The Changing Past

Not only is the past unchangeable. It might even be defined as that which cannot be changed as opposed to the present which is changing and the future which is to be.

Two sf nightmares are SM Stirling's Draka, the survival of slave-owning society, and George Orwell's 1984 where, to paraphrase, it is said that:

the past exists only in memories and records;
the Party controls memories and records.

(Present reality will hit the Party hard when its power sources run out or a comet strikes the Earth.)

However, our knowledge, interpretation and understanding of the past continually change:

"New data and insights often cause us to revise our ideas about the past and even the present."
-Poul Anderson, Orion Shall Rise (London, 1988), Author's Note, p. vii.

A few passages in Poul Anderson's Technic Future History connect with his fictional works set in historical or prehistorical periods, e.g., here. Another is when van Rijn quotes:

"'Like they used to say in Old Norse and such places, 'Bare is brotherless back.'" (David Falkayn: Star Trader, p. 431)

I remember this phrase from somewhere in Anderson's historical fiction but does anyone know where?

When reminded that he is praying to a mythical figure, St George, van Rijn replies:

"'Bah,' said van Rijn loftily. 'They got no faith. I need a good fighting saint, who says God can't improve the past and make me one?'" (p. 393)

In his Introduction to "Death and the Knight," Anderson quoted St Thomas Aquinas as declaring that even God cannot change the past. However, Anderson himself could write different pasts - and, in some of them, gods and miracle-working saints do exist.