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"I'm not sure we have made much progress on the problem, teacher," said Thorn of Dawn. "The circular dilemma still remains."
Stone Beneath silently agreed. They have come to the most important part of their mission. To create a communication device that will let the native intelligent species of this planet to get in touch with them was the easy part. Nor was it difficult to find a cave where this device could be sealed off for the next few millennia. The natives, currently in the Neolithic, won't be able to find it until they could understand interstellar flight. But to plant the clues that would cause them to look for this device here when the time comes - that was the real challenge.
Thorn of Dawn was thinking out loud. "If we carve an image of our communication device into the rock outside the cave, the future generations of natives will assume that it was their ancestors who made it. They might be puzzled as to what it means, but they will unfailingly conclude that it's some religious symbol. Even if the eccentrics among them might suggest that the image was left by an otherworldly civilization, the scientists will reject it out of hand. Because that would mean to assume the conclusion."
Stone Beneath, the mentor of Thorn of Dawn, braided and unbraided zir tentacles in pensive acknowledgement. Zie had been on enough missions to observe the development of nascent civilizations that zie knew there were ways around this dilemma. But zie wanted to see what the student would come up with.
Thorn of Dawn splayed out against the roughness and coolness of the rock and lazily wove a tentacle through a group of stalagmites. Zie said: "Perhaps the picture should be mutable. It could start out as a mythological scene. It might remain so for many eras. But when the natives start getting close to being able to understand us, the picture would start changing. It could morph into something that shows - well - some kind of concept that's a prerequisite for understanding our technology."
Zie fell silent. "I'm listening," said Stone Beneath.
"The image would not have to be carved out of the rock," said Thorn of Dawn. "I mean, we will still need to carve out the basic outline, but we could layer smart dust on top of it to make the details. And the details will be mutable. The dust will eventually reconfigure it into a different picture."
Stone Beneath privately admitted it was An Idea, but said nothing, waiting for more details.
"But what kind of image could start out as a mythical or religious scene, and morph into an advanced spaceflight concept?" said Thorn of Dawn and laughed. Zie spread out zir tentacles 180 degrees, surveying the cave floor. The dry, limestone floor was strewn with a thin layer of smart dust in which the two of them had drawn designs of possible images. Their initial brainstorming had generated phantasmagorical creatures that could definitely be considered mythical by the species that populated this planet. It's that second part that was difficult to make happen.
"Oh, how about this?" Thorn of Dawn pointed out a drawing of two somewhat abstract animal heads in profile, facing each other. Both had their tongues out. The tongues were long, tapering towards the end, and they touched, forming a parabola.
"When the natives discover general relativity, the smart dust will start morphing the image," Thorn of Dawn explained. "The beast heads will shrink and disappear. The tongues will be covered with a grid, representing spacetime. The grid will become more and more distorted towards the end of each tongue. So they will look like two halves of a wormhole. Their touching point will become a passage. And the scientists of this planet will understand what this means."
"You know, this might succeed," said Stone Beneath.
They sat in silence for a while, contemplating it.
"This could also fail in many ways," said Thorn of Dawn. "For example. How would the smart dust know that the indigenous population has advanced enough that it's time to start revealing the real image?"
"Ah, that's the easiest part," said Stone Beneath. "That's why smart dust is dust! It gets everywhere, right? Over the millennia, the natives will come and rub the image, and the dust will come off onto their hands and travel home with them. It will then monitor them and their progeny for thousands of years. Dust is indestructible, you know."
"Why are you so sure they will be drawn to rub the image? As opposed to holding it a taboo?"
"Thorn, think about what that image represents," said Stone Beneath and snickered.
"Oh!" said Thorn of Dawn with an embarrassed chuckle. "The touching tongues. It has erotic and fertility significance, right?"
"That's right," said zir mentor. "They'll rub the point where the tongues touch for their fertility rituals. I can already see it! All civilizations have rites like that."
"And one day that point will represent the wormhole passage," said Thorn of Dawn dreamily. "A point that they will literally touch when they travel to us."