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After the time travel designs from the CASH WHEN YOU NEED IT YESTERDAY business was stolen by a corporate spy, Amazon now has time travel capabilities. Of course, saying that they could deliver the packages yesterday puts them in the same legal kerfuffle as the cash loans company, so they decided to go with instant delivery. Here's how it goes;

  • Bob the customer clicks the purchase button for something that is bought off Amazon, choosing instant delivery.
  • Bobby the delivery driver gets a thing saying 'deliver this thing to Bob at this time' on their delivery schedule.
  • Bobby hops in the special time travel truck with the package.
  • Bobby drives to Bob's house, taking 1-2 days like normal shipping.
  • Bobby pulls up to Bob's house and pushes the time travel button.
  • Bobby exits the truck with the package 5 minutes before Bob clicks buy.
  • Bobby knocks on the door after Bob clicks buy and delivers the package to Bob.
  • Bobby goes back to their truck and pushes the time travel button again, travelling to three seconds after they left.
  • Bobby drives back to the Amazon warehouse for their next package or continues on their route.

Now Amazon can't be causing paradoxes left and right because that's bad for business and they might get in trouble, so they need to make sure this delivery method doesn't cause any paradoxes. Amazon also wants to avoid secret code sending to your past self, like was mentioned in a few of the cash loans answers.

Does Amazon have a good delivery system for avoiding paradoxes and secret messages?

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    $\begingroup$ A time travel truck seems like an unnecessary and massive liability - if someone else hops in the truck while Bobby is delivering the package and manages to time travel, Bobby has no hope of getting that truck back. Why not just send a message back in time, deliver the item(s) to a local facility, and then just have protocols to make sure it doesn't get delivered before it is ordered? "Same hour delivery" would still be quite popular. If you really want "instant delivery", you'll need to beef up your security protocols. $\endgroup$
    – Rob Watts
    Commented Oct 20, 2020 at 19:53
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    $\begingroup$ The thing about paradoxes is not just that they shouldn't happen, but that they shouldn't be possible at all. Even if a particular way to use time travel wouldn't cause paradoxes, you'd probably still have to deal with the possibility of someone using it in a paradox-causing way, either by somehow making sure it's not possible or finding a way for time to correct itself. $\endgroup$
    – NotThatGuy
    Commented Oct 21, 2020 at 10:32
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    $\begingroup$ +1 for Rob, and it's gotta be cheaper to time travel messages rather than trucks. $\endgroup$
    – Jontia
    Commented Oct 21, 2020 at 13:01
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    $\begingroup$ Writing paradox- or exploitation-proof time travel is almost impossible. Just accept that you'll need to fudge/handwave somewhere $\endgroup$
    – Dragongeek
    Commented Oct 21, 2020 at 19:03
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    $\begingroup$ It's good to keep in mind that some of the best time travel stories cough BttF don't make a lick of sense of you actually think through the time traveling. $\endgroup$
    – eps
    Commented Oct 22, 2020 at 19:08

18 Answers 18

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This is totally doable! With of course the usual mind-bending that comes with time travel.

So the fun thing about paradoxes is that they only happen if you allow them to happen. However, if your time travel system disallows them, then by definition they won't happen.

This requires closed-loop time travel. This simply means that any form of backwards time travel has already happened and its effects are already seen. This means, that, for example, you cannot travel back in time to kill your previous self (at least not successfully). This is because for your future self to exist, you must have avoided getting killed up to that point. Clearly, since this is a universe where your future self exists, this must be true.

There are some ramifications about fatalism and the effect of choices in a universe like this. Because time travel is allowed but paradoxes are not, the amount of things that are possible go down. If you get handed a million dollars by your future self, you know with absolute certainty that you'll survive, acquire a million dollars, and travel back in time at some point in your future. Although there are still a bunch of other fun uncertainties to play around with.

Back to your specific example: this works really easily for a delivery service! You simply have an app that queries if the package is nearby the moment the user tries to order it. If it's not, we already know the package must have been lost before the customer even orders it! So you can just disallow those purchases with an "out of stock" or "internal error" or what have you on the app itself.

Building a world like this just requires reordering your steps:

  1. Bobby2 appears seconds before the delivery date with a package delivery.
  2. Bobby2's time machine truck notifies headquarters of arrival.
  3. Bob tries to order a fancy pair of mittens.
  4. The ordering software checks and sees that Bobby2's truck is in position with the package.
  5. Bob's order goes through.
  6. Bobby2 walks up and delivers the package to Bob.
  7. Simultaneously, Bobby1 gets an order to get a package of fancy mittens, gets in his time travel truck, take it to Bob's house, and time-travels back to the exact time the software has given to him.
  8. Bobby1 takes his time doing all of this.
  9. Simultaneously, Bobby2 is allowed to take future orders or do whatever he wants.
  10. Eventually, Bobby1 travels back in time, becoming Bobby2.

Note that this means that there will be 2 Bobbys walking around until Bobby1 finally time travels. From his perspective, he goes back in time, but from time's perspective, there's simply a span of time where 2 Bobbys (and 2 time travel trucks!) exist at once.

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    $\begingroup$ I think you mixed up Bob and Bobby. $\endgroup$
    – Paŭlo Ebermann
    Commented Oct 21, 2020 at 22:40
  • $\begingroup$ Showing an error if the package isn't already nearby may be correct, but building the system to work that way puts in a natural failure mode - the package isn't ready to deliver because the error message prevented the order from being placed. To work reliably, the system must be designed to deliberately cause a paradox if delivery fails. It will never succeed in doing so, of course, but if successful delivery is the easiest way for cause and effect to avoid the paradox then that will be the most frequently occurring solution. $\endgroup$
    – Douglas
    Commented Oct 22, 2020 at 4:38
  • $\begingroup$ I attempted to fix the bob/bobby mixup. Pretty sure i got it right but you welcome to re-edit if I got anything wrog! $\endgroup$
    – EveryBitHelps
    Commented Oct 22, 2020 at 17:51
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It will create a nightmare contagion tracing feedback loop.

Let's say Bob orders a package to be delivered, and over the day or two it takes for Bobby to drive to Bob's place, Bobby is exposed to a contagion (Covid, chemical attack, zombie virus. Let's go with zombie virus as it's more fun). Bobby then travels back in time, dragging the virus back in time as well. Now the contagion outbreak event occurs three days earlier than it did in the first loop.

Bob, happy with his purchase, immediately pops over to the shop and interacts with another delivery driver who is just about to travel back to deliver a package to Bob's neighbour Alice three days before then. Now the zombie virus is even further back in time. Alice's outbreak actually makes waves in a bad way, and Bob never got round to making that initial order.

Two questions result:

  • Does the zombie virus outbreak still occur?
  • How do medical personnel track, trace and more importantly contain the origin of the time-shifting outbreak?
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    $\begingroup$ Although I really like the answer, this is solved by strict contamination procedures. They can bend time and space. Making sure a package is contamination free seems trivial by comparison. Delivery guy can wear a hazmat suit. Although I'm wonderingbwhy the package can't be delivered by time travelling drone or that time and space is messed with, so it's instantly in your mailbox. $\endgroup$
    – Trioxidane
    Commented Oct 20, 2020 at 20:24
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    $\begingroup$ @Trioxidane, you would think so and yet... current day researchers are developing vaccines at breakneck speed to reduce the risk to the world's population from our current contagion, wearing a mask to potentially reduce contamination seems trivial in comparison... $\endgroup$
    – EveryBitHelps
    Commented Oct 20, 2020 at 20:41
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    $\begingroup$ Any procedure's effectiveness is only as strong as its weakest link, and unfortunately Human error is the weakest link. $\endgroup$
    – EveryBitHelps
    Commented Oct 20, 2020 at 20:45
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    $\begingroup$ so just have a time travelling drone truck. Remove the weakest contaminated element and then decontaminate the truck and drones all the time. The time travelling might already sterilise the truck. $\endgroup$
    – Trioxidane
    Commented Oct 20, 2020 at 20:49
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    $\begingroup$ If you avoid the obvious paradox by ensuring that the package is NEVER delivered before the order is placed, then you can argue that either the recipient will not contract the virus (because he didn't), or that he was always going to the contract it, and does so. Alternatively, of course, the act of going back in time creates alternative timelines, and there is one in which the recipient gets the zombie virus, and one in which he didn't. The choice is up to you. $\endgroup$
    – WhatRoughBeast
    Commented Oct 20, 2020 at 22:43
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How do you deal with holdups? And the duplicated packages?

Say Bob orders the item, Bobby drives to Bob's house, does the time travel and hands him the package, then returns to normal time an drives off. Problem is Bob now knows that in a couple days Bobby will be driving up with a package containing the item he ordered. What happens if he steals the item before Bobby delivers it? For the time it takes Bobby to deliver the package, there are two instances of the package in existence.

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    $\begingroup$ Presumably Timeazon sends route data to the drivers, accounting for traffic and weather. This is pretty normal in the present day, but could be supercharged with data from the future. Obviously this would be highly confidential information otherwise it'd be quickly invalidated and/or lead to a bunch of paradoxes as people discover they're about to be in traffic accidents... Most likely you'd just receive a waypoint list and follow it blindly around any bad traffic. $\endgroup$
    – Ruadhan
    Commented Oct 21, 2020 at 10:43
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    $\begingroup$ @ruadhan "hold-ups" as in at gun-point - not based on traffic. Traffic wouldn't matter, they could drive to the location at any random point in the future after clicking buy and then time-travel back to the correct time. Bob (indoors, why use two names starting with B?) won't know when the Bobby is going to turn up in "realtime", it could be years in the future. If the realtime-delivery is robbed before travelling back, just send another delivery. $\endgroup$
    – fdomn-m
    Commented Oct 21, 2020 at 16:05
  • $\begingroup$ @freedomn-m Interesting point about sending multiple drivers. And the years in the future possibility. $\endgroup$
    – IronEagle
    Commented Oct 21, 2020 at 17:21
  • $\begingroup$ One assumes that Timeazon is fully aware of whatever happens to their drivers prior to actually sending them. In normal circumstances, you submit an order, and then a moment later the present-day timeazon servers receive confirmation from the driver that the parcel was delivered on time. If that confirmation doesn't show up, something happened to the driver. They then find out what happened to the driver ahead of dispatching him ("Employee safety is our top priority!") and take steps to prevent it from happening. Most likely they simply cancel and refund the order, $\endgroup$
    – Ruadhan
    Commented Oct 22, 2020 at 11:31
  • $\begingroup$ [cont] By cancelling the order, they then produce a new future where their employee didn't get mugged and the customer simply got their order refused. "please try again later!" $\endgroup$
    – Ruadhan
    Commented Oct 22, 2020 at 11:33
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Minority Report style predictive delivery

As mentioned by others, there's no way to muck with causality and avoid paradoxes. Even the simplest "send one bit back in time" leads to all manner of weirdness. Fiction writers rectify this by all manner of handwavium, but personally I'm a hard-sci-fi fan and like to minimize the unobtanium and contradictions.

Don't bother with sending the whole truck back in time - send information instead. Just employ precogs which are so good at knowing users' needs, they can dispatch the delivery before they decide to order.

"Precog" as a time-travel mechanic has a wonderful anti-paradox escape hatch built in. It's imperfect. So if you ever deliver a package that was never intended to be ordered, Amazon can just say, "Are you sure you didn't actually want this? Given how easy it is to return stuff anyways, it's trivial to just send the delivery back.

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It causes a paradox if Bob does not then click buy five minutes after recieving the package. Would be better and non paradox causing if Bobby time travelled to five minutes AFTER (or immediately after) the package was ordered.

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    $\begingroup$ Bobby just waits on the porch for 5 min until Bob hits the buy button, then he knocks and hands over a package. amazon can't charge as much for 5-minute delivery as instant delivery, or at least as soon as you can get to the front door delivery $\endgroup$
    – Ceramicmrno0b
    Commented Oct 20, 2020 at 19:59
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    $\begingroup$ @Ceramicmrno0b what if the sound of the truck pulling up causes the buyer to get distracted, and either change his mind or delay the purchase significantly? $\endgroup$
    – Ryan_L
    Commented Oct 20, 2020 at 20:01
  • $\begingroup$ @Ryan_L, Bobby only knocks on the door after he knows Bob clicked buy, if it takes to long he hops back in his truck and leaves $\endgroup$
    – Ceramicmrno0b
    Commented Oct 20, 2020 at 20:04
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    $\begingroup$ Downvoting because OP stated "Bobby knocks on the door after Bob clicks buy and delivers the package to Bob" $\endgroup$
    – crobar
    Commented Oct 21, 2020 at 15:54
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    $\begingroup$ Many people have live streaming security / door cams. You see a delivery guy turn up at your door before you've even opened the website to order (if you know what you want can take only a few seconds to order+pay) - then you go down to the door and open it (without having made the purchase). Not sure why he needs to be there 5 mins before; the description appears to indicate time-travel is instantaneous so just go back to the exact time the buy button is clicked. $\endgroup$
    – fdomn-m
    Commented Oct 21, 2020 at 16:00
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Yes, yes

The paradox seems pretty much solved. Unless the customer is actively trying to break the universe, like hanging a camera to see if the guy is going to bring the package, it should work fine. Even so I would recommend to have the delivery truck arrive at the moment the message arrives on the servers. This way the act can't be taken back. A paradox can still happen if the guy tries to mess with the delivery chain, but should be difficult to do for most persons.

The secret message thing is also solved with the above. If it only arrives after your click, you can't give yourself information about the future.

But if you still have the package delivery arrive earlier, you can have a message already. As in 'only buy stock x when truck arrives'. Just wait behind the computer with stock information and Timazone open for the purchase. The moment you spot the truck, the message is received. 5 minutes later whatever you check on will go up/down. Get or sell the stock at that moment, buy the thing 5 minutes after.

Currently there are traders making millions on being the first to know/do something in miniscule timeframes. Milliseconds and less. Imagine what you can do if you have information about 5 minutes ahead.

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    $\begingroup$ Yall dont get what Causality means right ? The Radiation from the Body of the dude would break the Universe. $\endgroup$
    – Erik Hall
    Commented Oct 20, 2020 at 20:22
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    $\begingroup$ @ErikHall depends on what kind of time travelling is working and if the world is truly, fully deterministic. Maybe free will exists and because of that they correct any causality change. A person wanting a sandwich isn't suddenly wanting kebab. Thus they move in the same patterns and the world in practically the same way, not altering any choices. $\endgroup$
    – Trioxidane
    Commented Oct 20, 2020 at 20:30
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    $\begingroup$ Well the world is not deterministic in the classical sense. Kind of depends how you look at it. Thats not how Causality works. The core problem is that your changes in the past prevent you from comming from that Future. Dosnt matter if there is free will or not tbh. Even if it does, that dosnt change anything. Stuff still happens without a Cause. You think to larg. Causality is not about having a Sandwich or Burger. It is about everything at the smallest scale having cause and effect. And you cannot prevent a Paradox with that in mind. Even if it effectivly changes nothing, it still does $\endgroup$
    – Erik Hall
    Commented Oct 20, 2020 at 20:34
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    $\begingroup$ @ErikHall I'll try once more with a different perspective. For many years it was thought liquids only flow down to gravity. Liquid helium however potentially flows up. Gravity from Newton is an old theory. These aren't laws. They are our best worked out theory how everything is working. We know way too little about time to understand it. It might break/be impossible according to most theories, but in the end we don't know. And please check up what a paradox is, or send me a citation where you got yours. Causality might break because of one, but it certainly isn't the same thing. $\endgroup$
    – Trioxidane
    Commented Oct 20, 2020 at 21:09
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    $\begingroup$ Fair to say that Anthropic principle is at work. If time-travel could break causality and end the universe, the story wouldn't happen. It does happen, ergo causality is safe. QED. $\endgroup$
    – Ruadhan
    Commented Oct 21, 2020 at 15:46
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Another problem, which isn't really a paradox, is that the time-traveling delivery guy can't know whether the road is clear in the time he's traveling to. What if he drives into an empty parking spot, but that spot is not free when he hits the time travel button?

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  • $\begingroup$ More sensible would be to simply have an allocated spot at the depot or somewhere nearer the destination where he can be confident nobody will be standing. Some route-scheduling can ensure that he doesn't try and use it at the same time (hah) as another delivery driver. For that matter, it may well be that if there's lots of deliveries to make at the same rough time and area, the same driver might encounter himself in traffic... $\endgroup$
    – Ruadhan
    Commented Oct 21, 2020 at 15:21
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Yes, there is a problem, it is inevitable that a someone will figure out a system to encode information in the delivery request so that they can make a killing on the stock market, or whatever they need to do.

How would this work? You need an insider who is able to view the delivery request in the past before it is shipped, this could also be the buyer, or they could be partners. They could be monitoring a portfolio of stocks for movements and when a big event happens, immediately make an order that encodes how to bet on the stocks. This could be based on what items they are buying or the address, or the 'notes to the driver' or whatever. This information will be transmitted into the past where the insider see it and takes the appropriate action 1-2 days before the event happens.

Now, forget money. What if the insider is a plant by the CIA or some other nation state who want to monitor world events in the same way and prevent the future occurring as it should. It seems far too tempting for someone to do this or similar schemes.

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  • $\begingroup$ The only catch is that the delivery orders are coming from the same past the messages go to. So essentially for your scenario to work, the Client makes routine Timeazon orders every day (or more frequently) and your Insider, working at the Timeazon Depot, sneaks notes into the parcels with data from the future. The one-man variant is for a person who actually works for Timeazon to place orders on their days off and know that they'll be in the depot to insert the future-information in a couple days subjective time. Essentially sending themselves info from their own future. $\endgroup$
    – Ruadhan
    Commented Oct 21, 2020 at 15:40
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    $\begingroup$ @Ruadhan, no, the insider must only look out for, say, a parcel for a certain address each day. No parcel for that address means no action. The information in the future could then be encoded in a number of ways, e.g. a simple note, or by what the parcel size is, or perhaps a part of the address (a number or house name). If the insider can actually look in the parcel there would be a variety of ways to pass the information. The Client could get it engraved on a pen or whatever. $\endgroup$
    – crobar
    Commented Oct 21, 2020 at 15:46
  • $\begingroup$ That's pretty much exactly what I just said I think. $\endgroup$
    – Ruadhan
    Commented Oct 21, 2020 at 15:47
  • $\begingroup$ @Ruadhan, actually no, your point was that they have to make routine purchases every day (the "catch"), and I explained why that's not true. $\endgroup$
    – crobar
    Commented Oct 21, 2020 at 15:49
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    $\begingroup$ I may have gotten mixed up on your original description of the scenario. Re-reading, I think we've managed to re-state one another a couple times! $\endgroup$
    – Ruadhan
    Commented Oct 21, 2020 at 15:56
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Any time travel is open to paradoxes and that is fine

Just go to the TV Tropes page of any movie involving time travel and look at the Headscratchers and Fridge Logic sections. There is always some theoretical possibility that lets the world building seem lacking at first glance.

However, your goal as a story teller isn't to world-build a time travel mechanism that works flawlessly in all possible situations. It is to create one that doesn't leave any obvious loopholes/headscratchers in the situations that your characters face in the course of the story.

So in your case the question is, what role will instant delivery play in your story? If it is just an excuse to get stuff to the characters fast, it is definitely good enough as you described it. If for example a character plans to hijack instant delivery for a heist, you need to harden the mechanism for that application.

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Rule #1: There are no universal laws for time travel, only mathematical speculation.

Rule #2: The rules of time travel are (thanks to Rule #1) 100% in your control.

Rule #3:: If you choose to not set rules, then there is no such thing as time travel that doesn't cause paradoxes. One could reasonably assume that it's possible to minimize paradoxes through protocol and procedure — but it's impossible to avoid them all.

You'll notice that I've not said a thing about your presented scenario. I don't need to. It'll work if you want it to. It won't if you don't set time-travel rules to fix the problems. But I will use an example of what can go wrong. You have the following four steps:

  1. Bobby pulls up to Bob's house and pushes the time travel button.
  2. Bobby exits the truck with the package 5 minutes before Bob clicks buy.
  3. Bobby knocks on the door after Bob clicks buy and delivers the package to Bob
  4. Bobby goes back to his truck and pushes the time travel button again, travelling to three seconds after he left.

Where can things go wrong? At any time during steps #2 and #3. Bobby has days worth of time where he can go do anything he wants - including leave for Europe and never be seen again. Pardoxes abound!

Unless you don't want them to.

  • You could declare that time is self-healing. After step #1 there are two bobbies. Who cares? If they meet, so what? The fact that the recipient got his package is now a part of history and if future-Bobby convinces present-Bobby to not go to work that day, what really happens? Nothing.

Although nobody having told Amazon that the package had been delivered would technically cause Amazon to send a second package back in time. Hey... you could make a fortune by killing Bobby at the door and selling future-truck and each succeeding copy of the product you receive! Bobby keeps showing up. You kill Bobby. Amazon isn't informed so they send another package.... And since nobody in the recipient's timeline is missing Bobby....

There's no paradox because time is self-healing. But your world (or a portion of it) might find itself stuck in a time-loop until the recipient gets bored with killing Bobby.

  • You could declare that time abhors a vacuum. In other words, whether Bobby hits the button or not, he (and the truck) will eventually be yanked back to the future because something's missing in the future and the future wants it back! Dang, that means the package will be yanked back, too. OK, this one doesn't work. But it would stop Bobby from heading off to Timbuktu to do what he wants for two days.

  • You could declare that time stops until the loop is resolved. There is no progressing future until Bobby hits the back-to-the-future button. This would work no matter how many trucks are in motion. Unless the recipient kills Bobby. Then there's no restart to the future. I'm thinking the Langoliers here leading to a new Big Bang. Maybe that's too much power to give to a recipient who's frustrated with Amazon's shipping policies.

On the other hand...

Why are you trying to remove all the paradoxes? The paradoxes are the point of every time-travel story because they always engender moral and ethical crises. If we're assuming you can't avoid the paradoxes, then you're left with Rule #3, right?

  1. Amazon's trucks are outfitted with time-stamped GPS-enabled transponders that know how to account for the shifts in time. In other words, when one of those trucks pops up in the past, the then-present Amazon knows this! Ah-hah! Somebody's about to push the "Buy!" button! But it also means Amazon's Department of Temporal Protocol and Affairs is now watching Bobby very carefully — because the last thing Amazon wants is for Bobby to think he can pop off to the local gentleman's club for a quick glimpse on company time — because all time is company time (and it's Amazon's!). In other words, Amazon's DTPA will be quick to get on the walkie-talkie and remind Bobby to push the darn button! Time's a-wastin'!

  2. Amazon has an entire warehouse full of the most secured computers in the universe that manage and handle tracking of time-travel. Bobby isn't allowed to move that truck two inches after traveling to the past. In fact, Bobby has a chip behind his left ear that lets Amazon track him, too. No grandfather paradoxes here, thank you very much! Truck... button... door... Thank you!... truck... button... or we hunt you down like the mangy dog that you are and nail you to a sour apple tree, Bobby!

  3. Of course, nobody knows how Amazon is achieving their incredible record for on-time (literally!) deliveries — except maybe Ford Auto, from whom Amazon buys its trucks. Ford started wondering when Amazon's truck purchases suddenly went up 20X. When Ford investigated, they discovered that trucks that normally took 20 years to look run down, beat up, and over-repaired where rolling out of Amazon's fleet control center looking that way after only a couple of months. So, Ford knows what's going on, which is why Amazon's shipping costs went up — to pay the extortion bill.

And we won't even mention why Bobby, who was once thought of as an intelligent young man, would allow himself to work at a job that would age him 50% faster than normal.1

  1. Which is why Amazon modified their trucks so that the whole truck doesn't go back in time. Only the package and a drone do. Bobby pushes the button and the package disappears and the drone shimmers for a moment — the drone having dropped the package off and pushed the door bell, then returned to the future. The drone company's on the take now, too, but that's offset by the reduction in pay to drivers for the fact that they were dying at age 50 rather than age 90.

Conclusion

The truck and Bobby don't travel through time. A drone, which is controlled in the past by the Amazon DTAP, and the package do. Bobby just waits for the drone to shut down in his truck and drives away.

1 OK, not 50%, but faster. You know what I mean. Who would take a job that shortens their life?

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In short, it creates a lot of problems and wouldnt work.

In long: There is no way, not a single possibility, not the sightest of changes to travel backwards in time in or Universe with or Laws. The main reason for that is not even Causality although it is a big part. It is Energy. If you go from the Future, your Present, to the past, you add Energy to this new time while removing it from the present. This breaks conservation of energy as the Energy dosnt get converted, it is just gone. And then comes out of nowhere. Which again, cant happen. So this puts a hard stop to the idea right of the bat.

Causality then punches you while down, even the Atoms of your Body interacting with the Air around you break Causality. Because an Event from the Future changed an Event in the past. So you just standing there breaks it.

I suppose you could go back in time if you had no Matter or Energy and wouldnt ever interact with anything at all.

To make Time Travel work you have to change the Laws of Physics. Which honestly isnt that hard to do because you can just bs something at the Macro scale that Magically solves your Problem. Idk, say that something like Retrocausality exsists. Like you can go back in time and do everything, dosnt matter. Causality now works both ways. It dosnt here but hell, if it would we wouldnt know.

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    $\begingroup$ I think that the assuption "time-travel is possible" was part of the suspension of disbelief in this scenario $\endgroup$
    – Gilsido
    Commented Oct 21, 2020 at 10:10
  • $\begingroup$ its possible if you are allways traveling backwards in time entropy looks weird for you, but then you dont know any different $\endgroup$
    – Ewan
    Commented Oct 22, 2020 at 12:33
  • $\begingroup$ I mean, imagine writing these comments instead of searching through for ones by you and untyping them like normal people!! $\endgroup$
    – Ewan
    Commented Oct 22, 2020 at 12:36
  • $\begingroup$ > This breaks conservation of energy as the Energy dosnt get converted, it is just gone. And then comes out of nowhere. Which again, cant happen. So this puts a hard stop to the idea right of the bat. Why not some kind of entanglement where the particles are exchanged? $\endgroup$
    – user772
    Commented Oct 22, 2020 at 16:07
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I think your biggest problem is taht your customer will always cancel their order.

"Instant delivery" definitely isn't free for your company, time travel probably costs energy, so your customer has to pay extra for this instant delivery and certainly will cancel it if it doesn't arrive on time.

So now we have to ask how time travel works in your universe, as far as I know there are mainly three ways:

  • Directly affecting the past (Back to the futur I): there's an "original reality" that you change by traveling back in time, but in that original reality your customer didn't receive their package on time, since you hadn't traveled back in time yet, so they cancel their order
  • Alternate realities (Back to the futur II / Terminator): you don't change your universe but create an alternate reality, or simillarly traveling back is just hoping to a parrallel universe that lags behind yours in time. In that case you're immune to paradoxes anyway but there has to be a "first universe" and we fall back to the previous case, at least in that case only your first-universe-company will fail, the others will have some time when they indeed work
  • Loops (Harry Potter 3, and a bit weird so only makes sense with magic): there is only one reality and time-travel is possible only if it creates a more stable loop than a universe where it doesn't happen, so in your case the loop decribed (order by the customer -> instant delivery -> order stays up) is stable, the problem is that the customer canceling their order also is a stable loop and doesn't require time-travel (in which case there is actually no loop)
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    $\begingroup$ Well then don't allow them to cancel their order :) When they hit Instant-Buy, there's a little tickbox confirming that they waive their right to cancel delivery due to the unusual nature of it. The only hangup is cases where the delivery driver got caught in traffic or had an accident. But that's okay, because all that happens is the client hits "Buy" and the parcel doesn't show up.. Or it does with a diferent driver. The company has unlimited time to arrange an alternative delivery driver anyway if the time-travel can go further than a few days. $\endgroup$
    – Ruadhan
    Commented Oct 21, 2020 at 10:34
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    $\begingroup$ [Cont] The real kicker is if the driver tasking app reports that Driver A didn't make his delivery... does Driver A still set out on his run knowing that he's going to be in a potentially life-threatening traffic accident? $\endgroup$
    – Ruadhan
    Commented Oct 21, 2020 at 10:35
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    $\begingroup$ Heck. The company could easily just wait until the injured driver is well enough to resume his rounds. He could convalesce for months or years if necessary! He'd have a bit of a backlog of trips, but with time-travel, he can easily make sure all of them are on time with no break of service as far as clients are concerned. $\endgroup$
    – Ruadhan
    Commented Oct 21, 2020 at 10:37
  • $\begingroup$ @Ruadhan just to be clear you're only answering the "loop" scenario here? $\endgroup$
    – Gilsido
    Commented Oct 21, 2020 at 15:10
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    $\begingroup$ it's the only one where a business-model predicated on time-travel makes any sense :P Flinging inventory, time-machines and personnel into parallel timelines with no means of actually profiting or even getting them back is definitely a failing business model. Sort of an Anthropic principle applied to time-travel $\endgroup$
    – Ruadhan
    Commented Oct 21, 2020 at 15:14
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Yes, it can be done and it will be (mostly) safe.

First, forget about sending Bobby back in time. The machine is really big, not to mention expensive to give a portable! one to every delivery guy working on minimum wage (and Amazon wouldn't be able to ensure they don't telly anyone about the time-travel nor they actually take advantage of that for personal reasons).

Instead, Amazon will have the purchase request travel back in time. As the won't receive it before ordering, the timeline is preserved, avoiding paradoxes. Today it's 22nd October and the customer wants to buy the iphone xxi. This would usually have taken 5 days to deliver. In fact, Instant delivery wasn't offered as an option for the first 5 days. When the customer clicks Purchase:

  • The request travels back in time to 17th October
  • A normal delivery is routed to the customer house, taking 5 days
  • Bobby has no idea that the request originated in the future
  • Bobby will stand at the provided position by the delivery with the iphone xxi. When the customer completes the request his pda/smartphone will update with the details and Bobby will just ring customer bell seconds after purchase. An excellent so that customers don't have time to change their mind!

Another answer mentioned that this could be done with precogs, but there is no need for that. In fact, Amazon is already doing this. When their AI algorithms/precogs/time-travel purchase order state (or "estimate") that there will be X purchases of iphone xxi from your zone, they will distribute their stock accordingly, so they will try to have that in a warehouse near your region/city.

That they have a delivery guy standing next your door or already on your neighborhood is just the next step. For anyone questioning it, Amazon official reply is that they use state-of-the-art Artificial Intelligence algorithms to predict the sites where a new iphone xxi will be bought and provide a better experience for their customers.

From the delivery department, it doesn't really matter from where those predictions come, as long as they are accurate:

  • they might have a complete profile on you and, knowing you better than your own mother, know that as soon as you receive your check (at day X), you will be buying the new iphone X
  • they may have time-travelled the purchase order
  • they could know your intent (or wishes) to buy an iphone xxi from conversations between third parties that they eavesdropped their Home Assistant used, in combination to many other data downloaded from the cloud.
  • Amazon could have just showed the gullible customer, as planned, an ad for the iohone xxi

Just remember, if anyone asks, it's just an AI. Customers will find creepy the accuracy of our results, but we have here another order they perform in an hour, showing they will continue working with us.

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The amount of paradox in this situation is endless:

Bob the customer clicks the purchase button for something that is bought off Amazon, choosing instant delivery. Bob could changes his mind, cancel his order and go for a walk, encountering the delivery worker who just got out of his truck is one.

Bobby the delivery driver gets a thing saying 'deliver this thing to Bob at this time' on their delivery schedule. Unless the delivery event info is stored locally on his device, it will not exist in the delivery database when he travels back in time. Also, this does not account for past Bob changing his mind and cancelling the order, also leading to problems.

Bobby drives to Bob's house, taking 1-2 days like normal shipping. Wait, we've normalized time travel but not teleportation? Pretty sure we'd have teleportation or at least drone delivery perfected by now...

Bobby pulls up to Bob's house and pushes the time travel button. This very action and all those following are now creating endless alternate theoretical futures. Bobby may accidentally scare Bob's cat, Roberta, by materializing into existence in front of her. She stops chasing the mouse she had been chasing which had accidentally ingested a diamond, and said mouse would have been a ritual gift to Bob, who, upon investigating, would have found the diamond inside, taken it as a sign that he can now propose to his girlfriend, now having a stone to set in a ring he added to his wish list but had decided against, and this delivery was the ring he had chosen instead. Bob's girlfriend, Gertie, instead of giving up on Bob because he's taken too long to commit, decides to marry him and start a family...and that is only one of an infinite list of alternate future universes for only one individual this action would affect, not to consider the ecological impacts if Bobby accidentally materializes on top of the last endangered Gypsy Moth, wiping the species out forever...

Bobby exits the truck with the package 5 minutes before Bob clicks buy. Bob cancelled his order 4 and a half minutes ago, but Bobby doesn't know yet because the delivery order can't be updated from the past due to certificate errors with the delivery API due to time differences.

Bobby knocks on the door after Bob clicks buy and delivers the package to Bob. Bobby has had 5 minutes to create more paradoxes and butterfly effects than any current supercomputer could track...

Bobby goes back to their truck and pushes the time travel button again, travelling to three seconds after they left. If a random hit and run driver totals Bobby's truck, he's not getting back to the future any time soon, and by this time the universe has imploded because Bobby cannot exist in the past without changing the future. Any sustenance he takes will not be available for the individual it was 'destined for', leading to even more butterfly effects, not to mention any personal interactions he may have such as meeting someone new and causing new and complex relationships to both cease to exist or come into existence.

Bobby drives back to the Amazon warehouse for their next package or continues on their route. Oblivious to the irreparable damage he has done to the present and future, Bobby continues on his way, never meeting his future wife, Gertie, because she never moved to San Francisco after deciding Bob had gotten cold feet and was never going to commit. The Gypsy Moth, having been mankind's last hope for cloning a new generation of species that depended on the moth's special genetic traits, is now gone, and in despair, one of (insert nuclear superpower here)'s top generals convinces one of his fellow generals to help wipe mankind out by initiating nuclear missile launches.

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How about combining doing the shipping in the past with making the time travel system unreliable? Then you get a list of potential orders, which may or may not be clicked, but you don't know whether they would have if you hadn't shipped them. And then compensate for the unreliability with informing the driver at point of delivery whether to deliver the package, using fast but causal communications. (For all practical purposes, 5min = instant when we are talking delivery). The extra undelivered parcels would be compensated for by the drop in failed deliveries due to people not being home.

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I still say that any time travel method that allows the past to be changed, WILL NOT be invented.

Not that I think its impossible, simply that it will inevitably destroy itself.

If you CAN go back in time and change the past, then inevitably at some point, somewhere in the universe it will be used to change the past such that time travel is never invented. As long as time travel exists, its containing universe is unstable. As soon as time travel's changes cause time travel to never be invented, the universe solidifies into an existence where time travel is never invented.

So. The above scheme does include the possibility of a disgruntled delivery worker going to the past and preventing the invention of the timetravel, thus until that happens that universe is unstable.

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Yes, it does cause paradoxes. But we can avoid paradoxes, we can make time-travel technically possible, but practically of no effect.

Frame challenge. Businesses that claim to utilize time-travel for delivery are scams. Time-travel is technically possible, but practically useless, as it happens only on quantum level, under very obscure circumstances. The general public doesn't understand these nuances, they are overwhelmed with cheap sensationalism about proven possibility of time travel and strong associations of time-travel with "Back in the Future"-like shenanigans. This lead to creation of all kinds of unsavory businesses that claim to utilize this revolutionary technology, but in reality just scam people.

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Bob can (slowly) destroy the Earth!

  1. (Evil) Bob moves to the top of a high cliff
  2. Each day at mid day, Bob orders as many heavy as he can afford.
  3. Bobby arrives from the future with the heavy packages
  4. Bob immediately drops the packages off the cliff. Accelerating the earth towards the sun!
  5. 2 days later Bobby drives up to the top of the cliff with the heavy packages, But crucially it's not midday and the Earth is 2 days further along in its orbit of the sun. The Earth is pushed in a random direction.

Essentially here your time travel machine glosses over the problem of appearing in the same place when it travels backward in time. In doing so it violates conservation of momentum and hence energy.

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  • $\begingroup$ Couldn't you do that exact same thing without time travel? Just order near your cliff and drop everything at midday the next day. But your limitted time and money makes it impossible in practice $\endgroup$
    – Gilsido
    Commented Oct 22, 2020 at 7:59
  • $\begingroup$ no the earth would just wobble. say you take a bucket of moon dust from the dark side and carry it around to the light side of the moon. you push the moon towards earth when you pickup the bucket, but in walking around you pushl it away again, so no change. If you instead teleport, you skip the pushing away step $\endgroup$
    – Ewan
    Commented Oct 22, 2020 at 12:13

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