Santa is a time traveller. How does he not violate causality?

People wonder how Santa can deliver all his presents in one night. It's, simple: he's a master time traveller. (This post will form the base of a long series of questions).

Here's how Santa's usual Christmas goes:

1. Santa spends Christmas with his family
2. After Christmas, all the presents are in all the homes
3. Santa begins travelling backwards in time
• According to Norad, Santa's sleighs max speed is "faster than starlight". According to special relativity, this is equivalent to travelling backwards in time.
4. Santa breaks into and steals the presents from a bunch of houses (this is logically necessary, since by the time he reaches the beginning of Christmas, there should be no presents in their homes).
• Since Santa is doing this in reverse, it's equivalent to him giving them presents.
5. Once it hits the beginning of Christmas, he starts going forward in time again, and gives the presents he stole to different homes then he stole them from.
6. Once he reaches the end of Christmas (having delivered all the presents he stole), he goes back to step 3.
7. He is careful to never visit the same home twice.

At the beginning of Christmas, no homes have presents. At the end, all homes have presents. This is logically consistent because if

• If Santa visited your house going forward, you started with no presents at the beginning of Christmas, and Santa gave you presents, and then you had presents by the end.
• If Santa visited your house in reverse, you have presents during the end of Christmas, Santa in reverse stole them, and you have no presents at the beginning of Christmas.

Therefore, we do not have a paradox, yet.

To an outside observer, it appears that there are millions of santas and reverse santas (see One-electron universe).

• At the beginning of Christmas, millions of santas and reverse santas pair produce. Every santa and reverse santa is fully stocked with presents.
• The santas deliver presents to the children like normal.
• Reverse santas also deliver presents, although it appears that they are stealing presents, but in reverse.
• After Christmas, every santa annihilates with a reverse santa. They will do so with a different reverse santa then they were produced with.
• Actually, one of the reverse santas annihilates with the santa from last year, leaving one santa to become the santa for the year to come. This new santa will have the combined experiences of all the santas and reverse santas.

If it seems confusing, it's because it is.

My question is, how does Santa avoid running into himself or otherwise causing temporal paradoxes. Given how many times he has to go forward and back, it seems likely that he could cause a temporal paradox if he is not careful.

• Why would he have to avoid temporal paradoxes? If the universe he lives in can accommodate paradoxes or automatically avoid them via the Novikov self-consistency principle, he could just ignore them and let the consequences sort themselves out. Which is to say - how do paradoxes work in your universe? – user867 Dec 24 '15 at 1:31
• @user867 It has Novikov self-consistency (it sets all paradoxes to probability zero, and renormalized (which basically means a bunch of random and apparently contrived coincidences occur to prevent paradoxes)). The problem is, if a time traveler keeps almost causing paradoxes, the probability of them never time travelling again (by say, dying in a freak accident, or the apocalypse) shoots way up. – PyRulez Dec 24 '15 at 1:34
• @user867 (He's already pushing his luck (literally) since he is basically ensuring all the homes have presents via consistency.) – PyRulez Dec 24 '15 at 20:40
• I thought Santa is his own reserve Santa? Guess I'm wrong but one thing is for sure his mass is definitely from the elves! – user6760 Dec 25 '15 at 3:48
• @user6760 He is all the santas and reverse santas. It just looks like there are a bunch of him. – PyRulez Dec 25 '15 at 13:25

He uses his magical invisibility/phasing abilities.

Lets face it: both normal Santa and reverse Santa need to be able to break into people's houses, deliver presents, and get back out without being seen. This means he must be invisible (or have some way of making himself invisible) and the easiest way of getting in/out is to have some form of phasing / teleportation in order to get to the christmas tree without having to damage anyone's property.

Therefore the natural way for Santa to avoid running into himself is for him to turn on his phasing and invisibility whenever he isn't inside someone's house. Then he literally can't run into himself, or accidentally see himself!

Novikov consistency maintained, paradoxes avoided, presents delivered, and no accidental Santa Sightings!

• It would be funny of changing time travel direction required turning phasing of. (Imagine a child seeing a santa and reverse santa annihilating each other.) – PyRulez Dec 24 '15 at 15:20

I know that the quantum physicists are going to crucify me for this one, but it is still worth a try...

It is an as-of-yet undiscovered physical law of the universe that identical entities, co-existing in any given moment of time, cannot interact with each other in any way, including paradoxically. That's just the way the universe works. Two Santas can face each other from an inch apart, screaming at the top of their lungs and swinging their arms violently through the space that the other is occupying, and neither will ever know that the other is present. They are both completely real and interactive with the rest of the universe, but they are completely intangible and unperceivable to each other.

I have no proof that this unusual law actually exists, because I have never been in two places at the same time. But you can't prove that my law doesn't exist, because neither have you.

• I have not, but the electrons have. – PyRulez Dec 24 '15 at 19:33
• I disagree the answer defines two Santas whereas the one-electron interpertation, by definition allows for the existence of a single electron. But there is a deeper problem, provided we do not abandon logic and resort to magic, quantum mechanically speaking for the Santas to not interact will take till beyond the expected end of the universe. – Athena Widget Dec 25 '15 at 2:30
• @AthenaWidget, please explain the time requirement. I was assuming that each Santa could interact with all of the Reverse-Santas and that every Reverse-Santa could likewise interact with all the Santas. Only Santas that share the same spin can't interact. With that amount of limited interaction, the recursive gift cloning and massively parallel distribution of delivery-work, should still be possible... -ish. – Henry Taylor Dec 25 '15 at 5:03
• @HenryTaylor Santa can interact with himself anytime he likes, its just that doing so increases the risks of paradoxes. – PyRulez Dec 25 '15 at 13:27
• @PyRulez, i was working towards a universe which allowed for time travel but didn't have to worry about paradoxes. Take the idea of "out of phase" and bring it in as a anti-paradox defense. That is not perfect alone. Time-travelling Santa can always hire someone to kill his father, thus paradoxing all of the Santa's out of existence; but smarter minds than mine can figure out how to defend against that. – Henry Taylor Dec 26 '15 at 0:00

That doesn't make sense. What is the point of stealing-in-reverse, since the house only has a present because Santa delivered it? You're re-arranging them, but not providing a way for presents to come into existence. Or, stealing in reverse is the same as delivering except for the conscious activity of Santa himself, so from our pov he's just making 2 delivery runs.

• Stealing in reverse is the same as giving. If I steal from house A in reverse, and then give to house B going forward, it's the same as if I had given them both presents (they both start with no presents, and end with presents). – PyRulez Dec 25 '15 at 18:03
• But where did the presents come from? I think A will be an anti-present when time reversed. – JDługosz Dec 25 '15 at 20:24
• see the observer interpretation above – PyRulez Dec 25 '15 at 21:04
• So long as the presents appear de novo, you're good to go. But the observer interpretation does not explain when the toys were manufactured or where their materials came from, But that's part of Santa's magic. – WhatRoughBeast Jun 12 '17 at 19:19

First point: Christmas is not celebrated in all places (only about half to three quarters the world's population actually celebrate it) second point: Christmas is not celebrated at the same time all over...

The first point is easy, Santa will only have to deliver gifts to half the children in the world.

The second point hinges on differing traditions; yes, if all the world woke up to gifts on the 25th of December, he'd have a hard time... However, where I come from (Denmark), the gifts are traditinally given on the evening of the 24th. Some eastern countries wait until January 6th

At least this gives Santa a bit more time to distrbute his gifts

Santa breaks into and steals the presents from a bunch of houses (this is logically necessary, since by the time he reaches the beginning of Christmas, there should be no presents in their homes).

Is a present, once opened, still a present? Logically, Santa would be unable to give your kid that brand-new 3DS he wished for, as he re-uses gifts.

Then you come with this:

Once it hits the beginning of Christmas, he starts going forward in time again, and gives the presents he stole to different homes then he stole them from.

How does that explain how I got my Gameboy Color when it was just released? (it came out late november 1998)

If Santa visited your house going forward, you started with no presents at the beginning of Christmas, and Santa gave you presents, and then you had presents by the end. If Santa visited your house in reverse, you have presents during the end of Christmas, Santa in reverse stole them, and you have no presents at the beginning of Christmas. Therefore, we do not have a paradox, yet.

YES WE DO... forwards or backwards, in order to cover up his thievery, he has to be in the same place twice (or hire his brother, Jeff). Also this presents either a paradox of changed history OR item duplication.

Lastly, Santa WOULD need to go to the same house at least twice (forwards and backwards)

Actually, if you count in different traditions across the world, Christmas is comparatively like a busy work day for Santa