Lebedev, Petr Nikolaevich

(b. Moscow, Russia, 8 March 1866; d. Moscow, 1, April 1912)

physics.

Lebeve’s father, Nikolay Vasilievich, a prosperous merchant, tried to guide his son into the same path by sending him to commercial school and then to a Realschule. After completing his secondary education, Lebedev entered the Moscow Highest Technical School. Convinced, however, that the career of and engineer was not for him, in 1887 he entered one of the best schools of physics in western Europe, at Strasbourg University, to work under A. Kundt. In 1888 Kundt moved to Berlin University; Lebedev could not follow him because he lacked the prerequisite diploma from a classical Gymnasium. At the suggestion of Friedrich Kohlrausch, Lebedev wrote “Ob izmerenii dielektricheskikh postoyannykh parov i o teorii dielektrikov Mossotti-Klauziusa”(“On the Measurement of the Delectric Constants of Vapors and on the theory of Dielectrics of Mossotti and Clausius”), in which he experimentally confirmed this theory. In the fall of 1891 he returned to Moscow; and at the invitation of A.G. Stoletov, then head of the department of physics at Moscow University, became a teacher there. In 1900 he defended his dissertation, “Eksperimentalnoe issledovanie ponderomotornogo deystvia voln na rezonatory”(“An Experimental Investigation of the Ponderomotive Action of Waves on Resonators”), for which he was awarded the doctorate in physical and mathematical science. Lebedev subsequently became professor of physics at Moscow University.

In his dissertation Lebedev posed the problem of studying the action of light waves on molecules in the simplest form. In view of the impossibility of investigating the effect on individual molecules, he tried to observe the influence of electromagnetic waves on a schematic molecule, using a resonator suspended from a twisted thread, which had its own period of vibration. Lebedev extended this research to hydrodynamic and acoustical waves. It appeared that the ponderomotive actions of waves completely different in nature were subject to the same laws. In investigating electromagnetic waves Lebedev encountered their similarity to light waves. He succeeded in constructing and extremely small vibrator, which allowed him to obtain waves from four to six milli meters long—100 times shorter than the waves studied by H. Hertz and ten times shorter than those studied shortly before by Righi. The small size of the apparatus allowed Lebedev to study, for the first time, the double refraction of electromagnetic waves in natural crystals of rhombic sulfur. His work “O dvoynom prelomlenii luchey elektricheskoy sily”(“On the Double Refraction of the Rays of Electric Force,”1895), established him as and excellent experimenter. Lebedev’s success in obtaining short waves was not surpasses until thirty years later, by A. A. Glagoleva Arkadieva and, independently, by M. A. Levitskaya.

As early as 1891 Lebedev became seriously interested in the pressure of light. He turned his attention to the fact that since the force of gravity is proportional to the volume of a body whereas light pressure must be proportional to its surface, it may be asserted that in a particle of cosmic dust the forces of light pressure pushing the particle away from the sun will be equals to the force of gravity attracting it toward the sun. Lebedev used this theory to explain why comets’tails always point away from the sun. His hypothesis was considered correct until the discovery of the solar wind, which creates substantially greater pressure than the sun’s light.

Around 1898, Lebedev began experimental research on light pressure. Although its presence had been predicted by Maxwell’s theory, it had not been detected experimentally before Lebedev. He first undertook research on the pressure of light on solid bodies. Because of the weakness of the effect itself and the considerable number of possible side effects, this experimental problem presented very great difficulties: if a body that is supposed to react to light pressure is placed in a gas, the warming of the body by the light will inevitably cause convection currents and thus set the body in motion. If the body is placed in a vacuum (in practice, in gas at very low pressure), the so called radiometric effect will occur. As a result of the uneven warming of the front and back of the body, the molecules of gas hitting the body from the front will be repulsed more forcefully than those striking the back, thereby exerting greater pressure. By extremely ingenious methods Lebedev succeeded in completely eliminating these side effects and not only detected the pressure of light but also measured it and showed the correctness of Maxwell’s quantitative theory. “Opytnoe issledovanie svetovogo davlenia”(“An Experimental Investigation of the Pressure of Light”) was read by Lebedev at the International Congress of Physicists at Paris in 1899 and was published in 1901.

In connection with the study of processes in gas, Lebedev first considered the advantages of measuring radiant energy with vacuum thermocouples (1902).

He began the second, more difficult, phase of his research by studying the pressure of light on gases and completed this work in 1910. The attempts of other scientists to repeat Lebedev’s research on light pressure were unsuccessful until and 1920’s, when physicists developed new vacuum techniques.

Lebedev subsequently began to study the origin of terrestrial magnetism, seeking to relate it to the earth’s rotation. He did not live to complete this and some other research.

Lebedev was one of many professors who in 1911 left Moscow University in protest against the illegal violation of the university’s autonomy by the minister of education, L. A. Kasso. Deprived of his laboratory, he was offered chairs at a number of universities, including some abroad, and a campaign was begun to gather private funds to build him a laboratory. But the shock of these events affected his health and led to his sudden death.

Lebedev’s activity was not limited to scientific research. He created and important school of Russian physics, which produced such scientists at P. P Lazarev, T. P. Kravets, S. I. Vavilov, and V. K. Arkadiev, who have played a major role in the development of Soviet physics.

BIBLIOGRAPHY

Lebedev’s writings have been brought together in three books: Sobranie sochineny (“Collected Works”; Moscow, 1913), with “Biografichesky ocherk”(“Biographical Sketch”), pp. vii–xxiii; Izbrannye sochinenia (“Selected Works”), A. K. Timiryazev, ed (Moscow, 1949), with Timiryazev’s foreword and biographical sketch, pp. 11-32; and Sobranie sochineny, in the series Klassikov Nauki (Moscow, 1963), which includes T. P. Kravets,“Petr Nikolaevich Lebedev,”pp. 391-405; and N. A. Kaptsov,“Rol Petra Nikolaevicha Lebedeva v sozdanii nauchno-issledovatelskikh kadrow”(“The Role of Petr Nikolaevich Lebedev in the Development of Personnel for Scientific Research”),pp. 406-412.

J. G. Dorfman