End Of Classical Physics
By the close of the nineteenth century, many physicists felt that the accomplishments of the century had produced a mature and relatively complete science. Nevertheless, a number of problem areas were apparent to at least some of the community, four of which are closely related to developments mentioned above.
New rays and radiations were discovered near the end of the century, which helped establish (among other things) the modern model of the atom. These included the discovery (by William Crookes and others) of cathode rays within discharge tubes; Wilhelm Conrad Röntgen's finding, in 1895, of X rays emanating from discharge tubes; and Antoine-Henri Becquerel's discovery in 1896 that uranium salts were "radioactive" (as Marie Curie labeled the effect in 1898). Each of these led to further developments. In 1897, Joseph John Thomson identified the cathode rays as negatively charged particles called "electrons" and, a year later, was able to measure the charge directly. In 1898, Ernest Rutherford identified two different kinds of radiation from uranium, calling them alpha and beta. In 1902 and 1903, he and Frederick Soddy demonstrated that radioactive decay was due to the disintegration of heavy elements into slightly lighter elements. In 1911, he scattered alpha particles from thin gold foils and explained infrequent scattering to large angles by the presence of a concentrated, positively charged atomic nucleus.
The study of blackbody radiation (radiation from a heated object that is a good emitter) yielded results that are crucial to the early development of quantum mechanics. In 1893 Wilhelm Wien derived a promising "displacement law" that gave the wavelength at which a blackbody radiated at maximum intensity, but precision data failed to confirm it. Furthermore, classical theory proved unable to model the intensity curves, especially at lower wavelengths. In 1900 the German theoretical physicist Max Planck (1858–1947) derived the intensity curve using the statistical methods of the Austrian physicist Ludwig Eduard Boltzmann (1844–1906) and the device of counting the energy of the oscillators of the blackbody in increments of hf, where f is the frequency and h is a constant (now known as "Planck's constant"). Despite achieving excellent fits to data, Planck was hesitant to accept his own derivation, due to his aversion for statistical methods and atomism.
It is doubtful that Planck interpreted his use of energy increments to mean that the energy of the oscillators and radiation came in chunks (or "quanta"). However, this idea was clearly enunciated by Albert Einstein in his 1905 paper on the photoelectric effect. Einstein explained in this paper why the electrons that are ejected from a cathode by incident light do not increase in energy when the intensity of the light is increased. Instead, the fact that the electrons increase in energy when the frequency of the light is increased suggested that light comes in quantum units (later called "photons") and have an energy given by Planck's equation, hf.
Electromagnetic theory, though one of the most important results of nineteenth-century physical theory, contained a number of puzzles. On the one hand, electromagnetism sometimes gave the same result for all reference frames. For example, Faraday's induction law gave the same result for the current induced in a loop of wire for two situations: when the loop moves relative to a stationary magnet and when the magnet moves (with the same speed) relative to a stationary loop. On the other hand, if an ether medium were introduced for electromagnetic waves, then the predictions of electromagnetism should usually change for different reference frames. In a second paper from 1905, Einstein reinterpreted attempts by Henri Poincaré (1854–1912) and Hendrik Antoon Lorentz (1853–1928) to answer this puzzle, by insisting that the laws of physics should give the same results in all inertial reference frames. This, along with the principle of the constancy of the speed of light, formed the basis of Einstein's special theory of relativity.
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——. The Newtonian Revolution. Norwalk, Conn: Burndy Library, 1987.
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Hankins, Thomas L. Science and the Enlightenment. Cambridge, U.K., and New York: Cambridge University Press, 1985. The best place to start for the general reader.
Harman, P. M. Energy, Force, and Matter: The Conceptual Development of Nineteenth-Century Physics. Cambridge, U.K., and New York: Cambridge University Press, 1982. A useful book but compact and difficult for the beginner.
——. The Natural Philosophy of James Clerk Maxwell. Cambridge, U.K., and New York: Cambridge University Press, 1998.
Heilbron, J. L. Electricity in the Seventeenth and Eighteenth Centuries: A Study of Early Modern Physics. Berkeley: University of California Press, 1979. A particularly good survey of both the conceptual and the institutional development of physics.
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——. Intellectual Mastery of Nature: Theoretical Physics from Ohm to Einstein. Chicago: University of Chicago Press, 1986. A detailed study of the conceptual and institutional development of theoretical physics as a subdiscipline.
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——. The Copernican Revolution: Planetary Astronomy in the Development of Western Thought. Cambridge, Mass.: Harvard University Press, 1971.
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Mach, Ernst. The Principles of Physical Optics: An Historical and Philosophical Treatment. Mineola, N.Y.: Dover, 2003.
——. Principles of the Theory of Heat: Historically and Critically Elucidated. Edited by Brian McGuinness. Boston: D. Reidel, 1986.
——. The Science of Mechanics: A Critical and Historical Account of Its Development. Translated by Thomas J. McCormack. 6th ed. LaSalle, Ill.: Open Court, 1960. Mach's books are guilty of "presentism," the tendency to judge past science in terms of current knowledge. Nevertheless, his work should be studied by the more advanced student.
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Park, David. The Fire Within the Eye: A Historical Essay on the Nature and Meaning of Light. Princeton, N.J.: Princeton University Press, 1997. A good popularization.
Pullman, Bernard. The Atom in the History of Human Thought. New York: Oxford University Press, 1998. Guilty of "presentism," but nevertheless a useful survey.
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——. From X-Rays to Quarks: Modern Physicists and Their Discoveries. San Francisco: W. H. Freeman, 1980.
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——. Force in Newton's Physics: The Science of Dynamics in the Seventeenth Century. New York: Elsevier, 1971.
——. Never at Rest: A Biography of Isaac Newton. Cambridge, U.K., and New York: Cambridge University Press, 1980.
Whittaker, Edmund. A History of the Theories of Aether and Electricity. New York: Humanities Press, 1973.
Williams, L. Pearce. The Origins of Field Theory. Lanham, Md.: University Press of America, 1980. This classic study focuses on the work of Michael Faraday.
G. J. Weisel
Science EncyclopediaScience & Philosophy: Philosophy of Mind - Early Ideas to Planck lengthPhysics - Middle Ages, Sixteenth And Seventeenth Centuries, Eighteenth Century, Nineteenth Century, Causes Of Motion: Medieval Understandings