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In the novel Anathem by Neal Stephenson a spaceship appears in orbit around the protagonists planet. The character of Orolo observed it for a while and explains that he believes that the ship is not send from another star system because it is too small and slow to make that journey and therefore it comes from a different universe.

It turns out he is correct about his conclusions but when it's explained how travel between different universes is accomplished by someone from the ship I was left confused: The ship apparently left its version of earth using its nuclear pulse drive and completed a journey around the entire circumference of the universe which leads to the same place but in the past. In order to prevent temporal paradoxes the ship arrives in a different version of the universe it started in rather than the original. But given that the universe is implied to at least look cosmologically similar to ours (and one of the parallel universe the ship passed through is implied to actually be our world) this journey would be over many billions of lightyears at the very least. So doesn't this invalidate Orolos original reasoning? But where was he wrong?

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    Where does it talk about traveling the circumference of the universe? – Daniel Roseman May 25 at 15:04
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The character of Orolo observed it for a while and explains that he believes that the ship is not send from another star system because it is too small and slow to make that journey and therefore it comes from a different universe.

It would help if you could provide this quote, because it doesn't match what I see in the book. Orolo, shortly before his death, declaims his knowledge of the ship:

"I remind you that I was Thrown Back before I saw the last picture that I took of the alien ship."

"Of course. But I assumed you had taken other pictures before then - had been taking them for a long time."

"Streaks and blobs!" Orolo scoffed. "I was only learning how to capture a decent image of the thing."

(page 528)

You may be thinking of Barb's commentary on the ship, instead:

"A piece of the PAQD ship is missing!" Barb announced.

"That structure you've been studying - "

"It's where the missing piece used to be attached!"

"What do you think it was?"

"The inter-cosmic transport drive, obviously!" Barb scoffed. "They didn't want us to see it, because it's top secret! So they parked it farther out in the solar system."

(page 685)

But that's all conjecture on Barb's part, not backed by anything else we find out in the book. We never get other mention of it, but since we know that the PAQD dropped capsules onto Arbre to infiltrate the Matarrhite's Concent, that's a likely explanation for the missing equipment.

It turns out he is correct about his conclusions

Again, not sure which conclusions here, but it's not necessarily correct that the Daban Urnud isn't sized correctly to do what it was designed to do - travel between solar systems within a single narrative. And in fact, in the home Narrative of the Urnudans,

[The Daban Urnud] was designed to send a colony to a neighboring star system, only a quarter of a light-year away.

(page 709)

It simply hasn't done so because they keep crossing narratives instead of going somewhere in one narrative.

"The first Gan of the Daban Urnud was entrusted with the responsibility to establish a colony on another star system," Gan Odru continued....

... "Late in [the third Gan's] career, he became aware of your summons, and made the decision to alter the trajectory of the Daban Urnud so that it would - as he conceived it - fly into the past."

(page 827)

And when they do cross narratives, they seem to show up near planets, bypassing the interstellar aspect upon arrival:

"But when they reached the end of that journey, they found themselves, not in the past of Urnud, but in an altogether different cosmos, orbiting an Urnud-like planet --"

"Tro," said Arsibalt.

"Yes. This is how the universe protects herself - prevents violations of causality."

(page 709)

The ship apparently left its version of earth using its nuclear pulse drive and completed a journey around the entire circumference of the universe which leads to the same place but in the past.

Anathem never suggests they circumnavigated the universe, and given the number of years they've been traveling - roughly a thousand - we know that's not possible, since the most they can do is relativistic velocity.

The only technical specification we get is that they be able to travel far and fast enough:

"Geometrodynamics!" said Suur Moyra.

"If the equations of geometrodynamics are solved in the special case of a universe that happens to be rotating, it can be shown that a spaceship, if it travels far and fast enough --"

"Will travel backwards in time," said Paphlagon.

(page 709)

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