Mainstream And SF

Mainstream fiction deals with human beings. 

More specifically, mainstream fiction deals with human beings on Earth in our timeline. Mainstream fictional characters can see heavenly bodies but not travel to them and can refer to alternative histories but not experience them. If interplanetary travel does begin at last, then and only then will it be incorporated into mainstream fiction but history cannot change. If the Germans won World War II, then they did it in another timeline, not in this one.

Sometimes fictional characters look beyond the confines of their familiar context, e.g.:

"'Do please remember that battles have gone wrong, wars have been lost and the history of the world has been changed, because valour has outrun discretion and men have lost touch.'"

-Dornford Yates, Perishable Goods (London, 1938), CHAPTER IV, p. 105.

Poul Anderson fans immediately think of the Time Patrol intervening on battlefields in "Delenda Est" and The Shield Of Time. More generally, sf readers think of entire novels set in alternative histories.

Sf deals with the future, space travel, time travel, alternative histories etc, everything other than Earth in our timeline. Unfortunately, genre sf has developed cliches and is not necessarily good at dealing with human beings although it should show the impact on human beings of space travel etc.

Anderson's Mirkheim is a good novel because it shows its characters aging and changing, a good political novel because it shows social conflict and change and a good sf novel because it shows the consequences of an astronomical hypothesis. Anderson uses but transcends cliches like hyperspace.

Maybe mainstream fiction needs to acknowledge the cosmic setting of human life. And sf needs to deal with human beings in the same depth as mainstream fiction.