It must be noted that we are referring here to the curvature of the whole four-dimensional world constituted by space and time. The close link between space and time was established, as is known, in the special theory of relativity by its founders, Lorentz, Poincare, Einstein, and Minkowski (1905-1908).
Besides the three effects derived from the general theory of relativity, Einstein's theory also predicts some other phenomena, the most important of which may be considered the possible existence of gravitational waves, propagated with the speed of light and apparently carrying energy.
Gravitational waves may be in principle revealed either by means of a suitable detector, directed at the Sun or other astronomical objects capable of emitting relatively large pulses of such waves, or else by building some type of powerful gravitational-radiation generator in the laboratory.
In the opinion of J. Weber (U.S.A. ), who constructed the first operating setup (1962), the best way of detecting gravitational waves is to use electric quartz crystals, in which the waves would produce a strain resulting in electrical polarization. V. B. Braginskii and G. I. Rukman (Moscow) recently suggested that the nonstatic character of gravitation maybe disclosed by ing for gravitational effects in suitably rotating masses, and came up with a new design for a piezoelectric generator of gravitational waves, based on the use of quadrupole oscillators. It may be noted that some points in the matter are still not very clear, due to the complexity of the nonlinear equations involved. At any rate, the problem of discovering gravitational waves is now placed for the first time on firm experimental ground and is mobi lizing the powerful resources of modern radioelectronics and other techniques.
If gravitational waves carrying energy are detected, a new form of matter will have been discovered; this is bound to have many physical implications and might eventually give rise to technological applications. Some experiments have also now been set up for the first time (Fairbank, Nordsieck), designed to display the rotational effects predicted by Einstein's theory (Thirring and Lense, Schiff).
Recently, two interesting experiments were performed which did not yield any basically new findings but provided a more precise means of investigating gravitational phenomena. V. B. Braginskii and G. I. Rukman
showed the absence of any screening of gravitation, and thus disproved the older, less precise, observations of Majorana. Further refinements are none the less expected to exhibit some screening effect. At Princeton (U.S.A.), Dicke demonstrated the equality of inertial and gravitational mass, with a precision fifty times higher than that of the classical Eotvbs experiments. Some interesting proposals have been made of trying to detect antigravitation in anti-K-mesons (Podgoretskii, Okonov, and Khrustalev in Dubna).
The success of Einstein's theory, the first to have refined the concept to gravitation since Newton, naturally roused hopes for the possibility of reducing the electromagnetic field, as well as all other fields and properties of matter, to the geometrical properties of space-time.
In 1918 an article by Weyl was published which considered the construction of a geometrized world picture, and it was followed by a large number of works on this same subject.
However, in spite of the enormous number of papers produced on the subject, including many by Einstein himself, and by other eminent theore ticians and mathematicians, which no doubt substantially contributed to the development of higher geometry, the attempts to construct a geometrized unified theory of matter did not achieve any material success.
No new physical results have been obtained (except for one minor achievement, the derivation of the Klein-Gordon equation in five dimensions) which go to establish a relativistic quantum field theory and which are directly applicable (as it later turned out) and K-mesons. It should be noted, furthermore, that the aims of such authors were too broad, as they were trying not only to work out a unifield field theory (comprising the gravitational, the electromagnetic, and later also the mesic fields), but also to derive the existence of particles and even of quantum phenomena from geometrical schemes. It has now become clear that this should be attempted on the basis of quantum theory, but in a considerably generalized form. It is therefore not surprising that the flow of papers devoted to a geometrized field theory subsided in the 1920's, when physicists were beginning to develop quantum mechanics, turning in the thirties to the intensive investigation of the atomic nucleus and later of the elementary particles.