Now when no particular observer is specified, we must of course assume an observer connected with the train, or with whatever the body mentioned. To that observer it doesn't make the slightest difference what the train does; it may stand at rest with respect to some external system or it may move at any velocity whatsoever; its length remains always 1,000 feet. In order for this question to have the significance which its propounder means it to have, I must restate it as follows: A train is 1,000 feet long as measured by an observer travelling with it. If it passes a second observer at 60 miles per hour, what is its length as observed by him? The answer is now easy.]* [According to the formula the length of the moving train as seen from the ground will be $$1,000 \times \sqrt{1 - (88)^2 / (186,000 \times 5,280)^2} = 999.999,999,999,996$$ feet, a change entirely too small for detection by the most delicate instruments. Examination of the expression K shows that in so far as terrestrial movements of material objects are concerned it is equal to 1]232 [within a far smaller margin than we can ever hope to make our observations. Even the diameter of the earth, as many of the essayists point out, will be shortened only 2 1/2 inches for an outside observer past whom it rushes with its orbital speed of 18.5 miles per second. But slight as the difference may be in these familiar cases, its scientific importance remains the same.]* RELATIVITY AND REALITY [A simple computation shows that this effect is exactly the amount suggested by Lorentz and Fitzgerald to explain the Michelson-Morley experiment.]188 [This ought not to surprise us, since both that explanation and the present one are got up with the same purpose. If they both achieve that purpose they must, numerically, come to the same thing in any numerical case. It is, however, most emphatically to be insisted that the present "shortening" of lengths]* [no longer appears as a "physical" shortening caused by absolute motion through the ether but is simply a result of our methods of measuring space and time.]188 [Where Fitzgerald and Lorentz had assumed that a body in motion has its dimensions shortened in the direction of its motion,]220 [this very form of statement ceases to possess significance under the relativity assumption.]* [For if we cannot tell which of two bodies is moving, which one is shortened? The answer is, both--for the other fellow. For each frame of reference there is a scale of length and a scale of time, and these scales for different frames are related in a manner involving both the length and the time.]220 [But we must not yield to the temptation to say that all this is not real; the confinement of a certain scale of length and of time to a single observation system does not in the least make it unreal.]* [The situation is real--as real as any other physical event.]165