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In the dedication section in the beginning of Larry Niven's "The Ringworld Engineers", Niven writes:

Freeman Dyson (Freeman Dyson!) has no trouble believing in the Ringworld (!), but can't see why the engineers wouldn't have built a lot of little ones instead. Wouldn't it be safer. I hope the answer I've given in this book is satisfactory.

I read the book but I don't understand what is the answer. What is Niven talking about? What answer does the book give to this question?

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  • Hi, welcome to SF&F. Just to clarify, you're wondering what in The Ringworld Engineers answers Dyson's question about why build one large ring instead of many little ones, correct?
    – DavidW
    Commented Jul 5 at 20:09
  • Yes, precisely.
    – itai
    Commented Jul 5 at 20:10
  • Something something paging Dr. Freud.
    – Valorum
    Commented Jul 5 at 20:12
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    @EikePierstorff, both those problems go away if the sun is not within the ring. I assume Dyson had in mind rings 12 light-seconds across (so that rotation in one day makes 1g), orbiting the sun at around 1AU.
    – Anton Sherwood
    Commented Jul 5 at 22:07
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    @AntonSherwood Smaller rings rotating in one day are Ian M Banks’s Orbitals, and feature very prominently in his Culture series.
    – Mike Scott
    Commented 2 days ago

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The answer comes from physics.

  • A ringworld has to have the sun at its center (otherwise it would be vigorously unstable and you'd have strange lighting effects and no easy way for the sun shields to provide a day-night cycle.
  • The sun needs to be at a distance from the ring's surface that insolation is Earth-normal (otherwise you'd need to artificially heat or cool the ring, an expensive proposition and prone to disastrous consequences if it failed). Essentially, the ring has to be in the star's habitable zone.
  • It needs to be around a fairly bright star -- red dwarfs, which due to their lower luminosity would allow for asmaller rings, are prone to frequent, nasty, high-radiation outbursts.
  • Regardless of the ring's size, the sidewalls have to be the same height to keep the air in. Really small ringworlds would be peculiar. (And if they were covered to prevent the loss of air, you've basically got topologically complex O'Neil colony, not a ringworld. (I think this may be what Dyson was talking about, but it's no longer very much like Ringworld.)

The bottom line is that a somewhat smaller ringworld is certainly possible, but we're still talking planetary orbit sized. Little (i.e., small enough that we can vaguely imagine how we might one day make one) ringworlds would probably not be habitable without messy extras to make them work.

Having multiple ringworlds in one system would be very difficult, since they all have to be at about the same distance from the sun, and even having them in different orbital inclinations would probably not prevent some nasty instabilities. (Remember, that even Ringworld by itself itself was not stable without fine adjustment thrusters.)

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    This answers the question stated in the subject line, but the body of the question asks what Niven meant when he said "the answer I've given in this book" (Ringworld Engineers). The answer would involve quoting some passage from that book.
    – nebogipfel
    Commented Jul 6 at 0:22
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    Fair enough, but I don't think he gave the answer in the book, unless it was somewhere in the handwaving. (Ringworlds are unavoidably incredibly dangerous no matter how you build them, at least as long as you don't have access to miracles. But they're really neat.)
    – Mark Olson
    Commented Jul 6 at 0:33
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    @nebogipfel I think I tended to assume, after I first read the book, that Niven's implication was: "Yes, I am now admitting for the record that a Ringworld can drift off-center over time. With multiple Ringworlds surrounding the same sun, there would be a high risk of a catastrophic collision between two or more Ringworlds someday, and the Pak Protectors certainly wouldn't want that to happen to their future descendants!"
    – Lorendiac
    Commented Jul 6 at 13:16
  • As I mentioned in another comment, I now realize that the problem is that we're not sure what does Freeman Dyson's "lot of little ones" suggestion mean. Whatever it is, we must acknowledge that Dyson did know his physics, so it is unlikely that whatever he had in mind would have been less safe than the Ringworld described by Niven.
    – itai
    Commented Jul 6 at 19:57
  • @itai Dyson was thinking of something more like lots of small habitats independently circling the sun. As long as you don't demand that they be ringworlds, building them and keeping them stable and safe -- while still not easy -- would be much, much, much, much easier than building Ringworld. Requiring the habitat to be a ring around the sun, puts very severe constraints on it. Dyson was quite right that Ringworld is a much better means to a story, than a means to habitation.
    – Mark Olson
    Commented Jul 6 at 20:20
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The Ringworld Engineers reveals that the Ringworld was built by Pak Protectors; by bringing in the Pak, TRE also brings in the events of Protector, which explains what happens when protectors compete for resources and land for their descendants. Per TRE, the builders of the Ring intended to build a home for their descendants.

By building the biggest possible habitat (the Ring used essentially all of the non-star mass in the Ring's solar system) the time until the Ringworld is full, and protectors once again face a battle-royale of everyone against everyone else is maximized. If they could have made it even bigger, I suspect they would have.

Furthermore, smaller Ringworlds in the same solar system would compete for the remaining resources in the system, like solar energy or materials (to build the next small Ring), and any left-over asteroids would make great weapons to use against other small Ringworlds.

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  • I'm not sure this answer can be deduced from "The Ringworld Engineers". Possibly it can be deduced from other books in the series.
    – itai
    Commented Jul 5 at 21:08

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