1 5 4 D O C U M E N T 1 3 2 D E C E M B E R 1 9 2 5 always just the “thermal portions.” One must, of course, first demonstrate system- atically that these thermal portions, which should accordingly be regarded as the real quantities of the field, also possess the necessary relativistic tensor properties As soon as I have time I want to try and formulate a systematic matrix theory of the electromagnetic field. The above-said, that the zero-point portions should always be omitted from the field quantities (which initially looks very ugly), will, by the way, I believe, assume a quite natural and reasonable form if a formulation of ma- trix theory that has been worked out by Born in America with the mathematician Wiener is taken as its basis.[4] It permits an entirely uniform treatment not only of periodic but also of aperiodic motions (e.g., of the hitherto inaccessible transla- tion). Of course one will not doubt that such a theory is still far from the desired ideal of a theory of light: a more profound light-quantum theory in which a contin- uous world and continuous quantities generally do no longer occur at all. But I still think that some progress can be achieved with a matrix theory of the Maxwell equations. One must obtain the usual interference experiments as in the classical theory however, all the interference fluctuations would naturally be obtained dif- ferently. I also have a quiet hope that in the process some aspects will be revealed for the future light-quantum theory. Finally, as regards the zero-point energy for a crystal lattice, I would actually like to believe that it also makes no contribution to the inertia of the macroscopic crystal. In fact I had earlier considered some points that seemed to demand an inertial effect of the zero-point energy in a crystal (mainly because one can alter the number of degrees of freedom by evaporation) but those considerations no longer seem to me unobjectionable and I now feel somewhat more secure in the hope that it will not be possible to find an inertial effect of zero-point energy in the crystal, as well as in the cavity, by consistent application of our theory. The strange fact that one must reckon with all possible atomic states at the same time must probably be tolerated.[5] As regards the Coulomb forces, you have per- haps already heard that Pauli has supposedly been able to implement the theory of hydrogen using the matrix method.[6] Bohr, who is here in G[öttingen] just now,[7] is hoping very much that a consistent execution of the Uhlenbeck-Goudsmit idea of an electron magnetism by means of quantum mechanics will also explain the anomalous Zeeman effects and those mystical “ambiguities.”[8] You have hopefully, meanwhile, received the correction proofs of our three-man paper, Born- Heisenberg-J[ordan].[9] The diatomic molecules are supposed to be calculated now they obviously must be treated entirely as a three-dimensional problem an artificial transformation into a 2- or 1- dimensional one only works in the classical theory.[10] With kind regards, respectfully and sincerely yours, Pascual Jordan