JANUARY i, 19&4 By considering a radiating gas as a single quantum-mechanical system, energy levels corresponding to certain correlations between individual molecules are described. Spontaneous emission of radiation in a transition between two such levels leads to the emission of coherent radiation. The discussion is limited erst to a gas of dimension small compared with a wavelength. Spontaneous radiation rates and natural line breadths are calculated. For a gas of large extent the effect of photon recoil momentum on coherence is calculated. The effect of a radiation pulse in exciting "super-radiant" states is discussed. The angular correlation between successive photons spontaneously emitted by a gas initially in thermal equilibrium is calculated. " 'N the usual treatment of spontaneous radiation by .a gas, the radiation process is calculated as though the separate molecules radiate independently of each other. To justify this assumption it might be argued that, as a result of the large distance between molecules and subsequent weak interactions, the probability of a given molecule emitting a photon should be independent

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A leading physicist delves into relativity and experimental applicationsGravitation and Cosmology: Principles and Applications of the General Theory of Relativity offers a Nobel laureate's perspectives on the wealth of data technological developments have brought to expand upon Einstein's theory. Unique in basing relativity on the Principle of Equivalence of Gravitation and Inertia over Riemannian geometry, this book explores relativity experiments and observational cosmology to provide a sound foundation upon which analyses can be made. Covering special and general relativity, tensor analysis, gravitation, curvature, and more, this book provides an engaging, insightful introduction to the forces that shape the universe.

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to 171.67.34.205. Redistribution subject to AIP license or copyright; see http://rsi.aip.org/about/rights_and_permissions 268 R. H. DICKEThe work being carried on at present in this laboratory is a study of passivity of iron and other metals. Preliminary results show with iron a surface potential change of about 0.2 volt in the negative direction upon passivation in concentrated nitric acid followed by a reverse change when made active with hydrochloric acid. This is in the direction that would be expected on the basis of an oxide layer. However, activation by THE REVIEW OF SCIENTIFIC INSTRUMENTS dilute nitric acid gives a more negative value than the passive iron, a result which perhaps means the presence of a different oxide or possibly a basic nitrate.Acknowledgments are due Doctors L. P. Gotsch and Laubscher for suggesting the use of a pH meter for this purpose and Mr. Robert Busch for laboratory assistance. This project was supported in part by the Line Material Company.The connection between Johnson noise and blackbody radiation is discussed, using a simple thermodynamic model. A microwave radiometer is described together with its theory of operation. The experimentally measured root mean square fluctuation of the output meter of a microwave radiometer (OA°C) compares favorably with a theoretical value of OA6°C. With an r-f band width of 16 me/sec., the O.4°C corresponds to a minimum detectable power of 10-16 watt. The method of calibrating using a variable temperature resistive load is described.

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