The concept of relativistic spacetime is fundamental to the structure of modern physics. The measurement of time is no longer absolute, but is, according to special relativity theory, path-dependent. In Einstein's terms, an observer at rest in one reference frame measures his own proper time ("eigen-zeit") on a clock, while observers moving relative to him can calculate his proper time using their measurements of both distance and time. From Albert Einstein's general relativity theory—the relativistic theory of gravitation—it is possible to derive three standard models of the Universe that allow for both finite and infinite temporal developments, all starting from an initial singularity. In deciding between these models, quantum mechanics—the modern (nonclassical) theory of matter—must be taken into account. Newton's conception of time now remains valid only locally and approximately for velocities considerably lower than the velocity of light.