Two Major SF Themes

"There's a divinity that shapes our ends, rough-hew them how we will." (Hamlet, Act 5, Scene 2)

There is a gravity that draws us back to time travel and future histories, the two most fascinating themes in sf. Both involve time, more specifically human experience of time. It is consciousness that divides a temporal sequence into past, present and future. Without consciousness, there are days, years and relationships of before and after, although they remain unnoticed, but there is nothing to mark any place as here or any moment as now. Every moment is the present moment to any organism that is conscious in that moment. A time traveller enters his past which is the present of anyone living in it, possibly including his younger self. 

Poul Anderson excels at both of these themes, writing several, in some cases many:

single-volume future histories;

future history series;

circular causality time travel narratives;

causality violation paradox time travel narratives;

other aspects of time travel.

There is a future history triad:

Heinlein's Future History;

Anderson's Psychotechnic History;

Anderson's Technic History.

The Psychotechnic History was modelled on the Future History whereas the Technic History grew organically, without preplanning, into a longer, more elaborate Heinleinian future history series.

More later.