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In his thesis (1911) F. Harress reported the results of his experiments concerning the propagation of light within a rotating vitreous body. A similar experiment has been carried out two years later by Sagnac using air instead of glass as the propagation medium. The theory of the experiment published by Harress has been corrected by P. Harzer and A. Einstein respectively and the theory of both experiments has been correlated to those of Fizeau and Zeeman and exposed together by M. von Laue. Based on these theoretical corrections, O. Knopf has re-examined the results of Harress.

Originally published as "?ber die Wiederholung des Harress-Sagnacschen Versuches?, Annalen der Physik, V80, N51, pp. 217-231 (1926).

The problem addressed by the present work refers to a sequence of ideas, which have been worked out recently by Lenard, namely, whether it might be possible to demonstrate an absolute reference system, the primordial ether, using Michelson's interference experiment (M.I.). The negative outcome of Michelson's experiment in the usual setting, namely, with terrestrial light, has shown, according to Lenard's interpretation, that the ether carrying terrestrial light waves should be considered as taking part in the movement of the earth. This, however, needs not be the case with extraterrestrial light as Lenard has shown. It could be that light quanta from extraterrestrial sources running with the primordial ether were still linked on the earth to the primordial ether, which should lead to an at least partially positive outcome of the M.I. with extraterrestrial light. A negative outcome should permit elucidating the behaviour of light quanta entering the ether of different property. In the following I will report on experiments conducted with extraterrestrial light.

Originally published as "?ber das Verhalten des Lichtes auberirdischer Lichtquellen," Annalen der Physik, V73, pp. 105-126 (1924).

This paper was first published in 1898, but posthumously republished in 1917, as a challenge to Einstein's then new theory of general relativity, J. for Math. & Phys., Zeitschrift f. Mathematik und Physik, V43, pp 93-104.

If it is assumed that the hitherto unexplained advance by 41`` per century of Mercury's perihelion is caused by the delay of time spent for the spatial propagation of gravity, it follows that this value equals the velocity of light, of thermal radiation and of electric waves. Attention has to be paid to what can, on the one hand, really be proven by computation and observation and what, on the other hand, is presumed in the first place without any proof. If the gravity between two masses is transmitted from the first body to the second and back again with some lag of time, one finds that this necessarily gives rise to an advance of the planets perihelion. It is, however, impossible to prove that the actual value of the perihelions advance, although it could not be deduced from disturbances of any type, could not have another origin than the presumed time lag. If this presumed origin gave a value for the propagation velocity of gravity different from the velocity of light, this would have no further meaning. Just the coincidence of both velocities vindicates this presumption and, hence, the notion of a finite propagation velocity of gravity.

From "The Collected Papers of Albert Einstein", Volume 2, "The Swiss Years: Writings, 1900 - 1909". The lecture of 1909.

Among other statements, Einstein suggested the abandonment of Maxwell's electrodynamics with the following:

"Today, however, we must consider the hypothesis of ether as an obsolete standpoint. It cannot even be denied that there is a large class of facts concerning radiation which show that light is endowed with certain properties which can far easier be understood from the standpoint of Newton's emission theory than from the standpoint of the undulation theory. Therefore, it is my opinion that the next step in the development of theoretical physics will bring us a theory of light which must be conceived as some sort of fusion between emission and undulation theory. The following statements are intended to give reason to this opinion and to show that a deep change of our intuitions into the essence and nature of light is unavoidable."