EINSTEIN'S CONCEPTS OF MOTION, SPACE, AND TIME
The Newtonian theory of gravitation has been very successful in explaining most gravitational problems involving the material objects of the macroscopic world. As we have pointed out, in another world, the submicroscopic universe of the atom, nuclear and electromagnetic forces dominate the very weak gravitational force, and they determine form and behavior in that realm. But it is the movement of bodies at high velocities or in the presence of very strong gravitational fields, where Newton's theory of gravitation breaks down, that we are interested in at this point. Here relativity has changed cosmology profoundly, helping to explain cosmic reality.
The special theory of relativity of 1905 was not originally designed as a refinement of Newtonian gravitational theory but was addressed by Einstein to some aspects of electromagnetic radiation, that is, light. Special relativity theory deals with physical phenomena in which gravitational forces are not involved. By 1916 Einstein had worked out another, more comprehensive theory, which was an alternative to the gra\itational theory of Newton. He incorporated in his general theory of relativity a description of nopuniform, or accelerated, motion between observers
The general theory of relativity has important implins for our ideas of motion, mass, and the geomof space and time.
at is relative in relativity? Let us consider an example. If we walk down the aisle of a moving train car can be thought of as the frame of reference of our motion. Looking out of the train window, our frame of reference moving relative to the ground as another frame of reference. We know that ground moves relative to the center of the earth as er frame of reference. The center of the earth moves relative to the sun as still another frame of reference. And so it is on a larger scale-we cannot ascertais our possible absolute motion in the universe we can find no frame of reference that is absolutely still.
Einstein coupled the three dimensions of space together with that of time and taught us that when we measure space and time, we find no absolute results the answers are relative, depending on the observer. Two people who are moving relative to each other see the same events going on at different places erent times; each experiences the event in his own frame of reference. We need relativity in astronomy use we cannot drop anchor in the universe and pssively watch what happens around us: Each of us is observer and participant. The first truth in the relativistic universe is that all motion is relative; that is, there is no preferred frame of reference in which space and time are defined absolutely.
Newtonian space has three numbers, the spatial coordinates that describe where an object is: forward-backward, right-left, and up-down. Location has nothing to do with time. According to Newton, Absolute space, in its own nature, without relation to anything external, remains always similar and immovable .... "Absolute, true and mathematical time, of itself, and from its own nature, flows equably without relation to anything external. ... "
Einstein's theory on the other hand coupled space and time, making time a fourth dimension. In his general theory of relativity Einstein described the geometry of four-dimensional space-time, in which we, planets, stars, and galaxies exist. The space-time continuum shows deformities (that is, space curvature) in the vicinity of material bodies; the more massive the body, the stronger is this curvature. From Newton's viewpoint an object moves in a curved path in response to the gravitational force of a massive body. For Einstein an object moves in a curved path as a natural consequence of the space curvature produced by the massive body. Where no massive body exists, the space-time geometry is flat, showing no curvature, and the object moves uniformly in a straight line. In the Newtonian world it moves that way because no force is acting upon it.
