Inspired by this exchange in chat:

Jack B Nimble @ScienceFiction&Fantasy Brought you to by someone who doesn't understand probability.

phantom42 @ScienceFiction&Fantasy Zaphod just used the Infinite Improbability Drive at exactly the wrong time.

BESW @phantom42 This is my new third-favourite explanation for improbable things in fiction.

The Infinite Improbability Drive "passes through every conceivable point in every conceivable universe almost simultaneously".

In the books/movie, we see not only the Heart of Gold and its passengers affected, but we also see things outside the ship affected (e.g. missiles being turned into a sperm whale and a bowl of petunias).

Is there any indication of the maximum distance from the drive that reality or an object can be affected?

*Note: I am not asking about the maximum travel range of the ship/drive, but about how far an object can be from the ship/drive and be affected.

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    I'd say that "passes through every conceivable point in every conceivable universe almost simultaneously" is a rather good indication of what it can affect... – Deltharis Mar 4 '15 at 15:28
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    @Deltharis It may jump around everywhere, but can it leave a wake of sperm whales and petunias across the entire universe along the way? – phantom42 Mar 4 '15 at 15:30
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    Not "may jump". Jumps. On the way to anywere it passes through every point. Whether that means it may affect/affects every point depends on the principle behind affecting things, which is as far as I know in the realm of narrativum. – Deltharis Mar 4 '15 at 15:40
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    If I had to take a wild guess, I'd say 'infinite'. – Zibbobz Mar 4 '15 at 20:39
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    The affect on the missiles was explicitly called out as being because the drive hadn't been operated correctly, I think the phrasing was something like "without activating the protective shielding". So I'd say that large-scale effects like that don't ordinarily happen at all, or at least aren't supposed to. – Harry Johnston Mar 4 '15 at 21:33

When the ship picked up Ford and Arthur, there was a passage to the effect that the engine made molecules way way back in the past try to assemble themselves into different shapes and start making more of itself, and thus life was born in the universe:

A hole had just appeared in the Galaxy. It was exactly a nothingth of a second long, a nothingth of an inch wide, and quite a lot of million light years from end to end. […]

The nothingth of a second for which the hole existed reverberated backwards and forwards through time in a most improbable fashion. Somewhere in the deeply remote past it seriously traumatized a small random group of atoms drifting through the empty sterility of space and made them cling together in the most extraordinarily unlikely patterns. These patterns quickly learnt to copy themselves (this was part of what was so extraordinary of the patterns) and went on to cause massive trouble on every planet they drifted on to. That was how life began in the Universe.

The Hitchhiker’s Guide to the Galaxy, chapter 9

Despite the tongue-in-cheek nature of this passage and, well, just about everything else related to the Drive, this implies it's random effect range is just as infinite as it's distance range.

  • It says "a small random group," but does not say where that group was in relation to the Drive. The hole was many millions of light years long, but very thin. How far away from that hole was the group of atoms? – Dave Johnson Mar 4 '15 at 19:21
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    @Dave - Well, do you want that answer as a 4-dimensional vector, or shall we just call it timey-wimey? The ship's not even in the same epoch as it's effect. Distance is kinda relative at that point. And thank you alex, for doing my work for me. – Radhil Mar 4 '15 at 19:49
  • Wait. Isn't the Infinite Improbability Drive an invention by life? So the Infinite Improbability Drive caused not only life but also its own invention? – jpmc26 Mar 4 '15 at 23:17
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    @jpmc - Paradox is fun, isn't it? – Radhil Mar 4 '15 at 23:34

The Infinite Improbability Drive affects reality itself, greatly increasing the odds that something improbable will occur. As stated in the books, each time the drive is used, there is not only a major local event, but also random minor events scattered throughout the universe as well.

Douglas later specified that the drive generates a field, inside of which anything can happen. Although the major events seem to be confined to inside this shield (the general area around the drive), the random side effects can still occur anywhere in the universe.

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    @AndrewThompson - updated to 'universe'. Sorry, stuck in Star Wars mode. – Omegacron Mar 4 '15 at 18:03
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    "Sorry, stuck in Star Wars mode." LOL! I'll pay that.. ;) – Andrew Thompson Mar 4 '15 at 18:32

You can go anywhere, as long as you know the exact improbability of getting there. So the only limitation would be your ability to compute the improbability.

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    I'm not asking how far the ship/drive can take you - I'm asking how far from the ship/drive something can be affected. – phantom42 Mar 4 '15 at 15:23
  • Oh, I see. The same logic would still apply, though. You can cause any improbable event to happen, as long as you know its exact improbability. :) Maybe the improbability goes up as you move away from the ship. – Dima Mar 4 '15 at 15:25

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