D O C . 4 0 M A Y 1 9 1 9 3 7 3. I now consider the electron’s motion according to the equation (μ = 1–3 yield the equations of motion).[8] Electric terms are furnished for α = 0 β = 1, 2, 3, 4 and in reverse: , or . In order for this to comply with experience,[9] must be . The light-second being the unit, the right-hand side results in Thus results, taking into consideration the α quantity found under (1), . This horrendously large quantity creates the difficulties. 4. yields in the electron’s equation of motion the force term , which gives for the Earth’s field the order of magnitude . However, I obtain for the corresponding terms of electrostatic origin in a field of only the or- der of magnitude . It hence appears that the effects of are so enormous that they spoil every- thing. If, on the other hand, one wished to have small, then the electromagnetic forces cannot be explained [10] because of the quadratic character, these forces would evidently also act on “neutral” matter. Perhaps you will find a way out of this.[11] In any case, I am waiting with the submission of your paper until we have come to some resolution about this point. As you requested, I shall send your longer manuscript back to you.[12] With best regards, yours, A. Einstein. d2xμ⎧ ds 2 ---------- -⎨ αβ μ ⎩ ⎭ ⎬---------------- ⎫dxα ds - dxβ ds 0= 0β μ ⎩ ⎭ ⎨ ⎬ ⎧ ⎫ dx0 ds ------- --------- dxβ ds α dx0 ds ------- - fμβ dxβ ds -------- α------- dx0 ds - ε μ -- - ∼ 107. ∼ dx0 ds ------- - 1021 ∼ g00 00 μ ⎩ ⎭ ⎨ ⎬ ⎧ ⎫ dx0 ds ------- - dx0- ds ------- 1024 300-------- volt cm - 10 14– g00 dx0 ds ------- -