Compound Interest Time Travel Gambit

"Compound interest is the most powerful force in the universe."

- falsely attributed to Albert Einstein.

"A hundred dollars put into a savings account at 6% for 200 years will be worth two million, which, by then, will be worthless"

- Lazarus Long in Robert A. Heinlein's Time Enough for Love

It's the simplest way to turn time into money: drop a few dollars into a compound interest bank account, then hop into your Time-O-Matic or become Cryogenically Frozen and come back in a couple dozen decades to find that your pocket change has exponentially increased to a fairly tidy fortune.

Of course, this particular moneymaking scheme is dependant on a few variables remaining constant at both ends of your time journey. The first being that no world-spanning disaster occurs and renders all forms of money worthless. The second being that Ridiculous Future Inflation doesn't counteract the effects of compound interest, meaning that you only get slightly richer (or maybe a whole lot poorer) at the end. The third being that the financial institution in which your dollars are stored never becomes bankrupt or closes in the intervening years. The fourth being that, in your prolonged absence from reality, your assets aren't seized under laws of escheatment or eroded to zero by inflated service charges. And, of course, that said financial institution doesn't have a 'no time travelers' policy to prevent this kind of scheme from occurring. Sadly, once you add up all of the sneaky costs and charges, the average chequing account all too often pays the account holder less than zero.

A more secure way would be to invest in something that regularly outpaces inflation (like stocks or bonds). The best way (especially if you can control the time travelling), is to make numerous stops, investing in securities that have very high short-term returns (for example, a stock that jumps 20% in value in one day, but overall has average growth), and continually reinvesting the returns. This removes most of the problems above, but may get you arrested for insider trading.

Another variable that is rarely mentioned in fiction is the need for authentic-period currency. A dollar bill will obviously get you nowhere in Feudal Japan, but the more detailed design on a series 2004 $20 bill is likely to be considered counterfeit anytime before 2004 (depending on how much the other party is paying attention, but bank tellers would be the most likely people to spot that kind of thing; oh, and don't forget that most coins have the year they were made on them.). You do not, however, have to go that far back in time. Just grab what's in your pocket, remove everything printed in the last year, travel to one year ago, and invest that. You wouldn't get your money in your present, but you may get it in the way cooler than your present future. The reverse is much easier, though; a 1928 $500 or $1000 bill is still legal tender, although worth much more than its face value to collectors. This of course raises the possibility of gathering collectibles before the majority are destroyed or lost, but that opens a whole new can of worms in terms of authentication and ensuring your actions don't eliminate its future worth. Alternatively, a prospective time-traveler could get around the currency problem by using their seed money to purchase non-currency valuables, like jewels or precious metals, and then sell those at your investment-point to get chronologically appropriate tender. But watch out that they aren't Worthless Yellow Rocks in the future.

Anime

 * In Doraemon, Nobita has Doraemon do this in an attempt to get a better allowance; though he does earn a buttload of interest, the bills are in future currency, so it only nets him a modest increase when it gets exchanged for present day bills.

Comic Books

 * Mystery in Space #12 "The Richest Man on 9 Planets". Sam Drake invents a suspended animation drug. After bank-deposting the single dollar he has left after paying his bills, he goes to sleep for a thousand years. Waking up in his miraculously still-intact basement in clothes that are still in perfect condition, he goes to the bank and retrieves his now $250,000 balance. Worried about changing monetary values, Sam tells the bank manager to invest his money in gold and goes to sleep for another thousand years, only to wake up in a world where gold is now common due to a cosmic accident.
 * There's a variation in a recent Spirou and Fantasio story where the villain goes back to the nineteenth century to buy houses in the Marais quarter in Paris, then a common slum, now the most expensive part of the city.
 * A Disney comic had the Beagle Brothers (and their mom) travel back in time to take advantage of how much cheaper everything was then... and paying with present-day money. Naturally they get caught and accused of forgery, as all their bills have wrong yearmarks.
 * A comic story had Pinky and The Brain planning to use a time machine to explot the trope. They'd open a savings account in present time and then go to the future to collect their winnings. Their first obstacle was finding a bank that agreed to start a savings account with 37 cents. After they finally found one and opened their account, the bank was robbed and they went back in time to prevent their past selves from leaving their money there in the first place. Brain's worries about Never the Selves Shall Meet was justified as somehow it changed the world to have it

Fan Works

 * Doug Sangnoir in Drunkard's Walk tries to plan around a variation of this trope during his travels from universe to universe by carrying "trade goods". The first time he (or rather a friend acting on his behalf) tries to use them, in Drunkard's Walk II, though, it fails spectacularly because they offer gems (his initial and only trade goods at the time) to someone who can create dozens of diamonds for pennies using a nano-fabricator.  In later stories he also carries gold bullion, jewelry, and cash left over from previous worlds; when the cash can't be recycled in a similar world, he can usually sell the rest to a jeweler or even a pawnbroker or "We Buy Gold!" outfit.

Film

 * This is invoked by Idiocracy's Joe to convince Frito to lead him to the time machine. Frito doesn't quite understand how it works, so he thinks he'll get the money
 * Time Cop has time villains trying to buy Wall Street stock on the day of the 1929 crash, and cheap Hollywood land back when it was only worth a few cents an acre.
 * A scene in Back to The Future II strongly implies Doc Brown engages in this. Marty needs some money from the timeline they're in, and Doc opens a briefcase with neatly arranged stacks of bills of various colors and sizes.
 * In Time Chasers (spoofed on Mystery Science Theater 3000), the Corrupt Corporate Executive's wormy assistant points this out to the protagonist, who is showing off his working time machine. Of course, all our hero uses it for is taking his Love Interest on a date to the 1950s. Note that the entire conflict of the plot is kicked off because the Hero needs money and sells his invention to a Corrupt Corporate Executive.
 * Averted in Vanilla Sky, where Tom Cruise's rich character is revealed to be flat broke by the time.

Literature
"All you have to do is deposit one penny in a savings account in your own era, and when you arrive at the End of Time the operation of compound interest means that the fabulous cost of your meal has been paid for. This, many claim, is not merely impossible but clearly insane..."
 * Paying for a meal at the Restaurant at the End of the Universe is made easy this way:

"$100 invested at 7% interest for 100 years will become $100,000,000, at which time it will be worth absolutely nothing."
 * This is also how Wowbagger the Infinitely Prolonged makes money, by investing in long-term stocks and generally outliving the hell out of everybody.
 * In "John Jones's Dollar" by Harry Stephen Keeler, a man deposits a dollar in a bank, stipulating that it and its accumulated interest are to be paid to "no other person than [his] fortieth descendant ... along the oldest child of each of the generations which would constitute his posterity". By the time the fortieth generation rolls around, the payout is greater than the total value of the combined holdings of the entire human race.  Which is of course absurd on multiple counts, but then this is Harry Stephen Keeler.
 * In H. G. Wells's When The Sleeper Wakes the protagonist, after sleeping for 200 years, wakes to find his fortune has accumulated to the point where he owns more than half the world. The trustees of his estate are the world's rulers. He becomes the figurehead for an opposition party that wants to take control, but once they have, realises they're no better than the first lot. Despite his wealth there isn't a lot he can do to make things better, since he's just one man and doesn't understand the world around him very well.
 * Mentioned (thus becoming an Ur Example) by a friend of the Time Traveler in The Time Machine. The Writer shoots this idea down by pointing out that the investor might arrive to find that the future society has gone communist and abolished money.
 * The Sleeper's funds weren't all just interest. If I recall correctly, it was stated that one of the Sleeper's relatives as well as the man he fell asleep at became financially successful, and both chose to have the Sleeper inherit them. (The trustees of the estate are also protrayed as extremely aggressive businessmen, and the Sleeper's wealth owes at least as much to their finaglings as to the effects of compound interest.)
 * The German novel Eine Billion Dollar (that's a trillion dollars in English), by Andreas Eschbach, has this trope as a premise: John Fontanelli inherits the fortune, originally just 300 florins, which his ancestor in the 16th century left for him. Along with the prophecy that the heir will give mankind its lost future back.
 * The Price of Oranges, a short story by Nancy Kress. An elderly man discovers a time corridor, and uses it to bring back fine cigars and fur coats bought 'cheaply' in 1937. It never occurs to him to buy gold certificates which will be worth a fortune now, but as the story shows he's well-meaning, but not very bright.
 * This trope is mocked in one of the Lazarus Long books by Robert A. Heinlein:


 * Subverted by another Heinlein novel, The Door Into Summer where the protagonist is put into cold sleep for 30 years and wakes up penniless due to the company managing his finances having gone bankrupt..
 * He also takes advantage of a reverse
 * In Hyperion by Dan Simmons it's mentioned that laws are passed to prevent this occurring, so one character (Martin Silenus) put in suspended animation by his parents wakes up to find that all his funds have been confiscated, and he has to work as a common laborer.
 * Averted by the rich villain's plan in The Anubis Gates by Tim Powers. Scheming to travel into the past and invest his wealth there, he also intends to become immortal and watch over his investment, taking full advantage of his economic foresight, rather than trust in compound interest alone.
 * Thanks to his job requiring he travel at relativistic speeds for most of his life, Ender Wiggin is fabulously wealthy a thousand years after the events of the first book.
 * This one is more justified than many of the others; first, Ender started with a great deal of money (it was the pension he was granted for saving the human race from destruction). Second, the money was managed by investors until he reached adulthood, which ended up taking hundreds of years, and managed by a sentient computer program after that; it was never just sitting in a bank account gathering interest.
 * In Speaker for the Dead, he has Jane buy him a starship, which she does for 40 billion. When he asks, worriedly, if this will bankrupt him, she waves off his concerns, claming it's a "drop in the ocean" for him. Of course, Valentine's husband is able to buy a starship of his own thanks to the money he made from his fishing business. Given that his is a semi-luxurious yacht versus Ender's cargo hauler implies that he actually spent more money than Ender without the benefit of this trope.
 * In The Belgariad, Polgara does this the old-fashioned way. She puts a trustworthy person in charge of investing the token rents for land in her former duchy ... and then lives a couple thousand years. By this point, she could probably buy herself a kingdom if she felt like it, and may be one of the richest if not the richest person in the world. Too bad she has absolutely no interest in money. This may also not be an example, because the money isn't invested; it's simply the accumulation of small rents over the course of millennia. Her case is also aided by a severe case of Medieval Stasis and the fact that the tense relations with the Angarak nations prevent their massive deposits of red-gold playing a part in the Western economy.
 * Used by interstellar travelers in Stephen Baxter's Manifold Space, who are frozen in time while traveling through the portal network. It works at first, but laws are eventually passed and assets seized, leaving the travelers flat broke when they return.
 * Vernor Vinge's Marooned in Realtime begins with a 'colony' consisting of all the people that were in stasis during the 'singularity' that saw everyone else on Earth vanish in some unknown way. A pair of brothers are two of the travellers who were in stasis attempting just such a plan.
 * Clifford Simak's Why Call them Back From Heaven? is a variation: "Give us all your money, and we'll resurrect you in distant future when we'll invent time-travelling".
 * Another story plays the "anabiosis + interest money" scheme completely straight.
 * In The Dark Tower series, Eddie comes up with a plan to outmaneuver the Sombra Corporation's plot against the rose, by investing Odetta's capital from Holmes Dental in Microsoft and other up-and-coming businesses in 1977. Given such financial guidance, he predicts their Tet Corporation will one day be able to buy North Central Positronics out from under the bad guys.
 * Subverted in The Forever War, and hard. The soldiers get quadruple pay for being away in space in a combat zone for the equivalent of twenty years. But by the time the soldiers get back to Earth from their first tour of duty, their money is worthless due to hyperinflation and chronic shortages caused by the war and a world-wide famine. And the size of their compounded pay puts them in a 92% tax bracket.
 * It's given an offhand mention in The Dresden Files a couple times. But it's never just sitting around collecting interest, as the White Council has several staff members (pun not intended) that are tasked with managing the Council's finances.
 * Also part of the reason why, as she doesn't realize that Harry hasn't had a chance to shore up his accounts yet. Also, she's just a Rich Bitch that's Rich in Dollars, Poor In Sense.
 * Subverted in Anne Rice's The Vampire Chronicles, where vampires tend to be wealthy thanks to them not needing a lot of money and having trusted people manage their funds. Lestat has a number of brokers investing on his behalf and wiring him money whenever he needs it. Given that they're paid handsomely for their services, the brokers don't bother trying to figure out the strange habits of their clients.
 * In Tom Holt's Flying Dutch, the titular character had a life insurance policy which still paid off several centuries later-so much so that to cash it in would bankrupt the entire world economy,
 * In Tom Holt's Flying Dutch, the titular character had a life insurance policy which still paid off several centuries later-so much so that to cash it in would bankrupt the entire world economy,

Live Action TV

 * In a Red Dwarf episode, it is revealed that Dave left £17.50 in his bank account on Earth. Three million years have passed, and he now owns 98% of the world's wealth. He also left a sausage out on his table which went mouldy; now the mould covers seven-eights of the surface of the Earth. Furthermore, he left the lightbulb in his bathroom, racking up a gigantic debt to the local utility company, which has now become the ruling faction on Earth and whose battle fleet is rapidly approaching in an Attempt To Collect.
 * Mentioned in the American version. Lister's first words after being informed that he's been in suspended animation for almost three million years involve exclaiming that his baseball cards must be worth a fortune.
 * In the original version he also laments that he has an unreturned library book.
 * In the Doctor Who serial The Time Meddler, the eponymous meddler claims to have done this. Though given what had already been shown in previous serials, he would have had to withdraw his money in a year when Earth was a post-apocalyptic wasteland ruled by the Daleks.
 * This has been said of the Doctor himself, in the Fifth Doctor Missing Adventures novel The Crystal Bucephalus. He explains that to avoid drawing too much attention, he occasionally has to invest huge sums in doomed business ventures (such as the time-travelling restaurant of the title) and the British film industry.
 * In Goodnight Sweetheart, this gets tried using the portal that connects contemporary London with war time London, but the company that they invest in split in 50s and, without any contact from the investors, the bank puts the shares in the half of the company that folded.
 * A case of Did Not Do the Research: in real life he would have ended up owning shares in both spin-offs.
 * A non-time travel example: an episode of Highlander: The Series shows Duncan taking advantage of his immortality with this strategy. Thanks to his "great-great grandfather" depositing 10,000 francs, his account holds over a million.
 * Which does not take into account that in the meantime there was at least one devaluation of the Franc, at a cut of 100:1
 * Subverted in Star Trek: The Next Generation. A businessman who was cryogenically frozen during the 21st century arranged for a firm to manage his account, expecting to awake to a great profit. As it turns out, a third World War and conversion to a currency-less economy left any remnants of his accounts worthless.
 * While he's upset at this, he doesn't realize that he can get anything he wants just by talking to a replicator. In an Expanded Universe novel, he becomes the Federation ambassador to the Ferengi, given his rather unique skillset and understanding of the capitalist mind.
 * Played with in an episode of The Twilight Zone. Four thieves steal gold and freeze themselves for a hundred years, thinking they will be able to claim their riches after everyone has forgotten about the crime. Turns out that . Their intent is different (hoping to avoid interest rather than earn it) but the result is the same.
 * The titular character of Angel is notable for not attempting this. Many vampires and demons use their long lifespans and considerable powers to form and maintain successful businesses. By contrast, Angel has to charge the people he helps because, as Cordelia points out, he never bothered to make any investments over a 200 year life.
 * In an episode of Sabrina the Teenage Witch, Sabrina's aunts explain how they got rich: junk. When you live hundreds of years old, it's easy to buy worthless commercial junk made by the hundreds of thousands of units and put it away for a few hundred years until age and scarcity turn them into priceless antiques. They add that the Mickey Mouse plates they just bought will be worth millions in a few centuries.

Tabletop RPG
"To Art Dankwalther, I leave the sum of 34,586,224,739.58 in UCAS dollars. According to my calculations and accounting for conversion of the original currency, inflation, and 1 percent interest per annum, this settles my debt to your ancestor for the gold piece he kindly lent me for the last meal we shared."
 * Shadowrun supplement Portfolio of a Dragon: Dunkelzahn's Secrets. One provision of Dunkelzahn's will stated:

Video games

 * It's possible to pull this trick off in Animal Crossing. Deposit a reasonable sum of Bells in the bank. "Time travel" by setting the system's clock as far forward as it will go. Go to bank and check sum. Profit! Then you can travel back in time and safely keep your vast fortune. The downside? Your town will become a depopulated, weed-ridden wasteland, of course.
 * A strange case of banking and time travel is possible in The Legend of Zelda: Majora's Mask. Link has the ability to rewind time 72 hours before a certain point in-game time, and predictably, when he rewinds time he loses all of the money he accumulated during that 3-day period. Fortunately for Link, the hub city's bank is apparently temporally insured, since the banker stamps Link's account balance on his hand, and this stamp never fades or disappears with the rewinding of time. Realistically, this should mean the complete destabilization of the city's economy, since the money deposited at the end of the 3-day cycle technically does not exist at the beginning of the cycle, but most people are probably more worried about the moon falling out of orbit than they are about time-travel based banking scams.
 * As long as the banker had about 5,000 Rupees in reserves at the beginning of the cycle, Link would be able to withdraw it all without causing a bank failure. Whenever he resets time, the Rupees go back to the banker, so he's really just borrowing the Rupees; in three short days, the city's economy can't be ruined by just 5,000 Rupees.
 * Can be easily performed in Fable II right after the mains story begins. Rent out your caravan for a measly few gold, then set your clock forward a few decades. Use that money to buy more property, repeat, and you can own all of Albion the moment you lay eyes on it.

Web Comics

 * Dave Strider of Homestuck has the LOHAC Stock Exchange in his back pocket thanks to this trope.
 * Spinnerette has a time-traveling Benjamin Franklin use this as a means of obtaining start-up capital for the modern-day superhero support organization he runs in the present day. Interesting, its stated that the money wasn't that much, only enough to get things started, and most money comes from merchandizing deals.

Web Original

 * Dr. Steve tries a variant of this in The Adventures of Dr. Floyd. While he's always got a time machine and is trying to sell various things on ebay for profit, one time he simply bought 100 copies of Action Comics #1 and tried to sell them when he got back to the future.

Western Animation

 * Double Subversion on South Park, where the money saved this way was almost worthless in the future, but it was still better then the grinding poverty most people suffered from.
 * In Futurama, Fry becomes rich overnight when he goes to the bank for the first time in a millennium and discovers that he's rich beyond imagination, but he falls for an elaborate scheme to steal his money after he obtains the world's last can of anchovies, which would then be used to buy them off him. He ends the episode as broke as before.
 * To throw another monkey wrench into the works, he wasn't actually trying to get rich. He just needed 50 cents and realized he still had just under a dollar in the bank.
 * The Stable Time Loop Xanatos Gambit on Gargoyles that helped make Xanatos part of the Fiction 500 comes close to this trope. Xanatos went back in time and earned a practically worthless coin that he then had the Illuminati Society deliver to a David Xanatos 1,000 years later... when it was worth 20 grand. The rest was investments.
 * Which was also part of his lesser Xanatos Gambit to prove to his father that he was a "self-made man".
 * inverted in The Flintstones: Fred borrows 4 dollars on his paycheck so he can have a long weekend with Wilma and the Rubbles, thanks to the Great Gazoo, they get sent to a very Jetson-like future, while he's in the future, he visits his employer, and discovers that he owes his company 23 million dollars.
 * An odd variant in Back to The Future The Animated Series: Jules and Verne travel to 2091 and have to take a cab, but find out the cab fare is very high. The only money they have is a quarter; however, it turns out that a 1983 quarter is a rare and expensive collector's item by that point in time. The cab driver skips away in glee at his new fortune.

Real Life

 * Truth in Television, somewhat, in IRA retirement funds.
 * Those who opt to have their brain and/or body frozen after death. They accumulate a ton of money for the procedure, and the interest generated from that money is used to pay for the upkeep of their brain.
 * The Time Travel Fund is based around this idea. They are even taking Ridiculous Future Inflation into acount.
 * If you invest in the right assets, this can work. See the Louisiana Purchase, the Alaska Purchase, and probably most impressive, Manhattan, bought for about $1000 (2010 purchasing power).
 * Had the Native American's invested that $1000 in market rate investments it would have grown to something like 60 billion dollars by the 21st Century, indicating that their deal was not so foolish after all.
 * Do you reckon the total value of the real estate on Manhattan is worth more or less than $60 billion today?
 * Would it be worth that much if the local owners hadn't sold it to the settlers?
 * Actually, the initial payment was to tribes that didn't even own the land. They just had a deal with the actual owners to hunt there. A later, larger payment was made to the angered actual owners of the land.
 * Not quite time travel, but Benjamin Franklin left both the cities of Philadelphia and Boston money in the will, with the stipulation that they invest it for 100 years at 5% interest.
 * Most banks have an inactive account upkeep few of a couple of dollars a year. Before partaking of this scheme, make sure you have enough seed money to generate more interest revenue than these fees will deduct every year.
 * More importantly, don't forget Ridiculous Future Inflation. Bank accounts tend not to keep up with it, and there's no point in investment $100 and turning it into $1,000,000 if it's only worth $1.
 * A few people tried and failed at this. It turns out, people don't like the idea of someone investing money for so long that they control the economy, and they're willing to change the laws to stop it.