sn y™to˜er —nd xovem˜er IWHUD just over two ye—rs —fter the ™ompletion of his spe™i—l theory of rel—tivityD iinstein m—de the ˜re—kthrough th—t set him on the p—th to the gener—l theory of rel—tivityF ‡hile prep—ring — review —rti™le on his new spe™i—l theory of rel—tivityD he ˜e™—me ™onvin™ed th—t the key to the extension of the prin™iple of rel—tivity to —™™eler—ted motion l—y in the rem—rk—˜le —nd unexpl—ined empiri™—l ™oin™iden™e of the equ—lity of inerti—l —nd gr—vit—tion—l m—ssesF „o interpret (...) —nd exploit this ™oin™iden™eD he introdu™ed — new —nd powerful physi™—l prin™ipleD soon to ˜e ™—lled the ’prin™iple of equiv—len™e4 upon whi™h his se—r™h for — gener—l theory of reE l—tivity would ˜e ˜—sedF woreoverD with the ™ompletion of the theory —nd throughout the rem—inder of his lifeD iinstein insisted on the fund—ment—l import—n™e of the prin™iple to his gener—l theory of rel—tivityF iinstein9s insisten™e on this point h—s ™re—ted — puzzle for philosophers —nd histori—ns of s™ien™eF st h—s ˜een —rgued vigorously th—t the prin™iple in its tr—dition—l formul—tion does not hold in th™ gener—l theory of rel—tivityD gonsiderD for ex—mpleD — tr—dition—l formul—tion su™h —s €—uli9s in his IWPI Encyklopadie —rti™leF por €—uli the prin™iple —sserts th—t one ™—n —lw—ys tr—nsform —w—y —n —r˜itr—ry gr—vit—tion—l eld in —n innitely sm—ll region of sp—™eEtimeD ˜y tr—nsforming to —n —ppropri—te ™oordin—te system @€—uli IWPID pF IRSAF sn responseD su™h eminent rel—tivists —s ƒynge @IWTHD pF ixAD —nd even iddington ˜efore him @IWPRD ppF QW{RIAD h—ve o˜je™ted th—t — ™oordin—te tr—nsform—tion or ™h—nge of st—te of motion of the o˜server ™—n h—ve no ee™t on the presen™e or —˜sen™e of — gr—vit—tion—l eldF „he presen™e of — ’true4 gr—vit—tion—l eld is determined ˜y —n inv—ri—nt ™riterionD the ™urv—ture of the metri™F „he gr—vit—tionEfree ™—se of spe™i—l rel—tivity is just the ™—se in whi™h this ™urv—ture v—nishesD where—s the true gr—vit—tion—l elds of gener—l rel—tivity —re distinguished ˜y the nonv—nishing of this ™urv—tureF „his o˜je™tion h—s immedi—te r—mi™—tions for the ’iinstein elev—tor4 thought experimentD whi™h is ™ommonly used in the formul—tion of the prinE ™iple of equiv—len™eF sn this thought experimentD — sm—ll ™h—m˜ers su™h —s.... (shrink)

This paper provides new elements for reconstructing Heinrich Hertz's conversion from an electrodynamic concept to a concept of field theory, a process that took place between September 1887 and February 1888. First of all, it is argued that, contrary to what one could deduce from Hertz's own presentation in his Untersuchungen (or Electric Waves), paper N∘ 5 was actually written after the first publication of papers N∘ 6 and N∘ 7; this fact is illuminated by Hertz's unpublished correspondence (Section 1). (...) Following this reordering, an approximative process of Hertz's conversion based only on his published papers is described (Section 2). Second, by complementing Hertz's published Erinnerungen (or Memoirs, Letters, Diaries) with his unpublished Versuchsprotokolle (Laboratory notes), a much finer description of this conversion process is discussed (Section 3). It is finally concluded that Hertz's idea of “air waves” belongs at the very end of this process, and was mainly attained through experimental hints found within Helmholtz's theoretical framework. (shrink)