Letter from Planck – The letter excerpts from Max Planck at the start of this chapter combine phrases drawn from two actual letters he sent to Einstein in 1907 [L-Planck, 1907a,b]. Planck’s early and sincere recognition of Einstein’s work gave a tremendous boost to Einstein’s growing reputation within the scientific community.

Letter from Minkowski – In October 1907, Hermann Minkowski wrote to Einstein, praising his work on special relativity [L-Minkowski, 1907]. Later that year, Minkowski introduced the revolutionary idea of four-dimensional space-time in two lectures [Minkowski, 1907a; 1907b]. Although no further letters are recorded, it is plausible that Minkowski would have written to Einstein again, perhaps sharing drafts of his lectures; the letter in the novel is a fictional but historically reasonable extrapolation.

1907 yearbook article – That same year, Einstein published a comprehensive review article for a scientific yearbook: On the Relativity Principle and the Conclusions Drawn from It [Einstein, 1907b], [Isaacson-1, p. 148]. In it, he summarized the key ideas of special relativity and made his first tentative explorations toward a broader theory—planting the early seeds of what would later become general relativity. This paper marked an important turning point in his scientific evolution.

Happiest thought – Einstein called the insight that led him to general relativity the “happiest thought” of his life. In a 1920 article for non-specialists [Einstein,1920], he used this phrase for the first time, recalling his sudden realization that a freely falling person would not feel any weight—a simple but profound insight that became the basis for his theory of gravitation.