Dr. N.L. Kachhara
In physics, the concepts of space and time are so basic for the description of natural phenomena that their modification entail an alternation of the whole frame work we use in physics to describe nature. The concepts of space and time underwent radical modification from the Aristotle 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 Aristotle 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 relativity theory, space was not three dimensional and time was not a separate entity. Both were intimately connected and formed a four-dimensional continuum- "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.
Einstein's earlier theory of time and space, special relativity, proposed that distance and time are not absolute. The ticking rate of a clock depends on the motion of the observer of that clock, likewise for the length of a "yardstick". Published in 1915, general relativity proposed that gravity, as well as motion, can affect the intervals of time and of space. The key idea of general relativity, called the equivalence principle, is that gravity pulling in one direction is completely equivalent to acceleration in the opposite direction. A car accelerating forward feels just like sideways gravity pushing your back against your seat. An elevator accelerating upward feels just like gravity pushing you into the floor.
If gravity is equivalent to acceleration, and motion affects measurements of time and space, then it follows that gravity does so as well. In particular, the gravity of any mass, such as our sun, has the effect of warping the space and time around it. For example, the angles of a triangle no longer add up to 180 degrees, and clocks tick more slowly the closer they are to a gravitational mass like the sun.
Many of the predictions of general relativity, such as the bending of starlight by gravity and a tiny shift in the orbit of the planet Mercury, have been quantitatively confirmed by experiment Two of the strangest predictions, impossible ever to completely confirm, are the existence of black holes and the effect of gravity on the universe as a whole.
Beginning in 1917, Einstein and others applied general relativity to the structure and evolution of the universe as a whole. The leading cosmological theory, called the big bang theory was formulated in 1922 by the Russian mathematician and meteorologist Alexander Friedman. According to the big bang theory, the universe may keep expanding forever, if its inward gravity is not sufficiently strong to counter balance the outward motion of galaxies, or it may read a maximum point of expansion and then start collapsing, growing denser and denser, and gradually disrupting galaxies, stars, planets, people and eventually even individual atoms. Which of these two fates awaits our universe can be determined by measuring the density of matter versus the rate of expansion. Much of medium cosmology, including the construction of giant new telescopes has been an attempt to measure these two numbers with better and better accuracy. With the present accuracy of measurement, the numbers suggest that our universe will keep expanding forever, growing colder and colder, thinner and thinner.
In the opinion of Einstein the space and time are products of our imagination, the same way as the colour, shape and from of objects reside in our thoughts produced by our consciousness. Space is nothing more than the order in which the things are arranged (in space) and time is nothing more than the interval of time between events. The space and time do not have a real objective existence and yet the four dimensional continuum of space time is supposed to be an objective reality. Time and space are separate subjective entities but when considered as four dimensional continuums they describe the real world.
Some scientists do not agree with the above proposition. They do not oppose the physical principle of four dimensional continuums of space and time but do not accept the view that space and time are dependent on the observer and do not have an independent objective existence. Hans Reichenbach, in his book "The philosophy of space and time", has deliberated on the philosophical as well as the physical aspect of the theory of relativity. In this book Reichenbach has proved the real independent existence of space and time mathematically and logically. According to him the mathematical space has many modes one of them supported by observations and experiments can be assumed to be the real space. Reichenbach has also refuted the wrong beliefs about time. Many scientists have started believing that time which is the fourth dimension, is like one of the dimensions of space. But this is not true. Combination of space and time in the four dimensional continuum of space and time only means that we describe any event in terms of four dimensions, three of space and one of time. Consideration of time as the fourth dimension does not change our concept about time. Accepting the theory of relativity, Reichenbach has shown that space and time are independent objective realities.
Henry Margenu also accepts the objective reality of space and time. He says that any thing that is relative does not become unreal. The theory of relativity does not accept an absolute space, but that does not mean that space is not real. It must be remembered that a mathematical theory describes a particular aspect of reality (if at all it does) and it does not represent the reality in totality. It shall be unfair to deny or accept the existence of a reality based on a mathematical equation, howsoever powerful. However, the scientists are still divided on the issue of reality of space and time.
