In fact, as early as 1934, Popper started criticising the increasingly more accepted Copenhagen interpretation, a popular subjectivist interpretation of quantum mechanics. Therefore, in his most famous book Logik Der Forschung he proposed a first experiment alleged to empirically discriminate between the Copenhagen Interpretation and a realist interpretation, which he advocated. Einstein, however, wrote a letter to Popper about the experiment in which he raised some crucial objections[1] and Popper himself would have declared that this first attempt was "a gross mistake for which I have been deeply sorry and ashamed of ever since".[2]