My world has a large cash loan company(we'll call it quick cash) which has a time travel device at each of its branches. when somebody comes in saying they need a loan, the employee grabs the cash they prepared yesterday, and then hop into the time machine. They travel to yesterday, tell past them the amount of money to prepare for a customer tomorrow, then drive over and deliver the cash. Simple? No.

Then this is where it gets confusing. If the person never comes in because they got the cash they needed, how does quick cash make them pay back the money? If they get the cash before they ever signed anything or asked for it, then they could argue that since they never signed anything in that timeline that they don't have to pay quick cash, and if they do end up needing cash they could argue that they would go to a different quick cash type store. How to make sure quick cash gets their money back after giving someone money yesterday when they will need it today but haven't signed any legal things in the timeline they get the money yesterday?

  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – L.Dutch
    Commented Sep 12, 2020 at 5:40
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    $\begingroup$ Wait - the lending doesn't involve any time travel? The only time travel is to ensure that the company itself has enough cash to make the loan? $\endgroup$ Commented Sep 12, 2020 at 9:00
  • 3
    $\begingroup$ It is not possible to travel back in time. But money can already travel in time, it is called interest, a very old concept. $\endgroup$
    – checkmath
    Commented Sep 12, 2020 at 18:35
  • 2
    $\begingroup$ This question is basically "How do I make payday loans even more exploitative with time travel", isn't it? $\endgroup$
    – nick012000
    Commented Sep 13, 2020 at 6:19
  • $\begingroup$ Do you also break their knee caps last week if they don't pay up? $\endgroup$ Commented Sep 13, 2020 at 7:53

17 Answers 17


Your primary concern is easily resolved.

Signature on delivery. All the paperwork is signed in the past when the money is received, you just need to remember to go into the office the following day to request it so you don't cause a paradox.

The bigger legal concern is that for the duration between delivery and despatch, the cash you're holding could be considered counterfeit as it's a duplication of currency already in circulation.

  • 17
    $\begingroup$ "Qwi€ash is not responsible for universe-destroying paradoxes which arise as a result of customer failing to maintain consistency of the timeline" 😃. This sounds dangerous, though; what if the customer dies in the meantime? Oh, and as for the counterfeit thing, OP said they travel back in time and then put the cash together; the loaned currency isn't from uptime. $\endgroup$
    – Matthew
    Commented Sep 10, 2020 at 14:21
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    $\begingroup$ You don't even need to go into the office (potentially, depending on your view of time travel paradoxes). The office can send money back in time because your paperwork says you were going to show up, even if you don't. $\endgroup$
    – Cadence
    Commented Sep 10, 2020 at 19:36
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    $\begingroup$ "I won't sign! I didn't agree to anything!" "Suit yourself, sir, but your future self handed us a vintage watch as collateral, and we're not giving it back to him if you don't agree to the loan." $\endgroup$ Commented Sep 10, 2020 at 20:12
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    $\begingroup$ @EmilioMBumachar Now I imagine a wealthy customer coming to the company to make them deliver ransom money he failed to come up with quickly enough and then the employees of the company have to convince him they're not in on the ransom. That'd make a good scifi novel story base, imo. $\endgroup$
    – Magisch
    Commented Sep 11, 2020 at 11:57
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    $\begingroup$ @Colm, sorry, but there's no reason to believe that a paradox or an inconsistency would cause the "collapse" of reality, whatever that means. For all we know, nature might love paradoxes. $\endgroup$
    – cowlinator
    Commented Sep 11, 2020 at 21:29

They make you use the time machine first.

  1. Walk in. "I needed money yesterday"
  2. "Excellent sir - come see our money lender yesterday."
  3. Walk through a portal that's permanently open to yesterday.
  4. "Welcome to yesterday! Sign here!"
  5. Money appears in your account, in the same timeline that you signed the contract. They notify past you of the money arriving.
  6. The only way out of the office is through the time portal back to the future.
  7. You come out, and the loan you just signed is one day old.
  • No duplicate you existing, no paradox.
  • You signed the loan papers at the time you got the money.

Of course, if I had a time machine to yesterday, loan-sharking is not what I'd do. Stock market and lottery numbers.

  • 8
    $\begingroup$ Lotteries don't work when everyone checks the numbers and goes back in time to buy the ticket, stock markets become ... complicated, but loan sharking always works. You'll still need to change your model to a loan fee as everyone will pay it off the same day they took the loan to avoid interest payments. $\endgroup$
    – Separatrix
    Commented Sep 10, 2020 at 12:12
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    $\begingroup$ But what timeline do you go back to in step 7? You start in the timeline where you're broke, then travel to yesterday's timeline where you sign the contract (which is now different). In the end, do you go back to the timeline where you're broke and it's actually another you on another timeline that gets the money? If you just jump forward a day in the contract timeline, you'll still be in a timeline where you didn't need the loan at all. $\endgroup$ Commented Sep 10, 2020 at 19:13
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    $\begingroup$ Doesn't it also mean that at that point yesterday, you're now in 2 physical locations at once? You're at the lender collecting the money, but at the same time you weren't originally there yesterday, you might have been at home or work. So does that mean you were in 2 places at one yesterday? $\endgroup$ Commented Sep 11, 2020 at 1:10
  • $\begingroup$ Yes there are two yous while you are in the past, but the only exit from the office is to the same future you left, and when you return, you'll be the only you there. $\endgroup$
    – Ash
    Commented Sep 11, 2020 at 3:55
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    $\begingroup$ There is a paradox. You get the money in the past thus never need the money in the future so why apply for the loan? $\endgroup$
    – Thorne
    Commented Sep 11, 2020 at 6:27

First of all, most customers won't need their money yesterday. You may be in a sore need of money, right. But you can usually forecast your need at least a few days in advance before you are broken.

So why does your QuickCash company offer this service if few people would actually use it? Marketing. Offering to lend you cash even before you know you need it is a advertising line. This actually drives more people to your services. They could borrow \$10000 from StingyLender a bit cheaper, or from QuickCash. However, QuickCash will lend you \$15000 today if tomorrow you discover that you actually needed a bit more. This is mind-blowing, but customers like to have that extra safety, and do pay for it (or actually, they agree to pay in the future for that assurance that they would be given such money in the past). There are even insurances whose policies cover up to \$AMOUNT of QuickCash lending in the past.

So, how does this work? The new customer comes into QuickCash, meets with one of their representatives and explains how much they need, why they need it in the past, the circumstances that caused that sudden need, why they will be able to repay, etc.

When both parties agree on the terms, the customer signs a notice template that will be sent to the customer past self. This form is sent back in time (ads make it look like there are time-travel machines on every office, and they sent people back. Actually it's more like a Fax machine, and only a few sites do support it, other offices route their past-messages through those which do. Million times cheaper this way).

An office staff from the past picks the documentation from the future office and visits the customer in the past timeline. Your future self has asked us to lend you $10000, since you will need them in the next 24 hours. The past of the customer may read the agreement, and QuickCash reputation precedes them anyway. Two options here:

  • Customer in the past agrees that he will need it, thanks QuickCash and signs the contract in the past. No big difference here.

  • Customer does not believe he might need such amount tomorrow, he has enough money, a nice home and a lot of fields, this must surely be a trick. Then the QuickHash representative goes all in. He offers a (slightly more expensive) lending arrangement. QuickCash is a really nice company, and would not like to see a future customer struggle for a money he didn't want to loan in the past. So they will lend him those \$10000 anyway for 24 hours. If the customer returns them the next day, there will be no penalty and, moreover, if he doesn't borrow anything from anyone in the next 30 days (thus showing the loan was clearly not needed at all), QuickCash will let the customer keep half the amount it had lent him needlessly. So in this case, that would be \$5000 for free if the past customer was right. Easy money. The trick is of course in that the staff at the future made sure beforehand that customer will indeed need that much money and would not be able not to take a loan. When the customer faces \$tragedy next day, he happens to have enough funds, thanks to QuickCash and the contract they signed the day before (thinking he would never need it).

  • $\begingroup$ Good cleanup if the OP's implementation of the details (sending a message, etc.) and interesting idea in the last part. But the parts that are conditional on not taking a loan from anyone else are probably hard to enforce. (You'd have to prove loan vs. gift for money from friends / family members; remember that normally it's only up to people making loans to worry about how to get their money back. AFAIK most countries don't let lenders go prying into people's finances to the level of turning up possible gifts / other loans; sharing financial details is done voluntarily to quality for loans.) $\endgroup$ Commented Sep 12, 2020 at 0:54
  • $\begingroup$ Thanks. The point is that you do not get external financial help and yet claim that they did not have any special need that might require a loan. As long as they don't claim that get-to-keep-part-of-the-amount clause, they could go change their future mind and get the money in a different way. Most people would not bother to do that when they already have an acceptable loan solving their immediate needs. $\endgroup$
    – Ángel
    Commented Sep 12, 2020 at 1:15

It's not the legalities that are complicated here, it's the logic.

If you have a time machine, why not just send the customer ahead to the point at which the loan would be due? If his future self has the money, and is willing to give it to the present self, the money is all set, even if the future self has to give it to the company who will give the past self equivalent money from the past day; if the future self doesn't have the money or isn't willing to give it up, why would you lend him anything? (Then send the present self into the past to pay off whatever needs to be paid off yesterday.)

A loan is a clumsy and ineffective way to make use of future money, what with credit checks, signatures, the risk of death, and all that. Selling trips to the future is an adroit and effective way.

  • $\begingroup$ The issue with this is that a person needs to get from points A -> B -> C to be able to have the money to pay back. If past you needs the money to get to point B, then just saving money to pay back the loan does not work. $\endgroup$
    – Nosajimiki
    Commented Sep 10, 2020 at 14:47
  • $\begingroup$ Sure it does. You have the money at point C, you give it to yourself at point A, your self uses that to make it to point B and then to point C, saving at every point after B. $\endgroup$
    – Mary
    Commented Sep 10, 2020 at 14:49
  • $\begingroup$ You cant use tomorrows money to give to yourself yesterday, because yesterday the money was owned by someone else. Hence the quick cash employee travels back in time to make them give you yesterdays money and avoid the double spending. $\endgroup$
    – Daniel
    Commented Sep 10, 2020 at 16:08
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    $\begingroup$ In which case it's not a loan, it's just telegraphing money -- you give them the money in the future and they pay you an identical value in the past. $\endgroup$
    – Mary
    Commented Sep 10, 2020 at 16:16

They don't pay you, they pay the person you owe money to.

  1. You try to pay your rent for the month, you don't have enough money.
  2. Your Landlord gets paid, but you don't know that yet.
  3. You are informed by your Bank that you did not have sufficient funds for the transaction.
  4. You go to QuickCash ("Funds delivered at 88mph"), and take out a loan for the missing amount.
  5. QuickCash go back in time with the signed loan agreement, and give the Bank the money. This is paid to your Landlord in Step 2.
  6. The Bank send you a notification of insufficient funds, which you receive in Step 3.

If, after receiving a notification of insufficient funds from your Bank, you don't visit QuickCash — or a rival company, such as GalliPay ("Home of the TimeLoans") — then you will instead later receive a notice of non-payment (or even eviction) from your Landlord. Unfortunately, by this point the loan companies are unable to help you.

So, the benefit of visiting QuickCash (other achronal loan companies are available) is that you do not have any late-payment issues with your Landlord.

Of course, this is all basically just a fancy Overdraft.


As with any financial service, you have to sign up for it and have credit checks run and so on and so forth. This way, there are no surprises when the Qwi€ash truck shows up at your location. You may not yet know why you need the money, but the packet will include the standard reminders about the nature of transtemporal financial services (e.g., Qwi€ash shall not be held liable by the present-customer for the past-customer's mismanagement of the money). Furthermore, the cash packet will likely include a note from present-self to past-self regarding what the money is for.

I'd also suspect that such an operation will have all kinds of legal boilerplate regarding use and misuse of the service. One key point that comes to mind immediately is that a customer can not send money into the past beyond the point in time when Qwi€ash was constituted. Very likely, loaned moneys can't be used to play the stock market or other forms of gambling. There may also be loan amount limitations.

As for being paid back, the loan shall have been on the books since the date it was delivered to the past-customer. Since the cash packet has to be signed for in the past, that will serve as a countersignature to the loan application made in the present.


You don't need the money yesterday, you need it right now ! When your landlord ask for the rent, when your loan shark comes knocking on the door... you press a button on your app which registers the need.

Regularly in the future, they check your bank account to see if you have enough money. When they see that you have enough to reimburse them, they collect the money and send a message back into the past BEFORE you press the button. Procedures are launched, money is moved and in the same instant that you pressed the button, money is credited to your bank account or someone with a load of cash enters through the window (someone is already at the door, remember).

If you don't get the money when you press the button, it means that you never could reimburse the loan in the future.

This way, each loan is a self validating temporal loop, the bank is making money with no risk at all and there is no paradox nor alternate universe.

The only things unknown are :

  • Would you have been able to pay the loan if you didn't have the loan ?
  • Isn't the reason for which you couldn't pay the loan the fact that you didn't get the loan ?
  • What makes some loop loan-possible and some not ? Nobody knows, but that's just how time works.
  • $\begingroup$ Perhaps you did not get the loan retroactively because you died between today and tomorrow? For some reason, you could not go into the store the next day. I was thinking about a similar scenario in my second answer - you realize you need money today, you say to yourself you must go into the loan store tomorrow to get a loan, and then check your bank balance. If the money is now there, you know you went into the store tomorrow and were (will be?) successful. Plays havoc with causality. $\endgroup$ Commented Sep 11, 2020 at 15:11
  • $\begingroup$ But what is not clarified in your answer is the legalities. When is the loan contract signed? It would seem that, at some point, you would have to sign for the loan, not just press a button. $\endgroup$ Commented Sep 11, 2020 at 15:13
  • $\begingroup$ @JustinThymetheSecond You install the app, you sign some forms... When you press the button, it is legally binding and you give the right to the bank to take money from your account in the future to send it to you now. $\endgroup$
    – Jemox
    Commented Sep 11, 2020 at 17:06
  • $\begingroup$ Isn't that how credit cards work now? $\endgroup$ Commented Sep 11, 2020 at 18:15
  • $\begingroup$ Kinda. But the banks is reimbursed "before" (in the future) they pay you back and there's also the guy with the briefcase full of cash going through the window. $\endgroup$
    – Jemox
    Commented Sep 11, 2020 at 22:50

This is too easy. First, instead of bringing money back in time (basically printing free money, as there will be (People taking out loan * loan amount) extra money in circulation every day), and, instead of sending people back in time (unavoidable paradox, there will always be two of the same person), just send the loan paperwork through the portal, pick it up yesterday and transfer the money (which you already did, yesterday), and if the person doesn't pay back the money (assuming on the day after the loan is requested, and two days after the money was given), just simply send a note back to yourself to deny that persons loan. You'll have already gotten the note, already denied the person, and have not lost any money since you read the note and denied them the loan.

  • $\begingroup$ Interesting. So the ultimate approval process is that you do not get the loan unless you have already paid it off? Risk-free lending. 'No you do not qualify for the loan, not ony can you not afford to pay it back, not only have you not paid back your loans in the past, but you haven't paid THIS loan back in the future.' $\endgroup$ Commented Sep 11, 2020 at 2:35
  • $\begingroup$ Yup, that's what I had in mind. To me it seems "self-healing",if the person never planned on paying back, a note would already be there from tomorrow(s) warning you. Like-wise, if someone needed more time to pay the loan, the teller would pretty much already know exactly how long they needed, and can adjust rates accordingly. $\endgroup$
    – Werlious
    Commented Sep 12, 2020 at 18:46
  • $\begingroup$ Also, I just kind of thought of this, if the portals are locked to within 24 hours, a kind of message system can be made by putting a "target date" on the note and have it keep getting sent back (send it through your portal to yesterday, yesterday you already put it in yesterdays portal for the day before, etc) $\endgroup$
    – Werlious
    Commented Sep 12, 2020 at 18:49

Okay, I thought about it again, and I realized that neither the money, nor the person, needs to go back in time. The trick is that the only thing that needs to go back in time, is the information. No 'time travel' necessary. The 'time machine' in the QuickerThanQuickCash store branches is just a computer linked to computers in the past. And in fact, since the computer data connections and technology would all be made originating in the present and extending into the future, for future use, no changes would ever have to be made 'in the past'. It would all be 'from this day forward'.

That is, the future 'you' deposits money through automatic banking into your account today. It is all done by computer. Only the information about the deposit has to be made 'retroactively'.

If there are enough QuickerThanQuickCash store outlets, then everyone would know about it. They would not be suprised that money showed up in their account, as everyone would be doing it. In fact, you might expect to get cash from your future 'you', just like today you might get cash form a relative, depositied into your account, without expecting it. The money would just show up. But just because 'you' got the cash today, 'you' would still have to go into the QuickerThanQuickCash store tomorrow to arrange the loan. Failure to do so means that the cash does not show up in your account today.

It's all about information and data traveling through time, not physical objects.

Even though 'you' got the money today, the loan is still made tomorrow, and the documents are signed tomorrow. 'You' are still legally accountable for the documents you sign tomorrow, in all of the days after tomorrow. There is always only one 'you' at any one time, and there is always only one timeline. The only real difference between on-line banking under the reality of today and this scenario, is that 'you' would just assume the money that showed up in your account today was a result of 'you' taking out a loan tomorrow, instead of some mysterious benefactor puting the money into your accounnt today. In fact, the deposit could have a notation that indicates it was a deposit from the future. Interest would be paid from the date of deposit, of course, by contract, but 'you' would be well aware of this when 'you' took out the loan, 'you' agreed to it at the time the contract was made. It would be entirely legal and enforceable from tomorrow on through eternity. It would not, of course, be enforceable 'today', because the contract was not signed 'today', but this is of no consequence to QuickerThanQuickCash store in the future. They really do not care about today, that is in the past.

Really, it would be like sending a money transfer to another person today, except that it would arrive yesterday, and the money would be available yesterday, not today. Since it is not really physical money, but a number on a spreadsheet, there would not be any timeline paradox. When 'you' take out the loan tomorrow, 'you' already know it has already shown up in 'your' account today, 'you' just did not know today that 'you' took out the loan tomorrow, until it shows up. Nothing really changes in time. Nothing changes in the past, because of a change made today, so nothing changes today because of a change made by someone in the future going back into the past. Just an accounting procedure made retroactively.

It does play havoc with our current notions of 'causality', and about things in the future causing things to happen today, but this is sci fi, after all. Granger causality is in some ways just as weird. But still, even though it happened today because of something in the future, it still happened only in our timeline, and our timeline only occured once. What happens, happens, irregardless of when it is caused.

I am not really sure what the economy would look like, when money could routinely be depositied into an account in the past, but it would be interesting. I suspect, at the least, that convention would demand no money would ever be transferrred back into the past beyond the time that the technology was first made available, because that would really botch up the bookkeeping. In fact, since the technology did not exist further back in time than when it was developed, I see no way money could be transferred back before the technology was implemented. But it would be fun to hope, today, that someone from the future just might send you money today.


The customer pays you back up front

It's not clear when the loan is due for repayment, but the simplest solution seems to be that the customer walks in to QuickCash, asks for a loan yesterday, and as part of the process immediately repays you the loan and interest. You then travel back in time and give the customer the loan.

This still works if the loan is needed for a longer period than a day. Customer walks in today and tells you he needs money yesterday. Customer then goes about their business. At some point in their future, they walk back into QuickCash and pay back the loan. You (or a colleague) then sends confirmation back to "now" you that the loan has been repaid, and only after receiving that do you go back to yesterday and extend the loan. The customer doesn't even really need to come in and ask for the loan, since you'll know from the repayment confirmation that you need to go back and give it out.

That way you're always guaranteed to be repaid, and as a bonus you don't even need any cash reserves in the first place (if the customer has to pay it today - if they're paying it back on the future you'll still need reserves to cover the difference, unless you can just send the cash back to today from the future when you get the repayment).

It also resolves some potential paradoxes and timeline issues - you will give the customers the cash, because you already have. And this all happens in one timeline, since the customer who pays you back has to be the one who you gave/give/willan on-give the cash to. The only outstanding question is the old one of where does the cash come from, since it only exists in that closed loop, that's the bit you need to handwave.

  • $\begingroup$ So this is just a money transfer service, except that it transfers money to the past? You need to have the money when the transfer is made, in order to transfer it? And a fee would be charged by the store for making the transfer. Not really a loan service. $\endgroup$ Commented Sep 11, 2020 at 15:47
  • $\begingroup$ From your point of view, it's a loan. In your timeline, you get money from them up front, get the use of it for a period of time, and then pay it back with interest. The fact that they don't give you the money until they've received it back is irrelevant from your POV. Surely that's all any loan is, the use of someone else's money for a period of time? Once you introduce time travel, every loan is effectively a money transfer service. $\endgroup$
    – Mohirl
    Commented Sep 11, 2020 at 15:53
  • $\begingroup$ Added a section on how a longer term loan could work, thanks $\endgroup$
    – Mohirl
    Commented Sep 11, 2020 at 16:03
  • 1
    $\begingroup$ Hmmm. A cash advance paid off in forward arrears. Yes, I can see how it could be construed as a loan. Cash Stores, I suppose, do something similar when you take out a loan against a cash garnishee of your future paycheck. You have already paid back the loan when you take it out, through your future earnings which you never see. Loans on your future but calculated income tax rebates operate similarly. $\endgroup$ Commented Sep 11, 2020 at 16:40
  • $\begingroup$ That 'not needing any cash reservres in the first place' is downright scarry. You lend out money you do not have, knowing that you already have it back, and just collecting the fee for service. I can see it growing the GDP of a country exponentially. $\endgroup$ Commented Sep 11, 2020 at 16:49

Signature on delivery.

All the paperwork is signed in the past when the money is received. When your past self signs, that's all the authorization PREDAY LOANS needs. They'll use that signature to take care of everything, and if they require collateral, they pick it up when the "past self" signs.

Paradox avoidance

If the customer's past self refuses to sign or fork over said collateral, the company rep simply leaves. It's true said loan office would have never made the trip if the customer's future self hadn't asked. If the time travel changes the customer's behavior such that they never contact PREDAY LOANS, the company will ensure no paradox happens of their own accord.

At the appointed time, if the would-be customer shows up, trying (in vain) to send themselves money they already refused to accept, that would be ... pretty weird, but not problematic in the paradox sense.

If the customer no-shows, the company sends itself a paradox-avoidance order at the same time they would have sent it at the customer's behest. In this case the rep will still have the details of the "original" offer, and probably won't even know it was refused (to avoid weird meta-influences).

In this way, smartass pranksters cannot cause a grandfather paradox by saying to themselves "Okay! If P.L. doesn't call me today, I go into their offices tomorrow and send myself a hundred bucks. And if they do, I'll refuse to sign and then make sure I don't go into their offices tomorrow! I ain't afraid of no time cops!"

You might think this would result in very strange orders sometimes just ... happening, for seemingly no reason. That's absolutely true. Breaking causality (the idea that cause always predates effect) has very strange consequences. Events can happen because they cause themselves to happen, and for no other apparent reason.

Other paradox issues

This company can really only give loans. It cannot be in the business of sending messages back in time. That's a big no-no, it's far too likely to cause paradox problems. People might try to use P.L. as a signalling service, by sending coded messages in the timing and amounts 'borrowed'.

Suppose my friend wants to give me an illegal insider trading tip. He contacts me, then I go borrow a hundred bucks via P.L. As soon as I'm done signing the company's paperwork, that means now is the time to turn around and sell my stock in XYZ corporation, making a killing.

This poses a much bigger risk of causing a paradox (because the effect precedes the cause in a way that impacts a lot of other people). From a legal perspective, not only will P.L. ban this in their terms of service, it will probably literally be a criminal act to use their services to send messages (by morse code or any other kind of steganography where you hide information in the fact that a request was sent and in its particulars). (In this specific example, insider trading is already illegal, and you better believe the SEC will know this trick - they'll heavily scrutinize any trade made by you or an associate during the interval between when the request was received and is later sent.)

Biggest paradox risk is P.L. going under

The company going under obviously poses a fairly large risk of causing a paradox. A bigger paradox concern is people who (for example) on Tuesday send a loan officer to themselves on Monday ... so they can mug her. (Or something nefarious).

Shenanigans similar to how the company avoids the "hiring P.L. changes the customer's behavior so they never hire P.L." paradox can avoid such problems. Besides which, making enemies of folks that can arrange for you to have never even existed in the first place is not a smart move.

Avoiding cash duplication

An interesting wrinkle is that if P.L. sends actual cash back in time, it might be technically counterfeit. There's a brief window of time during which the customer has bills which are an exact duplicate of other bills in circulation. Avoiding this is easy. The company doesn't send hard cash back in time; it sends itself instructions, according to which the (past) loan officer gets the client their money from a bank.


Existing answers are too overly-focused on the transactional side of the business. I help companies adopt new technologies, so let me tell you how I would sell this one and how it can be implemented profitably and with no paradoxes:

Sales & Lead Generation

All successful businesses are great at finding new customers. Pre-day lending is no exception.

So, a customer walks in to Quick Cash saying they needed money yesterday.

Customer fills out an application which includes detailed personal information, justification for the loan, phone number & best time to call yesterday, etc. There is no fee for applying (you'll see why, below). Quick Cash runs a background check, income verification, etc, then retro-emails the information to the office yesterday. What does the office yesterday do?

They forward the email to the sales team

It is the salesperson's job now to reach out to the Customer Yesterday and explain their loan they haven't applied for yet has been pre-approved (should we say, post-approved?), and all they have to do is come sign the paperwork and get the cash. The Customer Yesterday may accept or decline.

What happens to the Customer Today in the original timeline?

The original timeline is always rewritten.

If the Customer Yesterday declines the loan, the Customer Today has no reason to apply for a loan he JUST declined, and is anyway blacklisted from applying for a loan for another 24 hours. It's just a record in the computer, from the past. No paradox.

If the Customer Yesterday accepts the loan, the original timeline is rewritten and the only evidence of an alternate timeline that remains is a record in the computer dated yesterday that says, call this guy at 3pm and offer him a loan. Also, no paradox, it's an ordinary loan without any legal complications, and Quick Cash makes a profit from the loan fees.

Why don't they charge an application fee?

If you use this model of time-travel, where paradoxes are avoided by rewriting the timeline, there is no stable timeline in which Quick Cash gets to keep the application fee from Today. The fee has to be charged or agreed to Yesterday when the cash is picked up. This insight is key to profitability leveraging this technology.


There is no way to say for sure since different countries have different laws, and there is no way of predicting for sure how courts and future legislators will rule, but I can think of a few possible issues:

If you mean literally "yesterday"

Offering yourself a loan yesterday using time travel does not make a lot of sense. Financial institutions already offer grace periods for things; so, for financial adjustments over such short periods of time, we already have systems for doing that without actual time travel. So, for the remainder of this answer, I will assume you mean some time far enough in the past where time-travel actually becomes meaningfully necessary.

It could be challenged as Ex Post Facto in some jurisdictions


Most places today have laws that prevent retroactive legislation; however, only some nations extend that law to civil situations such as contracts, debts, civil suits, etc. Places that have more absolute interpretations of Ex Post Facto law may prohibit this practice because you are assigning a debt to someone retroactively.

Most places require loan agreements to be signed and dated as of the date of commencement

Because the date of agreement is a material component of loan agreements, many jurisdictions can nullify a loan agreement if the date does not agree with the context of the loan. So a loan that is signed and dated in 2105 but begins collecting interest in 2100 it would be indistinguishable from lender fraud. Moreover, if you sign it in 2105, then no notary in 2100 will not have a record to confirm against; so, there is no proof of a legally binding agreement in 2100. Then there is the time paradox issue. If I receive money in 2100 and it alters the course of my life, then there is no guarantee that I will walk back into the bank in 2105 to sign the paperwork which (for some versions of time travel) would leave the lender without any proof that I ever agreed to a loan. Lastly, if you go back in time to 2100 to sign and date it before a 2100 notary, then there are two distinct versions of you in the same timeline meaning that you would most likely be treated as a separate legal entity from your past-self (like a twin or a clone, you are no longer materially the same person).

It could be challenged as insider trading/civil damages

Older you may not be able to obligate a younger you to a debt; so, the bank instead just extends a line of credit to the younger you which he may or may not choose to accept.

Let's say in 2100 you had a really low credit score. You wanted to buy a house but could not; so, you rented instead. But, then you got your life together and started saving a lot of money/paying off debts, etc. Then in 2105 your debts are all paid off, and your credit score looks great. You could then go to the bank and say, "Hey, I want to apply for a house loan in 2100." They could then send your current financial information back to 2100 and use that to prequalify the younger version of you for a loan. So, the 2100 version of you is looking at a pile of bills, losing sleep at night, etc. when you get a call from the bank saying that you've been. PrePost-Approved for a home loan. It is then up to the younger version of you to decide if he wants to sign up for the loan or not.

This may sound like a nobody gets hurt scenario, but it's not. The owner of the apartment you were going to live in for the next 5 years just lost out on rent money because the bank engaged in a sort of insider trading. Now there is the risk that the Landlord could sue the bank for damages.

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    $\begingroup$ Among the poor, "pay day" loans are often for very short periods. These can be crucial when you need the money NOW. $\endgroup$
    – Mary
    Commented Sep 10, 2020 at 19:18
  • $\begingroup$ @Mary I agree, but there is generally no need for time travel to do that. If your car payment was due on the 3rd, and you pay on the 4th, the money is retroactively received as if you paid on the 3rd for a small late fee. Now if you waited 3 months to pay, and your car is now being repossessed, having money 1 day sooner is not generally going to save you since the paperwork to repossess it has already been in the works for days if not weeks by the time your car gets hauled off. $\endgroup$
    – Nosajimiki
    Commented Sep 10, 2020 at 19:50
  • $\begingroup$ Then at some point in those three months, there was one day in which you could pay, and another in which it was too late. $\endgroup$
    – Mary
    Commented Sep 10, 2020 at 20:03
  • $\begingroup$ Yes, but why mess with time travel when you could solve the same problem with a payday loan. If you are a lender trying to solve people's short term money problems, why punch a hole through space time and settle for a 1 day window of opportunity when creative bookkeeping can give you weeks? If the lender thinks a 14 day payday loan is costing him important business he could just as easily make it a 15 day loan instead. Also, if a person is 90 days behind on a loan, but they will somehow have the money on day 91, that can usually be fixed as easily with a phone call to your lender as not. $\endgroup$
    – Nosajimiki
    Commented Sep 10, 2020 at 20:59

Let me see if I have this right.

A man with a broken arm and leg goes into a QuickerThanQuickCash store to borrow the money he needed yesterday, so he could pay off the loan shark, so he does not end up with a broken arm and leg.

So he completes the loan, goes back in time to get his money, and indeed pays off the loan shark. The loan shark is happy, and does not break his arms and legs. The man from yesterday is really, really glad to see himself from the future come in with the money to pay off the loan shark. But he has not yet borrowed the money from QuickerThanQuickCash store to pay off the loan shark, so he still has to go into the store the next day to borrow the money. Otherwise, he can not pay off the loan shark, and he has his arms and legs broken. So he goes into the QuickerThanQuickCash store the next day, without a broken leg or arm. He gets the loan anyway, even though he does not need it today, but definitely needed it yesterday, and goes back in time to collect the money to pay off the loan shark. Since the loan shark has not been paid back yet, the loan shark is happy to get the money, and does not break the man's arms and legs. But he still needs to borrow the money the next day so he can pick it up yesterday to pay off the loan shark so he does not get his arms and legs broken. So the next day he goes into the QuickerThanQuickCash store to borrow the money. Seems to me that he is now in an infinite loop. But at least his arms and legs are not broken. Unless he fails to go into the QuickerThanQuickCash store the next day to borrow the money yesterday so he can pay off the loan shark. If he fails to go into the QuickerThanQuickCash store, he does not go back to pay off the loan shark, and he gets his arms and legs broken. When that happens, the next day he definitely goes into the QuickerThanQuickCash store to borrow the money yesterday to pay the loan shark so he does not get his legs and arms broken.

Except, how does the QuickerThanQuickCash store get their money back, if he is caught in this infinite loop?

And why, if he got past this loop, would he not just go into the QuickerThanQuickCash store the NEXT day, to borrow the money yesterday (today) to pay off the loan at the QuickerThanQuickCash store, as soon as he took out the initial loan? It would seem to me that, when he went into the QuickerThanQuickCash store, he would be in danger of meeting himself from tomorrow, comng back to collect the cash to pay off the loan he had not yet taken out, but will take out yesterday?

It would seem to me, that there would be no room in the QuickerThanQuickCash store, because it would be filled with an infinite number of the same people coming in from the future to borrow money today to pay off the money they just borrowed today to collect yesterday.

Steven Hawking once famously said that he absolutely knew there would never be time travel, because if there was, our world would be saturated with tourists from the future.

Time travel works when you are the only one. And what is the profit for the QuickerThanQuickCash store, in that?

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    $\begingroup$ the customer never goes through the time machine. $\endgroup$ Commented Sep 11, 2020 at 11:46
  • $\begingroup$ Please make that clear in the question, because many posters are assuming that the customer does go through the time machine. The paradox is, do the customers know, when they go into the store, that they have already received the money yesterday? Or when they go into the store, are they unaware that the money was lent to them yesterday? That is, does the past change between when they go into the store, and when they come out of the store? $\endgroup$ Commented Sep 11, 2020 at 15:04

When you travel back through time and alter the past (as in giving yourself a loan yesterday), you cause a divergence in the timeline. This would either cause a paradox, blowing you (everything?) out of existence, or it would cause a split in the timeline (multiple realities).

Assuming there is no paradox ending reality, then there are two possibilities - You travel back to the same reality you came from (where it would appear as though nothing had changed - your loan to yourself never occurred), or you would travel back to the alternate reality, where your past self received the loan, however this has diverged from reality and you are now a separate being that has essentially popped into existence out of nothing (your alternate self still exists and is living their life).

In the first instance, whereupon on your return your past self did not receive the loan, you have not benefited from the loan, however you did still take it out and are obliged to repay it.

In the second instance, whereupon on your return your past self did receive the loan, but you are now an extra entity that has popped into reality out of nothing, the loan was never given and there is no obligation to repay it. However in this reality, the money from the loan is essentially counterfeit (the loan was never given and the money used for the loan now exists in multiple locations, in the same way you do)


Simple: Reality cannot contradict itself.

Receiving the money is real. Sending the money is also real. Receiving the money happens exactly when the sending also happens. And there are only two valid solutions: Either the lender receives the cash yesterday and returns today to send it, or they didn't receive the cash in the first place.

Under the "reality cannot contradict itself" regime, pretty much all the time travel paradoxes disappear. At the same time, it still allows for interesting plots to happen, where the future influences the past and current. It only restricts the things that have happened to be influenced by the things that are actually going to happen.

  • $\begingroup$ So the person going into the store already knows that they got the money yesterday? In which case, their future is already determined. If they have the loan yesterday, they HAVE to go into the store and apply for the loan today. They HAVE to be approved for the loan today. No choice. Oh, and they COULDN'T have died between today and tomorrow, if the money appeared today. Weird flex, but okay. $\endgroup$ Commented Sep 11, 2020 at 15:20
  • $\begingroup$ @JustinThymetheSecond Almost. One loophole is left open: Someone else deposited the money tomorrow, in which case the recipient can have died yesterday. It's not uninteresting :-) And it is even possible that yesterday they didn't know the circumstances that made them send the money tomorrow. Nevertheless, the fact that the money arrives yesterday requires that the money's sent tomorrow. $\endgroup$ Commented Sep 11, 2020 at 16:34
  • $\begingroup$ @JustinThymetheSecond Btw, this is the way time travel works in the Harry Potter universe. It's the only way to make time travel logically sound, while still being weird and brain-knotting. In fact, without this realism, time travel is generally just a license to retcon. With this realism, time travel becomes mentally challenging both to the author and the reader. Because now the ends have to meet to create stable circles or spirals of causation, which may only reveal themselves in hindsight. $\endgroup$ Commented Sep 11, 2020 at 16:40
  1. Customers comes to the branch and asks for a loan.
  2. Loan is authorized, customer signs a 2-steps contract confirming he agrees to receive the money yesterday, and leaves a copy of an ID as a reference.
  3. Employee grabs the money, the contract, and the copy of the ID, travels back in time, and delivers the money, asking the customer to sign again the 2-step contract, confirming he actually asked/will ask for the money and that he is indeed receiving it on time.
  4. Profit.

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