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Microcosmology : Atom of the cosmos is due to the two other substances viz media of motion and rest. Without these two, the systematic structure of the cosmos would have been a chaos.
SPACE-TIME
Physics, the concepts of space and time are so basic for the description of natural phenomena that their modification entails an alteration of the whole framework we use in physics to describe nature. The concepts of space and time underwent radical modification from the time of Aristotles to the present time.
In Newtonian physics, matter particles moved in a three dimensional absolute space, filled with ether (medium of motion). It was an absolute space, always at rest and unchangeable. All changes in the physical world were described in terms of a separate entity called time, which again was absolute having no connection with the material world and flowing smoothly from the past through the present to the future. These concepts of space, time and ether were the basis of physics for almost three centuries.
Both Aristotles and Newton believed in absolute time. That is, one could always measure the interval of time between two events and that it would be the same whoever measured it. Time was completely separate from and independent of space. This commonsense view worked well when dealing with apples or planets that move slowly but they don't work at all for things moving at or near the speed of light.
According to Einstein's relatively theory, space was not three dimensional and time was not a separate entity. Both were intimately connected and formed a four-dimensional continnum"space-time". Furthermore, there was no universal flow of time. Concepts of an absolute space and an absolute time were, thus, abandoned and became merely elements of language for describing observed phenomena. Concept of ether was also given up.
Einstein's theory, moreover, says that three-dimensional space is curved and the curvature is caused by the gravitational field of massive bodies. Thus according to this theory, the universe is finite with nothing beyond it.
