Gaining on Gravity Waves

Abstract

In 1916 Albert Einstein published his theory of general relativity. In one of its major aspects this is a theory of the nature and operation of gravitational forces with which Einstein intended to replace the classical theory devised by Isaac Newton in the 17th century. Einstein's theory makes a number of predictions that are radically different from those of Newton. One of the most striking of these is that gravitational forces should be propagated in waves in a manner similar to the way electric and magnetic forces are. These gravitational waves should consist of cyclically fluctuating gravitational forces; they should carry energy from place to place, and they should cause minute fluctuations of the surfaces of objects they encounter. Any accelerated body could be a source of gravitational waves, but in practice physicists look to large astronomical bodies such as oblate stars or binary stars. The prediction was that gravitational waves would be extremely weak: For a cylinder a meter long the amount of surface disturbance would be a fraction of the diameter of an atomic nucleus. For 40 years no one seriously looked for gravitational waves, but in the late 1950's Dr. Joseph Weber of the University of Maryland began to develop equipment he thought would do the job. As receivers Dr. Weber uses aluminum cylinders of about a ton's weight, and he has developed piezoelectric sensors that can record fluctuations in the surfaces of these cylinders amounting to fractions of a nuclear diameter. In 1969, after about 10 years of effort, Dr. Weber announced that his equipment had recorded gravitational waves (SN: 6/21/69, p. 593). Since then he has been subjected to criticism, based mainly on his statistical analysis of the data. Throughout the last year he has maintained that further observations and more rigorous statistics support his original assertion. In spite of the critics, confidence in Dr. Weber's work has led other people to enter the search for gravitational waves. About half a dozen experiments are now in progress or in prospect in both the United States and the Soviet Union. Most of these seek to make the detectors more sensitive or to design new kinds of detectors that will record frequency ranges other than the one1,660 cycles per second (hertz)-that Dr. Weber has pioneered. Detectors for the waves can be designed either as broad-band receivers that respond to a range of frequencies or as narrow-band receivers that are excited only by a single frequency. Dr. Weber's cylinders are narrow-band receivers. To supplement the solid-bar type of detector that Dr. Weber used, Dr. David Douglass of the University of Rochester proposes development of a second class of narrow-band receivers, composed of hollow squares, hoops or U-shapes. For this second class of narrow-band receivers he predicts important practical advantages in searches for gravitational wave signals at low frequencies. Any individual detector of either of these two classes will respond to a particular resonant frequency determined by its size. For the first class of detectors the critical dimension is the length of the bar, and the resonant frequency will be inversely proportional to it. To reach low frequencies extremely long bars would be needed. For the hollow shapes the critical dimension is the length of a side or a diameter, and the resonant frequency 40~~~