprint. Amherst, N.Y.: Prometheus Books, 1999. Discusses in largely nontechnical terms a variety of topics, including the history of quantum theory, the Copenhagen interpretation and some of its critics, and the role of modern physics. -
Pagels, Heinz R. The Cosmic Code: Quantum Physics as the Language of Nature. New York: Simon & Schuster, 1982. A theoretical physicist’s popular, readable, and reliable account of the development of relativity and quantum theory, research into elementary particles, and the nature of the scientific investigation of the physical world. Provides interesting insights into the personalities involved and includes many illuminating examples and illustrations of the major issues. Includes bibliography and detailed index.
Rutherford Discovers the Proton
Discovery of the Compton Effect
Pauli Formulates the Exclusion Principle
Gamow Explains Radioactive Alpha Decay with Quantum Tunneling
Anderson Discovers the Positron
Yukawa Proposes the Existence of Mesons