The Roving Mind by Isaac Asimov

Author:Isaac Asimov [Asimov, Isaac]
Language: eng
Format: epub, mobi
Publisher: Prometheus Books - A
Published: 2010-08-15T22:00:00+00:00

Can we ever need four numbers? Yes, of course. If we are locating a fly in a room, or an airplane in the atmosphere, or a satellite in orbit, three numbers will do only if we consider a particular moment in time. If you don’t do that, then by the time you get the location and look for the located object, the fly or the plane or the satellite is no longer there. It has moved. You need a fourth number to give you the particular time at which the other three numbers are valid.

In that sense, time is a fourth dimension and, in Albert Einstein’s view of the universe, time must be taken as an integral part of space; so we speak of four-dimensional space-time.

Time, however, differs in fundamental ways from the other three dimensions.

The dimensions north-south, east-west, and up-down are interchangeable. Suppose you have a cubic box and wish to locate a point within it. You don’t have to keep the box in some fixed position. You can move it so that what was north-south becomes east-west and vice versa; or so that what was east-west becomes up-down and vice versa.

For that matter you can make the three dimensions arbitrary; just draw any two sets of lines at right angles to each other, and a third set at right angles to both the first two sets. It doesn’t matter at all if all the sets slant in such a way that are none are exactly north-south, east-west, or up-down. Orient the cube in that fashion and the sets of lines will still do to provide three numbers to locate a point.

The dimension of time cannot be handled in this way. No matter how you twist and turn a cube, the east-west line never becomes the yesterday-tomorrow line and vice versa. Nor can the north-south line or the up-down line become the yesterday-tomorrow line.

Then, too, we needn’t move in the north-south line, or the east-west line, or the up-down line if we don’t wish to. We can stay at rest with respect to them. Or we can move, quickly or slowly, as we wish.

In the case of time, on the other hand, we cannot rest; we cannot remain at one point. We travel always away from yesterday and toward tomorrow, all of us and everything, at what seems to be a fixed speed.

Consequently, we can speak of time separately from the other three dimensions. We can say that four-dimensional space-time consists of time and of three “spatial dimensions.”

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