He can't make liquids or gasses, but he can make a solid that's invisible in air, yeah. He'd have to touch it to craft it out of that state again, and he might be able to craft something smaller by touch than by sight but there's still a perceptual limitation, he's pretty sure he couldn't craft something down to the size of a grain of sand no matter how he did it. With enough practice, maybe.

They think about this for a minute.

"Can you make a box that looks like air, and if you put a thing inside you can't see the thing?" the woman questions.

No, making something invisible in air is just making it transparent to visible light - he can do infrared and ultraviolet and things too, but they don't usually matter for this - and then adjusting how it bends light to match how air does. He can do water, too, but that's a different degree of light bending.

He takes a little bit of crafting material off of his chair, expands it into a cube a few inches tall, adjusts all but a grey sliver on the bottom and top to be invisible in air so that it looks like the top is floating, and gives it to them to look at while he makes another one like water. (He'd want to adjust it in a bowl of water to make it perfect, he'll mention, but it's pretty close as is.)

The three of them pass the cube around, holding it up to different light sources and feeling out its contours.

The woman with the tablet bites her lip, and spends a moment conferring with her partner about different glyphs.

"Light bends in clear things because it has a speed," she settles on. "And when it hits at an angle, one side of the light gets slowed down first, so it bends. Light goes slower in water than in air; light goes faster in a box with no air in it. Can you make a box that makes light go through it faster than that? It would look like light bends the other way from how water bends it, by a lot."

After all — ansibles are already possibly faster-than-light-in-a-vacuum. Although they haven't gotten the chance to actually measure that yet, and more likely they just rely on some form of signalling the Crafters haven't detected.

She's sort of getting a lower and lower opinion of their science the longer she talks to Traveler, but that's not really fair. Science is a communal endeavor, and even though crafting should make it a lot easier to do basic experiments, they don't have the advantages of being able to work together on large projects. Also, if she's continuously updating in a direction, she should just jump to her final conclusion and skip the wait.

He's never heard of that; it could be that it just hasn't been discovered, though, or wasn't recognized as interesting when it was.

She considers telling him that this one should be safe to test — but she doesn't actually know that. Maybe the universe would enclose the object in an event horizon, to prevent detectable FTL signalling. Maybe it just can't be done with crafting. Either way, he's said that he doesn't want to experiment, and any tests should really be done in a lab.

"I will have more questions about crafting, but I want to think about what you've told us," she tells him, and then relays a different question:

"What are you looking forward to seeing in the city?"

He's really curious about how cities work! Crafters don't get together in nearly that big of groups; having six dozen people at a public meeting place at once only happens when there's a big event on. He's wondering what the locals are doing differently to be able to handle groups so big.

They flip through the dictionary in search of relevant vocabulary, and come up bereft.

"There are inventions that are (ways to act in a group and knowledge of how that works) instead of physical objects," they eventually settle on. "We think the first people had to work together to avoid dying, which forced them to invent the first ways of being in a group. But there are lots of advantages to being in a group, so once the first people were in groups, they continued to invent and learn and grow, until the world was full of people who were suited to being in groups and all the children learned how to be in a group when they were little."

Their grammar gets a little shaky as they try to collectively compose a complex sentence.

"None of us are (learning about how to be in groups) people — we are tiny-lightning-machine workers and a healthy-teeth worker — but the simple version of the best (learning about how to be in groups) knowledge that everybody knows is: design your groups and cities so that when everybody does what is individually best for them, this is mostly what is best for the whole group and stable over time. That isn't always possible, but for lots of important things it is. Not everybody can live in a city designed like that, so people who can't go and live outside the city."

"Would an example be nice?"

It would, if they have one in mind, but he's going to go see the city tomorrow, too.

Well, they're still not social science people, but there's an obvious example to use. It requires a long explanation to do it justice, though, so they spend a while composing a long sequence of sentences.

"Well, when people first started living in cities, one thing that became a problem was fires. Anybody's house can catch fire, but when houses are close together, fires can spread. Any fire in the city risks a lot of people's houses. Trying to make your own house not burn down is hard — you can make it out of things that don't burn as well, and design it so if it does burn people don't get trapped. But there is a limit. Things still burn."

"But fires are easy to put out if you get to them when they're small. So eventually we made a special group that always has at least a few people watching the whole city. When a fire starts, they put it out. Since people know they can rely on them, we worry less about fires. And in order to make sure there are people willing to do fire-putting-out, everyone in the city gives them a tiny amount of stuff each year. Giving a tiny amount of stuff is much better than your house burning down, so the system is better for people living in the city. Getting a tiny amount of stuff from everyone adds up to a lot, so it's better for the people who put out fires, since they can have nice things."

"But there is more: when everyone worried about fires, houses couldn't be too big or too close together. It was better for each person if their house was away from the others. So the city was more spread out, and harder to walk. It was a little harder to trade for things, and less convenient to go places. Once there was a system for dealing with fires, the city got denser, and living there got a little nicer."

"There are lots of things like that. If you can figure out how to make it so that a system is better for each person to have than not have, it is stable. And frequently those systems have other effects that are good. Add enough of them together, and living in a city becomes very convenient."

"There are systems that are stable but make things worse, too. Our living-in-groups people learn about this and try to make sure that we end up with good systems and not bad systems."

That makes sense, yeah, for people who can't craft and need to be around other people to get what they need. It's very different when everyone can craft, of course, and the territoriality instinct is a big factor too, so it's not intuitive to him. He was actually just in a big forest fire - that's part of how he got here - and it was kind of awful but not because his house might have burned, and having other Crafters around wouldn't've made a difference to how bad it was.

The woman with the tablet gets a thoughtful look.

"Why was a fire a problem for you at all?" she asks. "Couldn't you just make your house stay cold, and stay inside until it burned out?"

He can do that, yep. The actual problem was the smoke and bad air; he can filter some of it out, but it's dangerous to rely on that for a really bad fire, if the bad air builds up too badly it can kill you before you realize what the problem is. He converted his house to an airship and tried to hover over it instead, but when he came back down he was here.

Oh, and he can't craft gasses or liquids, so he couldn't just make more air. That makes sense.

"We have machines that filter out bad air. If you want that, you should ask," she advises.

"Can you make a very strong box and put lots of good air in it with a pump?"

It'd be nice to have a machine like that, yeah. He's not aware of a pump design that'd let him store enough air to wait out a forest fire in a reasonable volume of space, but it's been a while since he checked in with the space travel people, maybe they've figured something out.

All three of them perk up at the mention of space travel.

"What space travel have Crafters done?" she asks. "We have put some machines up, and sent some people, but establishing a territory in space is really hard."

They haven't done that much yet, they haven't figured out how to solve the thing where you run out of air if you go too high. They have figured out that if you set an unmanned ship up to blow stored air out behind it you can get it up past the point where there's enough air to push a ship with a propeller, and sometimes if you do that it doesn't come back down at all which is pretty interesting but hard to do much with yet. Last he heard they were working on better sensors to figure out what's going on past the top of the air, plus the air pump thing and better steering for their ships.

... right. Well.

"You should send the space explorers a book on orbital ballistics," the woman with the tablet recommends. "It is the (kind of learning) about putting things up high so that they don't come back down. It also includes knowing where they're going to end up, and also how to fall back down safely. We have figured that part out; the part that is hard for us is that it is hard to lift things that high, and people need so much stuff to live. Maybe with crafting we could send people with less stuff, and it would be easier to lift."

Also, she's pretty sure that self-contained perpetual motion machines and a modern understanding of materials science should let you make a carbon scrubber with an indefinite lifetime. She doesn't know how exactly — she's a nuclear engineer, not a chemist — but it seems pretty likely. She's going to have to see whether the future-infrastructure-prediction people have figured that out yet, or whether she can make a bit of extra money off of the prediction markets for the first lunar colony.

Her partner relays a question to her, and it's one that somebody probably ought to have asked before this. Maybe the Emergency Services people did, and they just didn't publicize it.

"Do you know how many Crafters there are on your planet?"

Not offhand but he can estimate it, hold on while he does some math...

Seven or eight million, he thinks.

Ah, okay. That really re-contextualizes their civilizations' relative levels of engineering knowledge.

They briefly debate whether to give him þereminia's corresponding population statistics; one of them worries that it will cause a diplomatic incident, but the others point out that he could easily infer as much once he gets a look at the city.

"There are about a billion of us," she writes to him. "A little less than a million people live in Largest City. I'm sorry, I was comparing your knowledge of things to ours and judging you, because you can do so much with Crafting, but it seems like your people know less. I shouldn't have judged, but it is hard not to, since knowing these things is so common on our world. But when we had only eight million people, we had not invented machines other than wheels and pumps, and we knew much less. That was many thousands of years ago. We only managed to put things up high enough to not come back down about a gross years ago. Probably in another gross years, Crafters will know a lot too, even if you didn't find us and give us the chance to help teach you things."

Wow, yeah, that's a lot of people. He thinks they have more incentive to mess around with things to see what they can discover, too; Crafting makes a lot of things easy, so there's rarely a need to invent things, and people just do it for fun or because their soul calls them to.

They all nod.

"That makes sense," she agrees. "And even if we didn't have a need, we'd still have twelve times more people who have souls for it."

"If people don't need as many things because of Crafting, what do you do with your time?" she asks, relaying a question from the third woman. "I know you have books; do you have lots of games? Amazing artworks?"

Lots of people do artwork, or challenges, or study things happening in their territories, or invent new machines, or breed or train animals, or things like that. Group things like games or singing are less popular but not uncommon, and so are megaprojects. Designing complicated pebbleclinkers has gotten popular within his lifetime; they're too difficult for most people to understand but the ones who can seem to really like them.

"What do you specifically do?"

He travels and writes about things in different places - how people live, or what the animals are like, or different megaprojects sometimes, or whatever he finds that's interesting. He'll be writing about them next obviously but he'd been working on one about how lake ecosystems differ from place to place. He also raised three kids up to toddlerhood, though he wound up passing them off to their sires when they were old enough to make the switch; he liked it, but he doesn't do well with staying in one place for that long.