Multiverses

The multiverse is both theoretical and fictional. Frank J. Tipler tells us that the multiverse of quantum theory includes only universes consistent with the laws of physics (see Limits To The Multiverse) whereas a philosopher can contemplate any logically possible universe, even one allowing magic. I have postulated a megamultiverse incorporating, among its multiple dimensions, not only the multiverse of Poul Anderson's Old Phoenix stories but also the mutable timeline of his Time Patrol series and the immutable timelines of his other time travel novels.

Tipler explains that, in the pre-Copernican conception of the universe:

sublunar objects were composed of varying combinations of the Classical "four elements";

heavenly bodies were composed of the "fifth element";

these two realms obeyed different laws;

this is wrong because "...all reality obeys one, and only one, set of laws." (The Physics Of Christianity, II, p. 16)

Since the Old Phoenix multiverse includes magical realms, it should also incorporate worlds where Terrestrial matter is composed of the four elements and extraterrestrial matter of the fifth element. I would expect such worlds to include the Carolingian universe. Holger applies scientific knowledge in that universe. See Ariel And A Nixie. Following Larry Niven's lead, I speculated here that the mutable timeline guarded by the Time Patrol might culminate in a pre-quantum mechanics timeline. This is comparable to the pre-Copernican universes.