This is “the happiest thought in my life.” -Einstein on his equivalence principle
THE BIRTH OF GENERAL RELATIVITY: FIRST PRINTING IN EXTRAORDINARILY RARE ORIGINAL WRAPPERS OF ONE ONE EINSTEIN’S MOST IMPORTANT PAPERS; containing the beginning of general relativity, the derivations of the equivalence principle, gravitational redshift, and the gravitational bending of light.
“Einstein’s road to general relativity began in November 1907, when he was struggling against a deadline to finish an article for a science yearbook explaining his special theory of relativity. Two limitations of that theory still bothered him: it applied only to uniform constant-velocity motion... and it did not incorporate Newton’s theory of gravity.
“‘I was sitting in a chair in the patent office at Bern when all of a sudden a thought occurred to me,’ he recalled. ‘If a person falls freely, he will not feel his own weight.’ That realization, which ‘startled’ him, launched him on an arduous eight-year effort to generalize his special theory of relativity and ‘impelled me toward a theory of gravitation.’ Later, he would call it ‘the happiest though in my life.’
“The tale of the falling man has become an iconic one, and in some accounts it actually involves a painter who fell from the roof of an apartment building near the patent office... Einstein refined his thought experiment so that the falling man was in an enclosed chamber, such as an elevator in free fall above the earth. In this falling chamber (at least until it crashed), the man would feel weightless. Any objects he emptied from his pocket and let loose would float alongside him.
“Looking at it another way, Einstein imagined a man in an enclosed chamber floating in deep space ‘far removed from stars and other appreciable masses.’ He would experience the same perceptions of weightlessness. ‘Gravitation naturally does not exist for this observer. He must fasten himself with strings to the floor otherwise the slightest impact against the floor will cause him to rise slowly towards the ceiling.’
“Then Einstein imagined that a rope was hooked onto the roof of the chamber and pulled up with a constant force. ‘The chamber together with the observer then begin to move “upwards” with a uniformly accelerated motion.’ The man inside will feel himself pressed to the floor. ‘He is then standing in the chest in exactly the same way as anyone stands in a room of a house on our earth... The man in the chamber will come to the conclusion that he and the chest are in a gravitational field. Just then, however, he discovers the hook in the middle of the lid of the chest and the rope which is attached to it, and he consequently comes to the conclusion that the chamber is suspended at rest in the gravitational field.’
Einstein observed that inertial mass always equals gravitational mass and through his thought experiments concluded that “From this correspondence it follows that it is impossible to discover by experiment whether a given system of coordinates is accelerated, or whether... the observed effects are due to a gravitational field.”
“Einstein called this ‘the equivalence principle.’ The local effects of gravity and of acceleration are equivalent...
“In 1907, working against the deadline imposed by the Yearbook of Radioactivity and Electronics, Einstein tacked on a fifth section to his article on relativity that sketched out his new ideas... He also came up with many predictions that could be tested, including that light should be bent by gravity and that the wavelength of light emitted from a source with a large mass, such as the sun, should increase slightly in what has become known as the gravitational redshift...
“It would take Einstein another eight years, until November 1915, to work out the fundamentals of this theory and find the math to express it. Then it would take another four years before the most vivid of his predictions, the extent to which gravity would bend light, was verified by dramatic observations. But at least Einstein now had a vision, one that started him on the road toward one of the most elegant and impressive achievements in the history of physics: the general theory of relativity” (Isaacson, Einstein, 145-49).
Weil, in his bibliography, also notes that “On p.443 are probably the first explicit statements both of the equivalence of inertial and gravitational mass and of the equation for mass in terms of energy now regarded as the theoretical basis for the release of atomic energy.” Weil *21.
Although Einstein submitted the paper on 4 December 1907, it wasn’t published until the January 22 issue of the Jarbuch. A short “Correction” was issued in a later issue (not included here).
IN: Jahrbuch der Radioactivität under Electronik, Vierter Band - 4. Heft, No. 16, pp. 411-462. Leipzig: S. Hirzel, 1908. Octavo, original wrappers. Boxed with the three preceding issues of the journal. Lighe pencil ownership notation on wrappers; only very minor wear; largely unopened. A FINE COPY IN SCARCE ORIGINAL WRAPPERS.