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Considering a distant future where mankind has spread and inhabits lots of star systems. Would there be a justification for inter-system trade or exchanges ? FTL travel may be considered, but with at a significant energy cost.

Most materials being available everywhere in most systems, and production capacity being likely to be roughly the same everywhere, interstellar travel seem of no use in this kind of setup. Any suggestion to make this more exciting ?

Edit : As the topic may interest other readers, I complete the question with some of the information I gathered.

This Princeton paper (written by a to-be Nobel prize!) consider interstellar non-FTL travel and the return of interest calculation. Non-FTL implies that time flows differently depending whether or not you stay on a planet or you board the ship. A 10 year travel at 0.99995C will make on-ship time will appear 100 times slower : roughly 5 weeks. Interest rate will follow the same logic...

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  • $\begingroup$ Your (ship-measured) travel times assume instant (or bone-crushing) acceleration. Most non-ftl travel schedules presume standard flight-plans: accelerating for half the trip and then decelerating at 1g. Reaching high fractional c would take years. $\endgroup$ – Serban Tanasa Mar 22 '15 at 17:24
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"Unique" Commodities

Sometimes you just want a unique thing. Consider, if you would, Emmental Cheese (Emmentaler Käse for the german speakers). There are a few certified areas of the world that actually produce Emmental Cheese. Every other cheeses, even if they are really similar, are simply not Emmental Cheese by virtue of where it was made. If you want "the real stuff," you need to go to the official places.

Perhaps there is a market on some planet for Tauntauns meat, but Tauntauns are not native to this planet and the planet is too warm for them, or the planet's local fauna love eating them so you cannot have a sustainable population. Someone will import them and there is your intergalactic/interplanetary economy. If there is a simple demand for something unique, someone will eventually supply the stuff.

There are some special instances where you need a particular bacterium or fungus, such as with bread. You may think yeast is yeast, but that is not true. While each type of yeast could be capable of raising bread, there are some that are desired for their unique properties, which can only be made in a particular environment. Replicating such environments may not always be practical.

Even those people who buy yeast from San Francisco thinking they will make an authentic starter at home are deceived. The yeast will adapt to their kitchen's environment, making them the same as local yeast after the second batch of bread. (See "Local Breads" from Daniel Leader.) So some things just need to come from the correct source.

Some Economics

It may be cheaper to manufacture some item on one planet and then ship it another. For instance, it would be much easier (once some initial hurdles are overcome) to manufacture steel on Mars than on Earth, and send it to Earth. This is due to the huge resources available to do so on Mars. (The thing is as close as we're going to get to a giant iron ball!)

There is also the issue of specialization and comparative advantage. Maybe one planet or system specializes in a thing, and they simply make that thing better or cheaper than other systems. It could be more economical for other systems to ship that thing in in return for whatever they specialize in.

Summary

You would trade between systems because:

  1. That thing you want cannot be made in your system, due to biology/environments/regulations.
  2. That thing you want is not worth the time and effort of making it in your system, because you specialized in something else.

...which seem to be the same reasons why globalization is a thing on Earth, but I fail to see reasons why these would not apply to a galactic-scale economy.

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    $\begingroup$ Final cost at Y = local cost at X + transportation cost XY. If the second is high enough, even a zero local cost cannot overcome that. I believe that is the case in most physical-matter interstellar trade. $\endgroup$ – Serban Tanasa Mar 22 '15 at 17:06
  • $\begingroup$ @SerbanTanasa this comment is just an alternate, perhaps more detailed, formulation of #2 in the summary. Should I add that for more clarity? $\endgroup$ – PipperChip Mar 22 '15 at 19:08
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Most authors who have interstellar trade simply make it work like current or medieval trade and ignore the details.

The big problem with realistic interstellar trade is the energy cost. It's certainly possible to have a star without heavier elements. Heavier elements come from previous stars that have died. So stars located away from other stars likely have fewer heavy materials (e.g. iron). The problem is that for the energy cost of bringing iron from another star system, you could transmute lighter elements into iron. Since power comes from fusion of light elements together, power's going to be available anywhere.

So what can be traded? Information (new discoveries, blueprints, schematics, movies, music, etc.). Unique items. This would be similar to medieval times. Although it was possible to ship food locally then. But this is consistent with shipping coffee, tea, spices, etc.

Note that you need FTL to make even that trade work. Trading via generation ships has several problems. First, they're slower than light, so you could have finished your information trade sooner by radio. Second, the exporter has to pay to create them but has no guarantee of a return (also a problem for radio, but less risky). Third, it's hard to motivate future generations by commercial promises made before they were born. So generation ships might bring things to trade, but they are unlikely to be built for the purpose of trading.

So you can have FTL trade similar to the medieval trade between Europe and China, expensive and only good for luxury goods. Given FTL, that seems reasonable. Ships would buy goods in one system and try to trade them in another.

If you really want trade more like modern trade, where we ship out of season vegetables from South America, then you'll probably have to handwave it. The closest to a realistic way to do this is folded space, where you take shortcuts. And even that requires space to be folded in the right way first. If you need to use energy to fold the space in the first place, it's probably a no go again.

The problem with comparative advantage here is that some things just aren't worth trading. If I'm only willing to pay 100 for something that costs 1000 to make, then you can't trade with me. Trading generally requires that shipping costs be less than the worth of the final item. To trade iron from Mars, we'd first have to justify the cost of shipping iron from Mars. Since iron is cheap and easily available on Earth, this seems difficult. It's also worth noting that the asteroid belt is going to be cheaper in energy terms. Less gravity trumps more distance.

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Raw materials may be available everywhere, but maybe the best FTL drives are built in the Aldebaran system, the best conventional thrusters are built around Barnard's Star, the asteroid mining equipment industries still concentrate in our solar system, and the best computers are built at Betelgeuse.

Note also that if a new invention is made in one star system, they will likely keep the workings of the invention secret as much as possible, so that, at least for some time, they are the only provider of that invention. This is what happened in the past with porcelain: The means to produce it in Europe were always there, but the method was kept secret by China, so for a long time China had the monopoly on it (which of course also meant porcelain trade between China and Europe, which also wasn't exactly cheap).

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