802 



NATURE 



[February 17, 192 1 



character. This renders the existence of charged 

 material particles possible without requiring an 

 immense mass-horizon as in liinstein's cosmology. 



At first the non-inicgr ability oj the transfer- 

 ence of distances {Streckeniibertragung) aroused 

 much antipathy. Does not this mean that two 

 measuring-rods which coincide at one position in 

 the universe no longer need to coincide in the 

 event of a subsequent encounter? Or that two 

 clocks which set out from one world-position with 

 the same period will possess different periods 

 should they happen to encounter at a subsequent 

 position in space? Such a behaviour of "atomic 

 clocks " obviously stands in opposition to the fact 

 that atoms emit spectral lines of a definite fre- 

 quency, independently of their past history. 

 Neither does a measuring-rod at rest in a static 

 field experience a congruent transference from 

 mornent to moment. 



What is the cause of this discrepancy between 

 the idea of congruent transfer and the behaviour 

 of measuring-rods and clocks? I differentiate 

 between the determination of a magnitude in 

 Nature by "persistence" (Beharrung) and by 

 "adjustment" (Einstellung). I shall make the 

 difference clear by the following illustration : We 

 can' give to the axis of a rotating top any arbitrary 

 direction in space. This arbitrary original direc- 

 tion then determines for all time the direction of 

 the axis of the top when left to itself, by means 

 of a tendency of persistence which operates from 

 moment to moment ; the axis experiences at every 

 instant a parallel displacement. The exact oppo- 

 site is the case for a magnetic needle in a mag- 

 netic field. Its direction is determined at each 

 instant independently of the condition of the 

 system at other instants by the fact that, in virtue 

 of its constitution, the system adjusts itself in an 

 unequivocally determined manner to the field in 

 which it is situated. A priori we have no ground 

 for assuming as integrable a- transfer which 

 results purely from the tendency of persistence. 

 Even if that is the case, as, for instance, for the 

 rotation of the top in Euclidean space, we should 

 find that two tops which start out from the same 



point with the same axial positions and encounter 

 again after the lapse of a very long time would 

 show arbitrary deviations of their axial positions, 

 for they can never be completely isolated from 

 every influence. Thus, although, for example. 

 Maxwell's equations demand the conservational 

 equation dejdt^o for the charge e of an 

 electron, we are unable to understand from 

 this fact why an electron, even after an in- 

 definitely long time, always possesses an 

 unaltered charge, and why the same charge e 

 is associated with all electrons. This circum- 

 stance shows that the charge is not determined 

 by persistence, but by adjustment, and that there 

 can exist only one state of equilibrium of the 

 negative electricity, to which the corpuscle adjusts 

 itself afresh at every instant. For the same reason 

 we can conclude the same thing for the spectral 

 lines of atoms. The one thing common to atoms 

 emitting the same frequency is their constitution, 

 and not the agreement of their frequencies on 

 the occasion of an encounter in the distant past. 

 Similarly, the length of a measuring-rod is obvi- 

 ously determined by adjustment, for I could not 

 give this measuring-rod in this field-position any 

 other length arbitrarily (say double or treble 

 length) in place of the length which it now pos- 

 sesses, in the manner in which I can at will pre- 

 determine its direction. The theoretical possi- 

 bility of a determination of length by adjustment 

 is given as a consequence of the world-curvature, 

 which arises from the metrical field according to 

 a complicated mathematical law. As a result of 

 its constitution, the measuring-rod assumes a 

 length which possesses this or that value, m rela- 

 tion to the radius of curvature of the field. In 

 point of fact, and taking the laws of Nature indi- 

 cated above as a basis, it can be made plausible 

 that measuring-rods and clocks adjust themselves 

 exactly in this way, although this assumption. — 

 which, in the neighbourhood of large masses, in- 

 volves the displacement of spectral lines towards 

 the red upheld by Einstein — does not appear any- 

 thing like so conclusive in our theory as it does 

 in that of Einstein. 



The Relativity of Time. 



By Prof. A. S. Eddington, F.R.S. 



THE philosopher discusses the significance of 

 time ; the astronomer measures time. The 

 astronomer goes confidently about his business 

 and does not think of asking the philosopher what 

 exactly is this thing he is supposed to be measur- 

 ing ; nor does the philosopher always stop to con- 

 sider whether time in his speculations is identical 

 with -the time which the world humbly accepts 

 from the astronomer. In these circumstances it 

 is, pot ^rprising that some confusion should have 

 arjsep. 



.I;n many globular clusters there are stars which 

 oscillate in mtrinsic brightness ; let us select two 

 such stars from different clusters and invite all 

 NO. 2677, VOL. 106] 



the astronomers in the universe to measure the 

 true interval of time between the moments of 

 maximum light of the two stars. They must, of 

 course, make whatever measurements and calcula- 

 tions they consider necessary to allow for the 

 finite velocity of light. It may easily happen that 

 the astronomers on Arcturus report that the two 

 maxima were simultaneous ; whereas those on the 

 earth report an interval of ten years between the 

 same two maxima. There is here no question of 

 observational error; the recognised terrestrial 

 method necessarily gives a discordant result when 

 used on Arcturus, owing to its different motion. 

 Our first impulse is to blame the astronomers. 



