Collected by Alfred 6. Garrett
Ohio State University Columbus
I I
The Flash of Genius, 16
Quantum Theory:
111 the fimt part of the 1800's John Dalton gathered the then known information to show that matter is not continuous stuff but is discontinuous and made of disercet pnrtiwhich he called atoms. His were drawn from rather simple observations about the solubilities of gases in liquids and from quantitative relat,ians in chemical reactions. But what could be said shout the other great entity of the universe, energy? I t is of interest in that about 100 years later a German scientist, ,Max Planck, gathered the then known informathat it too collld be tion %bout radiant and treated as though it were discontinuous stuff and nas composed of little hunches of energy which he called gzranta. His c6nclusions were drawn herause scientists had failed t o e x ~ l s i nthe cause
Max Plan~k
"When I recall the days of twenty years ago, when the conception of t,he physical quantum of 'action' was first beginning to disentangle itself from the surrounding mass of available experimental facts, and when I look back upon the long and tortuous road which finally led to it, the whole development strikes me as a new illustration of Goethe's saying, that 'man errs, so long as he is st,riving.' All the mental effort of an assiduous invest,igator must indeed appear vain and hopeless, if he does not occasionally run across striking facts which form incontrovertible proof of the truth he seeks, and which show him t,hat-after all-he has moved a t least one s k p nearer to his objective. The pursuit of a goal, the brightness of which is undimmed by initial failure, is an indispensable condition, though by no means a guarantee, of final success. "In my own case the long-time goal was to solve the problem of the distribution of energy in the normal spect,rnmof radiant heat. Gnstav Kirchhoff's discovery (in an enclosure surrounded by emitting or absorbing bodies at the same temperature, the quality of the heat radiation produced is entirely independent of the nature of those bodies) established the ezistence o f n universal function which depends only zrpon the tcmpcrature and wavelength and i s entirely independent f!f the particdar properties of the substance. The discovery of this remarkable function promised a deeper insight. into the
relation between energy and temperature. To understand this relationship is the problem of thermodynamics and therefore also of the entire field of m~lecularphysics. TO find such a function meant to search among $1 the different bodies occurring in to select one which had known emissive and absorptive powers, and to the energy distribution in the heat radiation that was in equilibrium with the body. This distribution should then, according to ~irchhoff'slaw, be independent of the niture of the body." Planck's first attempt was to derive a mathematical equation of two terms for radiant energy, but he was concerned about the lack of physical meaning of the equati~n.~ "Even if this radiation formula should prove to be absolutely accurate i t would, after all, be only an interpolation formula found by happy guesswork, and would thus leave onerather unsatisfied. I was therefore occupied from the day of its origination with the task of giving it a real physical meaning. This question led me along Boltzmann's line of thought to consider the relationship betweenentropy and probability. After some weeks of the most intense work of my life, clarity dawned, and an unexpected view was revealed in the distance." P!anck describes in general terms some of his reasoning in deriving the equation, E = h ~ . ~ "Much less simple than the interpretation of the Roltzmann const,ant was the interpretation of the second universal constant of the radiation law which, as the product of energy and t,ime (amounting on a first c~lculationto 6.55 X 10W2' erg-sec), I called the elementary quantum of action." Then he became concerned about tests of his new formula and he said, "While this constant was absolutely indispensable for the attainment of a correct expression for entropy-for only with its aid could the magnitude of the 'elementary region' or 'range' of probability for the statist.ica1treatment be determined -it obstinately withstood all attempts a t fitting it, in any suitable form, into t.he frame of classical theory.' "The first advance in this field was made by Einstein, who on the one hand pointed out that the introduction of the quanta of energy associated with the quantum of action seemed capable of explaining readily a series of remarkable properties of light action discovered experimentally, such as Stoke's rule, the emission of electrons,
' P r . a w ~ ,M., "The Origin and Development of the Quantum Theory," Nobel Prize address delivered before the Royal Swedish Academy of Science nt St,oekholm, June 2, 1920, Oxford, Clarendon Press (1922).
DAREOW,K. K., Sci. Am., 186, No. 3 pp. 47-54 (1952). PLANCK, M., Ann. Physik., 4, pp. 553-,563 (1901). PLANCK, M., Prtwisehe Akademie der Wissensehnftem, p. 723 (1911,IIj.
simple one, E = hu. In 1900 three interesting problems were being studied: the structure of atoms that Dalton had postulated; the cause of the lines that Bunsen discovered about 1860 in the spectra of elements; and the nature of radiant energy. Planck tackled the third problem. Up to that time several mathematical equations had been derived to attempt to describe tho distribution of radiant energy from a. hot body, hut no oquation was satisfactory over the entire energy range. In his 1920 Nobel Prize address Planck described his work as follows:'
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/ Journal of Chemical Fducafion
and the ionization of gases, and on the other hand, by the identification of the expression for the energy of a system of resonators with the energy of a solid body, derived a formula for the specific heat of solid bodies which on the whole represented it correctly as a function of temperature, more especially exhibiting its decrease with falling temperature. 4 number of
questions mere thus thrown out in different direotions, of which the accurate and many-sided investigations yielded, in the course of time, much valuable material. It is not my task to give an even approximately complete report of the successful work achieved in this field, hut to give the most important and characteristic phase of the progress of the new doctrine."
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