Reference: The Nature of the Physical World
This paper presents Chapter III (section 5) from the book THE NATURE OF THE PHYSICAL WORLD by A. S. EDDINGTON. The contents of this book are based on the lectures that Eddington delivered at the University of Edinburgh in January to March 1927.
The paragraphs of original material are accompanied by brief comments in color, based on the present understanding. Feedback on these comments is appreciated.
The heading below links to the original materials.
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The Four-Dimensional World
I do not know whether you have been keenly alive to the fact that for some time now we have been immersed in a four-dimensional world. The fourth dimension required no introduction; as soon as we began to consider events it was there. Events obviously have a fourfold order which we can dissect into right or left, behind or in front, above or below, sooner or later—or into many alternative sets of fourfold specification. The fourth dimension is not a difficult conception. It is not difficult to conceive of events as ordered in four dimensions; it is impossible to conceive them otherwise. The trouble begins when we continue farther along this line of thought, because by long custom we have divided the world of events into three-dimensional sections or instants, and regarded the piling of the instants as something distinct from a dimension. That gives us the usual conception of a three-dimensional world floating in the stream of time. This pampering of a particular dimension is not entirely without foundation; it is our crude appreciation of the absolute separation of space-relations and time-relations by the hour-glass figures. But the crude discrimination has to be replaced by a more accurate discrimination. The supposed planes of structure represented by Now lines separated one dimension from the other three; but the cones of structure given by the hourglass figures keep the four dimensions firmly pinned together. (In Fig. 4 the scale is such that a second of time corresponds to 70,000 miles of space. If we take a more ordinary scale of experience, say a second to a yard, the Seen-Now lines become almost horizontal; and it will easily be understood why the cones which pin the four dimensions together have generally been mistaken for sections separating them.)
A location has no dimensions. A location continually extended in a direction forms a line. A line continually extended in another direction forms a surface. A surface continually extended in still another direction forms a solid. A solid is a 3-dimensional object.
Now we have run out of new directions in what we know as space, but directions should not be limited to space. We can now extend a solid in a new direction known as time. This forms a history. A history of something would be a 4-dimensional object. We trace the history of a person from his birth to death. This will give us a 4-dimensional object.
We are accustomed to think of a man apart from his duration. When I portrayed “Myself” in Fig. 2, you were for the moment surprised that I should include my boyhood and old age. But to think of a man without his duration is just as abstract as to think of a man without his inside. Abstractions are useful, and a man without his inside (that is to say, a surface) is a well-known geometrical conception. But we ought to realise what is an abstraction and what is not. The “four-dimensional worms” introduced in this chapter seem to many people terribly abstract. Not at all; they are unfamiliar conceptions but not abstract conceptions. It is the section of the worm (the man Now) which is an abstraction. And as sections may be taken in somewhat different directions, the abstraction is made differently by different observers who accordingly attribute different FitzGerald contractions to it. The non-abstract man enduring through time is the common source from which the different abstractions are made.
What new direction can we think of now? We can take any object and conceptualize it and apply that concept in a much wider domain of abstraction. Mathematics and philosophy have been doing that for a long time. So, when we look at the existence of the object in the dimension of abstraction it gives us a 5-dimensional object.
When we consider the universe, it not only has an existence in physical space with a long history, but it also extends in the dimension of abstraction as well. The ultimate abstraction of the universe may be called God.
The appearance of a four-dimensional world in this subject is due to Minkowski. Einstein showed the relativity of the familiar quantities of physics; Minkowski showed how to recover the absolute by going back to their four-dimensional origin and searching more deeply.
The dimensions are not just limited to four, as shown in the comments above. The dimensions help us organize our perception of the universe.
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