One day, a portal, several hundred meters across, appears without warning above the Atlantic, roughly around the Bermuda triangle region. It hovers in mid air, immobile, 200 meters above the surface of the water. Space in and around it is distorted in such a way that makes it hard to directly see the other side without going through the portal, but by sending drones and eventually aircraft, it is quickly established that on the other side is one ocean of an alien planet. It's similar to Earth and has humans living on it, whose civilization is a few centuries ahead of our own in terms of technology.

Contact is established rapidly and linguists from both sides are sent to learn to talk to each other. Within weeks, the other side constructs infrastructure around the portal that can be used to transfer even the largest Earth ships through. They are clearly much more technologically advanced - this would be a megaproject for us, but it's not a big deal for them. They are generally peaceful, more so than us - while they have enough firepower that any Earth nation would be hopelessly outmatched (imagine modern Earth in contact with medieval Europe), they would for the most part prefer to trade with, learn from, and coexist with Earth. They will avoid transferring technologies that might be used for war, though. (Of course, not everyone might have such scruples.)

The question is: how exactly might trade work?

For the sake of the story, I want contact to be an extremely disrupting event for Earth humanity. On paper, their economy dwarfs that of the Earth. A back of the envelope calculation I did says it would be about 10 to 100 thousand times larger.

Therefore, in theory, they would have no problem making investments that would seem absurdly large by our standards. For example, the equivalent of a trillion dollars is not something that their governments would find all that expensive, generally. Of course, they don't have a trillion dollars: they have their own currency, and therein lies the problem. How do you buy things on Earth with alien money? How soon might people start to accept it? How would that process work? I imagine exchanges would be set up eventually, or they might trade some of their advanced technology for dollars or yuans, but it seems... hard to sell a trillion dollar's worth of stuff, no matter how fancy, for example.

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    $\begingroup$ I would reckon with their economy being so large they probably have food in abundance. Feeding Earth might not even break the margin for lost production. $\endgroup$
    – BMF
    Mar 27, 2023 at 15:38
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    $\begingroup$ For a parallel from our world, check the history of the similarly asymmetric USSR-West trade relations during the Cold War, where one side owned a currency of no real monetary value to the other. $\endgroup$
    – biziclop
    Mar 27, 2023 at 15:38
  • $\begingroup$ google "comparative advantage" $\endgroup$ Mar 28, 2023 at 16:46
  • $\begingroup$ I would suggest a very careful "it depends on what they want to buy". Because after initial excitement of the possibilities pointed out in other answers I found a way in which this could go HORRIBLY wrong. Think Nesdley(fictional company that does not exit in our reality) selling a significant amount of earths water. So there is very significant danger that should be considered. The question is would it and how? To which I have no answer any lawyers? Worst case they have replicators like in Star Trek and water has by mass the same value as gold. Go through the math of your region yourself. $\endgroup$
    – Daniel
    Mar 29, 2023 at 18:14
  • $\begingroup$ As it’s currently written, your answer is unclear. Please edit to add additional details that will help others understand how this addresses the question asked. You can find more information on how to write good answers in the help center. $\endgroup$
    – Anonymous
    Mar 29, 2023 at 18:35

12 Answers 12


As quickly as trades can be made.

The aliens want to buy a block of Manhattan. The owners of that block would like, oh, a couple billion dollars or whatever.

How can the aliens get a couple billion dollars? By providing goods and services, wherever they can see a need. Maybe they go up to Elon Musk and tell him they will gladly give him the design for a battery that has 50x the capacity of his best batteries, plus a few prototypes, and he gets full rights to produce and sell them on earth, and they'll do it all for $4 billion. Really I have no idea if that's a good trade but Elon Musk could come up with that kind of cash if he thought it was, and then the aliens can buy their block of Manhattan.

So getting cash is just a question of finding something humans want, and doing some quick exchanges.

I expect alien cash exchanges to take more time. The aliens could offer to buy that block of Manhattan for 23 million Xeptos, which, really, is quite a lot of Xeptos. You could buy a small moon with that. But the question will be "what good does that do me", i.e., what's my access to Alien Amazon and Alien Toyota or whatever? I don't want to be stuck in a "company store" type situation where the aliens give me money that I can only exchange for goods with those exact aliens. But if I can casually take the alien portal to the universe then those Xeptos might look even more attractive than dollars.

The ways in which this can ruin our economy are plentiful. We have pretty well ruined the ability for Africa to ever have a thriving textile industry because we "donate" so many clothes and it just floods their market with mass produced goods that they can't compete with. Imagine if aliens could sell you a laptop that was thousands of times more powerful than anything you could buy on earth, and they'd sell it for about $10. The whole earth hardware industry would be destroyed. Possibly all our industries could be destroyed by an alien economy that is largely post-scarcity. It would be the car replacing the horse, except across the whole of society.

I wouldn't underestimate human ingenuity to bring things back in line (or make a real effort with the aliens at the start to avoid this kind of flooding) but in the near term there is plenty of room for this to wreck us. ("Ha ha, no, we will not limit access in any way. Also, here's a machine where if you push a button, you get a day's worth of food. It also reads minds to figure out what you want. We're already teleporting them into everyone's homes right now. You're welcome!" And so the human farm and food packing industries are put out of business overnight.)

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    $\begingroup$ Yeah, that's exactly what could happen technologywise. The other country - I mean planet - could flood your market with cheap technology good until your country's - I mean planet's indigenous technology industry would be utterly destroyed and you'd be utterly dependent on them (or be pushed back to the telegraph age). Good thing nothing like that is happening for real! $\endgroup$ Mar 28, 2023 at 4:56
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    $\begingroup$ Of course, the common people of earth are now completely obsolete except for their ability to enforce property rights on the behalf of the aliens who now own most of Earth. The aliens would rather have humans keeping other humans off their property than do it themselves, and it doesn't need alien tech because it's only humans they're fighting. Most dictatorships seem to have a structure like this. $\endgroup$
    – user253751
    Mar 28, 2023 at 9:50
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    $\begingroup$ It's great that you pointed out that you don't even need malevolence or indifference for this to happen. You can ruin a place's economy while genuinely thinking that you're helping. Of course a well-meaning AND highly advanced civilisation might be more careful and cautious, they may even have principles of non-contact similar to what human anthropologists have adopted. $\endgroup$
    – biziclop
    Mar 28, 2023 at 12:30
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    $\begingroup$ This exact scenario (risks of contact with a non-malevolent post-scarcity society) is an important aspect of 'Singularity Sky', a sci-fi novel by Charles Stross. It is basically 'be careful what you wish for' but turned up to 11. $\endgroup$
    – Laura
    Mar 28, 2023 at 13:29
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    $\begingroup$ @biziclop I did get that Moana song stuck in my head. "What can I say except 'you're welcome!'" We like to think aliens are smarter than us, but they might just be richer and more powerful. $\endgroup$
    – JamieB
    Mar 28, 2023 at 13:30

The question can be rephrased:

How do you create an exchange rate with gods?

You can't.

You're at the level of a parent trading with a toddler. The toddler has nothing to trade with. The parent might give the kid some candy for a macaroni picture, sure, but that's charity, not true trade.

Picture the scene.

Aliens land, check our prices for land and resources.

They give us ten metric tons of rhodium beads, and ask for Manhattan.

And we give it to them. Everyone who lived there is now very, very rich indeed, but with the cold sense that "rich" no longer means ANYTHING.

You see the problem:

  • If they don't maintain artificial scarcity, nothing they offer us has any value: the value of any good tends to zero as supply grows infinite.
  • If they maintain scarcity, they can buy everything we have for beads.
  • We get positive or negative inflation depending whether they spend it locally or not.
  • They're rich enough to buy the whole planet and everything and everyone on it multiple times over, so they get to decide how it plays out. Either:
    1. they quickly own everything, we're left enslaved with a pile of worthless beads, or
    2. we trade them macaroni pictures for candy.

Assuming they can immediately provide large amounts of priceless stuff, they're immediately able to trade on any market as soon as they land. Rich people will loan them however much macaroni art as they need, happily trading on the futures of what amazing beads they might suddenly poop out.

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    $\begingroup$ tbf, we kind of did have a means to "create an exchange rate with gods". That's what many religions are about, after all. If the communication ability and culture between aliens and us are not so significant that trade is still possible, then presumably aliens can still feel nice having a city / journal / online community / etc. singing their praises $\endgroup$ Mar 29, 2023 at 13:07
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    $\begingroup$ "You can't have an exchange rate for the gods", then proposes a reasonable scheme to trade with the gods :) $\endgroup$ Mar 29, 2023 at 16:31
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    $\begingroup$ Don't forget about firepower. They can trade Manhattan for not making into a radioactive pit. $\endgroup$ Mar 29, 2023 at 16:31
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    $\begingroup$ I like option 2 in the last bullet. I make a great pet. $\endgroup$ Mar 29, 2023 at 17:43

Most Countries Accept US Dollars, Because Stuff They Want Can Be Bought With Them

We have a real-life equivalent to your scenario, with a country whose economy dwarfs many others, and which possesses a sizeable military and proprietary weapons tech that would discourage attempts at conquering them.

Their currency is accepted in many places, because stuff you want to buy, even stuff that country isn't personally selling, can be purchased using their currency. Plus (until recently, maybe), their country has generally been better than most about not overinflating that currency.

If your other-side-of-the-portal people have a currency that permits you to buy even better stuff, and which promises a more stable repository for value, it won't be long before this-side-of-the-portal folks will be clamoring for the new currency.

(Unless people begin to wonder what happens with that new money if the portal snaps shut again...)

In the meantime, there's always gold.

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    $\begingroup$ Better at not overinflating the currency . . . when that currency was defined as representing a certain fixed amount of gold. Otherwise, not so much. The inflation of the US dollar from 1950 to the current day is over 1,200 %; that is, you would need more that 1,200 of our debased US dollars to reach the equivalent purchasing power of one single 1950 US dollar. $\endgroup$
    – AlexP
    Mar 27, 2023 at 17:10
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    $\begingroup$ The bold title is generally an answer but here it seems like a hint, and also suggests the aliens would use dollars (until you're 3rd para clarifies that). Maybe a title like: "Alien money would quickly be accepted the same way US dollars are accepted in remote places today" ? $\endgroup$ Mar 27, 2023 at 23:50
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    $\begingroup$ @AlexP Since it's 1200% you'd need over 12 of today dollars to reach the purchasing power of a 1950 dollar, right? Or is one of the numbers incorrect? $\endgroup$
    – LordHieros
    Mar 28, 2023 at 10:58
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    $\begingroup$ @LordHieros: About 12 dollars and 50 cents. Find an old 1950s episode of Perry Mason and be astonished at the prices of stuff and value of attorneys' fees. Sorry for the typo. $\endgroup$
    – AlexP
    Mar 28, 2023 at 11:02
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    $\begingroup$ 1200% over 70 years does not sound like overinflation at all. It's only 3.57% per year. Reality is actually probably a fair bit worse than that (politicians have been playing games with the "basket of goods" since the idea of tracking inflation was conceived). $\endgroup$
    – Ben Voigt
    Mar 28, 2023 at 16:49

They trade the way trade existed before money (or when a country is not connected to the international monetary system) - give me X of that and I will give you Y of this.

So, the real question here is "what on earth would these aliens want that they will give something for?" Here you get to decide the values of those aliens.

In human societies, people pay extra for unique and exotic items. Thus, the art piece that was shredded in the middle of the auction is worth a bunch because of its uniqueness. Is that a value to those aliens? Or will there be chemicals produced by some organism on earth that gives them pleasure and they will give what we value in exchange? (See how the East India Company provided opium to China to trade for tea.)


(Questions in title and body are contradictory, so I will only answer the title question; I lack reputation to comment)

It can be fairly quickly

The aliens don't need to actually sell anything. They can just send a representative to any major bank carrying a cubic meter of gold, ask for a 1 billion dollar loan with the gold as collateral, and start spending as soon as the loan is granted. If they want still more Earth money, they can go to a different bank with a cubic meter of platinum, then another bank with a tanker truck full of printer ink, and so on.

So in principle, they can start making major financial transactions as soon as they understand enough Earth language to enter into contracts, as soon as they find a lender that is willing and able (politics permitting), and as soon as the lender can ascertain the value of what the aliens put down as collateral.


The government sell useful trinkets that won't reveal much to the natives.

There are many medieval societies out there. We mostly trade simple trinkets like fire starters, guns, cheap vehicles to such people, and in return they have a bit of cash and they sell interesting cultural curiosity.

The advanced government would probably ask for some large amount of cash from the local government in return for a few simple and easy to make trinkets, like nanites that would repair someone to peak condition and cure all diseases.

The government sells cash to any tourists

Nanites that make someone immortal might cost five or ten dollars of local advanced tech money, but they might be worth hundreds of millions of dollars on earth. The government can use that money and sell it to local tourists, with them promising to not sell their more advanced tech to the locals. The advanced civilization tourists can each get a couple million dollars in cash for spending.


If you want it to be disrupting, they give some people something, something that is much like glass beads and trinkets, very desirable in the context, but completely useless in practical sense: fancy entertainment, boost to self image, anything like that.

In return they get to own things whose ownership matters in the long run: land, shares, political influence etc.

If they do this at any point, they will be aiming at it as soon as possible. Breaking language barriers would be needed first, so that the immaterial things could be negotiated effectively.

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    $\begingroup$ I'm having flashbacks of some random "aliens" who actually did purchase Manhattan in exchange for a collection of beads and baubles. $\endgroup$ Mar 28, 2023 at 0:02
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    $\begingroup$ @PieterGeerkens according to newamsterdamhistorycenter.org/education/schaghen.html the "trade goods" would have also included "textiles, especially wool blankets; metal utensils, such as hatchets, knives, pots, and kettles". $\endgroup$
    – RonJohn
    Mar 29, 2023 at 14:25

The limiting factor is likely reliable two-way communication.

Assuming we have anything the aliens want, they (not us) will be begin negotiations with parties of their (not our) choosing as soon as they can establish rudimentary communication.

The chosen party(s) will be have the resources to procure whatever they need: it could be a government, a wealthy individual, or a corporation. The party will then be able to act as their proxy as soon as an understanding is reached, regardless of any recognized legal standing. For this reason, I'd expect an individual or corporation over a government, for the ability and willingness to act on direct behalf of the aliens with fewer strings and less oversight.

These negotiations can be begin as soon as we have basic communication, and a limited early consensus wouldn't take long while finer details on a larger arrangement are hammered out as linguistic ability improves. They can also be based on expected — rather than immediate — benefit from the aliens to the human party: someone willing to take a risk in exchange for a high potential payout.

With the scenario described in the question, I'd expect several large multinational corporations to form "Alien Relations" divisions fairly quickly, with the explicit goal of securing alien investment. They have the earth-based resources to do what the aliens want, and the ability to process useful things the aliens could provide in exchange, such as raw ores.

In fact, if you want disruption, this event could possibly just about shut down mining operations on Earth as we know them, as ore is a likely initial exchange medium and the aliens could provide it on much better terms than any mining operation. Clean energy would be one of the next items up, though it will take a little longer to adapt their production abilities for our consumption patterns.

That puts a lot of people out of work. Fortunately, the aliens are still acting through Earth corporations, and these corporations need to staff whole new divisions. Like similar events in history (industrial revolution), it's an upheaval creating transitory hard times for many individuals, but a significant net win for society.


Them being so much more technologically advanced could possibly make this very easy for them. They could probably just make money out of thin air. And I don't mean it physically. In today's society most money is digital. They could probably hack in our bank systems or fool our bank systems into thinking they are receiving genuine money. They could forge digital bank transactions in such a way that it can't be distinguished from real transactions by us. And in turn, this of course will ruin our economy. All digital transactions will be worthless because there is no way anymore to properly validate the transactions.


Obviously it's hard to say just how much more advanced the other humans' technology is. But if we suppose that the portal was opened intentionally by them, and enough of them knew that the portal was going to be opened, I think the answer to this question is on the order of a few minutes.

The thing is, large financial transactions on present day Earth are all electronic. Nobody spends $4 billion in cash; big trades like that happen on stock exchanges, using computers and networks. And electronic stock exchanges are a big opportunity to make money if you have better technology than the other traders. Contemporary traders spend huge amounts of money purely on building fast computers and putting them as close as possible to the exchanges, and they spend even more money on developing smart algorithms that can make profits by trading. High-frequency trading is profitable, but it's much more profitable if you're ahead of the competition.

So imagine you have 24th century technology, and you know in advance that you're about to get an opportunity to trade on 21st century stock exchanges. You probably have your tech stack set up already before the portal is opened, ready to scan the airwaves for radio signals or any other form of wireless communication where you can identify where the sender and recipient are and how they communicate. The 21st century encryption on the packets is trivial for a post-quantum world's technology to break.

Once you can talk to networked devices, you find an online trading platform. The time it takes to create a user account on this platform is the bottleneck, since you probably have to provide some sort of proof of identity, and it takes a few minutes for the broker to check it. But you have all of your credentials ready to go, because you found ID documents and photos of real 21st century people from the Internet Archive and you don't mind doing a little identity theft (just to get started). Once you have a margin account approved, you can borrow money and start making trades. Seconds later, your 24th century algorithms have made you rich.


It maybe makes sense to assume that gate operation depends on earth willingness to operate it - as in - there probably should be mechanisms to inspect whois approaching and shut the gates off. This gives a more balanced situation as this way earth even being technologically inferior would be able to fend of incoming invasion. This enforces to some extent that interchange benefits both sides.

They cannot transfer dollars unless they have a bank account that they manage on earth that holds those dollars.

Thus they need to open an account. This is fairly straight forward process. Even if they do not have legal right they can find someone who would be willing to cover for them likely easily as they have enough knowledge that they can potentially sell.

It's important that they can established encrypted communication channel with their representatives. Ideally also another encrypted communication channel with another party who will monitor the representatives. Of course earth governments will need to agree that gates allow encrypted communication channels.

Legal aspects are important. Ideally when you start dealing transactions you want to be sure that legal laws won't screw you up and won't close your account. While it is easy to open unofficial bank account authorities may decide to close it and freeze the money if they decide that such activity is illegal.

Once account is up and running they need to get income somehow. Establishing business that operate on profit thanks to superior knowledge is one way. Selling knowledge another. They may not be keen to just give away knowledge so easily and instead they may be interested in schemes where they design something based on requirements - for instance 10% better computer chip - without exposing the process.

When alliens transferring money, what are they paying for exactly? What do they need from earth? Assuming they are technologically more advanced. Running unethical experiments on earth? For that kind of things they may want to stay in low profile and unknown possibly establishing encrypted communication with more nefarious players on earth who in exchange for profit will be willing to run unethical experiments - players like North Korea come to mind. In such situation they may be less interested in money transfers as transferring big money makes you needing to stay on legal side of things. Instead they may chose to collaborate with nefarious players based on trade - nefarious players run experiments for them - in exchange gain knowledge that is profitable on earth.

Even if aliens are highly ethical beings and have highly functional government of their own it is likely that they too have less ethical individuals or organizations who might run shady unethical deals in undercover.


The basis of trade is that each side has stuff the other does not.

So... drugs. Movies. Music. Books. That is, things we would have or varieties of things we would have that they do not.

I am curious, though: how are you going to trade with us for knowledge?


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