I have a cosmos where FTL travel is via an alternate dimension. The problem is when you return to our universe, you find yourself at the right destination, but a completely different parallel world from where you started.You can't control which parallel universe you end up in, so you can never go home and never go back. Technological advancement is stifled because no one wants to spend a lot of research money inventing stuff that may have already been invented elsewhere. Currency is worthless and you can only bring valuables you can keep on your ship as you journey. What easily transportable valuables would people want retain between various Earths or colony worlds and be paid in?

  1. Assume most ships are large and cheaply made, intended to establish colonies and dump excess population. Alternatively, some are self-financed by rich starting crew members who live permanently aboard. Mineral resources are an obvious choice, but our crew has a smaller ship originally built to survey new stars and is small.
  2. Patent laws would be unenforceable, since the travellers would never go back to where the things were invented. Most places would have similar technology after the initial flurry of interplanetary cultural exchange. I was already assuming geological surveys wouldn't vary much and the crew could go back to a place over and over selling the same data until other surveyors saturated the market.
  3. Panspermia: There is little alien life, and what there is would be simple and much like primitive Earth life. On the upside. worlds are not severely alien, and don't require extensive terraforming.
  4. Close similarity. Most worlds are similar in basic physics and at least related historically. Sure, here the Nazis won WWII, and here the soviets ruled the world while in another the only differences are cosmetic - you can meet yourself and swap stories about your childhood, never even figuring out the difference.
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    $\begingroup$ i dont have an answer yet but wow this is an interesting thought provoker $\endgroup$
    – Topcode
    Apr 3, 2020 at 21:56
  • $\begingroup$ I hear that magic beans are popular. I know of a fellow who traded an entire cow for just three magic beans, and got the best of the deal. $\endgroup$
    – user535733
    Apr 4, 2020 at 1:15

3 Answers 3


Precious Metals.

In order for someone to be able to exist at all in different universes, the laws of that universe have to be the same (or very, very, very close) so that the approximate elemental abundances will remain roughly the same from one universe to another.

Elements which are rare (really rare) will retain their value from place to place. Elements which are relatively short lived radioactive (e.g. half lives in the tens of thousands of years order of magnitude) will retain a good value while also being reasonably stable in the short term as a medium of exchange.

This is essentially what we already do on Earth. Commodities are traded on a very organized market in things like precious metals.

So this is pretty straight-forward.

Now a cautious traveler would probably not use radioactive elements. In many places on our Earth (practically all of them) possession of many radioactive elements is criminal without government permission. So a traveler popping into the wrong universe is going to find themselves treated as e.g. a terrorist.

So probably the things to have in stock are essentially the same as Wikipedia list here :

  • Rhodium
  • Platinum
  • Gold ( big surprise there :-) )
  • Palladium
  • Iridium
  • Osmium

Roughly in that order of preference.

You can't really go wrong with Gold, although value fluctuates from time to time. It's also dense and easy to transport in that sense.

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    $\begingroup$ I suppose everything else would be subject to variability in manufacturing. Metals are heavy, but might/could work as 'coin'. I'll have to come up with a trade question (the 'answers' answer is good for trade). I had also decided to put radioactives in the 'trade' column rather than the 'coin' one. $\endgroup$
    – DWKraus
    Apr 4, 2020 at 12:18
  • $\begingroup$ Rhodium would be more valuable than gold because it has a lot more practical uses than just being a luxury item. For example, if aluminium was rarer than gold, then it would be found on expensive goods. Aluminium would be imitated with tin, which is more expensive than aluminium in our world. $\endgroup$
    – Galactic
    Apr 5, 2020 at 19:17

People would be given transferable credit between dimensions based on uninvented technologies or entertainment that they bring back from alternate dimensions. Imagine traveling to a world where the Star Wars series don't exist. A person who brings back copies of those films could receive large amounts of credit.

Ships would try to bring as many popular items with them as they can when they travel, because in the case that they do not exist in the alternate dimension, they would make large fortunes. We would probably see an appraisal system when experts in the field in which the new objects classify in would judge the worth of the technology. Credit would be given based on how "good" they judge the items to be. After the government judges the item, it will bid off the licensing rights to companies who are interested in acquiring it.

All credit from one dimension would be usable in another dimension, subject to purchasing power parity adjustments. The credit would likely be paper money or coins made out of inexpensive metal rather than credit/debit cards (because bank accounts are not trackable interdimensionally). Coins made out of precious metals would not work because the price of those metals would vary from dimension to dimension.

To adjust for purchasing power parity between dimensions, a government signed certificate with a proprietary hologram would indicate the current prices in credits of about 120 consumer goods (based on the UBS's basket to calculate purchasing power parity between countries) in one dimension (ex. price of 100 grams of rice). This document will be required to be brought each time a person travels to another dimension to allow the new dimension to calculate how much "new credit" the person will receive.

Because people in a different dimension would not know of events happening in another, extensive background checks would become mandatory before a person could enter a ship to travel into another dimension. Without these background checks, a person with a previously good financial record could get a loan in one dimension and hop into another and get away with it. People would only be allowed to move to another dimension if they do not owe any money (since you can't go back to the old dimension to pay it off). A bank account must be closed in one dimension and all the money taken off of it before traveling to the next dimension (since interdimensional monetary management is not possible).

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    $\begingroup$ I hadn't considered an interdimensional credit system - good thought. But I'm not sure how to enforce any of it. Every planet in every universe is kind of an island. How do you keep the Nazis from cheating and printing a billion credit notes? How do you prove potentially counterfeit notes supposedly from world (A) really represent value? I did consider gold, since every goldmine in every universe would be known in every other due to shared surveys and worked. Potential supply would be fairly constant. Decided it was too heavy. $\endgroup$
    – DWKraus
    Apr 3, 2020 at 23:51
  • $\begingroup$ This reminds me of the Eyre Affair when the dimension hopping father brings his daughter a Beatles album from 1974. $\endgroup$
    – Willk
    Apr 4, 2020 at 2:18

Assuming that all of the alternate dimensions are physically similar to ours and differ only in “detail” of civilisation / history then the obvious currency is physical gold.

Gold was used historically as a currency and under pinned the whole financial system up until 1971. The gold that the Pharaohs dug up thousands of years ago is still valuable today whereas paper money eventually becomes worthless.

  • $\begingroup$ But the amount of gold would vary from dimension to dimension and wouldn’t have the same value in each one. $\endgroup$
    – Galactic
    Apr 4, 2020 at 20:43
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    $\begingroup$ "Close similarity. Most worlds are similar in basic physics and at least related historically. Sure, here the Nazis won WWII, and here the soviets ruled the world while in another the only differences are cosmetic - you can meet yourself and swap stories about your childhood, never even figuring out the difference." The value of gold might well vary but it does in our world. Any drastic variation would not create a world that was historic $\endgroup$
    – Slarty
    Apr 5, 2020 at 12:09

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