R. H. Dicke

The Measurement of Thermal Radiation at Microwave Frequencies

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 (0.4°C) compares favorably with a theoretical value of 0.46°C. With an r-f band width of 16 mc/sec., the 0.4°C corresponds to a minimum detectable power of 10-16 watt. The method of calibrating using a variable temperature resistive load is described.

The Lunar Laser Ranging Experiment: Accurate ranges have given a large improvement in the lunar orbit and new selenophysical information

The lunar ranging measurements now being made at the McDonald Observatory have an accuracy of 1 nsec in round-trip travel time. This corresponds to 15 cm in the one-way distance. The use of lasers with pulse-lengths of less than 1 nsec is expected to give an accuracy of 2 to 3 cm in the next few years. A new station is under construction in Hawaii, and additional stations in other countries are either in operation or under development. It is hoped that these stations will form the basis for a worldwide network to determine polar motion and earth rotation on a regular basis, and will assist in providing information about movement of the tectonic plates making up the earths surface. Several mobile lunar ranging stations with telescopes having diameters of 1.0 m or less could, in the future, greatly extend the information obtainable about motions within and between the tectonic plates. The data obtained so far by the McDonald Observatory have been used to generate a new lunar ephemeris based on direct numerical integration of the equations of motion for the moon and planets. With this ephemeris, the range to the three Apollo retro-reflectors can be fit to an accuracy of 5 m by adjusting the differences in moments of inertia of the moon about its principal axes, the selenocentric coordinates of the reflectors, and the McDonald longitude. The accuracy of fitting the results is limited currently by errors of the order of an arc second in the angular orientation of the moon, as derived from the best available theory of how the moon rotates in response to the torques acting on it. Both a new calculation of the moons orientation as a function of time based on direct numerical integration of the torque equations and a new analytic theory of the moons orientation are expected to be available soon, and to improve considerably the accuracy of fitting the data. The accuracy already achieved routinely in lunar laser ranging represents a hundredfold improvement over any previously available knowledge of the distance to points on the lunar surface. Already, extremely complex structure has been observed in the lunar rotation and significant improvement has been achieved in our knowledge of lunar orbit. The selenocentric coordinates of the retroreflectors give improved reference points for use in lunar mapping, and new information on the lunar mass distribution has been obtained. Beyond the applications discussed in this article, however, the history of science shows many cases of previously unknown, phenomena discovered as a consequence of major improvements in the accuracy of measurements. It will be interesting to see whether this once again proves the case as we acquire an extended series of lunar distance observations with decimetric and then centimetric accuracy.

The Surface Temperature of the Sun and Changes in the Solar Constant

The solar distortion telescope has been used to measure the limb shape and latitude dependence of the limb brightness during the summer months for a 5-year period - a time base sufficient to detect solar cycle trends in the data. Comparison of these observations with spaceborne measurements of the solar constant suggests that a significant part of the solar cycle variation in irradiance is a result of temporal changes in the latitude-dependent surface temperature of the sun.

Pulse Techniques in Microwave Spectroscopy

Methods are described for exciting gases to states from which they emit coherent spontaneous radiation in the microwave frequency region. The excitation is produced by the application of short pulses of microwave power. The power subsequently radiated by the gas is calculated for several cases, and experimental methods used to detect the radiation are described. The problem of sensitivity is discussed and compared to the sensitivity obtainable in a continuous absorption experiment. The operation of a high resolution microwave spectrometer, which produces lines substantially narrower than the usual Doppler width, is described. The method used to stabilize the klystron used in these experiments, which is a modification of the Pound i.f. method, is given.

Cosmology and Newtonian Mechanics

The general relativistic solution to the cosmological problem is often considered to be one of its most beautiful results. It is shown that Newtonian mechanics is quite sufficient to discuss the dynamics of the expanding universe. Furthermore, this is not a cleverly contrived classical model, or a crude nonrelativistic approximation, but a completely correct discussion of the dynamics of the expansion in a region where both general relativity and Newtonian mechanics are equally valid.

Observations of a solar latitude-dependent limb brightness variation

A small latitude-dependent photospheric excess-brightness variation was observed from 131 days of data obtained with the Princeton Solar Distortion Telescope. Using an analytic model to separate the influence of faculae from the brightness signal, the authors obtained, for the summer of 1983, a temperature difference of 0.6 + or - 0.1 K between the poles and the regions at + or - 53/sup 0/ solar latitude, with the polar regions being hotter.

Gravitational theory and observation

ON THE 50TH ANNIVERSARY of the publication of Albert Einsteins general relativity, his theory of gravitation, it is appropriate to reconsider the observational support of this theory and its place in contemporary physics. Within the framework of most of todays specialized physical disciplines, gravitation appears superficially to be of little significance. Nonetheless, some of the most enigmatic and puzzling of physical problems intimately involve this phenomenon.

Gravitation and space science

Abstract : The relations between relativity and gravitation are developed. It is shown how space science may someday improve knowledge of gravitation. Important relativistic questions are shown to be involved in considerations of cosmology and radio galaxies.

The variable oblateness of the sun: measurements of 1984

The solar oblateness measured in 1984 appears to be significantly less than the 1983 value. This is in turn substantially less than the 1966 value. The observations of 1983 and 1984 were made with a modified and improved version of the Princeton Solar Distortion Telescope used in 1966. Its design is discussed. The source of error and the analysis technique are discussed. The observed changes in oblateness of the Sun are believed to be real and significant.

On observing the absence of an atom

An atom is confined to a box in its ground state. An attempt is made to observe it in the left half of the box by scattering photons out of a photon wave packet passing through this half of the box. If no photons are scattered, the atom is missing. It is located on the right side of the box and its wave function is changed. The expectation value of the combined atom and photon energy is increased. For the other alternative, that the atom is found on the left side, the expectation value is decreased. By including both alternatives, it is shown that the mean energy is conserved.