D O C U M E N T 9 3 O C T O B E R 1 9 2 5 1 1 3 Let be any timelike vector and a vector pointing toward the backward lightcone. Then we can establish that the sign of the scalar should be the same as the sign of . Thus it is possible, e.g., to express the electric density (the density scalar) uniquely by the field. The sign of this scalar is characteristic of the sign of the electric charge density. On the grounds of the general theory of relativity, it is then easily possible to set up laws according to which positive electricity is in equilibrium in other configu- rations than negative electricity. One can achieve this, for example, by adding a term to the Hamiltonian function that is an odd function of the electric density sca- lar or of the scalar electromagnetic potential. We shall not discuss this in detail right here. The essential insight seems to me that an explanation for the unlikeness of the two electricities is only possible if one assigns a direction to time, and refers to it in the definition of the relevant physical quantities. In this, electromagnetism dif- fers fundamentally from gravitation. This is also why the attempt to merge electro- dynamics and the law of gravitation into a unified whole does not seem justified to me anymore. 93. To Adriaan D. Fokker [1] [Berlin,] 21 October 1925 Dear Mr. Fokker, I enclose for you here a little note for the Lorentz issue.[2] Although the topic treated there is very simple, it does show how far away we are from a true under- standing of electromagnetic phenomena. People here have now opened their eyes to the fact that Lorentz is being guided by the best and noblest of intentions.[3] I am glad about that. On the whole, it must be said that politicians are showing more understanding for supranational goals than academics.[4] Specialization has led to superficiality among the latter. Here’s to a happy reunion in December, your A. Einstein aμ li a gμνaμaν = [p. 334] liai