IMHO, "hard SF" has developed as a term for books that have "hard physics", everything else like biology, chemistry, sometimes even mathematics is usually lacki

IMHO, "hard SF" has developed as a term for books that have "hard physics", everything else like biology, chemistry, sometimes even mathematics is usually lacking. As an example: The Cheela, the microscopic intelligent life forms in the more famous hard SF Dragon's Egg, are described like eukaryotes with an eukaryotic life style, but they're so tiny that they don't need any of the complex "inventions" of eukaryotes, for example, they wouldn't need any "plumbing" in the body since they're small enough for diffusion to deliver nutrients. IRC their DNA is triple stranded; DNA is double stranded so that it's more safe from mutations as you can always read the opposite from the opposite strand, if you had a second backup in the form of a third strand you would have no stable chemical bonds between the bases. Triple helices exist but these doesn't help with stability and are implicated in transcription. Edit: Similarly, there should be massive problems in the later communication because the Cheela shouldn't have a base 10 mathematical system as they don't have 10 fingers, you could still display it as base 10 but there has to be a conversion (see https://www.quora.com/What-does-one-mean-by-Every-base-is-base-10 https://www.quora.com/What-does-one-mean-by-Every-base-is-ba... ).