56 DOCUMENT 48 JULY 1907 48. To Conrad and Paul Habicht [Bern, 15 July 1907] Meine lieben Habichte! Ich möchte Euch gern wieder einmal sehen! Könnt und wollt Ihr nicht bald kommen? Jetzt im Laufe des Juli könnte ich Euch beide zusammen beherber- gen, weil meine Schwester verreist ist. Noch hübscher wäre es, wenn Ihr An- fangs August nach Lenk im Simmenthal[1] kommen würdet, wo ich am 1. Au- gust mit Weib und Kind hingehe,[2] um ca 10 Tage Ferien dort zu verbringen da hätte ich eben den ganzen Tag frei. Herrn Paul danke ich bestens für den interressanten Brief. Ich habe noch eine neue Methode zur Messung sehr klei- ner Energiemengen gefunden. Beste Grüsse von Haus zu Haus Euer A. E. Schreibt bald! AKS (Walter Habicht, Rodersdorf, Switzerland). [70 300]. The verso is addressed "An die Herrn Konrad und Paul Habicht Fulacherstr. Schaffhausen.," and postmarked "Bern Brf.Exp. 15.VII.07.-1." [1]A village (1070 m) in the Bernese Oberland. [2]In a postcard written on the eve of their trip Einstein-Marid informed the Habichts that the Einsteins would go on foot from Zweisimmen (964 m) to Lenk (a distance of 13 km), where they would stay at the Hotel Hirsch (see Mileva Einstein-Marid to Conrad and Paul Habicht, 31 July 1907, Walter Habicht, Rodersdorf). EINSTEIN ON SUPERLUMINAL SIGNAL VELOCITIES I In six letters to Wilhelm Wien dating from summer 1907 (Docs. 49-53 and 55),[1] Einstein discusses the occurrence of velocities exceeding the speed of light in disper- sive and absorptive media and tries to answer the question whether such velocities are the physically meaningful signal velocities relativity theory requires to be less than or equal to the velocity of light. Though failing to convince Wien, Einstein's let- ters refine and expand upon his published treatments of the possibility of superlumi- nal speeds. Discussion of the existence of superluminal velocities within the framework of Maxwell's theory goes back at least as far as 1889, when Oliver Heaviside considered electric charges traveling at superluminal speeds.[2] As the various competing elec- tron theories were developed in the first years of the twentieth century, the question of superluminal speeds was taken up from the point of view of electron theory, in par- ticular in connection with the structure of the electron.[3] Through the work of Arnold