Lettre d'Albert Einstein à Louis de Broglie - 15 février 1954 (dactylographié en français)

February 15, 1954 Professor L. de Broglie Institut de France Academy of Sciences Paris, France Dear Mr. de Broglie, Yesterday I read a German translation of your essay on the question of quantum theory and determinism. I am familiar with this text and took great pleasure in reading your ideas that were very clearly set out. It is funny how everything looks more vivid and lively when it appears in my old familiar language. I'm writing to you now for a rather strange reason. I want to tell you how I was driven to my methodology, which sounds quite bizarre on the face of it. I must have appeared like the well-known desert bird, the ostrich, constantly burying its head in the relativistic sand so as not to have to face the evil quantum. In truth, like you, I am convinced that one must search for the substructure—the necessity of this is skillfully hidden in present quantum theory by applying the statistical form. However, I have long been convinced that this substructure cannot be found by constructive means from the known empirical behavior of physical things, because the necessary mental leap would be too great for human abilities. I did not come to this conclusion just as a result of many years of work, but also because of my experiences in gravitational theory. Gravitational theories were only determined due to a purely formal principle (general covariance), i.e. based on the trust in the greatest conceivable logical simplicity of the laws of nature. Since it was clear that gravitational theory is only the first step toward finding general field laws that are as simple as possible, it seemed to me that this logical path must be thought through before one can hope to arrive at a solution to the quantum problem as well. As a result, I became a fanatical believer in the method of "logical simplicity." The physicists of this generation are now convinced that such a way cannot lead to the theory of atomistic and quantum structure. Maybe they are right about that. Maybe there is no quantum field theory, for my efforts cannot solve the problem of atomics and quanta, and may not even bring us closer to a solution. However, this negative belief is based solely on intuition, and not objectivity. I also see no other clear path to a logical simple theory. This explains the head-in-the-sand effect. I thought this might interest you from a psychological point of view, especially as you have lost confidence in the finality of the statistical method once more. With cordial regards. Yours, Signature of AE For your convenience, I enclose a French translation of this letter.