Einstein completed the general theory of relativity in 1915 and published this first exposé the following year. The general theory overturned Newton's theory of gravity, which had been valid for two hundred and fifty years. In Newton's universe, gravity was regarded as an attractive force, which all massive bodies exert on each other. The planets moved in elliptical orbits about the sun because of its great mass powerful gravitational force. In Einstein's universe, gravity is not regarded as an exterior force, but rather as a property of space and time or space-time. Einstein's curved four-dimensional space-time continuum is often likened to a suspended rubber sheet stretched taut but deformed wherever heavy objects - stars, galaxies or any other matter - are placed on it. Thus, a massive body like the sun curves the space-time around it and the planets move along these curved pathways of space-time. As Einstein put it: "matter tells space how to bend; space tells matter how to move".
The general theory predicted exactly to what extent a light beam would be bent when it passes near the sun.
When forced to summarize the general theory of relativity in one sentence: Time and space and gravitation have no separate existence from matter.
Physical objects are not in space, but these objects are spatially extended (as fields). In this way the concept 'empty space' loses its meaning. The field thus becomes an irreducible element of physical description, irreducible in the same sense as the concept of matter (particles) in the theory of Newton.
A field whose components are continuous functions of four independent variables - the co-ordinates of space and time, represents the physical reality of space. Since the theory of general relatively implies the representation of physical reality by a continuous field, the concept of particles or material points cannot play a fundamental part, nor can the concept of motion. The particle can only appear as a limited region in space in which the field strength or the energy density is particularly high.
Physics constitutes a logical system of thought, which is in a state of evolution, whose basis (principles) cannot be distilled, as it were, from experience by an inductive method, but can only be arrived at by free invention. The justification (truth content) of the system rests in the verification of the derived propositions ( a priori /logical truths) by sense experiences ( a posteriori /empirical truths). Evolution is proceeding in the direction of increasing simplicity of the logical basis (principles). We must always be ready to change these notions - that is to say, the axiomatic basis of physics - in order to do justice to perceived facts in the most perfect way logically.