first, organic beings with a different kind of language, a different means of reproduction and the technological ability and collective motivation to spend millions of years remaking an artificially enclosed planet from molten core to atmosphere and hydrosphere so that their descendants will be able to inhabit it when their home planet, closer to its sun, has become uninhabitable;
secondly, matter and energy falling into a black hole interact with vacuum virtualities to generate quasi-stable quantum states mutated by uncertainty, perceiving sets of tensors as objects, in some cases intelligent and able to send information-bearing modulated photons past-wards through space-time warps caused by artificially large spinning nuclei, thus able to communicate with newly arrived human explorers because instructed by their future selves, possibly also imposing permanent traces on the vacuum, thus surviving death.
Finally, as an auctorial joke, the physical description of another starfaring race encountered by the Yonderfolk exactly corresponds to that of the Merseians, although with a different biology and sociology: this new race is thought to be descended from communal burrowing creatures with large numbers but few females.
I am rereading Starfarers to extract interesting details like these. Since I am about half way through, there should be more. In particular, the changed terrestrial society later in the novel should be compared with such societies in The Boat Of A Million Years and Brain Wave. But there are also fascinating surprises like the ingenious explanations for why von Neumann probes would not work.
I advise sf fans and anyone else who is interested in scientific and social extrapolation to reread Anderson.
