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The Theory of Relativity
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Above: a dot-man traveling from point A in a two dimensional world would go ahead for an infinite period of time in a straight line as shown.

Right: If the dot-man started from point A on a sphere and in what for him appeared to be a straight line. He would eventually return to point A

SPACE-TIME

The space-time of Einstein is called a four-dimensional continuum. It is also called a space-time continuum. The point of space and the instant of time at which any event occurs represents a single point in the continuum. The interval between any two events would be represented by a finite line (one with definite limits). A succession of finite lines of this kind would make up a record of the successive positions occupied by a particle in space-time. A complete record of this sort of the total course of the particle from the first to the last moment of its existence would be called the parti­cle's world line.

To understand the principle involved, let us suppose that the two-dimensional world of the dot-man is marked out in equal squares, formed by intersecting lines (see below). The dot-man is taking a walk. He starts at point A, at the intersection of two of the lines and goes straight ahead. At the end of an hour he reaches point B, where two other lines intersect. At the end of two hours he reaches point C, and so on. If we draw a line connecting all these points, it will represent part of a world line based on the two dimensions of length and breadth and the third dimension of time. It will provide information concerning a series of events -the arrival of the dot-man at one point after another.

A dot-man traveling in a two dimensional world goes ahead in a straight line from A to B; from B to C; from C to D; from D to E; from E to F. The line AF would represent part of the world line of the dot-man, the record of his movement.

WARPING SPACE_TIME

Matter will have an effect on the net­work of the space-time continuum. Matter distorts, the network. Without matter, the continuum would be Euclidean, and go on forever in straight lines in any direction. We can show this warping in three dimensions by imagining a network of rubber bands, criss-crossing like a fly screen. Without matter nearby, the rubber bands would be flat in two dimensions.

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