The article by Dirk van Delft on Paul Ehrenfest’s final years (Physics Today, January 2014, page 41) offers fascinating insights into the life of a remarkable man. It also offers a speculation as to why Martin Klein never wrote the second volume he had originally planned for his Ehrenfest biography. As Marty told me many years ago, he took a sabbatical in Leiden, the Netherlands, when he began his research for the biography, and he visited Ehrenfest’s widow, Tatiana, fairly often during that time.
At some point—after volume 1 had been written, I believe—Marty visited Tatiana again. During that visit she inadvertently opened a closet, and papers that she had kept hidden came tumbling out. It was my understanding that Marty abandoned the second volume because he did not have access to those papers, which he considered essential and which I assume are the letters acquired by the Boerhaave Museum. The article by van Delft offers abundant clues as to why Tatiana wanted to keep them secret.
