A human space civilization once established a colony on a planet. Now, several centuries later, the people on this planet are only at a medieval level of technology. Whether by accident or on purpose, I have yet to consider. The former beliefs, form of government, and borders of the space civilization may have changed in the meantime.

About once a year a ship of the space civilization lands on the planet unnoticed. They disguise themselves as local traders and trade with the inhabitants for a good price. The traders pay more than one would normally get. As a result, they are considered friendly and generous and are welcomed. They do this for at least a few decades, maybe longer.

The traders do not help the locals to advance technologically themselves. They do not completely hide the fact that they are technologically superior, but they do not teach the locals any technological advances. For example, they might trade slightly superior non-electronic tools, but would never use or trade electronics or projectile weapons.

The story takes place from the point of view of the medieval civilization. There, the traders always travel to the same city. It is possible that there are similar cities further away that are also visited by traders.

My question is now, why would they trade at all? What would be their long-term goal?

If you can mine or breed the resource, why not industrialize the mining or breeding? You could give the locals (your own species in the end) better knowledge and tools or use your own workers. The same goes for the manufacturing of goods, which could be industrialized. Living beings can additionally be bred on other planets. Knowledge can also be copied and used elsewhere.

My ideas so far:

  • This passage could work well as a later plot twist. I guess usually the first thought would be the resource is valuable and therefore traded. The traders might have a hidden agenda that is discovered only much later in the story.
  • It's not about the goods they bought, but the ones they sold that they leave behind.
  • Political reasons between two space powers (trade agreements, espionage, conservation).
  • The bought/sold goods are illegal or the deposit must be kept secret.
  • They are not concerned with trade, but with low-key interaction with the local population for some reason.
  • By extracting or producing the resource, another side process takes place. The actual goal is to promote that process. But then why not industrialize the process directly?

Unfortunately, I'm having a hard time finding concrete examples for the different approaches right now, or further reasons for the behavior of the traders.

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    $\begingroup$ Why does anyone trade with anyone? because they have something they want and can't (or just can't be bothered to) make (extract or harvest, etc) themselves .. what those items are, they why's, character decisions etc leading to that is all up to you as the author, so this is probably going to attract some VTC for being opinion and story based. $\endgroup$
    – Pelinore
    Commented Feb 20, 2023 at 2:16
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    $\begingroup$ ... Or maybe the goods they buy get a high price exactly because they are exotic and hand-made? Like Cuban cigars, Oriental carpets, French wine, washi paper (known as "Japan paper" in the trade)? Scottish whisky aged in hand-made oak barrels? Why is Russia the one and only exporter of legal ivory in the world? $\endgroup$
    – AlexP
    Commented Feb 20, 2023 at 2:56
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    $\begingroup$ Not sure if the questions are close enough to be considered a duplicate but you may fish for more ideas here: worldbuilding.stackexchange.com/questions/113292/… $\endgroup$
    – JamieB
    Commented Feb 20, 2023 at 4:07
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    $\begingroup$ Adding to the pond of idea-fish: What could a technologically lesser civilization sell to a more technologically advanced one? $\endgroup$
    – Stef
    Commented Feb 20, 2023 at 17:52
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    $\begingroup$ Does this answer your question? What could a technologically lesser civilization sell to a more technologically advanced one? $\endgroup$
    – user458
    Commented Feb 20, 2023 at 18:11

26 Answers 26



There would be intense scientific interest in watching a medieval society evolve. Human knowledge and understanding of such societies being limited to the lens of history and various archeological discoveries etc. But just as is the case with the few remaining archaic cultures on Earth history has also shown that contact and interaction between advanced and primitive cultures can have devastating consequences for the latter. So while there is great interest in studying the peoples of the planet there is also a lot of public pressure on the teams not to screw things up. Your advanced culture does NOT want to repeat the mistakes of the past.

On top of that there is a purely selfish motive i.e. the near certainly that introducing themselves as they really are to the local cultures would ruin their one and possibly only chance to observe a 'pristine' medieval culture developing.

So they devise a plan. They use these once a year low key 'trading missions' to interact and observe. And it doesn't hurt that the hand made goods the 'traders' receive in exchange for their own goods would be worth a lot of money to Universities for study purposes and to collectors.

For that matter the covert recordings they make during their trips would not only be of major scientific interest but once edited their sale to media outlets for public consumption would probably pay the cost of mounting the expedition in the first place! That being so your 'merchants' are all trained anthropologists, linguists, historians and 'minders' appointed by the Government to make sure none of the 'rules' are broken.

Finally as someone else suggested the yearly visits also allow the teams to recover data from hidden recording devices located at major sites of interest. All of which have been busily compiling data since the last teams visited.

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    $\begingroup$ "'minders' appointed by the Government to make sure none of the 'rules' are broken." Commissar is such an ugly word :) $\endgroup$ Commented Feb 20, 2023 at 13:12
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    $\begingroup$ @The Daleks Yes, even as I typed 'minders' I knew it might come across as to heavy handed. 'Guards' might have been a better descriptor but then I console myself with the fact that at least I didn't use the term 'lawyers'!! (God forbid.) Imagine a bunch of them sucking the life out of the entire expedition. $\endgroup$
    – Mon
    Commented Feb 20, 2023 at 14:26
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    $\begingroup$ Even if the goods received aren't valuable, an annual trading event like that would also be a fantastic time to hear all of the stories of the goings on in the medieval society. We have comparatively few direct records of our own past, the chance to go back and get context of what many different people and groups thought about their current events would be super valuable for spacefaring anthropologists. $\endgroup$ Commented Feb 20, 2023 at 16:06
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    $\begingroup$ @Mon, try "safety officers". And that sort of thing isn't just government-mandated. Adventure cruises to the Arctic and Antarctic typically have local guides and/or ship personnel to escort the tourists around. Officially, to provide information and to act as protection in regard to local dangers. In reality, to stop freaking idiots from getting themselves killed or screwing up things so badly the company will be in litigation and dealing with fines for years. $\endgroup$ Commented Feb 20, 2023 at 18:25
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    $\begingroup$ @Dast. I love reading SF, I have ever since I was a child, Now I have to look that book up. :) $\endgroup$
    – Mon
    Commented Feb 22, 2023 at 8:05

Comparative advantage would be stretched to the breaking point ...

The concept of comparative advantage explains why trade makes sense even if the more advanced civilizazion is better at everything.

  • Say the advanced civilization spends 1 person-hour to produce a leather belt, and 2 person-hours for a cup of porcelain.
  • The primitive civilization, being less efficient, spends 10 person-hours for a leather belt, and 100 person-hours for a cup of porcelain (getting the temperatures and glazes right is quite difficult for them).

So if a leader of the primitive culture insists on fine pottery on the table, he or she could go to a local merchant and pay the value of 10 leather belts, or to the alien trader and pay the value of 2 leather belts. And if a consumer in the advanced society wants a leather belt, he or she could go to a local merchant and pay the value of 0.5 cups of porcelain, or to the alien trader and pay the value of 0.1 cups of porcelain.

Both sides benefit when each do what they do better. For leather belts, the ratio is 1-10. For fine pottery, it is 1-50. Both sides benefit if the primitives do all the leather belts, and the advanced people to all the fine pottery.

Now get the traders in there. Imagine a market on the primitive world where a leather belt trades for 1 copper coin and a fancy cup trades for 10 copper coins. And a market on the advanced world, where a leather belt trades for 1 digital credit unit while a porcelain cup is 2 digital credit units.

The traders borrow 100 digital credit units on the advanced world. They buy 50 cups and travel to the primitive world. There they sell them for 500 copper coins and buy 500 belts. (The ratio between leather and porcelain on the primitive world is 1-10. 50 cups buy 500 belts.) Now they travel home and sell those belts for 500 digital credit units. 100 of those go to repay the principal of the loan. 400 digital credit units are left for interest, transportation, wages, and profits.

The traders might also ask for less than 10 copper coins for each cup, and offer to pay more than 1 copper coin for each belt, to assure that they can fill their cargo holds without waiting. The reverse on the advanced world. Repeat a few times, and potters on the primitive world become tanners and leatherworkers, tanners and leatherworkers on the advanced world become potters. A mutually profitable trade develops.

Note that this does not require that the primitives are better than the advanced workers at any one thing. It only requires that the ratios in which they are worse differs from one sector of the economy to the next, and that some of their products remain useful in the advanced economy.

  • 1
    $\begingroup$ Also, if experience is any guide prices are eventually going to adjust until there is no more economic profit to be made by this trading setup. :'-( $\endgroup$
    – Michael
    Commented Feb 21, 2023 at 18:18
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    $\begingroup$ @Michael, not quite. The prices will adjust until there is an stable distribution of the benefits between the producers on either side and the merchants. In the end, the combined economic zone of the two worlds produces more total goods by specialization, and likely everybody gets some portion of the added wealth. Some more, some less. $\endgroup$
    – o.m.
    Commented Feb 21, 2023 at 21:20
  • $\begingroup$ IIRC, doesn't zero economic profit still mean that a trader in this business would make a profit roughly equivalent to what they would have made investing in the galactic stock market through an index fund? $\endgroup$ Commented Feb 21, 2023 at 23:20
  • $\begingroup$ @DanielSchepler yes $\endgroup$
    – Michael
    Commented Feb 22, 2023 at 3:03

They're trading for things that are still valued in an advanced society.

Obvious things are fur, spices, foods, wines, timber, wool, gemstones like amber or opals, pearls, crafted objects such as art, musical instruments or even furniture and pottery.

Sure, most of these could be easily made by robots in factory, but hand crafted items are worth a premium.

Now the traders are swapping cheap mass produced stuff they buy for next to nothing for items they can resell for a premium.

A robot manufactured stainless steel knife would be worth its weight in gold compared to the hand forged knife made from impure steel. Ditto for cookware.

A extremely durable cold weather, water proof jacket is far superior to furs or woolen coats.

Pain killers and other medicines would seem miraculous to a primitive society, or if the traders are assholes, they could sell illegal drugs.

Both parties would feel they're getting the best of the deal and trade happily

Like all traders, their long term goal is to get rich. I would imagine the traders found some backwater civilization and are keeping it a secret in their own society so they can cash in. They sell their stuff quietly and refuse to say where it comes from. Quite possibly, contacting primitive civilizations is illegal so they keep quiet about everything on both sides.

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    $\begingroup$ +1 Rare metals and food were on the top of my list. $\endgroup$
    – JBH
    Commented Feb 20, 2023 at 5:08
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    $\begingroup$ @JBH Rare metals will never be the answer for a space faring civilization because asteroid mining would be cheaper and more effective. Timber would be worth more. $\endgroup$
    – Thorne
    Commented Feb 20, 2023 at 5:24
  • $\begingroup$ @Thorne we are a (primitive) spacefaring society, and nonetheless precious metals are a major source of trade between economically developed and economically undeveloped groups. Maybe these issues don't end up so readily solved by mining asteroids? $\endgroup$
    – James_pic
    Commented Feb 20, 2023 at 10:15
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    $\begingroup$ Timber? It's pretty cheap, and shipping it will add a lot to the cost. Spices on the other hand do make sense; those can easily be a million times more expensive. Spacefaring or not, the economics or long-distance trade are fairly standard: prices can vary geographically, which creates a trade opportunity, but this is capped by shipping costs which are usually weight- or volume-dependent. $\endgroup$
    – MSalters
    Commented Feb 20, 2023 at 10:21
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    $\begingroup$ @James_pic We trade metals between countries because shipping costs are cheap. We don't set up asteroid mines because shipping the metals to Earth would be hugely expensive. I'm not sure what technology would ever make moving metals from an alien world cheap without making mining our own star system even cheaper. $\endgroup$ Commented Feb 20, 2023 at 10:56

They are playing the long game

They have a several century long plan to groom this civilization from medieval to interstellar age in order to become a powerful ally to them. The beneficial trading is part of that plan to inconspicuously boost the prosperity of a specific culture which will in turn affect the history of the planet in the way they intend.

How can they pull that off? They are just that good historians. They studied the history of thousands of pre-space worlds and created sophisticated computer simulations to model historic developments with astounding accuracy. So they can predict how the future of a civilization can be shaped through small interventions like that and can use it to shape their future in any way they want.

For example, the aliens predict that in a couple generations a certain trade-good will become very important. Whatever nation can produce it most efficiently will become the dominant superpower in the next couple centuries and their sociopolitical philosophy will shape that of the whole planet. But right now this good seems rather unimportant. So the aliens start to buy that good from their preferred nation now, so they get a head-start in building the know-how and infrastructure for producing that stuff.

Why not just hand those nations anything they are going to need directly? Because then they would become complacent and reliant on the aid of the aliens. In the long term that aid would make them weaker, not stronger.

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    $\begingroup$ The advanced civilization might even have a history of more forcefully advancing less advanced planets, only for it to spectacularly backfire when the new ally either collapsed as soon as they had to stand on their own... or became a rival. $\endgroup$
    – Zwuwdz
    Commented Feb 20, 2023 at 21:38
  • $\begingroup$ The outsiders could also preferentially trade with political / social factions they hope to introduce to interstellar society a few decades / centuries hence: Their "temporarily backwater" allies will be in a more advantageous local economic position than their competitors when the interstellar federation admits the planet... and thus the outsiders gain an ally who already dominates the local planet. $\endgroup$ Commented Feb 21, 2023 at 22:08

Sentimental Value

The more primitive civilization can't produce anything better than the spacefaring one can; that's why they want to trade in the first place. However, people do not always buy things based solely on quality. There are also many soft, sentimental factors that go into things. Here are a few IRL examples that I think would hold true in your universe as well:

  • "Organic" produce. Your advanced civilization's hydroponic skyscraper farms produce all the food it needs, at low cost and near-perfect quality. However, some people are under the misconception that the synthetic soils and pesticides used in these factory-farms are harmful. Entrepreneurs encourage this delusion, touting how the primitive culture's produce is produced NaturallyTM; their apples may be wormy and partially rotten, but at least they won't give you cancer and make your kids autistic. The same thing goes for art; it's handmade (and therefore unique), not "mass-produced [expurgated]."

  • Beautiful art. Your ultra-modern space civilization is just that: ultra-modern. As a result, they've taken IRL modern culture's deadly relativism up to eleven. Their relativism poisons their art, such that most of it is either derivative[1], variously nihilistic, repetitive, absurd, or just plain ugly.[2] Your primitive, decidedly not modern culture has yet to go over the Line of Despair, so it continues to produce beautiful art and music.

  • Cultural crafts. Regardless of the quality of their own art, people from the space-civilization value the primitive civilization's jewelry and ornamental doodads. Note that they aren't valuing it because of any especially good craftsmanship, which would fall under #2; rather, they like it because it's unusual and "ethnic".

TL;DR: The spacefaring civilization values the primitive civilization's stuff because it is primitive.

[1] To clarify: by "derivative" art, I am referring to the sort of mass-produced stuff you see and hear in waiting rooms.

[2] Explaining why relativism and existentialism inevitably cause horrible art would roughly double the length of this answer. I'll explain in the comments if asked; otherwise, suffice it to say that it does.

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    $\begingroup$ I think the explanation for your second bullet point is very strange and I would replace it with just that people might want the art of another culture. IMO the answer is made worse by your dislike for postmodernism which isn't really that relevant $\endgroup$ Commented Feb 20, 2023 at 4:21
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    $\begingroup$ @BugCatcherNakata - I think it's an interesting point, made even better by a dislike of Postmodernism. $\endgroup$ Commented Feb 20, 2023 at 4:35
  • $\begingroup$ @BugCatcherNakata To clarify my point: modernism and postmodernism aren't incapable of creating good art ("Rosencrantz and Guildenstern are Dead", La Peste, and The Hitchiker's Guide to the Galaxy come to mind); it's just that a lot of it tends to be very chaotic and not very beautiful. In the words of a free jazz artist I know, "my goal is to show that music doesn't need to be pretty." In the end, it comes down to a worldview difference. $\endgroup$ Commented Feb 20, 2023 at 12:48
  • $\begingroup$ @BugCatcherNakata (cont.) On a fundamental level, a piece of art, sculpture, music, etc. is a representation of its creator's worldview. As a result, people with different worldviews tend to find different art beautiful. People who think that there is some sort of order to the universe (such as the predominantly Judeo-Christian or Christian-influenced artists prior to the 1900s) find beauty in the grand structure and patterns of the world, so you get Michelangelo, Rembrandt, and Murillo. $\endgroup$ Commented Feb 20, 2023 at 12:56
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    $\begingroup$ Beyond critiques of current art trends, A) an alien civilization will have radically different cultural ideas, so the art they produce will almost certainly be very different from anything already produced or seen in the purchasing culture, and B) "authentic art" that comes from a hard-to-source location will naturally command respect and be a good way to demonstrate one's wealth, access to resources, and worldliness, which will probably always be desirable attributes. $\endgroup$
    – Aos Sidhe
    Commented Feb 22, 2023 at 15:51

The easiest answer is that they have a need for something that is found only on that world and that the quantities they need are easily provided for by the manual extraction on the primitive world.

Consider a particular HandWavium Mineral - the locals using Medieval extraction methods are doing 20 KG of Mineral a day, that's about 7 Tonnes a year (ish) - this amount more than covers the amount needed by the Traders and by the local population.

The price is good, the location is relatively convenient and the effort to find the Mineral elsewhere is more time and effort than continuing with the trading arrangement.

If it ain't broke, don't fix it.

Then you can add in some other factors - like it gives historians a living view into primitive societies, pure curiosity on the part of the advanced civilization and a small sense of a duty of care to keep tabs on them.

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    $\begingroup$ The locals may also not have a use for the HandWavium (given their technology) or don't recognise its value. So they assume it's just waste from them mining other minerals. So the locals think they are getting a bargain when these traders are paying them for their waste. $\endgroup$
    – Rob
    Commented Feb 21, 2023 at 11:06
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    $\begingroup$ I think a plausible Handwavium answer could be biological product that can only be produced on the planet, perhaps requiring expertise, perhaps that can't be done in captivity. Could be any number of things like a desirable food product, a luxury good equivalent to ivory, a unique medicine, a living clothing fiber that repairs itself, who knows! $\endgroup$ Commented Feb 21, 2023 at 18:27


There seems to be nothing that would be of interest by the space faring civilisation. Any rare resource is so unlikely to be only available on that planet that it's nearly impossible. It looks like there is no real reason to go there.

In the real world we go to many places regardless. We are going there to look at the locals, the customs and culture. How they live. Take some souvenirs. Just being able to say they were there at that backwards speck of a planet.

To perserve this medieval planet, the tourism sector is only allowed to go to this one place. Otherwise it could lead to many negative effects of the tourism, like mass hysteria or rapid development. The souvenirs are traded for better versions, because they either do not care enough to make perfect replicas, or it's done illegally. The tourists get their souvenir to display at home for whatever reason, while the local gets a better tool and thus is willing to trade.

This sidesteps a lot of problems and is very realistic. The tourism sector doesn't have much of a goal, long or short term. Tourism does weird things and often is full with people skirting rules and regulations. Their visit doesn't need to make sense any more, they just want to visit, interact with the locals and then go back to their own lives. It also makes the visitors something different from the locals in many ways.


Redefining Post Scarcity

It might also be that while the colony is post scarcity, individuals aren't. They get a fixed amount of income from their duties that they can spend, and they use trading as a means of getting more.

For example, in Star Trek Voyager, the crew can literally replicate anything that they could ever want, but not as much as they might want, due to power rationing. The crew would replicate things and then trade them up on local planets.

They might use their replicator rations to make a bottle of expensive alcohol, and then trade it on a planet for a much larger quantity of cheaper alcohol.

Your colonist might do something similar. They could make complicated machinery in small quantities, but what they might want was something much simpler but much larger that their production allocation didn't stretch to.

Utilitarian Future

It might also be that the colony's automated production facilities were locked down to strictly necessary items. They could produce an unlimited amount of practical machinery, but not a purely decorative item. So they trade for them instead.

Romulan Brandy

In Star Trek the Next Generation, Enterprise crew members could get near limitless amounts of food and drink from the replicators, but it's commented on several times that what they get isn't real food. It's a nutritionally balanced substitute that's made to look and taste like real food.

It's also commented on several times in DS9.

Synthahol won't give you a hangover, replicated chocolate cake won't make you put on weight, and it never tastes exactly the same as the real thing. Which is why the senior officers always seem to have a bottle of contraband alcohol somewhere in their quarters.

Your colonists might be in a similar situation. They get supplied with healthy food, but they might want moonshine. So they trade for it.

  • $\begingroup$ This was one thing I like about the Borg... they might have been hell bent on assimilating everybody, but they each drew their power from their regeneration alcove just like the rest did, none of this contraband "tastes better" organic power sources. $\endgroup$
    – Michael
    Commented Feb 21, 2023 at 18:21

Goods laundering

On our technologically advanced planet, everything can be tracked. When a customer buys a good on Earth, they can scan it and the scanner will tell them which factory the goods come from. When you buy food, the scanner will tell you which crop or farm the food comes from.

This tracking is done via a combination of several technologies. For instance, all cows on Earth wear microchips to identify them. But the most important technology is being able to recognise the "signature" on an object. When you scan a cereal or vegetable, the scanner gets a lot of information about the way it was grown, including its DNA as well as the chemical composition of the soil where it was grown. All this information is easy to recoup to find the exact origin of this cereal or vegetable. Likewise, when you scan an item that was produced in a factory, you can identify the factory in which it was assembled, as well as the origin of the raw materials. This process is pretty similar to the way the scientific police can identify the gun that shot a bullet, except much more advanced and applicable to everything, not just bullets.

Because of all this tracking, if you have goods of dubious origin, it's hard to sell them or exchange them against legitimate goods. You need to "launder" these goods, first.

Goods of dubious origin may include counterfeit goods, stolen goods, as well as anything produced by a factory that doesn't uphold legal standards, or employs illegal workers, etc. In addition to these illegal goods, there are also "undesirable goods" that are legal but are hard to sell because of all the information provided by the tracking. For instance, if a worker from your factory joins a neonazi association, all your potential customers are going to know about that when they scan your products - this will have a huge impact on the price at which you can sell your products.

That's where the medieval world enters the scene. On the medieval world, there is no tracking at all; so you can exchange your dubious Earth goods against legitimate medieval goods and the medieval people won't complain. Then you can sell the medieval goods on Earth, and all the Earth scanners will say is "this comes from the medieval planet". No trace of an illegitimate Earth production.


They just pretend that they are trading

Let's be honest here. Apart from information (and medieval culture produce very little of that - mostly literature, and of dubious quality at that) there is little need for trade between same-tech level societies in different star systems. Any material thing can easily be produced in-system, and trade for raw resources is just ridiculous (a solar system contain far more resources than one can imagine) when you count logistic costs (in time and resources). And if FTL travel is very cheap, then the issue doesn't disappear as that would mean producing stuff is even easier locally (because of higher tech level in general).

So what are those "traders" doing then? Either they are studying development of primitive civilisation, or they are tourists. I would bet it would be quite lucrative to run a business where people can go and experience a medieval culture. Since that is on another planet even more so. Or maybe they are just checking their development progress, to see when they develop enough that more direct interference is not so dangerous for their culture.

  • $\begingroup$ The snag here is that this kind of trade happened in real life (and is still happening today) between different countries both with different technology levels and with similar technology levels. For example, Germany and Japan are both at the same level of technology to the US, but the US imports hundreds of thousands of vehicles form both countries every year. While both China and India are at a lower tech level than the US but the US imports billions of different things from both. $\endgroup$ Commented Feb 22, 2023 at 9:38
  • $\begingroup$ @AaarghZombies What makes you think China is at lower tech level than US? There are things that US cannot currently produce that China can... In general, the tech level is the same. Better real world example would be trade with stone-age tribes from Amazon. We don't trade with them, because they literally have nothing to offer. And historically we traded for resources when trading with less technologically developed partners. Civilization capable of traveling between stars there is no need for that, as a solar system has more than enough resources, which are easier to access to boot! $\endgroup$
    – Negdo
    Commented Feb 22, 2023 at 10:58
  • $\begingroup$ I'm not American, and the main reasons that people don't trade with tribes in the Amazon is that they live in difficult to reach areas and they have a tendency to be hostile to outsiders due to the actions of other outsiders. Millions of people go to Indian reservations every year and buy traditional products that they could buy from factories in China. $\endgroup$ Commented Feb 22, 2023 at 13:39


The advanced civilization needs to travel through the primitive civilization's territory and it's cheaper to stop for goods they can produce like food than store more of it from home.


Subjugation by friendly faces / not wanting to needlessly create enemies

Whether it be for anthropology, resources, breeding, or even future enslavement, it is always less costly to build strong friendships over a society to gain access to inside knowledge or connections.

Covert tourism

What it says. These traders are essentially in a living history museum from Professors to the Captain to the random midshipman, They'd all like a chance to experience something thought to be thousands, if not tens of thousands of years in the past in their own history.

When in their house, play by their rules

This one comes from basic interpersonal respect. Despite being able to totally crash the economy with an influx of valuable goods or being able to storm and take everything by force, the Captain and possibly also the crew has decided to respect their economy and resupply little by little, and maybe pick up a few oddities along the way.

Resources lost to time

In blade runner, a small wooden horse was estimated to be enough natural wood to make the main character rich out of his mind. Like how Thorne mentioned in their post human effort could cause a premium due to provenance or even just the rarity of human craft. I'd also pitch in that there would be things like animal and plant genomes lost to time that are still found in this medieval society and so trading for some wheat could have major scientific significance to where the crew can learn about and store the genomic information of wheat that might have been lost to time.

The crew's little paradise

No one, No one else from the crew's society has found this medieval society and the crew intends to keep it that way. Its their little resupply point, their little vacation island, their little scrap of paradise. An escape from their home society. So as much as they want to enjoy it, they'd also like to preserve and protect it.

Having good friends are good, no matter who they are.

Knowing people on this medieval world can help them make friends and therefore guides to help them navigate through to find resources or even just interact with this medieval society. Guides can help them from accidentally making enemies and what not. Despite the power balance from being ultra-modernized versus the medieval world this medieval society could hide the crew from intergalactic enemies if they accidentally tread on someone's toes. Regardless, there's a chance for a real underdog position for the medieval people here.

Normally unattainable resources

Reasons for trade are simple. They have something that you don't and vice versa. This could apply to the crew. They could have found that this world is rich in the resources that they need for their ship and it is mindbogglingly cheap or they desperately need some mineral to refuel or whatever. they need it, and they have things to trade for it. See TheDemonLord's post on this


Huckleberries and Champagne (Or: not all resources can be mass produced)

I would like to offer a reason why the planet could have genuinely unique resources, even to an advanced spacefaring civilization.

First, Champagne and other "Protected Designation of Origin" (PDO) goods are an example of goods which are significantly affected by the local conditions ("terroir"), such as the soil, water, climate, breed of animal, variety of plant, etc. On Earth, we sometimes consider a combination of these factors noteworthy enough to legally protect. Only a certain process in a certain geographic area with certain ingredients can legally be sold as Champagne. For inspiration, here is a list of such Earth goods.

Why would this matter to an advanced civilization? Presumably the planet does not have PDO, but the civilization might - in either the legal sense, or in the sense of valuing the incredibly variety of factors that makes the goods unique.

Why would this resist industrialization?

  1. For practical reasons, there is only so much land where you can do "real Champagne." If you go too far away, the terroir changes. If you terraform the landscape, the terroir changes. If you genetically engineer the organisms ... and so on. It may be wine, but it's not Champagne any more. There's a natural cap on production.
  2. Legal protection reasons, if the local or advanced civ has them. Sure, you can make Champagne-like wine, but it won't have the legal stamp.
  3. Cultural reasons - "If it wasn't grown that way, it's not real Champagne" - people want the real deal, not the mass produced or replicated version. (Even if the imitation version has other advantages!)

Second, not all goods can be farmed. Huckleberries are very picky about their conditions and will not fruit anywhere except where they are found growing in the wild. While it's possible that at some point humans will figure out what makes huckleberries fruit, right now we don't know. We cannot increase the production of huckleberries at all because we can't grow them. It's reasonable that even advanced technology won't make it possible to increase huckleberry production much if at all.

While both examples are food based, non-food goods can also fall under these categories, such as wood, fur, and other natural materials. Anything which has a "non farmable" or "location based farming only" good in its supply chain will also be limited/unique to the planet, and therefore worth trading for.

  • $\begingroup$ PDO products are primarily a trademark issue rather than that of a unique product. The 'terroir' is mostly marketing and enforced by treaty, which the low-tech culture will likely not have the means to enforce off-planet. The 3rd bullet point is quite valid though, as well as the non-farmable goods. $\endgroup$ Commented Feb 21, 2023 at 20:26

Rare materials

While the civilization lacks technology, they have immense numbers and rich resources, and can mine a lot of valuable materials like uranium, osmium, neodymium. They can use this to trade for bits of tech they need.

The kings of this society have a few missile batteries squirreled away, so if any alien attempts to force them to comply they can blast them.

The traders get cheap rare materials, the locals get a few cool items.


Unzip your Genes for Me.

The old civilization is genetically senile. Combine capitalism with fledgling bioengineering tech and you get a society that slowly poisons itself with new flashy genes that are not obvious until they have spread to the whole population.

One thousand years of microplastics, designer babies, foetal vaccination and general genetic hoo-hah has led to a society where everyone is allergic to milk and nuts and gluten. They cannot touch each other without getting a rash. They develop cancers in their teens. They all have chronic social anxiety.

They were advanced enough to cause a problem but not enough to fix it. Now they need new genetic material to paste over the ruined section of the genome.

The old civilization was originally from Earth, so the human genome is compatible. No one knows what the aliens look like. They wear full-body coverings slathered in Vaseline, with only a thin viewing slit. This is because of their sensitive skin and crippling anxiety.

They trade, not for the tools, but for the genetic residue on the tool handle. Since they pay a high price they can choose the buyer and choose whose genes they get.

  • $\begingroup$ Even if the genomes are not compatible, even if they do not need to fix anything - assuming that life in the Universe is rare - which it most probably is - DNA will be universal very high-value goods. $\endgroup$ Commented Feb 20, 2023 at 20:09

A significant tradeable good that's commonly overlooked is literature. It's almost completely independent of your technology level. We still consider the works of Shakespeare, Cervantes, Chaucer, Virgil, and Homer to be great works of literature, even though they lived in worlds that were significantly more primitive than our own. It's also something that's continually produced, giving you a reason to return periodically to scope out what's new.

A trader from a distant land paying a good price for a book they like would not be seen as unusual. It would also create a natural avenue for you to get more involved in the primitive community if you wish, by using your superior-quality goods to become a patron and fund the work of writers. You could even attempt to alter the course of that civilization from the shadows by feeding ideas and concepts to your writers.

  • $\begingroup$ It's worth noting that the same argument extends to most any other form of art, too. $\endgroup$
    – jtb
    Commented Feb 22, 2023 at 20:20

Trading is just a smokescreen

The spacefarers are using the medieval planet as a way station for smuggling. One ship leaves the goods and some time later another one picks them up. They hide the goods well so the locals have no clue of what's going on.

Or they are not smuggling goods but trafficking people, possibly ones who are sought by the authorities for one reason or another. These people live in the medieval community for a while under the pretense of being trade agents, until it's safe for them to continue their journey.


Overt spying.

I think this option has been missed, so I'll throw it in the pile: the advanced civilization actually has no interest in trading what-so-ever, nor do they have any nefarious intentions. They are simply spying, up close and personal, and rather than be sneaky about it, they just show up, bold as brass, and "trade".

They actually could not care less about the goods. Possibly they dump them overboard the moment no one is looking. They don't need the planets, so expansionism isn't a goal. Perhaps the planet falls within their territory, though, so they thought it best to keep an eye on it, and rather than setup covert listening posts and worry over being detected, they just waltz in like, well, a bunch of traders.

They might have legitimate concerns over pirates setting up bases there. Or some other civilization discovering the place and conquering it. They can't simply leave it alone. Maybe they'll try to gently nudge the civilization in some favorable cultural directions but really their goal is just to make sure this planet, in their back yard, doesn't get misappropriated, and stopping by every few months to get out and chat up the locals is a great way to keep tabs on it without having to risk anything or even bother being sneaky.

(I would also personally make it so that the trade ship is actually a Q-ship. One day, some pirates or alien invaders show up and find out the "trade vessel" that comes around periodically is actually a Mark VII Dreadnought with detachable cargo containers around it.)


They are looking for something very specific

There is some extremely rare mineral, found only on this planet and even there very rarely. This mineral has enormous value to the space civilization.

While the space faring civ can and does send out search parties and use scanners and other techniques to try to find these rocks themselves, the tiniest ones don't show up on scanners and there is no effective way to search except with pure manpower.

Some of the citizens of the primitive civilization might have collected some of these rocks just because they are pretty. The space civilization would be more than happy to trade the rock for a easy to make axe. The primitive civilization would greatly benefit from the high quality tools, and some of them would also go purposefully searching for more rocks to trade.

The space civilization might also trade other objects, just to make themselves less suspicious. However, what they are really after is the tiny blue rocks.

(Instead of a mineral, it could also be some kind of plant or animal, or other object)



Once a civilization reaches a certain level of advancement, we start to see a significant fall off in population growth or even population decline. It's already started here on Earth, and is likely an even bigger problem in more advanced civilizations. As a civilization advances, the value of a large family which provides manual labor to support you in your old age is replaced by pure liability as the socio-economics shift away from the family farm to government and corporate reliance, and the required level of education you need to even become a productive member of society able to support kids cuts more and more into how many years you actually can use to have kids.

Children are becoming more expensive to raise as medical and educational needs increase, and the payoff has disappeared as people in advanced nations rely on Welfare and Investments to cover the cost of aging out.

In highly developed parts of the world, the average person only has a 50% chance of choosing to have children, and those that do have an average of only 2.5 kids because they either wait so long to start that infertility becomes a major obstacle, or they choose to cut off for economic reasons. If everyone everywhere in the world followed this reproductive strategy, the human population would decline to less than 10 million by the year 2400.

Technology requires a larger and larger consumer base to cover the ever expanding cost of more complex technologies, but your advanced civilization has a rapidly shrinking consumer base among its own people. This means in order to survive as a civilization, they need to import people from worlds with positive population growth.

So, the advanced civilization intentionally maintains Primitive Worlds where the populations will grow, and then trades goods with them as fair payment for human cargo. While these slaves will be forced to fill the bottom echelons of the advanced society, over time their descendants will be properly educated and integrated into the main society and new slaves will be brought in to refill these bottom tiers.

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Maybe for surveillance and all their traded items are bugged.

As for why they want interaction: It could be the ultimate goal is to reveal themselves to the natives over many generations.

But then why not industrialize the process directly?

That's how you destroy societies.


Because they're traders

Other people have answered from economic points of view, but from a pragmatic point of view, and I'm basing this on the Stainless Steel Rat universe, it could be simple specialism.

The medieval planet has some goods that are of worth - see other answers for examples.

Why don't the advanced civilisation mine / breed / manufacture it themselves?

Because they're traders! They wouldn't know porcuswine husbandry from a hole in the ground.

Space is big. The techies of the advanced society have lots of other stuff going on. The traders find a planet that makes stuff that they can sell, they go down, trade, fly off again.

They want a nice easy job selling trinkets, not a tough job setting up and running a mine, I mean, gods, that sounds like far too much work and would severely cut into their "making money and spending it in the pleasure dens of Floogleflarp 7" time.

  • 1
    $\begingroup$ Any answer which mentions The Stainless Steel Rat is a good answer. Take my money! $\endgroup$ Commented Feb 20, 2023 at 13:05

They need the planet in the future.

The humanoid settlers are slowly destroying their own planet and this is a form or reconnaissance to see whether this planet would be a potential new homeworld for them if/once they destroy their current homeland. However, government officials are very paranoid about destroying this planet before getting a chance to use it or wants to keep it a secret as the last resort - or "break glass in case of emergency" plan if all goes to hell. As such, they are gathering information about this planet and its resources to ensure that it holds or does not hold the future survival of the species and do not want to interfere too much with native life unil they have to.


If you want to export industrialization, they will surely figure out rockets. Then, those same primitive natives might get some ideas about shooting your orbital ships down, instead of peacefully giving them a lot of stuff for a little bit of higher-tech junk. You could keep them in stone age if you wanted to, but as anything pre-industrialization works, you might grab the most advanced tech level that is safe, as that world would be the most productive for your needs. So - medieval level of technology. Far enough from industrialization their geniuses won't surprise you with rockets one day, more income of anything you want to get than say stone age technology.

So, space civs don't actually trade - from their perspective that is. They offer a bit of junk equivalent to colored glass to native Americans and receive valuables in return. There are many things a medieval civ would value, depending on when exactly you want to put your setting. As for what would the high-tech civ value - anything that has transport costs lower than production costs (remember, their "trade" is nearly free from their perspective). So, this becomes a matter of transport costs:

FTL that barely costs anything - your world can offer mass-market produce cheap enough to the point the space civilization is utterly dependent on such world(s). Medieval natives have no idea they are the crucial farmland of a powerful world and that slaughtering their own livestock and sowing fields would destroy that world (in addition to them, of course). Would robots tending livestock perform better? Perhaps, but why bother when a good enough solution is available already.

Intermediate FTL costs - higher value stuff that is for some reason worth more if natural. Say pearls are a viable candidate here. They "grow" in the sea so you can't simply come and get them all. Rarer food/vegetables are a viable candidate too (eg truffles). One obvious candidate here would be precious metals like gold. I believe these would not work. If space civ wants gold, they would find it much easier to quickly collect all available gold and then leave for a while (or even forever) - there is no incentive to stay. You need something renewable-ish to make the space civ want to keep trading for a long time.

High FTL costs - only uniques remain valuable enough to bother coming for and hauling back. Space civ trades to feed agents on the ground which are scouring the area for masterpiece art that would bring a lot of money back at home. And then trade for said art - either buying from an artist directly, or perhaps from the king currently owning the painting or whatnot. They require natives to keep producing this art, again making local presence needed.


Gravity wells are slums

Living at the bottom of a gravity well is economic suicide, because exporting anything physical costs insane amounts of energy. The life support system advantages of a functioning are nullified by the fact it is all entangled and unprotected; imagine putting a millions let alone billions of people on a rock that a single mid-yield cobalt bomb could sterilize?

There are enough people in space that the population of planets isn't important, they live in a disease-ridden pit, and anyone who has any get up and go has gotten up and went.

Now, most backwaters like this are offered many chances to get off-planet, and are either turned into exotic tourist destinations (see how your 100 times great grandparents lived!), evacuated, or left as preserves for people who don't want to leave.

Think North Sentinel Island. There might be some marginal benefits to dismantling the planet for raw materials, but it is a bit rude to intrude on them. So they are mostly left alone.

Anthropology, Tourism and Caretakers

The traders could be any of the three of Anthropologists, Tourists or people tasked with Caretaking of this backwards rock.

The fact they always go to the same location makes it seem like they are regulated to a certain extent.

You'll want to ensure the planet isn't headed towards a nuclear war. Maybe you'll want to offer the right to exit the planet if people want (keeping them trapped might be illegal). Some people will want to see it.

We can see all 3 of these happening today. In some areas, we don't have any contact (North Sentinel). Other places have Anthropologists visiting regularly, In other areas, tour groups show up and gawk; in some senses, some Anthropology are just tourism with higher standards (especially historical stuff!)

You trade to keep yourself welcome

Just showing up and not interacting makes you a threat. By being friendly -- trading in ways that the locals understand -- you make your visits less hostile. Maybe people even want the knick knacks you collect.

It is possible that some of the stuff you trade away has trackers on it, and it is used to model trade networks of primitive planet-bound people.


The Prime Directive

Because of their laws, ethics, or something else, the offworld traders consider strip-mining the planet just as bad as giving the natives cell phones. Sooner or later the natives will stumble across the mine and its steam-belching automated digging machines; besides which, the effect on the environment would be hard to justify.


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