I am reading The Hitchhikers Guide to the Galaxy. To my understanding, how the infinite improbability drive (IID) works is that it goes through every single (thus infinite) possibility in the universe to come up with the one they want, in this case, ending up at the other end of the galaxy.

While on the bridge, Trillian says that the ship picked them up, which is very improbable, but is possible because of the IID. If this is true, shouldn't they pick up every hitchhiker in the known universe? I thought that all of the improbabilities that they didn't want faded away?

  • 25
    Everything in the Hitchhiker's Guide works (or doesn't) at the whim of the author who never had any intention of obtaining internal consistency except in a limited way to pertained directly to the story. Seriously. Anyone who starts retconning that universe has missed the point. I mean it should have been obvious in the first book, but if you missed it then the bistromathic drive would have clued in a concussed duckling. Feb 9, 2014 at 23:43
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    @dmckee Also, on top of that the Douglas Adams made certain that there were inconsistencies between different revisions of the books, and the radio play, movie, TV series. So there is hardly a single canon to go on.
    – user20155
    Feb 10, 2014 at 1:07
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    This is why it's so good, you can argue canon all you want but, as @Legi Stromtrooper said, Douglas Adams built oddity and inconsistency into the fabric of his 'universe' so all arguments are moot really ! 42.
    – Pat Dobson
    Feb 10, 2014 at 8:05
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    @dreamwithinadream: "it goes through every single (thus infinite) possibility" - if it goes through every single possibility, then by definition, the possibilities aren't infinite. If they're infinite, it could never finish going through them, because they go on forever. en.wiktionary.org/wiki/infinite Feb 10, 2014 at 10:06
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    @PaulD.Waite You assume it takes finite time to "go through" a possibility and that they must be "gone through" serially. We don't know either of those things. Feb 11, 2014 at 0:44

3 Answers 3


Actually it's pretty consistent from a plot perspective. All of the main characters are intimately connected; Trillian and Arthur met at the same party that Zaphod picked her up at, Ford and Zaphod are distantly related and locating a survivor from Earth is manna from heaven for the mice (carried by Trillian) who encouraged Zaphod to steal the ship (so they could find Magrathea) in the first place.

The odds of all of those people happening to be in one place is infinitely improbable, which therefore makes it almost a certainty of happening when exposed to an Infinite Improbability Field.

The author hangs a lampshade on the likelihood of this happening;

“I didn’t pick them up.”[said Trillian] “What do you mean? Who picked them up then?”[said Zaphod] “The ship did.” “Huh?” “The ship did. All by itself.” “Huh?” “While we were in Improbability Drive.” “But that’s incredible.” “No, Zaphod. Just very very improbable.”


“Tricia McMillan?” he said. “What are you doing here?” “Same as you,” she said, “I hitched a lift. After all, with a degree in math and another in astrophysics what else was there to do? It was either that or the dole queue again on Monday.” “Infinity minus one,” chattered the computer. “Improbability sum now complete.” Zaphod looked about him, at Ford, at Arthur, and then at Trillian. “Trillian,” he said, “is this sort of thing going to happen every time we use the Improbability Drive?” “Very probably, I’m afraid,” she said.

  • But why didn't they, and some other things, disappear like all of the other improbable things that happened?
    – Beatrix
    Feb 13, 2014 at 3:53
  • @Beatrix - An excellent question. I've got no answer for you...
    – Valorum
    Feb 13, 2014 at 18:03

As I understood it, the drive works as follows: (note that some of this is speculation)

1) the ship is directed to teleport to a specific spot (an immensly large improbability but finite that is likely denoted short hand by infinity minus one against).

2) in order to reach that improbability, the drive must generate each lower finite improbability (1:1 to 2:1 to 1,000,000:1 etc.)

3) at a given improbability the "normal" outcome of doing something will be the outcome with the probability the drive says should happen. For example: at 1:1 things are actually normal; at 2:1 a coins could land on tails or a dice on an even number; at 10,000:1 the coin would land on its side; at 10^1,000,000,000,000,000,000 somehow the coin lands on fish, monkeys type hamlet, and an ocean forms that stays still while the land has waves. The actual events are chosen for humorous effect.

4) the hitchhikers arriving was at a specific improbability that was massive (infinity minus one) but less than the intended teleport. Teleporting to a random location in the universe is more likely than teleporting to a specific one so it teleports to all locations randomly.

5) once the improbability to go to that specific place is reached, they teleport to that location (quite easy iff that is the normal thing for the ship to do now) and start bringing the drive down.

6) as the ship returns to the true 1:1 "normal" (whatever "that" is anyways), most things that have transformed or materialized turn back or disappear respectively. Apparently these apparitions are still inherently unlikely so change back as the drive goes back. Some things are more improbable to transform back than remain as they are so are unintentionally still in existance when "normal" is reached. This includes (for instance) the redecorated ship, the passengers remaining on board, and the eggs and confetti remaining to land on the (nearly) deserted planet.

  • Canon support?? Feb 10, 2014 at 15:18
  • I don't have a book I can copy available but every example (besides the coins) I used is canon. The computer counts down (and i think up) to whatever improbability they are currently at. The sources used in the other answer (which is correct but doesn't answer Mr. Poe Quoter's question as to how the drive works) support my explanation of the hitchikers being collected at a specific improbability. A quote I can't fully remember from Zaphod said how the 7 digit phone number probability was too low for where they were at so the event had to be more improbable for some reason.
    – user22672
    Feb 10, 2014 at 15:33
  • i don't really get what you mean by #6. Care to delve deeper?
    – Beatrix
    Feb 15, 2014 at 23:02
  • The book does not explain why some things persist and why some don't. I took it to be whether the thing summoned is inherently unlikely and is, therefore, returned to normal when things become normal (like an infinite number of intellegent monkeys typing hamlet can't really exist). Alternatively the event that summoned it is unlikely but the object itself isn't inherently improbable (such as the hitchhikers or a different redecoration) so an unlikely event would be needed to remove them. Likely which category each fits into is soley dependant on usefullness to the plot and humor.
    – user22672
    Feb 18, 2014 at 14:27

Arthur and Ford were NOT brought onto the ship when the ship was at infinite improbability, but at a probability level of 2^2079460347:1. Very high, but not infinite. As the probability level reduced in the ship, the madness around them came and went - buildings lapping against the shore of the stoic ocean, penguin transformation and arm detachment, etc.

That specific probability was far higher than the odds of two random hitchhikers being beamed aboard - it was the odds of two hitchhikers being beamed onto the ship 29 seconds out of the thirty seconds they could survive in the vacuum of space after being cast out of the airlock of a Vogon constructor vessel, AND one of them was related to the pilot, AND the other knew the Earthling who the pilot took off the Earth, AND ALSO whose phone number was the same as that improbability measurement.

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