Nod.

"The design document idea raises some interesting possibilities," she adds, thoughtfully. "For example, if I were designing a world under these circumstances, I would give it expanding borders, although then we would need to figure out a patch for the lighting, since the current scheme doesn't scale indefinitely. I think it should be possible to add in a few tweaks along those lines without a significant extra cost in your time and attention. It might even make the original task easier, by giving you more to focus on. How thorough an explanation of the cosmology have you heard; should I fill in the details you're missing?"

"Not thorough at all, I knew it would have to be screwy if the creatures were right about the planet being flat but I don't know that much astronomy from Earth to compare."

"The world is a flat circle of land and ocean under a hemisphere of air, surrounded on all sides by void. The moon rises in the northwest and sets in the northeast, travelling eastward over the Kingdom of Night; the sun rises in the southeast and sets in the southwest, travelling westward over the Kingdom of Day. A naive rescaling of that would eventually lead to too much air between the lights in the sky and the middle of the circle, making everything uncomfortably dim."

"The moon gives off its own light? And do the sun and moon just go around and travel through void? The void is voidier than vacuum, right?"

"The void is very much voidier than vacuum. I have reason to believe that it's what people travel through to get between here and Earth, except that transit between here and Earth is safe in a way that contact with the void normally isn't. The sun and moon don't travel through it; they're... more like a projection on the outer edge of the sky than like objects moving through it. And yes, the moon gives off its own light, although it still has phases."

"I felt like things were grabbing me. Are there grabby thing in the void."

"I'm not sure there are things in the void. When I fell through, I felt... like I was in a current, but not a current of anything. But based on the stories I've collected from other visitors, I think the experience is different for different people, or different instances of travel. No one has ever come and gone and come back again, so I have no data on repeated travel by the same person. I do know that the people who leave get home safely, though."

"How do you know?"

"I can - catch glimpses - of Earth. It would not be a very effective spying tool if I wanted to use it for that, but it suffices to verify that someone is alive and healthy."

Nod.

"In order to make the current lighting scheme scalable to arbitrary world sizes, I think the sun and moon would have to be... less like light sources and more like optical illusions, looking a certain way depending on how far north or south the viewer is. "

May writes that down. "Is the sun responsible for temperature or is that handled separately? Tides?"

"The sun affects temperature somewhat, but a lot of the details of climate variation are handled by the, ah, grass hairstylist phenomenon. Tides are not directly affected by the moon but are on a related schedule."

Nod. Notetaking. "Is there any time stuff apart from the weirdness relative to Earth?"

She shakes her head. "No. All the parts of the world have the same flow of time relative to one another. A day is twenty-four hours and there are three hundred and sixty of them in a year."

"Is that a fundamental law or a convenient convention."

"The lengths of the day cycle and seasonal cycle are both fundamental law. I do like having seasons. In some respects it would be convenient for timekeeping if every day had twelve hours of sun and twelve hours of night, instead of the proportions varying with the season, but the variance is useful for climate purposes."

"No, I just mean that if you wanted to get back the void-eaten people and they are in fact thoroughly eaten the easiest way conceptually would be something in the neighborhood of informational time travel."

"...ah. Yes, I see what you mean. No, I don't know of any way to accomplish that."

"Damn."

"Yes."

"You could maybe do something with backwards extrapolation if the place is deterministic enough and computational load isn't a factor."

She shakes her head. "I don't think it's deterministic enough."

"Hrm."