NATURE 



[March 14, 191; 



is so in the case of light can be settled only by exiperi- 

 ment— by weighing light. It seems that it should be 

 just possible to do this, if a beam of light passes an 

 object which exerts a gravitational attraction, then, if 

 it really has weight, it must drop a little towards the 

 object. Its path will be bent just as the trajectory of a 

 rifle bullet is curved owing to the weight of the bullet. 

 The velocity of light is so great that there is only one 

 body in the solar system powerful enough to make an 

 appreciable bend in its path, namely, the sun. If we 

 could see a star close up' to the edge of the sun, a ray 

 of light coming from the star would bend under its own 

 weight, and the star would be seen slightly displaced 

 from its true position. During a total eclipse stars 

 have occasionally been photographed fairly close to the 

 sun, and with care it should be possible to observe this 

 effect. There is a magnificent opportunity next year 

 when a total eclipse of the sun takes place right in the 

 midst of a field of bright stars. This is the best oppor- 

 tunity for some generations, and it is hoped to send 

 out expeditions to the line of totality to weigh light 

 according to this method. 



In any rase, great interest must attach to an attempt 

 to settle whether or not light has weight. But there is 

 an additional importance, because it can be made a 

 means of confirming or disproving Einstein's theory. 

 On Einstein's theory light must certainly have weight, 

 because mass and weight are viewed by it as two 

 aspects of the same thing ; but his theory predicts a 

 deflection twice as great as we should otherwise expect. 

 Apart from surprises, there seem to be three possible 

 results : — (i) A deflection amounting to 1-75" at the 

 limb of the sun, which would confirm Einstein's theory; 

 (2) a deflection of 0-83" at the limb of the sun, which 

 would overthrow Einstein's theory, but establish 

 that light was subject to gravity ; (3) no deflection, 

 which would show that light, though possessing mass, 

 has no weight, and hence that Newton's law of pro- 

 portionality between mass and gravitation has broJ<:en 

 down in another unexpected direction. 



The purpose of Einstein's new theory has often been 

 misunderstood, and it has been criticised as an attempt 

 to explain gravitation. The theory does noi offer any 

 explanation of gravitation ; that lies quite outside its 

 scope, and it does not even hint at a possible mechanism. 

 It is true that we have introduced a definite hypothesis 

 as to the relation between gravitation and a distortion 

 of space ; but if that explains anything, it explains not 

 gravitation, but space, i.e. the scaffolding constructed 

 from our measures. Perhaps the position reached 

 may be made clearer by another analogy. Let us pic- 

 ture the particle which describes a world-line as hurdle- 

 racer in a field thickly strewn with hurdles. The par- 

 ticle in passing from point to point always takes the 

 path of least effort, crossing the fewest possible hurdles ; 

 if the hurdles are uniformly distributed, corresponding 

 with undistorted Minkowskian space, this will, of 

 course, be a straight line. If the field is now distorted 

 by a mathematical transformation such as an earth- 

 quake so that the hurdles become packed in some parts 

 and spread out in others, the path of least effort will no 

 longer be a straight line ; but it is not difficult to see 

 that it passes over precisely the same hurdles as before, 

 only in their new positions. The gravitational field 

 due to a particle corresponds with a more fundamental 

 rearrangement of the hurdles, as though someone had 

 taken them up and replanted them according to a law 

 which expresses the law of gravitation. Any other 

 particle passing through this part of the field "follows 

 the guiding rule of least effort, and curves its path, if 

 necessary, so as to jump the fewest hurdles. Now, we 

 have usually been under the impression that when we 

 measured distances by physical experiments we were 

 surveying the field,, and the results could be plotted on 

 , NO. 2524, VOL. lOl] 



a map; but it is now realised that we cannot do that. 

 The field itself has nothing to do with our measure- 

 ments; all we do is to count hurdles. If the only causae 

 of irregularity of the hurdles were earthquakes (mathe- 

 matical transformations), that would not make much 

 difference, because we could still plot our counts of 

 hurdles consistently as distances on a map ; and the 

 map would represent the original condition of the field 

 with the hurdles uniformlv spaced. But the more far- 

 reaching rearrangement of hurdles by the gravitational 

 field forces us to recognise that we are dealing with 

 counts of hurdles and not with distances ; because if 

 we plot our measures on a map they will not close up. 

 The number of hurdles in the circumference of a 

 circle ^ will not be rr times the number in the diameter ; 

 and when we try to draw on a map a circle the circum- 

 ference of which is less than n- times its diameter, 

 we get into difficulties — at least in Euclidean space. 

 This analogy brings out the point that the theory is an 

 explanation of the real nature of our measures rather 

 than of gravitation. We offer no explanation why the 

 particle always takes the path of least effort — perhaps, 

 if we may judge by our own feelings, that is so natural 

 as to require no explanation. More seriously, we 

 know^ that in consequence of the undulatory theory of 

 light, a ray traversing a heterogeneous medium always 

 takes the path of least time ; and one can scarcely 

 resist a vague impression that the course of a material 

 particle may be the ray of an undulation in five dimen- 

 sions. What concerns gravitation more especially is 

 that we have offered no explanation of the linkages by 

 which the hurdles rearrange themselves on a definite 

 plan when disturbed by the presence of a gfavitating 

 particle ; that is a point on which a mechanical theory 

 of gravitation ought to throw light. 



From the constant of gravitation, together with the 

 other fundamental constants of Nature — the velocity of 

 light and the quantum of action — it is possible to form 

 a new fundamental unit of length. This unit is 

 7 X 10-^' cm. It seems to be inevitable that this length 

 must play some fundamental part in any complete 

 interpretation of gravitation. (For example, in Osborne 

 Reynolds's theory of matter this length appears as the 

 mean free-path of the granules of his medium.) In 

 recent years great progress has been made in know- 

 ledge of the excessively minute; but until we can 

 appreciate details of structure down to the quadrillionth 

 or quintillionth of a centimetre, the most sublime of all 

 the forces of Nature remains outside the purview of 

 the theories of physics. 



UNIVERSITY AND EDUCATIONAL 

 INTELLIGENCE. 



Cambridge.— The syndicate appointed to consider the 

 Previous examination has issued its report. The re- 

 commendations involve changes which, if passed by 

 the Senate, will greatly alter the present character of 

 the examination. The syndicate advises the discon- 

 tinuance of Greek as a compulsory subject, and recom- 

 mends that every candidate shall be required" to take 

 at least one paper in natural science. It proposes that 

 the examination should lx> in three parts : (i) Lan- 

 guages ; (2) mathematics and natural science ; (3) Eng- 

 lish subjects. In part (i) it is recommended that Latin 

 should continue to be a compulsory subject, and that a 

 candidate should be required to take one other foreign 

 language, namely, Greek, French, German, or Spanish. 



3 A circle would naturally be defined as a curve such that the number of 

 hurdles (counted along the p.ith of least effort) between any point on it and 

 a fixed point called the centre is constant. To make the vague analogy more 

 definite, we may suppose that the hurdles are pivoted, and swing round 

 automatically to face the jumper; he is not a'lowed to dodge them, t'.e. to 

 introduce into his path sinuosities comparable with the lengths of the 

 hurdles 



