I understand that the Earth was a computer for deriving the Ultimate Question, (for which the answer was 42), and that calculation would take 10 million years.

What doesn't make sense to me is, once that computer was designed by Deep Thought and built by the mice/Magratheans, why any outside interference (such as Ford Prefect) were allowed on the planet and thus might disrupt the processing on it.

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    Presumably the computer program would be robust enough to handle aliens as easily as meteors and therefore random extinctions. What it could not handle, of course, was aliens intent on creating a route through their solar system... Commented Feb 29, 2016 at 14:24
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    It was built by the Magratheans, not the Golgafrinchans. The Golgafrinchans were the ones that arrived on the B Ark and caused the original inhabitants to die out, thus making the program go wrong. Commented Feb 29, 2016 at 14:37
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    Eek, my bad - sorry! I had to Google Golgafrinchans to spell it right, but apparently I didn't read what I copy/pasted -_-;
    – J.J
    Commented Feb 29, 2016 at 14:43
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    What if the answer was ultimately provided by an external entity rather than from the internal state of the computer? What if that was the point? What if what actually happened was always meant to happen as part of the calculation?
    – Moo
    Commented Feb 29, 2016 at 14:46
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    @Moo - then the universe is infinitely much stranger than it's portrayed... oh, forgot the subject, right then, carry on.
    – Radhil
    Commented Feb 29, 2016 at 15:22

4 Answers 4


Because nobody was looking after it.

The Magratheans went into hibernation after the Earth was made and so weren't in a position to protect it.

The mice, being pan-dimensional creatures were too busy on the 5D TV Chat Show Circuit while they waited for the result - as stated by Benjy Mouse in Fit The Fourth of the radio series.

Nobody else knew it was a computer, it was just a planet like any other. Indeed, that is the beauty of the thing.

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    Nobody else knew, or maybe they knew and simply did not care.
    – ventsyv
    Commented Feb 29, 2016 at 21:59
  • That chat show stuff is just a bad excuse. With all those dimensions, surely the mice could have spared a couple to keep an eye on earth. Commented Feb 29, 2016 at 23:23
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    They seemed to expect the answer from the experiment just before the Vogons arrived. Which means that, left to its own devices, the calculation was doing just fine. The Vogon visit notwithstanding.
    – Misha R
    Commented Mar 1, 2016 at 7:25
  • @ventsyv The Vogons were contracted to destroy Earth specifically so that the question would not be discovered.
    – Izkata
    Commented Mar 1, 2016 at 20:43
  • Beauty? That travesty was in the way of my highway construction project! Commented Mar 2, 2016 at 15:21

Nobody actually knows the Earth isn't a real planet.

Since the building of the Earth, the Magratheans had long ago gone into their self-imposed deep sleep and had faded into myth and legend. Most people in this enlightened age don't even believe that they exist, except for highly gullible people like Zaphod Beeblebrox.

As such, the purpose of the Earth has no doubt been long since forgotten, and as it looks like a planet, and moves like a planet, and acts like a planet, it might as well be a planet for all practical purposes, which makes it fair game for hitchers of all persuasions.

A fair question at this point would be, why didn't the mice construct some kind of shield to keep people out? The answer of course being that the only thing that sort of thing would accomplish would be to make people that much more determined to get in to see what all the fuss was about. And that would rather defeat the purpose entirely. Plus, that sort of thing would require a tremendous amount of power to keep going for ten million years, and who do you think wants to be responsible for that electric bill?

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    Your mention of defences making Earth look like a juicy target reminds me of the work on nuclear semiotics by the Human Interference Task Force. Essentially, they were trying to solve the same problem on shorter time scales, for nuclear waste dumps. How do you stop people in the future interfering with our nuclear waste dumps? They'll be dangerous for 10,000+ years and people 10,000 years in the future will be as different culturally, linguistically and in every other way from us as we are from people in the late Stone Age. Commented Mar 1, 2016 at 5:51
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    Well written answer. Your writing style hints that you are channeling Douglas Adams. Nice.
    – Stewbob
    Commented Mar 1, 2016 at 18:08

The start of the first book hints about why:

Far out in the uncharted backwaters of the unfashionable end of the western spiral arm of the Galaxy lies a small unregarded yellow sun. Orbiting this at a distance of roughly ninety-two million miles is an utterly insignificant little blue-green planet...

It appears that the planet-like computer was placed in an obscure and untrafficked part of the Milky Way galaxy. From this we might infer that they simply did not expect anyone from the outside to come upon it, and therefore left it to its own devices. This is careless and sloppy thinking of course but nonetheless not impossible.

Also do note that in the narrative the Golgafrinchans caused the demise of the original cavemen of Earth fairly quickly, the latter starting to die out within only a year of the former's arrival, which is hardly even a shake of a lamb's tail in the ten million years the program was running. So even if the mice were keeping an eye on it, unless they did so meticulously and with utmost scrutiny then it is not hard to understand how this glitch in the input data could have gone unnoticed.


Based on the fact that the Magratheans intentionally placed the Earth into a relatively well trafficked region of space, it would seem likely that a small volume of alien visitations ("teasers" and the like) aren't just tolerable, they're part of the normal lifecycle of a normal planet in the Milky Way galaxy.

The alternative; placing the planet into deep space or hiding it between galaxies would likely result in the inhabitants becoming warped and unpleasant, much like the inhabitants of the planet Krikkit or prone to self-destruction like the Golgafrinchans

What the mice couldn't have predicted is that instead of an inconsequential number of alien visitors dropping by (advancing the planet's technology and generally prepping us to become part of the wider Galactic community) a whole damn colonisation fleet turned up and killed the most second most advanced inhabitants.

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    The inhabitants of Krikkit weren't really warped or unpleasant from being in deep space, though—they were just manipulated by the remnants of Deep Thought. They were misunderstood! Commented Feb 29, 2016 at 23:20
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    Most advanced inhabitants barring dolphins, as I presume the mice esacped Commented Feb 29, 2016 at 23:27
  • @binderbound - Quite right. See edit
    – Valorum
    Commented Mar 1, 2016 at 6:54
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    @JanusBahsJacquet - Hactar had been conditioning them, sure, but hiding them behind a cloud was certainly the first step.
    – Valorum
    Commented Mar 1, 2016 at 7:04
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    "Well trafficed"? Eh hm... "Far out in the uncharted back-waters of the unfashionable end of the western spiral arm of the Galaxy lies a small unregarded yellow sun. Orbiting this at a distance of roughly ninety-two million miles is an utterly insignificant little blue-green planet...". I would say it was just the opposite: they stuck Earth in a place where they did not expect interference. Unfortunately they got it anyway. And as is eventually is revealed (SPOILER) the Vogon fleet arrives specifically to wreck the computer that is the Earth, not because of the location being attractive.
    – MichaelK
    Commented Mar 1, 2016 at 9:08

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