The fictional history of the OA setting spans over ten thousand years, beginning with the real-world present day; dates in OA are marked according to the Tranquility Calendar (which is named after Tranquility Base and thus started after the Apollo 11 landing). OA claims to adhere to plausible, or "hard" science fiction; that is, there are no human-like aliens, no literal faster-than-light travel or other violations of the known laws of physics, and no "naval analogy" space battles. Certain speculative technologies, such as the creation of "negative mass" (averaged null energy condition-violating) exotic matter and the manipulation of strange forms of matter, such as magnetic monopoles and Q-balls, on length scales much smaller than that of an atom, strong artificial intelligence and artificial life appear in the setting, distinguishing it from "ultra-hard" science fiction (which assumes only technologies proven to be possible at the time it is written).