Can time travel make us rich through trading, and is this a problem?

At different points in history, commodities have been at different levels of supply, and therefore have been at different prices. One good might be valuable in the present while another is nearly worthless; a century ago, this could have been switched. Therefore, any time traveler with an eye for making a profit could realize that there's an easy way to become rich:

1. Buy a cheap commodity in the present.
2. Sell said commodity in the past, when it was more valuable.
3. In that same time, but a cheap commodity that is valuable in the present.
5. Profit.

If this is done on a large enough scale by enough time travelers, it could wreak havoc on global economies throughout time. This could lead to shortages of goods in time periods when they could be needed desperately - or in the years after trading begins, when prices would have risen.

Assuming that the trading does not happen at what the Doctor in Doctor Who calls "fixed points in time" - in other words, assuming that this trading can change the past, present and future - then I have a few questions:

• Is this strategy for getting rich feasible?
• What would be the large-scale economic effects?

Miscellaneous notes:

• Time travel first arose around the present-day.
• Due to the nature of the mechanism, you can transport about one person and a suitcase at a time. One time travel machine is not great as a mass-transit device.
• I'm interested in how the economies of different eras would be affected as people slowly gain access to time travel after its invention. Growth is limited.
• Time travel was a government invention originally, but the secret was leaked pretty quickly. It was soon legalized for the public, but heavily regulated by most major governments. Certain companies - in the same vein as airlines or train companies - rent out machines for short trips, although there is a little bit of monitoring on their part. In other words, most people can use it - if you can afford a ticket of roughly \$10,000, for enough fuel to jump a total of perhaps 200 years (round-trip). • These same companies will monitor their customers, to try to make sure they don't lead to the end of the world. Minor changes to history might be allowed to slide. • In theory, you can travel as far back in time as possible, but you need more energy to do so, and more energy means higher costs. Very few people will be able to travel before 1,000 AD. • In response to some comments/answers regarding paradoxes: I think that if you take a suitcase's worth of a good that's cheap - and likely in good supply - the market will essentially damp any ripple or butterfly effects it could cause. It's the equivalent of a drop in a bucket. Because this isn't just an issue in one world - although I've described the specifications arising in the particular world I'm working on - I'm a bit wary of getting overly specific, but I hope the above constrains the question enough. • I think that the invention of practical time travel would change the world so much that the effects on commodities would barely even be noticed. Time travel screws with everything - for example, you say that it was developed in the present - but as soon as you develop time travel, you can of course invent it earlier... – Matt Bowyer Jan 6 '17 at 14:45 • There is one commodity, that is even more powerful, then any item or material - information. With the right information one can literally change the world, the past, the future, and time-travel itself. You will need to somehow describe, why one traveler cannot change everything know, or how government could not stop the leak by using time machine. – David162795 Jan 6 '17 at 14:47 • The answer to this question depends heavily on your particular time-travel paradigm, but at the very least this is adding an extra level of complexity to an already complex system, namely Economics. – Joe Bloggs Jan 6 '17 at 17:13 • @df86 That would cause a causality problem. If you go back in time to get an object before you took it originally, the first one you had would cease to exist. You can't create new matter or energy. – HDE 226868 Jan 6 '17 at 18:49 • Time travel first arose around the present-day. - wrong. Time travel, by its very nature, is invented simultaneously in all epochs. - Douglas Adams. – Mindwin Jan 7 '17 at 0:07 18 Answers If I understand you correctly, you want a loop system where a person takes a cheap commodity from the present to the past, sells it, buys a cheap past commodity, and brings it forward in time as it is worth big bucks in the present, correct? The problem here is the very small amount of "cargo" that a traveler can bring back, just a suitcase. So they are limited to extremely low volume, high dollar stuff. Like gemstones, spices, medicine, possibly technology. I think you would have decent success bringing modern stuff back to the past and selling it there (like diamonds, medicine, clothing dyes, spices) but I'm hard pressed to think of much the past could offer the future other than specific pieces of artwork or documents (imagine a copy of the bible signed by Jesus himself....) But you are causing NUMEROUS causality loops, as the material you are bring back to the past will alter events leading to the present, especially if this is a casual thing that you could do on a lark. Plus anything we remove from the past and bring to the present will have to be accounted for (i.e. taking all of Picasso's paintings as he makes them will drive an artificial scarcity process that eventually removes ALL of his stuff from history, and thus eliminates their perceived value entirely as history will never know and appreciate his artwork). Adrian beat me to it, but there would be a lot of value in bringing forward biological specimens. There are extinct species now but we could bring forward egg or young. Certain herbologists or flower curators would probably pay big money for samples of various plants through time. So a possible sustainable loop that would minimize paradox/causality would be bringing back highly liquidable commodities just as gold, spices, etc in order to finance expeditions to retrieve biological specimens (lots of that going on in the recent past). You can bring back medicines to enhance the expeditions success that wouldn't have to be explained. Then you bring forward samples/live animals. This shouldn't create too many waves in the past (lots of Dodo eggs at one time). What you really need is a series of parallel dimensions that we can exploit for resources, so what we bring back won't affect our specific dimension's history, and what we bring forward will only add to the resources we already have. But in your situation I imagine that past cultures, or at least select influencial members (kings, generals, etc) will quickly react to these "time gods" and cater to them, as they become dependent on the goods they bring. Entire societies could work to develop whatever the time god wants (like trying to recover the giant gemstones that lead to things like the Hope Diamond). But canny past rulers, especially if they are relatively near in the past (maybe just a few hundred years), could try to suss out a time gods parentage in order to hold their ancestors hostage, thus threatening the time god itself with oblivion in order to negotiate better exchange deals. Of course the time god could then go a little farther back and kill the RULERS ancestor, so you see where the paradoxes come into play. In a fixed time stream the time traveler is chained to the past but they are constantly rewriting it, even in small amounts. • There's no sensible way in which people in the past can threaten the parentage of a time god. Case 1: parallel dimensions, threatening time god's parents doesn't do anything because it's not really "his" parents, just a parallel copy. Case 2: causality loops, if all time incursions are part of causality loops that can't actually be changed, there's no way to carry out the threat, something will happen to stop you, or you incorrectly identified the time god's parents since clearly they end up existing. – Shufflepants Jan 6 '17 at 15:42 • Case 3: you can change things, there's no parallel dimensions going on, and not everything is a causality loop. In that case there's no need to threaten the time god, by merely setting foot in a time before their own birth, they have almost certainly caused enough change in the universe to at the very least ensure that their parents conceive a DIFFERENT child than them because a different egg is released or a different sperm manages to fertilize the egg. – Shufflepants Jan 6 '17 at 15:45 • Why do you think so many artworks have been lost? – Ángel Jan 6 '17 at 16:50 • it would be funny if the reason Dodos went extinct was because of time travelers bringing all their eggs to the future – Luke B. Jan 6 '17 at 17:57 • @JasonK: Michelangelo did not paint the Mona Lisa. – Eric Lippert Jan 6 '17 at 18:05 It's unrealistic that time travel would be legalized in any way, shape, or form. One easy example of why, is this: I buy an average,$500 hunting rifle. I take it apart and put it in a suitcase. I travel back in time to the 1800's. I assassinate Queen Victoria, because why not?

Or I leave a bomb buried somewhere where I know it will be discovered in the future. Or I conveniently arrange for an accident so that that one guy who bullied me in grade school will never be born (kill one of his grand parents in their infancy, for example). Or I pass myself off as a God to some ignorant savages and start my own creation myths, which then spark religious wars which will sweep a continent. Again, why wouldn't I? A lot of people would find it amusing, and damn the consequences.

The economy is really the last thing you need to worry about, although it too will be screwed. Why? Because I can similarly travel to the past with a military grade rifle, kill some king's tax collector, and travel back to the future with a suitcase full of gold. Or jewels. Or some priceless artifact.

Once time travel is out of the bag, as it were, even with government regulation, it's going to be nigh impossible to control what people do in the past. And although contraband might conceivably be caught, there's another sort of interference which is incredibly dangerous: information.

Imagine some ideologist going back and feeding critical information to their dictator of choice. For example, Hitler might suddenly make some brilliant military decisions. Or the Soviet Union might develop the nuclear weapon before the US. Are you prepared for the consequences?

There's just too many things that can, and will, go wrong.

• Which is why every time a scientist gets close to figuring out time travel, they die in a lab fire which destroys all their work. The TimeCops aren't really cops. They are assassins. – Henry Taylor Jan 6 '17 at 14:49
• @henrytaylor - it would be insane for them to be anything else, in all honesty. – AndreiROM Jan 6 '17 at 14:51
• Because why not? Because that's murder, obviously. And you're also suggesting fraud and theft, more criminal actions. The TimeCops wouldn't assassinate you, they'd notice that the queen was assassinated with a modern rifle, and give you a fair trial, where you travel to a nearby spot and watch you load the weapon, and pull the trigger, and then they'd reveal that they replaced the gunpowder in your ammo with salt yesterday. Then you'd spend time in a prison. – johannes Jan 6 '17 at 19:25
• Would have to, or would of already had done? ;) – djsmiley2k Jan 6 '17 at 19:58
• @BgrWorker A time traveller's memories should remain unchanged, because that's consistent with his history. Anyone in the time he came from would have their memories changed because their past had been changed. The time traveller may suffer the fate of returning to a time when he doesn't exist, or if he does then there could be aversion of himself who didn't need to assassinate someone in the past. Single timeline models tend to resist being changed. Not good for time travellers. – a4android Jan 9 '17 at 12:36

Why trade when you can invest?

Trading is slow, and difficult, you need to make multiple costly trips each time accumulating a small profit margin than accumulates as a multiple of the number of trips. The alternative is to travel into the past and use the wealth you take back to invest in other people's labour. Even at small ROI, you can accumulate large sums of money through compound interest.

Why invest when you can gamble?

Assuming that not enough people are time travelling to completely destroy the gambling industry, you can use your perfect knowledge of the future to beat the bookies, make winning stock-market bets, and generally profit from high multiple payouts on small initial investments. The great thing about this is that you can do it in the recent past where you can simply take the money into the future with you or even pay it into your own bank-accounts.

• Yeah, if you want a quick profit, you just go hit the lottery (assuming it's still running, which would be rather dumb in a world with legal time travel). Then you've got your initial millions and you can do whatever after that. – Geobits Jan 6 '17 at 15:32
• @Geobits The butterfly effect should mean that lottery results are different every time. Narrative imperative demands it at least. – Separatrix Jan 6 '17 at 15:37
• In a book, I guess, but that would seem to apply to any gambling effort. In "real life", I don't think travelling to thirty minutes before the lotto drawing several hundred miles away would have any effect on it. – Geobits Jan 6 '17 at 15:39
• The most valuable thing you can trade and also carry in a suitcase is information. – johannes Jan 6 '17 at 16:27
• @Separatrix One could rent an airtight room with Internet, travel from and to it, but the ticket online, then wait in the room. Butterfly effect averted. – Emilio M Bumachar Jan 7 '17 at 23:48

Buying commodities using time travel is dangerous because it changes the chain of ownership of that commodity, basically taking the purchased resources out of play in the flow of time until their present day redemption/sale. If the commodity in question was heating oil (for example) and a time traveler bought 100 household years worth, then 100 households would go without heat. The resulting head colds would change the work history of the members of those households, starting the ripples which would catastrophically change the present day.

Here is a much safer way to profit from time travel...

Join the archaeological forums and watch for an announcement of a new;y discovered tomb or treasure. Wait until the carbon dating results come in to find out what date you need to travel to. Load up your suitcase with weapons and jump through time. When you arrive, kill all the other time travelers who had the same idea; then plunder the treasure, putting the best future artifacts in your suitcase for the journey home. In this way, you are only stealing those items from the past which will not be missed in the intervening years. The fact that some present day archaeologist opens up an ancient Egyptian tomb and finds it full of dead time travelers instead of treasure, that is unfortunate. ...but it doesn't mess with the continuity of time.

• I'd like to argue against the first point. If you pick a cheap commodity, odds are that it's extremely plentiful, meaning that it's unlikely that on a small scale - remember, you're limited to a couple suitcases each trip - you could influence that many people. You simply couldn't remove enough to make a significant difference. – HDE 226868 Jan 6 '17 at 15:10
• That assumes that time is kind as is suggested in Dr. Who, where it happily adjusts to minor changes. What if time is not kind, but instead, when you sting it (even just a little) it scratches the sore spot, making the ramifications much worse than they could have been. The trope of stepping on a Jurassic butterfly having apocalyptic ramifications makes more sense in my "time scratches the itch" universe than it does in Dr. Who. I don't think there are any insignificant differences when time travel is involved. Or at least, by thinking that way, time travel can be a lot more interesting. – Henry Taylor Jan 6 '17 at 15:19
• Also, a suitcase full of commodity certificates can represent a whole lot of heating oil. – Henry Taylor Jan 6 '17 at 15:20
• I'd be inclined to just pick - as the worldbuilder - a universe where it squirms a bit in its seat but keeps sitting down, so to speak - but I see your point. And there's not really a clear boundary I can draw. – HDE 226868 Jan 6 '17 at 15:24
• The key point in your answer is "kill all the other time travelers" -- all of whom will, if they are smart, also be trying to kill all the other time travelers. This introduces risk to the trade, and if it is no longer a sure thing, then only those willing to risk their lives will attempt to make the trade. – Eric Lippert Jan 6 '17 at 18:02

Time travel breaks cause and effect. Making a profit depends on cause and effect.

As long as you're the only person doing this and you do it carefully on a small scale, you should be ok. Once it gets out that this is possible and people start doing it on a large scale, economies collapse, cause and effect collapses, the concept of "being rich" collapses, and ultimately the entire concept of one thing happening before another also collapses.

That's in a universe which allows grandfather paradoxes. In a fresh timeline universe at least the rest of us are protected from your madness. In a predefined, no free will universe, you did it because you always had done it, so you had to do it when the future came and nothing is affected.

Time travel is invented simultaneously at all points in history.

In terms of actually making your fortune, take crafted aluminium goods back to a point where aluminium is valuable, sell for gold. Ensure that you're handing small quantities of high value goods, you want to make as few trips as possible for the greatest gain, selling only to the richest people in society. You certainly don't want to be moving large volumes.

Buy land, buy it when you are. You should be able to pick up large swathes of now valuable, but then worthless, real estate in places like London and Manhattan for a song. Set up trusts that lease the land and allow it to be developed while keeping the freehold and taking ground rent. The present day wouldn't notice as most people are barely aware of who actually owns the freehold on a lot of that land.

• The first section - the point on small-scale trading - was my main hope here. Yeah, the butterfly effect is a thing, but I figured that there would be enough "damping" that small perturbations would be absorbed by the markets, and there would be no real problems. – HDE 226868 Jan 6 '17 at 15:03
• Your trusts will fall foul of rules against perpetuities, which have been relaxed in some states now, but were in force when aluminium was valuable. “No interest is good unless it must vest, if at all, not later than twenty-one years after the death of some life in being at the creation of the interest.” You can't set up a perpetual trust and benefit from it a couple of centuries later. – Mike Scott Jan 6 '17 at 15:42
• @MikeScott, that means I need to show up every few years and maintain the trust. Reports of my death are greatly exaggerated. – Separatrix Jan 6 '17 at 15:46
• I like the sell aluminion buy land but you can also invest in thecnologies. You can seek Bell, Tesla, Edison as an investor. You can even buy patents or even join Steve Jobs or Bill Gates as a big company founder. If you still wants to live in the present you can hire a lawyer/investiment firm to represent your interests for decades to come – jean Jan 6 '17 at 19:29

Time travel enables the end of the world (as we know it)

Eventually, everything goes nuts. As the Good Doctor says "Time is this weebly-woobly timey-wimey stuff". Given the description of how time travel works, where everyone is on the same timeline, pandemic chaos ensues...but probably no one knows it.

Non-time travelers Perspective

For someone who doesn't have access to time travel, time will pass just like usual. They will always be the result of the past, not the future. Even changes to the past that result in that person not existing anymore would be no different than a change that absolutely ensured they do continue to exist. For their perspective, the past always is.

Time Traveler's Perspective

Going back or forward is easy. Returning to where you were is hard and predicting what "here" will be like is extremely difficult. Profiting off information gleaned from the past may be incredibly difficult since that timeline may not even exist anymore. Similarily, profiting off of future information will be incredibly difficult because as soon as you bring back that information, it changes the future. The Butterfly Effect will kick your butt every single time.

The Really Smart Time Traveler's Perspective

Since there's so much change all over everywhere in the time stream, travelers will look for the most stable points on the time line that for whatever reason don't change, or change only a little. These would be equivalent to Doctor Who's fixed points. Perhaps points in the very distant past or very distant future where no matter what humanity may do, physics will lead to an inevitable conclusion.

The best place to setup this kind of a quiet spot would be around the first billion years of Earth's existence, the Hadean Era, because all rocks from that period have disappeared. Any manmade thing deposited in that era will have long since corroded away or been subducted into the mantle. Granted, it will take a lot of trips to build a base in this era because of the energy required to get back that far and care will need to be taken to get an atmosphere that's not outright hostile to human life.

From this stable vantage point, a traveler would have all of time to choose from. Money doesn't really matter anymore.

Our Privileged Position Perspective

Because we are outside the stream, we can see how much havoc is wrecked upon the time stream and all the causal chaos that results. We see world wars popping in and our of existence based on which time traveler assassinated which world leader at which time.

• Even ourselves are information (mass--energy), so just by existing at any given moment, we're already altering the future on that timeline. – noisypixy Jan 8 '17 at 16:00

For the sake of the market I am going to answer another question. You can travel forward at most 10 years and you can only observe. You go to future and find a stock that has skyrocketed. The plan is to buy up the stock in present time and simply wait 10 years in present time. Say your required return for investing is 10%. As you buy up the stock the price goes up so at some point you stop buying as the return drops to 10%. When multiple parties do this all stocks will have return more like a 10 year CD.

You may have an event that hurts a company but the stock price does not move as the even was already built into the price of the stock.

The only real winners are those that got to travel first and buy before the market adjusted to the information.

• And what happens when your stock has gained 10% in price but the dollars you get when selling it are worth 10% less due to inflation? You always want to remember you are attempting to increase value when trading, and not just number of dollars. – Mark Ripley Jan 7 '17 at 8:38
• @MarkRipley Then you would have a different required return. – paparazzo Jan 7 '17 at 11:07
• "those that got to travel first and buy before" -- except that with time travel, there is no real first and no real before. – LSerni Jan 8 '17 at 9:45

Time travel wracks causality. Either something would be done about that or the world be in utter chaos.

However, simply assuming that we can trade commodities through time would result in interesting picture. Throughout time, both supply and demand for various things changed.

High price basically means that something was hard to come by. If you have some cheap commodity that was expensive at some other time you can simply sell it there, making causing more efficient distribution of resources.

How much platinum can you buy today with suitcase full of pennicilin? What about 1800s? If you have two groups of people with different rates of exchange between platinum and penicillin, you can have mutually-beneficial trade between them.

Basically it would mean that prices are almost constant (because any disparity between supply and demand has been fixed), slowly creeping to accomodate major changes like discovering new continent with abundant gold.

Prices would not be exactly constant because transactions are not free - even if time travel itself was free, there's time travelling person who wants to get paid for his services too. The cheaper time travel is, the smoother prices are.

• One could introduce limited means of "time travel" without breaking casuality. For example, a machine that sends a limited amount of information backward in time, but with the proviso that if the information received from the machine might affect the information later put in, the machine might output any information which could have resulted from any information (not necessarily the same) the machine could have output. Anyone using the machine would be obligated to avoid breaking causality. It might be possible to cheat at some lottery games by sending numbers back in time, but... – supercat Jan 6 '17 at 19:56
• ...any games that were concerned about that possibility could alter the order in which balls are initially loaded into the machine, the timing of draws, etc. based upon what tickets were sold or other factors, thereby ensuring that anyone who tried to cheat would create a causal link between their purchase and the drawing result, thus making the result that was sent back in time useless. – supercat Jan 6 '17 at 19:58

When we do arrive at specifics, this question always gets cool -- so many problems to think through...

A lot of the ideas in here rely on 'go back and buy' concepts, but with what? No modern financial instrument is going to be negotiable in the past, you can't make a bet in the 1820's with a 20 dollar bill printed in 2012. Nor will any stock certificate issued in modern times be legal tender. Same holds for commodities.

Not too much you can fit into a suitcase is going to help you... technology transfers would be regulated, I'm guessing - to prevent time-machines-at-every-second problems noted above. This regulation is what starts to make the world interesting...

Your universe, which seems defined in a way that damage done to space-time by probabilistic changes carried on the quantum level does not destroy the future specificity entirely, as the butterfly problem suggests (besides, how would that be any fun at all?) but that you are regardless looping back on the one space-time-line. At 10,000 a pop (USD, is it? roughly present day) this is happening A LOT! I live in New York City, and know many people who pay that for rent! (not me, but - lots of people pay that a month here, and much much more) - so there would need to be an ARMY of regulators or the past would be lousy with tourists from your own time, and it would start to suck - like times square. At 10k, if it were safe I'd do it - if it were not safe, I still might.

In fact, yeah - if time travel were popularized, and normalized, and had gotten to the position in society where it were just accepted, you'd be looking at a kind of post humanity that just spends all of its present in the past - dicking around - trying to profit - on risky westworld-y adventures set up by pro-guides. the guide thing might emerge as a way to make money though - if it were more rarified, and heavily regulated. A person might be able to promise you safe passage somehow through his deep knowledge of an hour or so at the invasion of Normandy, and could take you there and get you out, perhaps..for a fee.

You can imagine time periods becoming trendy, like bars or vacation destinations are now, but why would anyone not go back and setting in the past, using what they know about the future to bend the world into profit for them - or godhood -

In other words, I guess unless you more fully describe the limitations on the time travelers in your world, or the tech they use to get around it's hard to say much, because the problems posed by the tech in the beginning are so paramount.

One thing's for sure, there'd be a bunch of "psychic" folks in little shops all over the city - staying in business somehow....hey wait a minute...

• " 'go back and buy' concepts, but with what?" Synthetic diamonds. The Excalibur sword made by using modern composite materials. Aluminium cutlery for Napolen III or modern blue textile dyes (when indigo was expensive). Matches for Nero to play with during his childhood. Small quantities, perishable goods which aren't going to change the history. – Adrian Colomitchi Jan 6 '17 at 17:13
• I'll buy that. Trade to get you into the coin of the realm and day. Enough to get you into the market with a position large enough to move the market . The motion created by a single investment/investor would be safe enough- a real problem might come from the broad effect of so many people appearing suddenly in the past. Economic impact of an arbitrarily huge number of people (all of the people that can time travel for the remainder of humanity's existence and/or interest in time travel) removing small amounts of value from history and taking into the future...the past could suddenly go broke. – case Jan 6 '17 at 17:24
• @AdrianColomitchi: "Matches for Nero". Now we know how that great fire started. – AlexP Jan 6 '17 at 20:22

If you are allowed to bring items back, you can obtain an unlimited amount of any quantity

• Take the most expensive transportable item you can think of, say the crown jewels.
• Go back in time and steal them, then bring them back to the present.
• Now go back to 30 seconds prior to your stealing them, and steal them again, and bring those back. Now you have two sets of crown jewels.
• Repeat ad infinitum. Economic collapse ensues.
• As I explained earlier, this is not possible. It would create a paradox. – HDE 226868 Jan 6 '17 at 20:36
• @HDE226868: The scenario in your question creates the same paradox. The same item exists twice in the same time (just older/newer). You've created matter. – BlueRaja - Danny Pflughoeft Jan 6 '17 at 20:42
• No, because each time, you're buying and selling different objects, not the same one(s). – HDE 226868 Jan 6 '17 at 20:43
• @HDE226868 It doesn't matter, it's the same paradox: you are still "creating matter" in exactly the same way. To illustrate this, imagine you take a jewel back and put it in a box. Now that jewel exists twice in that time (once in the box, one somewhere else). You can open the box in the future and bring it back again to create three jewels (two in the box, one somewhere else). Etc. – BlueRaja - Danny Pflughoeft Jan 6 '17 at 20:49
• "If you are allowed to bring items back, you can obtain an unlimited amount of any quantity" At a cost, of course. Since you are creating matter, you'll nee to inject in the process (and thus manage) huge amount of energy. – Adrian Colomitchi Jan 7 '17 at 0:10

You have, in effect, created a modified form of Aorist rod. Side effects may include large immigration flows of tourists, assassins, and/or entrepreneurs, with the first two proportional to how interesting your local spacetime is, and the last proportional to how cheap it is. In the latter case, supply and demand mean a flood of merchants from various points in the future will buy up all sufficiently inexpensive goods (relative to their own respective presents), trading them for, most likely, information and sufficiently advanced technology.

Due to network effects, there's some incentive to create "market years" where/when all the (peaceful) time travelers meet up to exchange knowledge, goods, and expertise. Due to the fact that time travel costs more the further you go, it's unlikely to be limited to just one market year.

The fact that we are not presently inundated with obvious time travelers suggests that prices here are not particularly low, nor events particularly interesting - market years will likely be the biggest targets. This is possibly due to prior successful actions on the part of more subtle and/or competent time travelers... who may or may not have assassinated or otherwise waylaid the first would-be visitors, just before they took their trips. All the groundwork's in place for someone to go back, after all, and if you can go back to the year 1917 from the year 2017, someone else can probably go back to the year 2017 from the year 2117. And if you're not careful about staying quietly anonymous for your foray into the past, well, they can bribe your bodyguards at any point in their lifetimes.

Thus, all history becomes dull and uninteresting: interesting historical figures (including would-be time travelers intent on making a splash) are quietly assassinated, blackmailed, or otherwise removed; interesting economic anomalies (anything becoming inordinately cheap, or expensive enough to justify bringing back in exchange for the local currency) are similarly dulled through market forces.

Destroyed art

It would make more sense to go back and fetch destroyed artworks than commodities. Show up the day of the fire or whatever and just steal them. The whole world gains a unique piece of art. And if you make a bit of money out of the deal, that's just gravy.

Crafted goods

The reason why this works is that art is worth more than its components. Often a lot more. Rather than commodities, move crafted goods. Use modern machining to craft goods better than what can be produced in the past. Sell those. You don't even need improved alloys, although taking a steel sword back to the bronze age is likely to provide good returns. A weapon fit for a king (or his bodyguard or best soldier).

Knowledge is lighter than commodities

Why carry goods? The easiest thing for a time traveler to take back in time is knowledge. It costs no mass, and it carries high value. As an example, it is trivial to look up the locations of diamond and gold mines. Or oil fields. Buy the land in the past with partners who can manage the actual mining or drilling. Set up a trust that invests the profits for you. Collect the money in the future after compounding has made it much more valuable.

You don't need time travel to carry wealth forward. Compounding value will produce better results. You only need to time travel wealth back.

This is by far not the best way to become wealthy. Since the Industrial Revolution, our total wealth has grown at exponential rates basically continuously.

The best way to increase your wealth would be to simply jump-start human civilization a bit earlier so that you are born later in our history. For instance, you could start the Industrial Revolution a hundred years earlier. Nobody in 1917 had living standards anything like we have now- and the same will be true for 2117. Of course, by the time you return, time travel will be cheaper, so you can go further back and kick-start it earlier, and earlier, and earlier.

Of course, that is small potatoes. You could go back a million years and jump-start humanity's evolution and civilization. We could be hundreds of thousands of years more advanced and prosperous by the time you return. All the money and technology that exists in today's world wouldn't satisfy a child for a day in the poorest nations.

Simply having the world of today but you have a few more dollars in your bank account is thinking far too small for what could be accomplished.

• Some time scientists believe this has already happened several times. – Marshall Tigerus Jan 10 '17 at 18:55
• End result is advanced space fairing society appearing across the universe as soon as it it is livable. After about 1million years from the big bang the colonies start appearing and soon you have a hyper advanced k3 civ watching the first stars form. – Donald Hobson Aug 5 '17 at 12:47

To stick to the originally stated scope of financial trading, I'll choose an appropriate example, which will apply to lots of different goods with high value density.

As everyone catches on, this will have the effect of smoothing out both the Dy price, and the Google price, since all that Dy is being bought in the present, and dumped in the past, and Google is being bought up in the past, up to the point where one can no longer make a profit on such a journey. Therefore, expect prices of each in the past to nearly approach their modern levels (allowing some gap due to the cost of transit). That will mean that this strategy won't be feasible for long, because arbitrageurs will take much advantage quickly. Furthermore, expect this same principle to apply to any two goods whose value ratios change widely over a short span of time.

The large scale economic effects would likely be beneficial, as commodities would show up where they are most needed. The above example is essentially like allowing the past to borrow dysprosium oxide from the future, which will bring down the price of magnets for power generation and electric cars in the past. "Sure", you may say, "but what about now? Won't that resource consequently be more expensive now?". To some extent, yes, but all the Dy that is being sent to the past is being put to good use where it is most valued. And things that get more valuable as time goes by (e.g. gold), will be flowing in the other direction, causing a shortfall in their availability in the time between, resulting in more mining, etc.

In general, anything that facilitates trade will be an economic advantage to both parties to the trade.

* This is maybe not even the best pair, since a peek at the europium oxide market shows that it may be an even better bargain today, and dearer in 2012.

• I said it before (see above) and I say it again. "Transport costs. You need to factor in the cost of time travel not merely the price differential between the goods you are trading." Also, the laws will be changed to prevent share trading. This is the equivalent of insider trading. It will be made illegal in a world where time travel is generally available. – a4android Jan 6 '17 at 23:27
• "(allowing some gap due to the cost of transit)". Insider trading is illegal only because it allows employees and management to take advantage of information before the trading public (not a great reason if you ask me). Obviously time travelers from the future will be party to relevant financial disclosures before past-dwellers, so it is plausible that trade in stocks will be outlawed (even though I think that would make little sense). But note that this can't possibly apply to metals, so you could still do gold-dysprosium. – guenthmonstr Jan 6 '17 at 23:35
• I know how & why insider trading is illegal. I said it's the equivalent of insider trading, basically illegal foreknowledge. Stock trading will need to be better regulated. I haven't checked the arithmetic of Au-Dy trading, but transport costs do need to be factored. The OP needs to give better information about the time travel costs. – a4android Jan 7 '17 at 0:03
• OP said 10k USD for 200yr round trip. That's little enough for lots of arbitrage on Au-Dy. Again, I don't see why stock trading needs to be "better regulated" since stock valuation on improved (i.e. definite future) information can only serve to improve the allocation of capital goods. – guenthmonstr Jan 7 '17 at 0:33
• To prevent time travellers from having an unfair advantage in the market. This does suggest an interesting scenario stock market-time travel scenario. – a4android Jan 7 '17 at 8:47

There are scenarios where it can work without changing the history: transport aluminium spoons and crowns, get back the DNA of Michelangelo and Nero - insert a passage of Newton being nagged by a microrobotic mosquito probe; or just fingerprints of past celebrities for collectors.

Speaking of DNA samples of the past: how about female/gynoid agents seducing and collecting genuine reproductive genetic material for those willing to pay enough to have a child of a male celebrity of the history? (maybe it can work for ova as well?) Any ethical conflicts for a story here? On the line of just the empty vanity of the customers, the children are missing the "nurture" side to realize the potential of the "nature"?

I would like to answer the question from the personal view of the trader. I assume the trader want to get rich with a purpose, maybe to live with the amenities of the rich. Then, why not use time-travel to get what you want directly?

Example: Buying status symbol in the "present"

• Get a loan
• Travel back one day with the money or gold

"What would be the large-scale economic effects?"

The longer time-travel exist the messier the future will get. Paper money will probably be forbidden. Gold will loose its value, if everyone could get it from the past or future. Suppliers maybe produce for customers that will come from the time-travel-past. Because you can´t create matter, the future could have a shorthand for many article´s. A consequence could be: The rich getting richer and the poor getting poorer.

Example: watch