He can! And soundless things aren't hard either, though very dense things take enough material to not really count as small at that size.

This gets an alarmed exclamation from one of them.

"How dense can you make things?" the woman with the tablet writes, and then on being prompted adds:

"If you don't know, don't find out now. I don't think a most-dense thing so small is dangerous, but there are most-dense things so big they eat light out between the stars, and those are dangerous."

It's not really practical to make things too dense, anyway. But he doesn't know there to be an upper bound, exactly, just that you can't make mass from nothing. If he wanted to collapse his house down to the size of an acorn he could do it.

"If you collapse the whole planet down to the size of a pin head, it becomes a most-dense thing," the woman explains. "The limit is very high, but there is a limit. What is the smallest thing you can craft?"

"Also — if you can make a thing that glows purple enough and does not get lighter, and magnets, you can make mass. Don't try it, though, because things that glow purple enough are killing you."

Yeah, every Crafter kid gets warned not to mess around with giving things random traits, there's definitely some deadly ones out there. People do it anyway, that's how things like ansibles got discovered, but it's got a mortality rate and he's not interested in taking that kind of risk at all.

He's not sure the precise limit for the smallest thing he can make, but it gets harder as it gets harder to see what he's doing. And crafting the whole planet down like that would be wildly impractical even if it wasn't an obviously bad idea for lots of reasons.

"Do you have to see a thing to Craft it? What if you make crafting material that is see-through and just like the air?"

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.