Many scientific subjects are described in detail in the letters, including network theory (or graph theory), astrophysics, cosmology, the unified field theory, biology, and evolution. Some of this information is thought to be dubious pseudoscience, but much of it is scientifically accurate. However, Jerome Clark (Clark, 1993) notes that Jacques Vallée argued that the scientific content of the Ummo letters was knowledgeable but unremarkable, and compared the scientific references to a well-researched science fiction novel—plausible in the 1960s, but dated by the standards of the 1990s. Controversy arose about one particular assertion of the Ummites. In 1965, they wrote they were coming from a planet orbiting Wolf 424, adding this star is 3.68502 light-years of the Sun. This was consistent with the distance estimated in 1938, but after some additional measurements, the estimate was revised to 14.3 light-years. Fernando Sesma asked then the Ummites about this apparent mistake. The Ummites replied in another letter the same year that the first measurement is the real distance measured in the "three-dimensional framework" while the second is "the apparent distance traveled by light".