Limits To The Multiverse

"The multiverse does not quite involve all logically possible universes; it involves only those that are consistent with the laws of physics. There is not, for example, a universe in the multiverse in which magic is allowed."
-Frank J. Tipler, The Physics Of Christianity (Doubleday, New York, 2007), II, pp. 14-15.

See my distinction between realistic and fantastic alternative histories here.

Logically possible universes include all those in which there are, or at least can be, red triangles whereas an example of a logically impossible universe would be one in which there are square triangles. "Is there a red triangle hidden somewhere in this room?" is an empirical question whereas "Is there a square triangle hidden somewhere in this room?" does not succeed in being a question.

Tipler's limitation on the multiverse rules out Robert Heinlein's Magic, Inc. and would have ruled out Poul Anderson's Operation Chaos if Anderson had not subsequently written the sequel, Operation Luna. See the blog search result for Rheatics.