RELATIVITY 



6549 



RELATIVITY: TIME, SPACE, & MATTER 



Professor H. Wildon Carr, D.Litt., Author 

 of General Principles of Relativity 



In connexion with this subject see the articles Energy ; Gravitation ; 

 Physics ; also biographies of Einstein, Newton, and other scientists 



The principle of Relativity is 

 a mathematical principle which 

 claims to be more convenient in 

 science, and to admit of much 

 greater accuracy in measurements 

 of physical phenomena, than the 

 principle in use in classical me- 

 chanics. It is the principle that 

 the velocity of light is constant 

 for all observers, whatever the 

 relative movements of the systems 

 of reference (the earth, for ex- 

 ample) to which they are attached, 

 and that the movements of these 

 systems are compensated for the 

 observers by changes in what in 

 mechanics are termed the space 

 and time coordinates. 



The principle we make use of 

 every day in the course of our 

 ordinary life is that distances and 

 intervals are invariable, and un- 

 affected by the particular objects 

 or events which occupy them. To 

 apply this principle we mark off a 

 certain length in a rigid material, 

 and it serves as an instrument for 

 measuring the three dimensions of 

 space ; and we mark off the 

 duration of the free swing of a 

 pendulum, and register it by 

 means of a chronometer, and it 

 serves as an instrument for mea- 

 suring time. From very ancient 

 times this principle has seemed to 

 mathematicians to suffer from a 

 serious theoretical defect, viz. it 

 is not a direct means of comparing 

 distance with distance and interval 

 with interval, and there is no way 

 of knowing, with the certainty 

 mathematics requires, that mea- 

 suring rod and clock do not alter 

 shape when they are moved. 

 Michelson-Morley Experiments 



Until recent years this defect 

 in the principle remained purely 

 theoretical. Nothing had occurred 

 in the observation of physical 

 phenomena, and in the progress of 

 physical science, to suggest any 

 practical doubt concerning it. In 

 the latter half of the nineteenth 

 century, however, a series of re- 

 markable experiments, culminating 

 in the famous one known by the 

 names of the experimenters, 

 Michelson and Morley, completely 

 changed the situation. It became 

 necessary either to remodel the 

 principle of invariant distances 

 and intervals, or else to formulate 

 a new principle. In 1905 Einstein 

 formulated the new principle, 

 now known as the principle of 

 relativity. 



In order to understand the 

 nature and significance of the ex- 



periments we must go back to the 

 time of Newton and to the origin of 

 what we now call the classical 

 mechanics. In 1675, Roemer, a 

 famous Danish astronomer, ob- 

 served a phenomenon which could 

 only be satisfactorily explained on 

 the hypothesis that light signals do 

 not arrive at the instant of emis- 

 sion, but occupy a definite interval 

 of time proportional to their 

 distance in space. What he dis- 

 covered was that the moons of 

 Jupiter required different periodic 

 calculations for their eclipses ac- 

 cording to the position of the 

 planet in its orbit, and therefore of 

 its distance from the earth. On the 

 hypothesis that the phenomenon 

 was due to the difference in the 

 interval occupied by the light in 

 reaching the observer, he calculated 

 the velocity of light with an ex- 

 traordinary accuracy, which later 

 measurements with precise instru- 

 ments have hardly modified. The 

 effect of the discovery was far- 

 reaching, and it led Newton to 

 postulate absolute space and time. 

 Velocity of Light 



With the rise of electro-magnetic 

 science in the nineteenth century, 

 the velocity of light became the 

 factor of prime importance, for 

 electro-magnetic phenomena are of 

 the same nature as light. Ex- 

 periments then began to be con- 

 trived with the object of measur- 

 ing the effect on that velocity of 

 movements of the system in which 

 the light whose velocity was mea- 

 sured had its source. No one 

 doubted that the movement of the 

 source must produce an effect on 

 the velocity of the propagation ; the 

 only difficulty seemed to be to dis- 

 cover the means of revealing it. 



To the surprise of the experimen- 

 ters, however, the results of all 

 experiments proved to be uni- 

 formly negative, that is, the ex- 

 periments did not fail, but they 

 did not disclose the calculated 

 effect, and at last the Michelson- 

 Morley experiment, by the direct- 

 ness of its aim and the large scale 

 of its apparatus, left no d< ubt 

 that the effect which was being 

 looked for did not exist at all. 

 It was an experiment with a care- 

 fully contrived apparatus designed 

 to show the movement of the 

 earth in space, and its direction, 

 by means of the change which the 

 velocity of this movement ought to 

 have produced in the velocity of 

 the light projected from the in- 

 strument and reflected back to it 



from a fixed mirror. The negative 

 result of this experiment amounted 

 to a new discovery. T 



The experiment was described in 

 The Philosophical Magazine in 

 1886, and during the next twenty 

 years mathematicians and physi- 

 cists were engaged in attempts to 

 explain the negative result. It was 

 supposed at first that it must be 

 attributed to some action of the 

 ether on the masses of matter 

 moving through it. Ether is the 

 medium which was then generally 

 admitted to be something which 

 must fill space in order that the 

 waves of light may be propagated. 

 It was suggested by Fitzgerald, a 

 professor of Trinity College, Dub- 

 lin, that if the ether be all per- 

 vasive, and therefore not dis- 

 placed when matter moves through 

 it, then its effect on the moving 

 matter may be to bring about a 

 contraction of the matter in the 

 direction of the movement, and 

 this would account for the experi- 

 ment being negative. About the 

 same time and independently, 

 Lorentz, professor at Leiden, sug- 

 gested that this contraction effect 

 might be a character of the elec- 

 trons which constitute matter. If 

 we suppose an electron at rest to be 

 spherical, and to be automatically 

 elongated when it moves, in the 

 direction of its movement, the 

 result would be a contraction pro- 

 portionate to the velocity of the 

 movement. 



Einstein, however, took a differ- 

 ent line. He ignored all theories 

 and based his mathematics purely 

 on the experiments. He accepted 

 the results without explaining them 

 or admitting that they needed 

 explanation, and formulated the 

 principle of relativity. It meant 

 the rejection of the Newtonian 

 principle that space and time are 

 invariable, and the affirmation that 

 velocity, that is, the ratio of space 

 to time, is invariable. As all o ir 

 observations of physical pher.om- 

 ena are dependent on light signals 

 it must follow that the velocity of 

 light is constant for all observers. 

 Results of the Theory 



The new principle rapidly won 

 adherents, but it was some years 

 before it excited an interest out- 

 side the domain of pure mathe- 

 matics. It was then seen that it 

 involved very disconcerting conse- 

 quences for some of the most funda- 

 mental ideas concerning the frame- 

 work of the universe. The strangest 

 result, perhaps, was that it showed 

 the impossibility of assigning any 

 absolute values to the time of an 

 event or to the place of its occur- 

 rence, since these depended on its 

 relations to other events, and these 

 relations were different for all 



