I recently picked up a copy of Heinlein’s collection The Past Through Tomorrow (gotta love used-book sales), and was naturally re-reading it. It occured to me that we’re rather taking the editor’s word that these stories all occur in the same universe. So I decided to go through and pick out all of the cross-references and see which were connected and which weren’t.
First of all, Harriman is mentioned directly or indirectly in “Blowups Happen”, “The Man who Sold the Moon” (of course), “Delilah and the Space Rigger”, “Space Jockey”, “Requiem”, “The Black Pits of Luna”, “‘It’s Great to be Back’”, “The Green Hills of Earth”, and Methuselah’s Children. That gives us a good base to work on. Further, “Misfit” and Methuselah’s Children are definitely tied together by A. J. Libby, and Lazarus mentions having consulted with Dr. Pinero, the hero of “Lifeline”, so we can fold those in, too, and several of Blind Rhysling’s songs show up in “Logic of Empire”.
“Logic of Empire”, meanwhile, contains the first mention of Nehemiah Scudder, the First Prophet. So that means that “If This Goes On–” is also in the Future History. In turn, Dr. Micah Novak, one of the leaders of the Cabal and later drafter of the Covenant, ties “If This Goes On–” together with “Coventry”. So that connects all of the stories in The Past Through Tomorrow, except for seven: “The Roads must Roll”, “The Long Watch”, “Gentlemen, Be Seated”, “’–We Also Walk Dogs’”, “Searchlight”, “Ordeal in Space”, and “The Menace from Earth”.
There are also other connections between the stories, of course: I believe there’s a mention in “The Man who Sold the Moon” to Erickson and Harper, inventors of the isotope fuels; “The Black Pits of Luna” and “‘It’s Great to Be Back’” both mention Rutherford radiation lab on the Moon; and “The Green Hills of Earth” and Methuselah’s Children both mention the patronage/indenture system on Venus. “Coventry” and Methuselah’s Children, meanwhile, both refer to the Interregnum of the Prophets and the Second American Revolution, and in Methuselah’s Children there’s also a skyscraper named after Novak.
But as for those other seven stories: The Harriman stories refer to the mechanized Roads, and “If This Goes On–” and Methuselah’s Children mention the ruins of the old Roadcities, with the latter also referring to the “Strike of '76”. There’s nothing firm to make an absolute connection (there might be other timelines with mechanized roads), but it’s pretty safe to say that “The Roads Must Roll” is probably part of the Future History.
I can find nothing in “The Long Watch” to connect it with the Future History, and in “Gentlemen, Be Seated”, the closest thing to a clue as to what timeline it takes place in is that there’s a Richardson Observatory on the Moon (which, so far as I know, isn’t mentioned elsewhere). “Searchlight” is very short on details of any kind (understandable, considering the length restrictions under which it was written), so about all we can say is that there are military bases on the Moon. And “Ordeal in Space” has nothing to connect it to the other Future History stories, but there are many clues that might help place it with some of Heinlein’s other lines: There are things called “drop chutes” which may possibly work by gravity manipulation, there’s a Three Planets treaty, Mars is inhabited by old, tired beings, one of whom is named “Knath Sooth” (I’m almost certain that I’ve seen that name elsewhere in Heinlein’s works), and the Valkyrie was built sometime before the discovery of artificial gravity.
The remaining two stories, “’–We Also Walk Dogs’” and “The Menace from Earth”, I do not believe are properly part of the Future History, and were presumably included due to editorial error. “’–We Also Walk Dogs’” is the most blatent: Earth is governed by a sovreign planetary administration, totally at odds with the independent countries of the other works; the inhabited worlds of the Solar System include Venus, Mars, Titan, Jupiter, and Callisto, whereas the Jovian satellites, at least, are uninhabited in the Future History; and it’s O’Neil who invents the gravity modulator, not Libby in “Misfit”. For “The Menace from Earth”, the evidence isn’t absolute, but neither of the first two starships was designed by the engineering firm of Jones and Hardesty (or Hardesty and Hardesty) or named Prometheus, and I find it hard to believe that Holly wouldn’t have managed to wrangle that contract for the first starship.
That covers all of the stories in The Past through Tomorrow; what about Heinlein’s other books? Time Enough for Love is obviously part of the Future History; less obviously, Farmer in the Sky is as well (one of the characters in that book has a verse from “The Green Hills of Earth” inscibed on his tombstone). Space Cadet is clearly in the same universe as “The Long Watch”, since Johnny Dalquist is one of the Four who are always called on the roster, but although the Venusians in Space Cadet are portrayed consistently with those in “Logic of Empire”, I remain unconvinced that these two stories are part of the Future History. Orphans of the Sky may or may not be; I seem to recall Lazarus referring to the events of that story in Time Enough for Love, but with the name of the ship changed.
Time for the Stars, Tunnel in the Sky, and Citizen of the Galaxy are definitely not Future History, as each has a different method of FTL, with no mention of the Libby hyperspace drive. Likewise, Rocket Ship Gallileo has a different account of the first Moon landing. The Moon is a Harsh Mistress, and, by extension, The Rolling Stones, is probably not in the Future History: In the Future History, the residents of Luna are all the best and brightest, which seems rather inconsistent with the Moon’s use as a penal colony. Friday, with a Balkanized United States, and The Puppet Masters, with an alien threat from Titan, can likewise be probably ruled out as being part of the Future History. Offhand, I can’t think of any other stories which I can definitively place one way or the other, although I have suspicions that Stranger in a Strange Land, Podkayne of Mars, and Red Planet are all in the same timeline, as well.
Any I missed?