There could be 4 main probabilities I see.
1. Future you decided not to write a letter or never wrote any letters.
2. You died- but you’d never know likely. If the prevailing rule is that your future self MUST write you a letter- then for you to receive no letter you must either die before you turn 18- in which case you won’t be alive to know you didn’t get a letter- or you must have died so close to turning 18 that you couldn’t make a letter. Or: died under circumstances where the letter was lost. In the most likely case you either would never know or would would only find out moments before your death.
But- who knows how to push your buttons better than you? Especially as we age- if we grow as people- we tend to know the pathologies and mechanisms that governed our behaviors. So perhaps your future self knew full well exactly what you’d likely do when you got a letter telling you not to do something- and was counting on it? Perhaps- you literally played yourself?
What is interesting though are the mechanics. Your future self sends a letter to your past self- so either they received the exact same letter and already reacted the exact same way and so the letter is by and large meaningless since you will do what they did and everything will go exactly as it already has before- or- they did not receive the exact letter you did- in which case, you will most likely NOT do exactly as they did- in which case you may not become the person who sent that letter. So then, in the future- you will write a letter to your past self which will likely not be the same as the letter you received meaning on your 18th birthday you would receive a different letter than what that future you would remember- and repeat. Or... well- since we don’t really understand time travel who knows?
But... it never specified that the letter 18 yo you receives is written specifically TO your past self. It could simply be any random letter from a collection of letters you have ever written. Thusly it MAY contain information on future loves- if the letter happens to be a letter to a future lover or about them. It could also be a random job related message or a letter to the IRS requesting a tax extension. So without context we can’t really say- and we must ask- if you got a random letter yourself wrote to another- did that person not receive it? So you may interpret a letter based on only what it contains and assumptions- wrongly- but in the future might you lose the love of your life because they didn’t get a critical letter? And when you reach that point again- guided by detail from the letter- will things just repeat since they never get it?
They won't though! Because if you get the letter, and then never have the letter written to be sent out then you never GOT the letter and that time line would literally be erased. This is literally a more annoying version of the grandfather paradox where if you killed your grandfather who killed him because you never were born- I DON'T KNOW GUEST THIS IS THE STUFF THAT CONFUSES ME IF IT IS A RANDOM AND NOT YOU YOURSELF WHO WROTE THE LETTER THEN HOPEFULLY YOU ARE NEVER COMPELLED TO WRITE A LETTER TO YOUR PAST SELF AND ALL IS WELL BUT I DON'T KNOW!
Lol. Sorry @Calvinoot. Not trying to break your brain. But- not to worry! It all depends on how we view the events. There are numerous theories that the universe hates paradox- and several proposed ones in which the mechanics of reality either “self repair” paradox- “work around” paradox- or simply will not allow for paradox to occur.
One such theory holds one simply cannot go back in time. It makes sense when we think about it. At least theoretically, and assuming time is a real and tangible thing- your place in time would be described as a set of coordinates- if we call every thing that happens an “event” each event in life takes place at a fixed point in time and space. The planet is moving at thousands of miles per hour through space, the universe itself is moving. I’ve moved thousands of miles since I began writing this post and you’ll move thousands by the time you’re done reading it. So even if we could stay at the same place in time- the actual point on space we occupy would not be the same. But...
If we come up with some theoretically far fetched idea that we could somehow return to an exact coordinate in space and time.... there’s another missing number. Well.... and inconceivably huge amount of missing numbers. The coordinates for everything else. Just because I have returned to a set of coordinates doesn’t mean everyone else has. These coordinates are relative- not fixed- at least in theory, to not just every “macro” thing in existence- but every sub atomic particle etc. then there is the question of things like tachyons which theoretically move backwards in time- meaning that in relativity- the gap between where you are and where they are at any moment in reference to a past event is made even greater by the fact you are “moving” in “opposite” directions through time.
And that still is only possible IF humans and other matter can even move more than “one direction” through time. BUT! Even if we COULD Return ourselves to a point where all information were fixed at the same points in space time in reference to all other information in the universe to be the exact moment as we’d already experienced it.... by its very nature it COULD NOT be the exact moment we’d already experienced. Why? Because to be the EXACT moment of “past” isn’t just a question of us occupying a set of coordinates we previously did- those coordinates are in relativity to all other matter and information. Is being their precludes that because we have brought information to that point which then changes the coordinates of that point in reference to the information that wasn’t previously there.
Our cells aren’t the same for instance- they previously didn’t exist in that “event” and thus it is a new set of coordinates. But let’s say we send only our mind to occupy our body? Even if we found a way- and even if we worked out how to imprint our unmodified mind into a brain that has a fundamentally different structure (the neurons, synapses- connections and pathways of our mind change with time and experiences- ones we didn’t previous have in the “past”) even if that works- we would be bringing “information” that wasn’t there before in the form of thought and memory. This information changes our coordinates so they are not the same.
Thusly- to succeed in arriving at the exact set of coordinates we previously occupied- we would need a way to transmit our minds as EXACT copies of the information state they were in at the very event we choose to travel to- in a way which isn’t additive of information in the form of even energy or sub atomic disruption which wasn’t present in the original event. The mechanism itself cannot imprint any information that previously didn’t exist. So even if ALL this was possible and doable- that would result in you- being in that moment- with the exact thoughts and feelings and memory you had in that moment.
It is then most likely that whatever actions you next took- would be exactly the ones you previously did- and thus the effect of this “time travel” wouldn’t change the events that come next. In fact- it would most likely be akin to death. You’d leave now- go back, re live things exactly as they occurred, catch up to the same “future” aka present- bring the exact information you did last time, make the same choice to go back, and repeat the process- effectively in a universal sense it’s no more paradoxical than dying- any information you would contribute to future events would be via things like cell decay or secondary effects imparted by your existence. This “loop” only effects you directly. If you were “brought back” and the “loop” broken- your consciousness would simply bring the same information to the “future” that was already there. It would be no different than “naturally” “moving into the future.”
Now- if we apply some type of “chaos theory” and we theorize that even with ALL information in the exact same state that somehow you might do something different- it still doesn’t create paradox. If something different can happen it could always have happened. New information is created but information that already exists is not destroyed. Say you take a video game like “animal crossing” which continues without the “player.” Say you save the game before making a choice. You then copy the information so you have two games with identical information. You make choice A on game A and save- choice B on game B and save. When YOU return to either save- the state of events will have progressed in your absence- but you can otherwise continue where you left off. So long as you do not exceed an amount of information which can be physically held by a system- you can always add new information.
Each event is new information in reference and relativity to existing information. As long as the universe has space for new information we can create new events with unique coordinates without erasing any previous information. All matter is theoretically capable of existing in multiple states at once- all information being true and false at once.
The “observer” doesn’t have to be a conscious entity- it’s an interaction. Where every action has an equal an opposite reaction- the state of an object in question is observed by that which has interacted with it. On a sub atomic level such interactions are always ongoing- the person outside the box may not know if the cat is alive or dead- lets even say the cat does not know of it is alive or dead- but the subatomic particles interacting on and with and within the cat must act in a way which is consistent with the state of the cat- thusly the state of the cat is imprinted upon the fabric or reality wether we can discern it or not. There is no model of reality we understand that doesn’t hold this true unless we were somehow able to “freeze” time- “freeze” all movement including the movement of the universe itself- not just within a space but as an entire system in relativity.
So take heart and soothe your pounding head. It is most likely that even with time travel- paradox effectively doesn’t exist. Either by the nature of reality it cannot exist as regardless of relativity all things move “forward” from their own perspective- or that paradox is simply self resolving or self preventing. That forces like entropy are at play at the conversion of matter, energy, and other properties of matter like inertia and whatever information is imparted and converted through matter simply will not allow a perpetual event such as paradox. The mechanisms of physics will “bleed” off such an event leaving a state conducive to operation of reality. That... or maybe we just get another Big Bang or something to “reset” the game board if there’s a big enough “glitch.” Even then it isn’t a paradox since such an effect would provide a “break” to the chain and continue things progressing in the direction which they move.
Well, that's a relief and sort of disappointing that thinking about them could essential be pointless. But by god is this stuff still confusing! I don't know how you still manage to think so in depth about nearly everything, it is a talent that still astounds me!
I know what you mean. Time travel theory and all its unknowns is one of those black holes that can quickly overwhelm. Thank you for the compliment. I suppose I have lots of time to think about things while I’m typing my extremely long posts! Lol.
1. Future you decided not to write a letter or never wrote any letters.
2. You died- but you’d never know likely. If the prevailing rule is that your future self MUST write you a letter- then for you to receive no letter you must either die before you turn 18- in which case you won’t be alive to know you didn’t get a letter- or you must have died so close to turning 18 that you couldn’t make a letter. Or: died under circumstances where the letter was lost. In the most likely case you either would never know or would would only find out moments before your death.