Einstein also predicts that the lines of any element in the solar spectrum should be slightly shifted towards the red, as compared with those produced in our laboratories. Different observers have investigated this, and so far they disagree. The trouble is that there are several other influences which may shift the lines, such as pressure in the sun’s atmosphere, motion of currents on the sun’s surface, etc., and it is very hard to disentangle this Gordian knot. At present, the results of these observations can neither be counted for or against the theory, while those in the other two cases are decisively favorable.
The mathematical expression of this general relativity is intricate and difficult. Mathematicians—who are used to conceptions which are unfamiliar, if not incomprehensible, to most of us—find that these expressions may be described (to the trained student) in terms of space of four dimensions and of the non-Euclidean geometry. We therefore hear such phrases as “time as a sort of fourth dimension,” “curvature of space” and others. But these are simply attempts—not altogether successful—to put mathematical relationships into ordinary language, instead of algebraic equations.
More important to the general reader are the physical bearings of the new theory, and these are far easier to understand.
Various assumptions which we may make about the motion of the universe as a whole, though they do not influence the observed facts of nature, will lead us to different ways of interpreting our observations as measurements of space and time.
Theoretically, one of these assumptions is as good as any other. Hence we no longer believe in absolute space and time. This is of great interest philosophically. Practically, it is unimportant, for, unless our choice of an assumption is very wild, our conclusions and measurements will agree substantially with those which are already familiar.
Finally, the “general” relativity shows that gravitation and electro-magnetic phenomena—(including light) do not form two independent sides of nature, as we once supposed, but influence one another (though slightly) and are parts of one greater whole.
XIX
EINSTEIN’S THEORY OF RELATIVITY
A Simple Explanation of His Postulates and Their Consequences