RELATIVITY 



observers attached to moving sys- 

 tems of reference, such as the earth. 



In 1908, Minkowski, a Polish 

 mathematician (who died in the 

 same year at the age of 45), pro- 

 duced a mathematically beautiful 

 construction of the universe con- 

 ceived as a four-dimensional con- 

 tinuum, the fourth dimension being 

 time. This universe (unlike Newton's 

 conception of masses in space 

 changing their relative positions in 

 time) is constituted of events. An 

 event is determined in relation to 

 other events by four coordinates 

 (length, breadth, depth, and time), 

 and these vary for every observer 

 according to the system of relative 

 movement to which he refers it. If 

 we imagine the track of an event in 

 this universe we have what is 

 called M " world-line." This uni- 

 verse is quite different from the 

 common-sense conception of a 

 world in space and time. For 

 example, we believe that whatever 

 happens at a particular moment is 

 simultaneous with what is happen- 

 ing at every point throughout 

 infinite space. But in the four-di- 

 mensional world there is no simul- 

 taneity at all. Any two events 

 which for one observer occur at one 

 instant may for every other ob- 

 server be separated by an interval. 

 Also there is no absolute place of 

 anything. Any two events which 

 for one observer happen in one 

 place for all other observers may be 

 in different places. Each observer 

 will naturally take his " world- 

 line " to be straight, and it will 

 then be for him the criterion or 

 standard, and all other " world- 

 lines " will be curved. 



Einstein's Calculations 



Meanwhile, Einstein had become 

 convinced that the principle of 

 relativity must have a much wider 

 application than that which re- 

 stricted it to the special case of the 

 velocity of light, and he turned his 

 attention to the problem of gravi- 

 tation. This was not a case where 

 any results of direct experiment, 

 positive or negative, had disap- 

 pointed expectation. There was, 

 however, one astronomical phe- 

 nomenon which had obstinately 

 defied all the attempts to interpret 

 it conformably with Newton's law 

 of gravitation. This phenomenon 

 is known as the progression of the 

 perihelion of Mercury. When the 

 orbit of Mercury is calculated and 

 allowance is made for all disturb- 

 ing influences, there is a progression 

 amounting to 40-2" in a century. 

 In 1915 Einstein announced a new 

 principle for the calculation of the 

 phenomena of gravitation, and this 

 was found to give (allowing for a 

 practically negligible margin of 

 error) the exact correction needed 



655O 



to reconcile the discrepancy in 

 Mercury's orbit. 



The great glory of Newton had 

 been the discovery of the law of 

 gravitation, and it had ever since 

 held its ground unchallenged. It 

 was now found that if the new 

 general principle of relativity 

 were adopted, Newton's law would 

 not undergo correction but have to 

 be as entirely set aside and replaced 

 as his absolute space and time had 

 been by the special principle. 

 Working out the gravitational 

 field of the sun by the new prin- 

 ciple, Einstein found that it showed 

 that if the stars could be seen near 

 the sun, i.e. in its gravitational 

 field, they would be found to be 

 displaced proportionately to their 

 distance from the centre of the 

 sun. The total eclipse of the sun 

 on May 29, 1919, would therefore 

 offer an opportunity of testing the 

 matter. The interest this prediction 

 awakened in scientific circles every- 

 where was intense. Two British 

 expeditions were sent out, and the 

 results obtained were announced 

 to the scientific world at a meeting 

 of the Royal Society in Nov., 1919. 

 The predictions of Einstein were 

 then declared to have been fulfilled. 



The new principle rejects the 

 notion of force and the idea of any 

 action, direct or indirect, of one 

 body on another. It declares 

 inertia and gravitation to be one 

 and the same phenomenon, either 

 being the equivalent of the other, 

 and it interprets the phenomena of 

 gravitation, not by the properties 

 of the masses, but by the geome- 

 trical structure of the space in 

 which they are moving. This space 

 in the neighbourhood of rotating 

 masses of matter acquires the 

 special geometrical character which 

 constitutes a gravitational field. 



Bibliography. General Principles 

 of Relativity, H. W. Carr, 1920 ; 

 The Theory of Relativity, W. H. 

 Pickering, 1920 ; The Reign of 

 Relativity, Lord Haldane, 1921 ; 

 Relativity, The Special and General 

 Theory, A. Einstein, 1921 ; Space, 

 Time, Matter, H. Weyl, Eng. trans. 

 H. L. Brose, 1921. 



Relativity of Knowledge. 

 Theory, variously formulated, that 

 knowledge is only relatively true. 

 (1) Positive knowledge is im- 

 possible, owing to the ever- 

 changing nature of the data of 

 sense ; we do not know things as 

 they really are, but only as they 

 appear to us. f This is the doctrine 

 of Protagoras : Man is the measure 

 of all things ; all knowledge is 

 relative ; each thing is for each 

 man as it appears to him. (2) 

 Our knowledge of particular things 

 depends upon the relations in which 

 they stand to other things. (3) 

 In psychology, the view that sen- 



REL.EASE 



sations only possess importance 

 when related to other simul- 

 taneous or immediately previous 

 sensations in the mind. 



Relator. English law term. An 

 action will lie at the suit of the 

 attorney-general for a tort (gener- 

 ally a nuisance) of a public charac- 

 ter, but the attorney-general pro- 

 ceeds not by writ but by in- 

 formation (q.v.) ; and he proceeds 

 either of his own motion, or is set in 

 motion by someone who really 

 brings the action himself, but pro- 

 cures the attorney-general's con- 

 sent to use his name. Such a per- 

 son is called the relator. 



Relay. Device used in electrical 

 engineering to make a weak cur- 

 rent close a path for a stronger 

 current. It consists of a delicate 

 electro-magnet which, when 

 slightly energised, draws an arma- 

 ture against a stop and completes a 

 local circuit of which the armature 

 itself usually forms part. In tele- 

 graphy, relaying avoids the em- 

 ployment of strong line currents 

 and thereby reduces insulation 

 troubles. 



The resistance of a conductor 

 varies directly as its length ; e.g. 

 a telegraph wire 20 m. long 

 has twice the resistance of one 10 

 m. long, the diameter being the 

 same in both cases. To send a 

 current of a certain strength 

 through the first may require a 

 pressure of, say, 20 volts. The 

 same result will be obtained if the 

 line be divided into two 10 m. 

 sections, each provided with a 

 battery giving 10 volts. The cur- 

 rent passed through the first 

 section operates a relay, which 

 brings the battery of the second 

 into action. In this way a signal 

 can be, and frequently is, passed 

 along through a large succession of 

 circuits. See Circuit, Electric. 



Relay Race. In athletics, a race 

 in which several runners compete 

 on either side, each covering a 

 certain distance, his place being at 

 once taken by a succeeding runner, 

 until the tape is reached. The idea 

 was possibly derived from the 

 torch-race (lampadedromia) of the 

 Greeks, in one form of which the 

 runners passed on a lighted torch 

 to a waiting comrade. A similar 

 procedure is sometimes adopted 

 for cycling and other races. 



Release. Term used in English 

 law to describe the discharge of 

 any right on any action. A release 

 of a debt is an extinguishment 

 of the debt, as distinguished from a 

 receipt, which is merely prima facie 

 evidence of payment. A release 

 may be of a right of property ; as 

 where a partner, on retiring, re- 

 leases all his rights in the partner- 

 ship property to the remaining 



