On the Meaning of Physical Theories

Abstract

Whoever regards in a detached way the development of the exact sciences must be impressed by two contradictory features. On the one hand, the whole of natural science exhibits a picture of continuous and healthy growth, of unmistakable progress and construction, evident as much in its inward deepening as in its outward application to the technological mastery of Nature. Yet, on the other hand, one observes at not infrequent intervals the occurrence of upheavals in the basic concepts of physics, actual revolutions in the world of ideas, whereby all our earlier knowledge seems to be swept away, and a new epoch of investigation to be inaugurated. The abrupt changes in the theories are in marked contrast to the continuous flow and growth in the realm of well ascertained results. We may give a few examples of such convulsions of theories. Consider the most ancient and most venerable branch of physical science, astronomy, and the ideas concerning the stellar universe, whose course we can follow through thousands of years. At first, the Earth is at rest, a flat disc at the center of the Universe, round which the constellations move in orderly procession. Then, almost simultaneously with the realization of the earth’s size and spherical shape, comes the Copernican system of the Universe, placing the sun in the center and allotting to the earth only a subordinate place among many other attendants of the central star. The beginning of the new era in natural science is marked by Newton’s theory of gravitation, which holds the solar system together, and which remained unchallenged for some two centuries. In our time, however, it has been dethroned by Einstein’s relativistic theory of gravitation, which completely does away both with the heliocentric system of planets and with gravity acting at a distance.