Publications

Einstein writes to his friend Zangger (1874-1957), professor of physiology at the University of Zürich, the day after he submits the final version of the general theory of relativity to the Royal Prussian Academy of Sciences. While Einstein speaks of the theory’s “incomparable beauty,” his judgments of people are dark. He complains bitterly of his separated wife’s malign influence on their children and of what he takes to be David Hilbert’s plagiarism.

This letter antedates by a year or two the “new” quantum mechanics of Werner Heisenberg, Max Born, Pasqual Jordan, and Erwin Schrödinger. Einstein begins with a response to a letter about Japan by Born’s wife, Hedi—a letter that has since been lost. But then, as he would for the next 30 years, Einstein launches into his differences with Max on the interpretation of quantum phenomena. If atomic phenomena are truly random, he grumbles, he’d rather be a croupier. Berlin, 29 April 1924

Every attempt to establish a unified-field theory must start, in my opinion,from a group of transformations which is no less general than that of the con-tinuous transformations of the four coordinates. For we should hardly be suc-cessful in looking for the subsequent enlargement of the group for a theory basedon a narrower group. It is further reasonable to attempt the establishment ofa unified theory by a generalization of the relativistic theory of gravitation.Such a generalization, which does not seem to have been discovered so far, isdescribed in the following.If we speak about a unified theory we have two possible points of view, whosedistinction is essential for the following:(1) That the field appear as a unified covariant entity. As an example I citethe unification of the electric and the magnetic fields by the special theory ofrelativity. The unification here consists in this that the entire field consideredis described as a skew-symmetric tensor. The basic group of Lorentz trans-formations does not enable us to split this field independently of the system ofcoordinates, into an electric and a magnetic one.(2) Neither the field equations nor the U1amiltonian function can be expressedas the sum of several invariant parts, but are formally unified entities. Alsothis (weaker) criterion of uniformity is satisfied in our example of the specialrelativistic description of Maxwell's equations.The theory we shall describe is unified according to criterion (2), but not ac-cording to criterion (1). Such a theory is to be considered unified only in alimited sense.

in a complete theory there is an element corresponding to each element of reality. A sufficient condition for the reality of a physical quantity is the possibility of predicting it with certainty, without disturbing the system. in quantum mechanics in the case of two physical quantities described by non-commuting operators, the knowledge of one precludes the knowledge of the other.

that a theory which accounts for the results of all experiments - as quantum mechanics could - is complete. But Einstein contended that a theory which gives no account of the real world, but only of our imperfect (probabilistic) knowledge ofthat world, is incomplete. Partly for this reason, the world's most renowned scientist, during the last 20 years of his life, felt almost completely isolated in his scientific work and goals. The situation began to change, however, shortly before Einstein's death in 1955. Today, the subjects of Einstein's own scientific efforts - the general theory of relativity and the unification of the various physical forces (gravitation, electromagnetism, and so on) in a single field theory - have gradually become two of the most important foci of attention in physics. More to the point of Einstein's 1927 essay, physicists since the early 1950s have been less and less joyful about the indeterminism of our most fundamental theory, more and more ready to declare this feature of quantum mechanics unsatisfactory.

Dear Born, You will have to be a little patient. My son-in-law is certain to read the play, and I will write to you. But the poor man has to economize with his strength, as his heart is in poor condition. I have reminded him again to give an opinion on the play as soon as possible. I liked the beginning of the play very much, and I think its impact will not be lost on him. Quantum mechanics is certainly imposing. But an inner voice tells me that it is not yet the real thing. The theory says a lot, but does not really bring us any closer to the secret of the “Old One.” I, at any rate, am convinced that He is not playing at dice. Waves in three-dimensional space whose velocity is regulated by potential energy (for example, rubber bands) … I am working very hard at deducing the equations of motion of material points regarded as singularities, given the differential equation of general relativity. With best wishes, Yours, A. Einstein