4^ PRESIDENTIAL ADDRESS SECTION . A. 



vided that all distances in the direction of r are supposed to 

 contract in the ratio V (i — v /€■"). (iij it this system at rest 

 is a material body, the second or moving system is a Dody moving 

 with velocity ( — v), whose bulk is that ot the tirst body modihed 

 by the above contraction. 



1 lie logical conclusion is that if a body is (gradually) set 

 in motion trom rest m the aether, this " l^itzgerald contraction ' 

 will take place automatically. The Newtonian *' rigid body ' is 

 not even approximately realized in nature, except when the 

 velocities are very small compared with that of light. 



Experiments by Lord Rayleigh and by Brace, to see whether 

 such contraction caused, as would be expected, double refraction 

 of rays of light, gave no trace of such effect. Therefore the 

 contraction theory must assume other modifications to netitralize 

 optical effects. Other experiments on electrical and mechanical 

 ■ lines also failed to give expected results of this contraction. 



The problem became more and more involved. 



At this stage Einstein suggested the " Principle of Rela- 

 tivity " as a new theory oi motion explaining these accumulated 

 difficulties. 



In its first simple form it pustulated as a new fundamental 

 principle of physical reasoning that the velocity of light must 

 be equal and constant in all directions (in free rether) when 

 expressed by means of any of the infinite number of si)ace and 

 time frameworks between wiiich Newton's laws and experiment 

 have failed to distinguish. The main efifect of this new view 

 is to lead us to expect phenomena to be all relative to the 

 observer: the path of the same ray of light may be accurately 

 described by one oliserver as straight, by another (employing a 

 different framework) as circular. " Simultaneous " occurrences 

 at two places cannot be regarded as a simple conception ; nor can 

 the length of a material l)ody ; and, strangest result of all. ijravi- 

 tatioii. as the phenomenon which is independent of all physical 

 conditions (except what we call mass), api)ears inevitably in the 

 analysis as a mere attribute of space-time represented l)y the co- 

 efficients which specify the framework of the equations, and 

 which vary with the system of co-ordinates used. Our old 

 co-ordinates x, y, c, t of space and time become four independent 

 co-ordinates on efjual footing : and we work as for a four 

 dimensional space. 



The mathematical theory for the simple case of uniform 

 velocity of the framework in a straight line was soon effected 

 in beautiful form by Minkowski, who introduces two forms of 

 "vectors" ( " 4-vector " and "6-vector"). the former corres- 

 ponding to the projections of a " straight line " on the four 

 axes, the latter to the projections of a " surface " on the six 

 axial planes. All, expressions that do not vary with a change 

 of co-ordinates are expressible in one of these forms. Einstein's 

 later developments have extended the Principle so that it postu- 

 lates that all laws of nature (not the velocity of light only) must 

 be " invariant " when the framework in 4-dimension space is 



