The theory of relativity brought about some revolutionary changes in the concepts of modern physics. Amongst them the most startling one was regarding the nature of space and time. Einstein, in the light of his theory of relativity, imagined the universe as a "four-dimensional continuum of space and time" According to Einstein, the three dimensions of space and one dimension of time are welded together forming a four-dimensional volume which he described as a "continuum." According to great German mathematician, Herman Minkowski, who developed the mathematics of the space-time continuum as a convenient medium for expressing the principles of relativity, "Space and time separately have vanished into the merest shadows, and only a sort of combination of the two preserves any reality."^{[1]}

It is very difficult to describe the new concept without the use of mathematics. But still the concept of "four-dimensional continuum" may be explained in simple language as follows: it is a simple fact that when a body exists in space, it occupies the three dimensions of space, on account of its length, breadth and height (or depth). Now, when it changes its position in space (i.e. when its motion takes place), it takes some time. Thus, to describe the motion of any body we should have the knowledge of two factors: its position in space and time required to change the position. The position is described by the three dimensions of space. Hence, time is referred to as the fourth dimension. Now, the theory of relativity welds together the three dimensions of space and the fourth dimension of time.

Minkowski showed that the theory of relativity required all electrical phenomena (all physical phenomena are considered to be ultimately electrical) to be thought of as occurring not in space and time separately as had hitherto been thought, but in space and time welded together so thoroughly that it was impossible to detect any traces of a join, so thoroughly that the whole of phenomena of nature were unable to divide the product into space and time separately.^{[2]}

Eminent scientist Werner Heisenberg, describing the new structure of space and time, writes: "When we use the term 'past', we comprise all those events which we could know at least in principle, about which we could have heard at least in principle. In a similar manner we comprise by the term 'future' all those events which we could influence at least in principle, which we could try to change or to prevent at least in principle. It is not easy for a non - physicist to see why this definition of the terms 'past' and 'future' should be the most convenient one. But one can easily see that it corresponds very accurately to our common use of the terms. If we use the terms in this way, it turns out as a result of many experiments that the content of 'future' or 'past' does not depend on the state of motion or other properties of the observer. We may say that the definition is invariant against the motion of the observer. This is true both in Newtonian mechanics and in Einstein's theory of relativity.

"But the difference is this: In classical theory we assume that future and past are separated by an infinitely short time interval which we may call the present moment. In the theory of relativity we have learned that the situation is different: future and past are separated by a finite time interval the length of which depends on the distance from the observer. Any action can only be propagated by a velocity smaller than or equal to the velocity of light. Therefore, an observer can at a given instant neither know of nor influence any event at a distant point which takes place between two characteristic times. The one time is the instant at which a light signal has to be given from the point of the event in order to reach the observer at the instant of observation. The other time is the instant of the observation, at which the light signal reaches from the point of the event. The whole finite time interval between these two instants may be said to belong to the 'present time' for the observer at the instant of observation. Any event taking place between the two characteristic times may be called 'simultaneous' with the act of observation".^{[3]}

Thus, in the theory of relativity space and time are related with each other, because of the finite velocity of light. The mathematical formula of Lorentz Transformations reveal that the velocity of light is the maximum possible velocity. It means that no body can travel faster than light, and hence, the knowledge we obtain through our senses or external equipment cannot be obtained faster than the velocity of light. Consequently, the time taken by an observer in perceiving any event cannot be lessened beyond a certain limit. That is to say that the definition of simultaneity depends upon the spatial distance between the point of event and the observer.

A | B |

Fig. 1 |

This relation of space with time can be clearly understood by the example. (See Fig. 1) A is the point of observer and B is the point of event (say, a collision between two cars). The instant of the observation of the event and the instant of the happening of the event are not the same, for the light will take a finite time to travel the distance A B. Thus, the time, which we term as 'present' in our daily practice, is not a single instant but comprises of the whole interval that has elapsed between the happening of the event and the perceiving of the event by the observer, and the length of the interval will depend upon the distance between the observer and the point of event. It should be noticed here that however small the distance A B may be, light, on account of its finite velocity, will take finite time to traverse it.

A practical astronomical example will reveal the importance of the new character of space and time. (See Fig. 2) If, in the above example, B is a star 50 light years'^{[4]} away from the earth and A is the observer on the earth making the observation of the events on the star through a telescope. Now, suppose that an explosion on the star is being observed by the observer. In our ordinary expression, we shall say that the event of observation on the earth and the event of explosion on the star are simultaneously taking place. But is it true? No. Since the star is 50 light years away from the earth, the light from the star will take 50 years to reach the earth. It means that the explosion which is being observed 'Now', had already taken place 50 years ago. Thus, it becomes clear that the term 'simultaneous' has not remained absolute, but it depends upon the distance between the two events. In other words, space and time are related to each other and the events of the universe are effected by them jointly.