3 4 8 D O C U M E N T 3 5 0 A U G U S T 1 9 2 6 The importance of this work, from the standpoint of the natural sciences, lies in the fact that it alone enables us to understand the accuracy we are to ascribe to New- ton’s laws. By a comparison of results obtained from calculation with those ob- tained by observation and experiment, it has been found that the theory agrees almost with experience with extraordinary precision. Only the orbit of one single planet shows a minuscule deviation from the results obtained by calculation— which exceed to a considerable degree the observational error limits. It is that of Mercury, the planet nearest the Sun. Observations show that Mercury’s major axis performs a slow rotation in the plane of its orbit—obtained from perturbation cal- culations according to Newton’s law—in the direction of the planet’s movement. This rotation amounts to something like 40 arcseconds in one hundred years that is, the discrepancy is so slight it would bring about a complete revolution of the ecliptic in not less than 30,000 years. All efforts to explain this deviation from the Newtonian theory satisfactorily have been unsuccessful. The theoretical investigations of the general theory of relativity showed, some ten years ago, that Newton’s laws cannot be strictly valid, but only provide a close approximation. The exact laws, obtained by speculative processes, show that the major axis of every planet’s orbit (barring the disturbing influences exerted by other planets) must make a slow rotation. For all planets except Mercury it is too small to be observed. But, with regard to Mercury, the calculation produced that re- sult of 40″ per century—which until then had caused so much perplexity.[5] Thus, the theory of relativity amended the work of perturbation calculations and brought about full agreement with experience. Kind regards, your A. Einstein 350. To Max von Laue [Berlin,] 16 August 1926 Dear Laue, I later also reconsidered the problem of whether the two conceptions of molec- ular light emission lead to the same result,[1] and likewise arrived at the outcome that, in principle, an observable difference does result in both cases. However, I do not see a way to obtain sufficiently homogeneous canal rays for such an experi- ment. In any event, I shall be glad to send your letter to Mr. Rupp,[2] which, know- ing him, he will hardly understand. It is better that you explain the matter to him more thoroughly. Best regards, your Einstein