# FTL travel is impossible. How is interstellar trade possible?

Assume a universe depressingly like ours, in which special relativity holds. No hyperspace, no warp drive, no wormholes, and even the limited get-out clauses offered by real world physics either don't exist at all or are not practical. [Added later in response to comment by Fhnuzoag: no ansibles either. Neither material nor information can travel faster than light.]

I can think of a few ways that trade between star systems might take place. Emphasis on the word "few".

Information can be sent by radio. Trade in knowledge between Earth's system and e.g. Alpha Centauri is practical, assuming that either alien evolution or a previous human colonisation effort has put anyone there to trade with. But it is still so slow that a human lifetime could get used up just straightening out the terms of the contract. Still, human institutions can outlive individuals. Possibly networks could build up where Star A talks with Star B which in turn talks with C and so on.

For anything other than very nearby stars even lightspeed information transfer takes anything from thousands of years to an arbitrarily high limit. So if the nearest star systems are not inhabited the only way information trade can take place is if the inhabitants and/or institutions are very long lived and don't mind receiving payment thirty thousand years later.

Trade in goods, or trade of goods for knowledge, has all the problems above multiplied a thousandfold. Certain authors have plausibly sketched out small payloads of high-tech stuff being sent out by automated Bussard ramjet, e.g. Larry Niven's A Gift From Earth, but that isn't really trade, as the title of the book implies.

Perhaps the most likely model is generation ships (or ships with long-lived nonhumans aboard) plying the spacelanes. The ship has a large population and the capacity to grow food, create technology and art, and to mine or otherwise gather raw materials. The ship as a society can trade with the people of the planets it visits. But that is only going to work if an implausible proportion of stars have inhabited planets. What if they don't? A generation ship that goes 1,000 generations without meeting anyone else is for all practical purposes as isolated as Earth is now.

I had some brief hopes for trade with hive-minds spread out over multiple star systems. It won't work. If the thought of the hive mind takes place at lightspeed or less, trade is constrained by the same problems as above. The thought of the hive mind cannot be instantaneous because (as I may have mentioned) faster than light travel is impossible.

Any better ideas?

• Paul Krugman actually wrote a short paper about the subject of setting interest rates on goods used for trade conducted through relativistic travel. It's really just a (phenomenally nerdy) critique of neoclassical principles (of course, this is Krugman), but might have some interesting tidbits for you: www.princeton.edu/~pkrugman/interstellar.pdf – user9659 May 15 '15 at 15:32
• Comments are not for extended discussion; this conversation has been moved to chat. – Monica Cellio May 17 '15 at 20:17
• Answer: suspended animation & local backwards time travel are both possible. – Joshua Oct 16 '15 at 15:23
• There's a very big difference between possible and profitable. It's possible as soon as you're capable of interstellar travel, it's profitable when it's cheaper (and quicker) to transport goods than to make them locally. – Separatrix Feb 13 '17 at 15:56
• The only reason to buy things from people is if doing so is cheaper than providing that thing for yourself. You need a reason why something that takes decades to arrive and at huge expense would be both unsubstitutable and cheaper than producing it locally - and also that this can't realistically change over the decades that the thing is en-route. (After the decades that the order took to arrive at the seller.) The only possible solution is Amazon Prime - guaranteed next day delivery dontcha know! – Grimm The Opiner Feb 15 '17 at 14:00

I think you've got yourself in a bit of a corner.

Without FTL, trading would be nomadic rather than what we think of today - think the old Silk Road, or traveling peddlers. It'll be people whose livelihood involves being constantly on the move, buying stuff here hoping it'll sell for more than that on the other end. (This of course assumes that we don't have transmutation or anything). It may be more feasible if it's combined with an entertainment troupe of some sort (circus being the classic, but even postWW1 barnstormers would fit the model).

But, this assumes that it's cost-effective to be roaming the stars in the first place, and that means while you don't need FTL, you do need some sort of high-efficiency power source and sub-light propulsion. Otherwise, it's not worth the trip at all for most things.

If we're use near-now tech, that means the only things that are even worth considering shipping is things that the destination planet can't get on their own. (Because if it's available on-planet or even in-system at all, it's cheaper to do it locally than it is to ship it). To my mind, that brings you to three things:

1. People - colonists, refugees. Yes, you can "manufacture" them on-site, but assuming the travel time is < 20 years (or just for bootstrapping a new colony), you have to get people from there to here.

2. Bootstrapping for a colony - basically, the stuff you need to build the stuff you need. Once a colony is established, this market dries up entirely, but for a colony that can't do X, a machine to let them do X is worth shipping.

3. Unobtainium. Which goes back to the "can't get it here" issue. If some colony finds Dune Spice and it's useful enough, there will be people willing to pay the transport costs to get it.

4. (Credit to R. in the top-post comments). Refugees - by which I mean folks who aren't trying to go somewhere, so much as needing to get away from where they started. In this context (esp. given the money needed to build this sort of thing), you're probably looking at some rich religion/orthodoxy that builds something relatively self-supporting so they can bugger off and find a brave new land (or Xenu or whatevs). It's crazy and most of them probably won't make it, but human history is full of people packing up and hoping for the best.

A note, though - except for point 1 (edit: and 4), I wouldn't expect any of these transports to be manned. The life-support costs are atrocious, and space travel is notoriously "straight". I would expect unmanned cans that are radio-controlled to get them going, they drift through the stars, and get picked up (and radio-controlled in) at the other side.

• Interstellar Circus is an interesting idea. Time-wise, hopefully you get to have at least one "official" showing during your lifetime as a performer. – DoubleDouble May 14 '15 at 20:51
• Since this question has already referenced Charles Stross, have a look at Singularity Sky, whilst it includes FTL in it, it also has an STL Interstellar Circus, and STL cargo ships carrying quantum entangled bits for plausible FTL comms. Also for nomadic traders using buzzard ramjets see A fire upon the deep by Vernor Vinge – Gordon Coale May 15 '15 at 8:40
• Larry Niven described this in one of his Draco's Tavern shorts - travelling aliens arrive to buy/sell stuff, one of the things they sell is plans for a launching laser to push a solar sail. Thing is, if you don't make the launcher, they drop a device that sends your sun supernova to give them the push to the next destination. – gbjbaanb May 15 '15 at 14:34
• I would say biologicals are a good unobtanium. We don't really know how to create biological life after all. Even if you don't have sapient aliens, just certain plants, fungi, or even bacteria may be very useful if it has an effect you need, such as being useful in producing a drug you need. Anything non-biological and you start to get in a situation of asking why this thing is produced in only one part of the world and can't be manufactured elsewhere. All the raw elements exist everywhere, it's not hard to combine elements into other chemicals. but evolution can produce complex things! – dsollen May 15 '15 at 17:28
• dsollen I would file that under "bootstrapping" - there'll be money in getting the first few specimens down, but after that it's far cheaper to do a breeding program on-site than to continue to import. – Allen Gould May 15 '15 at 17:56

This is well addressed by this excellent site for sci-fi writers.

Turns out, you're doomed:

For example, the most expensive substance on Earth is antimatter, which currently has to be manufactured atom by atom at a cost of $100 trillion/gram. Let's say we can make it for 1,000x cheaper and that the friendly aliens at Proxima Centauri are desperately in need and will pay full price. So you get in your ship and travel 174 years there and 174 years back (>100x faster than Helios II, the fastest man-made object ever, or an insane ~2.4% light speed). Turns out, if you had instead invested in a 2.00%-APR savings account, you'd be just as well off. [note: figures for Helios II are "18000+ years", "0.0234% light speed", and "far better off"] Except, it's probably only 3x cheaper, it will take 500 years each way, and investors get around 5.00% annually. So you're really more like$1034 in the hole for every gram. And in a millennium probably manufacturing antimatter will be cheaper and you'll also be dead.

And that's for antimatter. There definitely won't be any "Durrr, let's trade plutonium, people, computers, water, or any other worthless thing."

Sorry.

But check out that website or the other answers for plenty of optimistic hope.

• Right, trading water. Who would be crazy enough to suggest that? (cough V cough) – Luaan Aug 11 '15 at 7:41
• If you're trading in antimatter you can get there in 100 years. But what are you buying? – Joshua Jul 16 '18 at 22:00
• Not correcting because it's a quote, but the Helios B probe is .023% light speed. (0.000235c or so), not 2.4%. The math used by whoever wrote the quote was... wrong. – Andon Nov 10 '18 at 6:22
• @Andon I have augmented the quote with "sic" and brackets so that the original wording remains, but the error is highlighted. It should be noted, also, that the figure is dated—the Parker Solar Probe, in-particular, currently holds the record. The slower speeds, in-fact, make the quote's message much stronger. – imallett Nov 10 '18 at 7:41
• Andon, I think they're saying that their hypothetical trade ship travels 100x as fast as the Helios B probe. – tbrookside Nov 10 '18 at 12:32

This was discussed in my Answer and comments to another question not too long ago. Just replace mining with the more general trade. Any transshipment of goods at fractional c speeds is more expensive than you can even imagine.

Perhaps they trade information. Art, exotic cultural narratives, scientific exploration?

Charles Stross in Neptune's Brood makes an interesting economic model where new colonization is the generator of money. "Slow money is a digital currency backed by debt—the debt incurred by constructing a new interstellar colony."

• I suggested this in the main comments, but couldn't remember enough of the details to make a good answer. +1 – AndyD273 May 14 '15 at 16:12

There would be only minimal trade in "wares" simply because the cost of transport is so outrageously high.

To reach 0.8c a ship needs to burn its own weight in antimatter - providing you can use up all the energy to speed up the ship, you don't spend any energy on speeding up that fuel antimatter, or the matter to annihilate it with. And antimatter is about the only ware worth transferring directly; normally worth the price. Other than that, only luxuries of "emotional" value, whose price is only bound to actual desire of the extremely rich buyers, irrelevant to actual value of the item - signed physical copies of books of celebrity authors, invaluable collector items etc.

In the relatively narrow timespan transfer of seeds and embryos of new revolutionary species of plants and animals developed would occur. The timespan would be between the antimatter-powered space travel becomes even moderately viable, reducing transfer time to orders of half a century, and the time when biological matter can be synthesized on molecular level; "scans" of said seeds and embryos being sent by radio.

Everything else is cheaper to synthesize or obtain from within your own stellar system.

...but information travels at speed of light, and it is valuable. Trade would flourish for not only science, but art, entertainment, music - and technology, sending plans, projects, blueprints - including stuff to be 3D-printed, both of art and technology. A digital currency would be exchanged; contracts would be mostly arranged by representatives on the "destination world", with the "manufacturer world" adapting management to the fact it takes a decade for modified ToS to reach the representative - and insurance companies shouldering the burden of conditions changing over the decade or two in ways that make fulfilling a contract impossible. Escrow companies that hold the payment until conditions are fulfilled would abound.

Trade like this would be difficult, but not impossible. Think about trade with India before the Suez Channel. A round-trip would take time in excess of a year, but it was still flourishing. A decade or two with modern law, logistics and planning would be quite doable for virtual goods. Oh, and of course it wouldn't be like waiting twenty years for one transaction to complete before starting another. The flow would only be limited by the throughput of the link, new goods arriving nearly continuously, while new contracts/purchases are being signed; round-trip of a couple decades but no waiting for "ACK" before sending the new packet...

• "Think about trade with India before the Suez Channel. A round-trip would take time in excess of a year, but it was still flourishing." One year is still a small fraction of the life of a 40 year old. – RonJohn Nov 10 '18 at 4:49
• @RonJohn: 40 years is also a small fraction of lifespan of a man living in the interstellar trade era. – SF. Nov 11 '18 at 5:27
• But it's not compared to the thousands of years it takes to go from star-system to star-system. – RonJohn Nov 11 '18 at 7:28
• @RonJohn: With antimatter drives or beamed power the speeds of order of 0.5c are achievable. At 0.02 stars per cubic light-year in Sun's neighborhood, you get ~670 stars within 20LY - or 40 years of travel. The average distance between neighbor stars is 2.3LY (Earth is particularly unlucky with Proxima at 4LY). So the times would be more of order of 5-10 years, not thousands. – SF. Nov 11 '18 at 7:40
• I guess -- since it's fiction -- you can accelerate as fast as Plot needs you to. And have inertial compensators so it doesn't kill the passengers. But then, you should just have worm holes. – RonJohn Nov 11 '18 at 7:48

One way to get slower-than-light trading is to combine four (large) assumptions:

1. People are willing to create new interstellar colonies because they want to leave Earth.
2. Most planets are, sooner or later, doomed to have a large enough war to knock themselves back into the Industrial Era.
3. Someone is willing to continuously transmit a huge library of information: enough to get any civilization up to basic spacefaring technologies. This can be hilariously expensive if the information is broadcast, so the signal will probably be tight-beamed to all known planets, to save on energy costs.
4. Suspended animation is a safe way to spend decades.

Now advance time by several millennia: there's a smattering of colonies all over the place, at varying technology levels, with access to the language and culture in the universal broadcast. This is the background of A Deepness in the Sky.

One group of people has decided to never settle down, so they continuously broadcast the knowledge required to build a spacefaring civilization, and they wander from planet to planet as a survival strategy: spaceships don't suffer the same long-term instability problems that planets do. (If you have an irreconcilable disagreement on a ship, you just wait until you reach the next planet, build another starship, and split the population.) Every time they arrive at a planet, they will have a few technologies that the locals don't have, so they trade technologies for supplies and other technologies. If the locals just nuked themselves back to the stone age lately, the nomads go into suspended animation for a few decades until the locals can resupply the nomads' starships. At that point, the locals will resupply the nomads because the nomads have been accelerating the locals' development; the locals owe a lot to the nomads.

This isn't "long-distance trade," exactly, but it does have similar trappings: the nomads wander from star to star, and each time they arrive, they trade with the locals. Sooner or later, the knowledge on one planet will migrate to the other planets, but not as an explicit trade agreement between the two planets.

• "Most planets are" not even going to evolve life. – RonJohn Nov 11 '18 at 7:49

Niven had three other examples of this that I can think of off the type of my head.

The Outsiders

They had FTL, but for whatever reason didn't trust it. So they cruised around the galaxy on their reactionless drive, and traded knowledge with any species they encountered.

The Ringworld Civilization

This is only mentioned tangentially, I believe in the first novel? But the lady they meet, Priss, is a former harlot on a sub-FTL style trading ship that went between the Ringworld and the colonies. I don't recall if they mention what they trade - I think possibly they were scavengers, looking at the dead worlds and then returning to the Ringworld to trade. But it's been quite a while since I read the book last.

The Laser Drive guys

I don't recall the name. But I distinctly remember one of his short stories about an alien sub-FTL craft that arrives on Earth, sets up shop and does some trading - again, primarily in information and technology. But the primary plot of the book is about their propulsion system, and how they get around if they can't get the target culture to build them a sufficiently big laser (induced novas).

Commonalities

The common factor between all of these is that you don't have, say, a trade balance between two worlds. Niven apparently felt the sheer distance and time factor would render that impossible, and to some extent I agree with him. Instead, you have sub-FTL craft - kind of like generation ships - that cruise in a circuit between many worlds, trading as they go.

I think this makes more sense than trading over laser communications. I mean, think about talking to a world 10 light years away. You have a 20-year turnaround just to say "Hey, we have this neat new propulsion tech." And then you get back "oh cool, tell us more." and by the time you get your final response it's 40 years down the road, and they're all "Yeaaah... hey. So we somehow, totally coincidentally, developed that same new propulsion tech. Just now. Amazing coincidence, right?" And things get even worse the further you are away. Imagine trying to trade with a 200-year communication lag.

But if a ship arrives in orbit with that new propulsion tech, and they say, "Man, that's some nice nano-assemblers you've got there. Swap the designs?" then you can do actual trading with people on the spot, instead of just giving up and developing it yourself.

• Laser drive guys: Larry Nivin. Bartender gets a starship captain knowlege pill for free and discovers what happens if the client doesn't build a launch laser. – JDługosz May 14 '15 at 15:17
• The title of the "laser drive guys" story: "The Fourth Profession." Great little story. – ravron May 14 '15 at 21:01
• Ringworld assumed unrealistically efficient Bussard ramjets. Essentially ships were able to gather both fuel and reaction mass for free while en route. I think the rationale was that the original location had unusually dense interstellar medium or something. (It has been years, so could remember that wrong.) // But yes generation ships with a permanent nomadic culture that trades when it hits a system, is the only way I see as well. – Ville Niemi May 15 '15 at 10:02

Without FTL, it almost certainly is too expensive to trade using manned vessels and negotiate on prices and values. So if you wanted to have contact and exchange, it would be better to exchange "gift ships".

Each side would stuff a ship with art, crafts, writing and interesting objects more akin to 'time capsules' than a store. They send out the ship and when it arrives in 20 or so years, each side has a holiday opening up the stuff and seeing what the other society is like in detail. Repeat the exchange every 5 years or so and you could mingle your cultures to some extent. The 'payment' is the vessel the other side sends back. It could be a matter of pride to show your best to the other planets..

No need to make humans travel, reach near light speeds at all, if you are patient.

• That’s an interesting idea to avoid the negotiations across a vast communications delay. But it doesn’t address the problem of goods value relative to shipping cost. – JDługosz Feb 15 '17 at 7:02

Interstellar trade works just like in the Stone Age: Through a lot of intermediaries for a long time

In Neolithic Europe, ornamental shells and amber were traded across the continent. A decorated shell made in Greece could be found buried with someone in Germany. No single individual travelled from Greece to Germany to trade goods. Instead these non-perishable goods were bartered among a host of intermediaries. The goods may have travelled across the continent in a few years, circulated back and forth over decades like coins do today or been in use for a lifetime before being reworked and traded further.

To set up trade between say, Earth and Alpha Centauri, we have to assume that like in the Neolithic, there are many intermediaries. These would not be based on planets, but on wayward comets, generation ships, autonomous unmanned ships, etc. A precious object from Earth could be traded a dozen times over several centuries before making its way down to a planet in the Alpha Centauri system. This has several advantages:

• With many intermediate trades, nobody sits around for generations waiting to cash in on the trade.
• Like stone-age necklaces, many of the goods may actually be in use in transit and therefore not perceived as costly long-distance trades.
• With intermediaries in space, most of the trades do not involve going up and down steep gravity wells, cutting the transaction costs.
• Trade could happen between parties travelling at very different velocities. The cost of accelerating a parcel of goods to meet a ship passing through your system is a lot smaller than powering up a dedicated vessel to transport said parcel.

The trade in information would also be through intermediaries. From the point of view of Earthlings, there's a steady flow of trade only with the habitats orbiting earth, but the goods may originate from lightyears away. Because of the cost of travelling in gravity wells, we may even find that planet-space trade consists mostly of energy and information, with the physical goods more often being traded among fellow spacefarers.

• Nice ideas and good presentation! This would work along with the “dandilion seed” travel of asteroid habs simply leaving, at low speed. – JDługosz Feb 15 '17 at 6:57
• This would work very nicely, if the human lifespan were much larger than today, or even infinite. Then people might get bored of a planet after staying there too long and they want to go explore the universe! – M.Herzkamp May 14 '18 at 11:51

After some thought, I came up with

Think of it this way: humans on Earth have decided it's time to settle the galaxy. They pick some good-looking planets and send out some probes. When the probes return (or when they send back their data), the people on Earth get a pretty good picture of what's on each planet. At this point, they plan out the next few centuries. What will the colonization process need during its first few years? Building materials for settlements and terraforming? Send that with them. What might they need afterward? More building materials for exploration and space travel, and elements not found on their planet? Send that later. What will they find on that planet that the people of Earth would want? Rare elements not found in our solar system? Send that back as soon as possible, in a sufficient quantity to repay whatever supplies Earth would be sending. In this way, the terms of the trading are agreed upon while everyone's still on Earth, so no communication is necessary once everyone gets far away.

There are still some problems with this. For one thing, it's going to be hard to find people willing to invest in such a venture. However, to this I would say that it's hard enough sending a space ship to another planet, and if we can manage that I'm sure there will be people willing to wait for their payment. Also, a lot of people are talking these days about how humans don't have to age, so maybe by the time we get to other planets humans will be virtually immortal.

There's also the problem that once the colonists get to the new planet, they don't have to send anything back. They'll get supplies for a while before the people on Earth realize they've been stiffed, so they'll probably be able to survive without future shipments. The reason this probably won't happen is because, for one thing, planetary colonization should be done by the best humanity has to offer, so the leaders should be morally good enough to honor agreements. Also, the things they'll be sending back and forth should be sufficiently useless at their source, and sufficiently useful at their destination, thus making it nearly pointless not to send.

As for information transfer between planets, which seems to be a large part of your question, I would say that there wouldn't be conversations so much as newspapers. Each planet would send the others a burst of information about what's been going on- current events, scientific advancements, things that might be considered useful. If a planet really needs something from another planet, they're probably screwed long before anyone can send aid, so there's really no reason to ask for things. Thus, the only information worth sending would be scientific, technological, and cultural advancements that might help the people on the other end.

• Investing in a future that far ahead means allot of unknowns. that's a massive expense, with lots of risks of colony dying out or failing to mine or not able to produce the goods you need. The only way to make such a trade beneficial is a MASSIVE return on investment, which in turn leads to resentment of the people paying 10x markup for what they get; increasing the odds of their renigging. – dsollen May 14 '15 at 14:50
• @dsollen I agree, but this is assuming they make the colonies in the first place. I don't really see why we'd send people to other planets other than 'because it's cool', so if we ever get it done I'm sure it will be because there are people throwing money around who don't much care about getting it back. – DaaaahWhoosh May 14 '15 at 15:45
• @dsollen There might be reasons why we'd want to colonize the stars without it being an investment opportunity. Interstellar comes to mind. Spreading to the solar system isn't a bad idea, but if something happens to the resources of Earth any Mars or asteroid colonies are going to be really limited for a long time. Spreading out a bit might be important to humanities survival at some point. – AndyD273 May 14 '15 at 16:18
• Pre-designated trade routes would mean pirating will be a very real risk wouldn't it? Seeing how we just send off supplies and hope for the best. – DoubleDouble May 14 '15 at 20:59
• @DoubleDouble I'm kind of assuming there isn't much interstellar infrastructure. Kind of like the early Internet, where it was mostly just universities talking to one another. It might be easy to hack, but only if you scheduled time on the big computer in the basement, and even then you probably wouldn't get much for your trouble. – DaaaahWhoosh May 15 '15 at 3:11

What if they trade in very specific information, minds. There is really no reason to transfer matter. Given enough energy, if you can build generation ships you have energy, you can build nearly anything, Including bodies. What you can trade is experienced colonists, warriors, persons transfered by wire by entangled messages that destroy the original (in a sense teleporting him). If not teleporting moving with slow ships, what is time in stasis but then thats not trade but work force?

Perhaps you sell AI's. Or perhaps working AI needs constant new experiences in quantities that far surpass the need of humans. After all if you think infinitely faster...

You could trade in immaterial goods, films, music, designs. Science is another good perhaps its impossible to measure something in our gravity well... Perhaps these planets collectively search for alien lifeforms and sell their infrastructure as service for a cosmicaly sized telescope array.

They could sell other services too like message bouncing. Imagine istead of sensitive archives you bounce the message 50 lightyears away. No risk of being opened until it comes back. So history writers will have that info but its no risk to you. What about planets that specialize in archival that started allready thousands of years ago. They could sell you back history info...

• Actually, if cloning is socially acceptable, you could trade minds AND people: a new colony would acquire teachers and technicians and would exchange them for capable pioneers which would find a use on a new world - and all it would take is a couple scans and a DNA sample. If you need capable, proven, new colonists (and freshen up the DNA pool...) it shouldn't be too hard to find volunteers. Ships of this kind wouldn't need much storage space and could visit any number of colonies and keep trading and expanding their supply. And they will stay useful for a long time. – Peter S. Aug 21 '17 at 21:24

I agree with other people that without FTL you are doomed, it does not make sense to trade.

But why do you want to go out so far ? While it does admittely not answer your question, what about the Solar System ? It is quite complex: more than 60 moons for both Jupiter/Saturn and many known planets. Interplanetary trade is quite possible when high-tech makes travel cheap and many planets and moons are very interesting for abundant natural resources. Many human colonies with different cultures are possible, aliens with generation ships could live in settlements together with humans.

I'm going to throw in my two cents on FTL in fiction and maybe it helps you.

From a physics perspective, FTL travel has deep time travel implications. However, note that in the vast majority of FTL travel in fiction, going FTL does not entail any form of time travel. In some cases, we do see some uber-FTL travel result in time travel, but that is typically a separate, exceptional phenomenon. So time-travel aside, FTL exists in fiction precisely because of vast distance related problems such as what you are facing. The way I prefer to think of FTL as simply a way to get around space being too vast rather than something terribly unrealistic and unsettling.

For some concrete examples to maybe solidify what I am talking about, consider the warp drives in Star Trek. Warp 1 is the speed of light, but the real speed limit is Warp 10, which can't normally be reached, and if you do reach it then you get time travel. Also, real-time communication is possible across incredibly vast distances (via "sub-space", whatever that is). These things basically imply that the real maximum speed of travel is Warp 10, not $c$ = traditional speed of light = Warp 1. This kind of thinking is made explicit and concrete in Futurama, wherein scientists increased the speed of light in the year 2208. Again, this allows fast travel over interstellar distances while also avoiding time travel and making it plausible that space travel is not prohibitively expensive.

• Mostly irrelevant side note: Warp 10 is only the limit in some Star Trek implementations, and it's because of the new formula. The new formula is exponential up to warp 9, then uses a weird formula that's asymptotic to warp 10. This means warp 10 is, by definition, infinite speed. Under the old warp formula, you can go to any arbitrary warp number (TOS has instances up to warp 36). Also, while infinite-speed warp 10 used time particles, it wasn't time travel. It just let you jump instantly like a teleport. – MichaelS Aug 24 '15 at 8:00
• The question is “without FTL…”. Saying to just use FTL is not an answer. – JDługosz Feb 15 '17 at 7:00
• The Enterprise traveled in time on occasion, without being able to reach Warp 10. Given Special Relativity, FTL is equivalent to time travel. The reason it is generally accepted in science fiction is that most people don't grok relativity. – David Thornley Dec 11 '18 at 21:31

Okay here's some numbers that Larry Niven has come up with for interstellar, and in fact intergalactic, experiential travel times, they clearly assume fuel is free and infinite (which is theoretically possible but the tech involved could probably be used for FTL cheaper and easier) but they're for one G continuous acceleration flip over deceleration trips. The point is they make it clear that the crews are in for a long but survivable trip; 4 years to Proxima Centauri, 21 years to the centre of our own galaxy, 28 years to Andromeda. (As a note I'm reasonably sure that the longer trips at one G would actually violate the speed limit, C, but they're also unrealistic in the scope of the question in my view so I'm treating that as a non-issue for the purposes of this discussion). So crewing a STL trader is awkward but not too bad, so the question really is what kind of trade goods are valuable enough and keep long enough to be worth the hassle and good at the end?

I'm going to assume a "post-scarcity society" in which nanotechnology is limited and in particular biosynthesis is problematic, rare, expensive and/or nonexistent. I've also assumed that there are starships that can reliably bridge the gulf between the stars carrying worthwhile cargoes of goods. This leaves several categories of trade goods, Art and Historical pieces - the antique/museum trade, Information - this can be IP like the light speed datanet in Wil McCarthy's Lost in Transmission (the problem with this is that we don't really have a means of transmitting data across stellar distances, see this article, and in the novel it does break down) or it could just be the mail, and "Unreproducibles" - this covers a lot of ground (and technically includes art etc...) but it's things that we don't have the technology to make and that can only be found certain places; I've deliberately categorised Art and Historical artifacts separately because there's a very limited stock of such things. Unreproducibles are the most likely to be traded in large value transactions, this could be the Melange around which the Dune series centres, the exotic timbers of Plague Ship, or the gem trade of Uncharted Stars, the trade goods are complex materials who's unique physical characteristics can't be synthesised, that only occur on a single planet or a couple of planets, and which are stable over extended storage times. Potentially this may extend to extremely rare elemental raw materials over short distances from exceptionally rich sources to particularly bereft markets, but that's unlikely.

Provided that there is a demand market for these goods that is sufficient to pay for shipping costs over interstellar distances they can be traded at any speeds, but it's going to look a bit more like feeding frenzy than what we think of as a "market", with supply and demand. There is the possibility of a limited futures market in this format since the ship coming in will be able to broadcast their goods at lightspeed while arriving somewhat slower but the actual arrival of goods creates a supply in a demand market that hasn't had new goods for some period of time. Prices are going to be at their highest just before the arrival of new goods and are going to collapse for a time when new goods flood the market from a freshly arrived shipment. This turbulence in the market is going to be awkward for traders and buyers alike, especially with goods that are available from several places instead of just one, arriving just behind a competitor could be ruinous to a trade ship. So in effect any trade route is going to be a single shipper monopoly, not because of any deliberate conspiracy or controls but because anyone who comes in late is simply going to go bankrupt.

There is one other good that will always be worth something in any society, and is actually more valuable in post-scarcity environments; Time, any drug, any treatment, any anything that can give people a longer life is going to be worth it's weight, or probably much more, in any material you want to name, if such a treatment requires an unreproducible substance then you have a license to print money, if you can secure the source. There are any number of examples of this in fiction from Dune to The Collapsium, in James Blish's Cities in Flight series they use life extension as a currency in and of itself.

Which brings up an interesting point, there has to be a currency to trade in for all of this to be successful, ships that can cross the gulf between stars are going to be expensive in materials, maintenance, and crew. The trip itself may or may not cost a lot too, I would suggest that fuel is going to have to be basically free to make interstellar trade economically feasible but that's me you may go another way. All of this is going to have to be paid for one way or another; any currency being used is going to have to have purchasing power at both ends of the trip, so that what you, as a trader, earn at one end of the trip is usable for fuel and maintenance costs at the other.

Short version: You can trade in sufficiently scarce/unique goods at any speed provided you have a high enough demand for these goods, a currency to pay for them, and can ship them before they "go off".

• Note: Those travel times assume sublight travel, but take time dilation into account. Assuming free 1G acceleration, it will take roughly a year to get close to lightspeed, so except for short hops (like to Proxima Centauri), figuring that the ship goes at approximately light-speed is close enough for how much time passes on Earth (in Earth's reference frame). To find out how long it will take the crew, just do the calculations and ignore relativity, and you'll be close enough. – David Thornley Dec 11 '18 at 21:28

The main development that might resonably occur in the near future is that our lifespan will increase substantially. Whether due to the singularity, or simply that medical science is adding a year to the average lifespan more frequently than once a year - humans may soon find themselves living for hundreds or even thousands of years.

What is a few hundred years to a human that is going to live for a few thousand? Once we've gotten to that point, stasis of some form or another will likely be close behind, so one can sleep during such journeys if there's nothing more interesting to do - but keep in mind that humans are creating knowledge and entertainment much more quickly than any individual can possibly consume, so just download a couple thousand years worth of entertainment and learning to the ship, load up with whatever cargo you believe will be valuable to the distant cluster you're traveling to, and hope for the best.

Trade over short distances can inter-act with local economies on both sides and can be part of the financial strategies of individual short-lived humans. Because of the relatively short times needed for each transaction, all economic entities, from nations and corporations down to individual people can participate and benefit from trade.

As the distances and necessary times increase, the economic entities which are long-lived enough to benefit from each transaction decreases. When a one way trip between colonies takes more than half a human lifetime, individual people can no longer benefit. At 10 or 20 times that distance, families and smaller corporations fall off the list.

Eventually, you get to a distance where even nations and super-corps can't be certain that they will survive long enough to benefit from a single transaction. There is only one economic entity larger than nations and super-corps and that is the whole of humanity itself; and it is this entity which has the most to gain for extremely distant colonization and trade.

Ultimately, humanity has only a single reason to colonize the planets of distant stars and it is the same reason which ultimately drives any life form to every action which it pursues; the avoidance of extinction.

We will colonize Mars because if we don't, then our species will die when Earth dies.

We will colonize Alpha Centauri (whether there are suitable planets there or not) because if we don't, our entire species will be wiped out by anything which kills our sun.

As we reach out further into the stars, no matter how distant, humanity becomes more immortal than it was before.

So ultimately, distant interstellar trade will not involve raw resources or manufactured goods or even information. The most valuable commodity in the universe from humanity's point of view, is its own genetic code (or in a lower tech form, a few thousand breeding pairs of living humans). Periodically, each distant colony will send a ship out to Earth and to each of the other colonies, containing whatever is needed to re-establish humanity on that colony world, just in case something has wiped us out since the last transaction. In this way, humanity becomes a self-repairing network of intelligent life across an ever increasing sprawl of galactic real-estate. In doing so, humanity approaches true immortal.

It is Noah's Ark or An Economy of Survival - It all depends on how you look at it.

• But that's not trade. – RonJohn Nov 10 '18 at 4:43
• Why is it not trade? A merchant caravan only participates in trade if it happens to contain items that are needed where it travels. Repopulation ships would only participate in trade if they happen to reach planets where human life had become extinct. If humans still occupy the target planet then it is no different than a merchant caravan visiting a city which needs nothing they have to trade. The repopulation ship simply moves on to another planet. But if the target planet needs a new population, the ship "sells" its crew and passengers in trade for total ownership of the target planet. – Henry Taylor Nov 10 '18 at 8:59
• "the ship "sells" its crew and passengers in trade for total ownership of the target planet." Spaniards miffed at revisionist historians claiming that Francisco Pizarro and Hernán Cortés are Bad People would love your definition of the word "conquest". – RonJohn Nov 10 '18 at 9:06
• Along with total ownership of the target planet comes the right to put any spin on their arrival as they choose. History writers always favor the new management. The distance of years dulls the blade of conquest until both the conquered and the conquerors are seen as the indigenous "victims" of the next conquerors. "Bad" doesn't have much meaning in the study of deep history; which is probably why we keep repeating the lessons it teaches. – Henry Taylor Nov 10 '18 at 16:38

Would a starfaring society be as developed in other sciences as it is in energy generation and system engeneering? If true, is such society is as advanced in medicine/genetics/cybernetics as it is in space technology? It could have some very long-living beings, some kind of cyber-elves with lifespan around the dozens of millenia, due to their very advanced cybernetics, biochemistry and genetics. Then you combine that long lifespan with relativistic time dilatation and the cyber-elves could solve the time issues.

The second issue is the energy and resources spent on such travel. I have no idea of what could be worth the antimatter that will be spent during the travel but then, i'm not a cyber-elf. Maybe some very rare elements, like monopole magnets or other rare particles.

Deep space trade would itself be quite cheap.

An ION drive is not fast but more than capable of providing cheap transport long range.

While each drive will have a price tag associated with it, the trick is not to manufacture them in the Sol system, but to send out automated mining, refinement, and manufacture ships to uninhabitable star systems to cannibalise the system for resources.

The second trick is not to send one vessel every few hundred years, but to steadily send them out. This helps smooth out supply ensuring that a consistent income can be generated.

This would be quite expensive if we used chemical rockets to move trade-goods around a gravity well, but there are more efficient (and cheaper) alternatives:

1. Build the trade goods in higher orbits.
2. Use cheap transport technologies, and gravity assists.
3. Build a space elevator.

Building trade goods closer to their delivery point is just good business sense. It removes all of those transport costs. The raw materials to supply such manufactories could be sourced from deep-space trade itself, or from goods in lower orbits raised by efficient transport technologies.

In system transport can be supplied by using ion drives (or if it works an em drive) with a solar sail. Obviously quick transport costs more, and may be preferred by humans, but for goods these technologies are efficient and can be used even now. Additionally leverage the opportunities afford by gravity assists to alter course and speed efficiently to produce a very effective in solar system transport network.

Space elevators are very real. We can with today's technology construct a space elevator on the moon. Once constructed the cost of transporting goods to and from the surface will drop dramatically. It is not too far fetched to believe that in the near future materials science could make space elevators possible for Mars, Earth, or Venus.

Home System Market

Hello Dyson Sphere, Halo Rings, and Generation Ships.

These are not cheap nor quick things to build. While a solar system may have enough material to build a few of these things, populations will stretch and demand more. This will place a pressure on local governments to reserve many in system resources for meeting immediate needs, at the cost of longer range projects, particularly if those projects are meant to leave the system, removing resources.

This will provide a mass market for material, and a predictable one. While the automated mining rigs will provide a bounty of resources, what each inhabited system requires will vary, depending on their local super scalar projects. This will require trade.

Colonial Market

In the early days of colonisation the system will not have the manufacturing capabilities to implement many super-scalar technologies that would seriously benefit the colony. It would make sense for these technologies to be assembled in another solar system from imported resources and then sent to the new colony. These might include:

• Orbital manufacturing
• Asteriod Mining
• Solar System monitoring satellites
• Space Elevators
• Halo Rings

Not all of these trades will balance out immediately, but they rarely do even in our world. As this form of trade is more long term, running a large deficit for generations would be expected. The balance might be restored in a number of ways:

• Accepting immigrants
• Supply materials back
• On supplying another colony (essential transfer this systems deficit to the new colony).
• Selling information.

Trading companies would probably start out as governmental institutions concerned with supplying resources for colonisation, and super-scalar projects. This would shield such institutions from having to worry about an immediate bottom line while they established trade routes, mining operations, and long-term storage facilities in high system orbit. Eventually these institutions would be privatised in an attempt to make market competition and drive down prices.

Trust

Finally trade cannot exist without trust.

There would be many opportunities to weasel out of repaying or servicing a trade deficit, particularly when your local space region is rich in controlled automated mining, and anything resembling a Navy would take years, perhaps even centuries to arrive.

There would also be issues around ensuring resources are actually delivered. Space is a dangerous environment even for automatons. It is possible that some of these ships would die. Their resources might make it to their destination, but how would the sender know? The resources might fly off into deep space, so how would the receiver know that the attempt really was made?

At the end of the day, most (but not all) colonies would maintain the status quo on trade. Not because they wanted to, but because it would stabalise their own solar-system economies. Excess production can be dispatched to consumers, and conversely demand can be satisfied through importation.

I like the idea of a silk road, as Allen Gould mentioned. Traders on the silk road would rarely travel from one end all the way to the other with a carriage full of spices or gold. More often they would travel a stretch of the silk road, and trade their wares to the next trader along.

A large number of self-sustaining deep space waystations between stars populated by trader societies, each trading with the next on a long link between two stars would eventually allow for a flow of goods and wealth between them.

The aliens grow a plant that grants longevity (hundreds of years) to human life spans. Humans have something similar for the aliens. FTL is not possible so build robot ships. It may take one ship 300 years each way, but that does not matter. This does assume that interstellar space is mostly empty, so the probability that the ship survives such a long journey is reasonably good. The valued product is stored cryogenically. Some animals and plants can be frozen or dried and return to life after some time. With enough research it might be enough to survive the trip. Assuming that transport of living aliens is not an ecological disaster, it would be an amazing curiosity.

• Why not learn to synthesize it locally? E.g. see SF’s answer above. – JDługosz Feb 15 '17 at 7:12
• There are plenty of molecules that we cannot synthesize using current methods. Obviously, both sides will try. Spider silk is one such material. At least one company makes "synthetic" silk (Kraig Biocraft), but they use genetically modified silk worms. As far as I know we can make the proteins, but we cannot assemble them correctly to make silk. In Dune, why didn't they synthesize spice? A guild controlling rarity might find it useful to prevent a synthetic knockoff. Maybe most of alien life is a specific enantiomer of Earth life. The idea of DNA is the same, but the molecules are different. – TimothyEbert Feb 16 '17 at 19:48

Without FTL... and even with it really. The most precious cargo is information, art, and "history", because once you're in space the only reason you go back to a planet is because you personally might prefer to live on a planet, but the majority of people will live on space stations/habitats and low Gravity places where all the materials needed will be petty much freely mined from asteroids around the solar system. Anything that is missing can in theory, just be created, even if you're dealing with a situation where fusion is expensive, if you need something the expense is worth it.

Even with Earth based current technology we're on the edge of physical materials that aren't information, art, or "history" are pretty worthless because we are getting ever closer toways to experiencing more with less via Games, Movies, Books, and eventually VR.

So how would "Trade work". Not like the trade we see today where you are assuming a two way flow between nation states, but rather more like in the past during the 1500s and before. I want to be a trader so I look at cultural things that probably haven't developed on my destination or things that may carry cultural value. I also load up on as much data as possible, because even with STL travel anything you take with you will likely be of higher quality than a transmission simply due to the signal being too spread out and not to mention eventually having a satellite system always arranging itself to every possible node world would just get too much for a system to bare. The last thing I look at is with whatever cargo room I have left I bring raw materials and maybe a bunch of replacement parts for various ship types that fly around in the region. The parts might not get used for hundreds of years, but also those parts, due to the length of time that ship flies around will likely be valuable since especially is the Data for the 3d printers on those ships get corrupted/damaged and it is likely that even if your colony does have a data file on those parts you might have to venture into very old stored data that the comparable cost wouldn't be worth it... and those parts can be used as raw materials anyways so no real lose there.

The Raw Materials themselves are only worth as much as colony actually needs it. They still have to process and mine those materials so even if they have it they will still have a value of some sort, more for in those places that have less of it and less in places where they have lots, just like in regular trading.

You get to the planet and you post ship manifest to the colony's database and then people who see something they want they will mail you, visit you, or propose a trade. Sometimes that will include conversion of currency, sometimes it will just be barter trading of old.

This is nothing more than the ancient barter trading of traveling merchants of old. The only difference is that by the time you get home your home might as well not be and so you might as well keep traveling...You trade for what you need and for what you think will give you a better return on your next colony. If you are going between the same few planets you'd probably want to set up banks on those worlds to hold assets on them as they acrue intrest which will more profital than doing that in the past was because you'll likely be gone far longer.

Here's the very interesting part of this though... the most valuable thing and ironically the thing that will get you locked into this world. You'll end up living thousands of years from the perspective of regular world and you'll be like walking history of the time you originate from, and each planet you visit and see you'll be able to tell historians what it is like and compare the time periods which will be extremely valuable as trade commodity... So in a sense you stop becoming a trader and start becoming a story teller. The reason it locks you into the life style if you do it long enough is simply that the longer you go the more out of touch you will become...but that's somewhat a different topic.

So the TL;DR version is basically... the same way they did it in the past, just the time differences will be greater. There isn't nation to nation or company (as we think of them now) to company trading. Only individual to individual trading...

Instead of trying to reach the light speed limit,(and using up far more energy than whatever goods your are trading worth), simply have your protagonists live slower: have your protagonists being liquid hydrogen based life form living on a cold planet, like Pluto, where the gravity is far lower and the temperature is far colder. Then, slow down your major races’s metabolism, making a year to us be like a day to them.(for example, your protagonist(s) have an average lifespan of 70 Pluto years(365 earth years per Pluto year) and consequently moving and thinking 365 times slower than earth life, making a year’s wait a mere day to them) (Which you can also say that it is the consequence of living on a slow and cold planet)

It turns out, being slow is greatly advantageous to space travel: you don’t need a generation ship if your 100-light year away colony is a mere 8 month of traveling to our slow life: what looks like a year to us feels as merely a Day to them, and you doesn’t have to worry about economics: receiving a package shipped from Pluto to alpha Centauri that takes 8 years to arrive to them would feel just like receiving a fedex package from amazon to us that takes a week to arrive. And an hour of delay from Neptune to Pluto would be reduced to about ten seconds when your "second" is 365 times longer than a familiar second on earth.

What you effectively have done is that you have just increased the speed of light to whatever multiplier your races experience time when compared to us: if a year to us is a day to them, then a light-year of distance is merely a light-day to them.

On the another plus-side, if you have very slow metabolism, then your ships don’t have to go very fast: 3 percent of speed of light, then you still have to wait just 5 months traveling from Pluto to Alpha Centauri C, same time as going from earth to mars for humans, and at about the same cost(to a fusion mastering civilization) as well. Perfect for bulk shipping of goods. This is about the same perceived speed as the best alcubierre warp drive postulated by our modern understanding of physics.

Then, you can argue that since interstellar travel only make economical sense for such slow metabolizing life forms, have all your races being this same type of slow metabolizing organism, which means that you just need to swap out the water with the liquid hydrogen, the rocks and soil with water ice and tholin, the oxygen gas with helium, and the carbon base with orthocarbonic acid(dry ice), then argue that it have to be so slow because of the heat generated by the metabolism: a 62.4k planet radiates away heat about one 365th as well per square meter as the earth does, so a metabolism based on Pluto or titan fluids must be 365 times slower than the equivalent based on earth fluids in order to not boil itself or it’s surroundings.

Then, just tell your audience that your protagonists measured the speed of light as 1.095x10^11 meters per "second", your "water" boils at 27K and freezes at 22K, and you are all set for your scientifically plausible interstellar trade masterpiece!

slow metabolism.

if an alien experiences an earth standard year as the equivaent of one hour on their (slow rotating) homeworld, and lives their lives for 700800 years, while processing the same amount of information as an average human processes in 80 years, mere traveling at 5% speed of light allows them to cross 10 light years in 8.8 subjective days, and crossing the milky way would take at most 2.5 lifetimes, implying a (very huge) managable area for their possible empire, without the need for faster than light technology, relativistic travel or cryonics at all.

if ftl is impossible, then because of the timescale involved, no civilzation with regular, humanlike rate of metabolism will ever set foot out of their solar system, leaving the (largely uninhabitated) galaxy to the races that are much slower.

in fact ,the slower the metabolism for a given race, the larger the effective managable empire the race can colonize, and with dumb autonamous systems, no weakness through slowness would be realistically incured, as dumb robots clearly does not do politics nor trade, trading between the actual, slow living sentients would be about the same as the more traditional FTL worlds.

The dominant forms of life of the galaxy would also be unlikely to discover relativity, nor functional radio before they went to the stars, rendering them unresponsive and invisible to terrastrial seti efforts.

So everyone is talking about how the lack of FTL is basically the end of interstellar trade. I agree with them. So I started thinking, "What would interstellar trade look like? What would it have to be?"

With travel times in the ranges of thousands of years to a few million years, whatever trade good one is trading would have to be slow. Really slow. Your "pressing need" for something needs to be measured on the timeline of a generation ship taking a thousand generations.

This means the good you need has to be hard enough to obtain that it's worth trading it over the course of thousands of years. We're not talking antimatter, we're talking something far more valuable.

What if the trade is in life itself?

Evolution operates on a million year scale without too much trouble. What if the valuable resource you are trading is strains of life in different forms. Each one took millions of years to evolve on its home habitat, so they aren't very easy to come by. Transporting a good breeding stock over a few thousand years is actually quite a trick!

If you are the kind of civilization that looks ahead tens or hundreds of thousands of years, you could see a bespoke need for a particular strain, and put the call out to a nearby star for your need. They pack the goods up, and ship it on a generation ship in time to save your civilization. We're not talking Fed-Ex 2 day shipping here, but shipping on the scale of geological time!

And if you needed tension in a plot, this has it. You literally have an entire ecosystem at stake, as they eagerly await their transplanted strains of life.

• The problem with this answer is that trade is two-way. "You give me what I want, and I'll give you want you want of equal value. The original seller's species won't be around in the multi-million span it takes to make the transaction. – RonJohn Nov 10 '18 at 4:40
• @RonJohn If so, then that short-sighted species that can't even exist for millions of years probably isn't in the interstellar trade business. Its reserved for the species with the long-sighted vision needed to span these timelines. And remember, we humans invented the concept of species, not nature. – Cort Ammon Nov 10 '18 at 4:40
• I think you're trolling. – RonJohn Nov 10 '18 at 4:41
• @RonJohn No more trolling than anyone talking about Class II civlizations, or other obnoxious things that are clearly beyond any reasonable species... or maybe they aren't. We're really only limited by our imaginatino. – Cort Ammon Nov 10 '18 at 4:42
• If you think that's a strange one, I highly recommend "Manifold Time" by Stephen Baxter. That story deals with timescales that make a few million years look like a walk in the park. Well, not quite. More like a step in the park. Maybe just the first shifting of weight. The story includes timescales where proton decay actually starts to matter. – Cort Ammon Nov 10 '18 at 4:45

### It isn't practical and there won't be anyone to trade with anyway

There's zero practicality to trading anything other than information- but even that is impractical because we'll have to wait decades for responses that might not even transmit reliably. The number of inhabitable star systems within that distance is small and unlikely to have another intelligent species at a compatible level of technology. FTL is basically a must for any sort of interstellar trade.

### ... unless you happen to have a wormhole

The only way it could be remotely practical is if a wormhole connects the star systems of the two trading civilizations. That at least puts things in the manageable range of travel/transmission time. Message latency (speed of light) would be somewhere in the neighborhood of 6 hours and travel time would be a couple weeks to a few months round-trip, depending on where the wormhole is in each star system and how fast ships can go.

I have read a few stories set in such a universe. The most common answer seems to be nomadic "traders" (who are similar to vikings) using NAFAL ships. They wander from place to place following rumors and their gut instincts. If they find unattended natural resources, they collect them. If they find a weak civilization, they pillage it. If they find a strong civilization, they trade with it.

When traveling Nearly As Fast As Light, most of the logistic challenges involved in interstellar flight disappear. We're not talking generation ships or spending centuries in flight. At 99.986% of the speed of light a trip to Alpha Centauri would be about two months subjective time. Ten years objective time, but that doesn't matter much to nomads. I've seen theoretical discussions of solar sail designs that, combined with either nuclear pulse or antimatter drives, would possibly let a ship get pretty close to that, and that's without factoring in the interstellar currents that would exist if the "plasma universe" theory is correct. (Our space probes have already discovered that at least some of the predicted interplanetary currents are there, so there would seem to be a decent chance...)

• I think you underappreciate just how much energy it takes to get a craft up to that kind of speed. Yes, antimatter is good fuel, but it’s not a natural resource: you need primary energy source to make it before you depart. – JDługosz Feb 13 '17 at 4:09
• In any case, thus does not address the issues raised in comments and other earlier Answers: the resources they collect, pillage, or trade are not worth the expense of going there. – JDługosz Feb 13 '17 at 4:12
• The antimatter drive would mostly be for manoeuvring and course corrections. I've seen a few theoretical writeups on using solar sails to propel a vessel to a significant fraction of the speed of light using no fuel at all, and if the plasma universe theory is correct and the gravitational anomalies we've seen are caused by large electron currents instead of by "dark matter" then riding those currents could get you going insanely fast with the only fuel required being for shifting between currents. Just watch out for dust particles. – Perkins Feb 14 '17 at 21:45
• I don’t think solar sails get that fast, though laser beamed sails can. I guess your real point is “the story universe, different from the real one, provides handy shipping conduits that make trade practical” – JDługosz Feb 15 '17 at 6:54
• @JDługosz From the ship end, there's no difference between sunlight and laser light. The only question is whether your source provides sufficient power. I'd expect you'd have to choose your stars carefully. (Stay away from red giants.) We're not sure if the real universe provides this kind of "conduit" or not. They exist within our solar system, but so far none of our probes have gotten far enough out to report on whether they exist in interstellar space or not. Check out the plasma universe theory, it's really quite fascinating. – Perkins Feb 23 '17 at 19:11

Human Trafficking and Slavery

A timeless commodity that will inevitably persist well into the future.

Human populations will likely genetically diverge across planets across time. Humans could be sold into slavery as curios of odd traits or relics of human evolution. Kind of like keeping a caveman in a zoo.

They could even be useful in genealogical medical experiments trying to fight modern pestilence with different genomes.