This article is about a grandfather-like pseudo-paradox, and I say pseudo because it’s a little different. I would be grateful to receive opinions from any expert who is familiar with the physics and philosophy behind the subject.
Minimally invasive time travel or time-reversed communication
Originally, the Grandfather Paradox is a paradox because in his time travel (to the past) Bob (let’s call him Bob) does something that affects the events profoundly and pertinently enough to prevent his time travel in the first place: Killing his own grandfather he wouldn’t be born, hence he would never travel back in time, he would never have killed his grandfather, he would have been born and he would have time-traveled and killed his grandfather, preventing his birth, and so on, we know the drill.
However, what if in his time travel (to the past) Bob doesn’t do anything that would prevent his (future) time travel? Let’s say he only engages in minimally-invasive activities, temporally speaking. What happens then? (at least what’s your opinion to the best of your knowledge and understanding of the subjects involved).
Okay; to simplify, thought experiment:
We’re in 2019. Bob builds a rig to communicate with himself backwards in time from now on (Let's say that, for example, in 2022 he will be able to send a message to himself in 2019. He could never send a message to himself back in 2012, for example, since in 2012 he had not built the rig yet).
We’re in 2020. Bob and Alice live in TOWN A. Bob is in love with Alice. Alice is in love with Bob. None of them gets a chance to talk to the other.
We’re in 2021. Bob moves to another city (TOWN B). Alice gets sad and regrets not to have talked to him. Alice calls her best friend Sophie and starts crying because now she’s lost Bob and she’ll never know if he liked her nor she will ever have another chance with him.
Sophie, on the contrary, happened to know that Bob also liked Alice. So, Sophie calls Bob and tells him he’s an idiot because he never told Alice, Alice also liked him, but now he moved away and lost that chance.
We’re in 2022 (December 31st at 11:59 pm to be precise). Bob sends a message to himself backwards in time in 2020. He tells himself it’s him from the future and he should ask Alice out because in 2021 Alice told Sophie and Sophie told him that she likes him.
The younger 2020’s Bob has now two choices:
1) A PARADOX-FORMING CHOICE: Bob calls Alice right away. They end up together, so he never lives town, so Alice never calls Sophie crying, so Sophie never calls Bob, so Bob never sends the message to himself in the past, so Bob never asks Alice out… Grandfather-like paradox. Let's not even talk about that (headache).
2) A NON-PARADOXICAL CHOICE: Bob retains this information (remember we’re in 2020, he just received the message from his future self) but doesn’t act on that until the date of his time-reversed transmission to not to alter radically the so-called “timeline” (which, by the way, I don’t believe is a “line”, neither literally nor metaphorically. I believe it is more like a fox’s tail full of cross-interacting hairs, if I can think of a marginally better analogy and really must retain something of a "line").
He now (Dec. 31, 2020 and ahead, altered pseudo-timeline) knows that Alice likes him, he doesn’t do ANYTHING accordingly, he keeps living his ordinary life. In 2021 he moves to the other city (TOWN B), as expected. He receives the call from Sophie, as expected, but, before doing anything differently, he waits. He sends the time-reversed message to himself in 2022, as expected. The pseudo-timeline is preserved, no paradoxes here. He packs everything, moves back to the previous town (TOWN A), asks Alice out, and they get married and live together happily ever after.
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• Key-point #1: in the second option, yes, surely Bob altered the pseudo-timeline a little bit. Altered with respect to what? To the “original” pseudo-timeline in which he simply didn’t know that Alice also liked him. Indeed, after knowing (because of the time-reversed message received from his future self), even if any day he stopped for a second wondering about Alice, that changed something (at least the motion of the molecules of the air and of his body) with respect to the original pseudo-timeline. However, those changes, although potentially devastating via butterfly-effect to who knows who or what else in the universe, will not prevent Sophie’s phone call and Bob’s time-reversed messaging to himself. No paradoxes. Just alterations of the pseudo-timeline (with respect to the original pseudo-timeline), but no paradoxes.
QUESTION 1: To the best of your knowledge and understanding of the subjects involved, am I right? (about the no paradoxes thing)
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• Key-point #2: let’s talk about the “changes” (although non-paradoxical) in the pseudo-timeline caused by Bob’s time-reversed message to himself. If we (likely inappropriately) draw this story using the usual horizontal line which represents the “timeline”, putting the dates on it as little vertical pipes, we can see that, about the matter of Bob and Alice’s ignoring each other, Bob moving, Alice’s crying, Sophie’s phone call, Bob’s time-reversed messaging, etc., nothing changed. So, we could draw the “new timeline” with a different color but simply overwriting the original “timeline”. However, from the point of view of other matters and events (let’s say universally), for sure, from the moment Bob received the message from his future self and read it, he will start micro-changes that will inevitably amplify more and more throughout the “new timeline” towards the future, contributing “building” (or consisting in part of) that future.
QUESTION 2: To the best of your knowledge and understanding of the subjects involved, what happened to whatever and whoever underwent different “events” (at any scale) in the so-called “original timeline”?
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• Key-point #3: let’s talk, finally, about the subtle distinction between the POTENTIAL future (Bob and Alice never living together) that WOULD have happened (had Bob never sent the time-reversed message to himself) but that (important!) never had a chance to “start” (at least from the point of view of the time-reversed communication event), and the ACTUAL future in which Bob and Alice will live together happily ever after because of the time-reversed message he sent at a certain point. So, any possible action that scrambled up time (jeopardized its “integrity”, or whatever we wanna call it) via tinkering with time-reversed communication rigs happened BEFORE that “future” (let’s say from 2023 and ahead).
So, from the point of view of the events that happened WITHIN Bob’s time-reversed messaging (2022) and younger Bob’s receiving that messaging (2020), yes, there are TEMPORAL RIPERCUSSIONS in terms of “changes” in the pseudo-timeline with respect to what had “originally” happened in time. That’s WITHIN that time-frame in which the time-reversed communication (retro-causality) occurred.
However, BEYOND the time-reversed communication (after 2022’s messaging and on), nothing time-reversed (retro-causal) happened!
QUESTION 3: So, since ALL the events that happened in the “new” pseudo-timeline between 2020 and 2022 (BOTH those events that would have happened/did happen also in the “original” unaltered timeline and all those events which changed because of Bob’s time-reversed messaging) chronologically happened BEFORE the “new” timeline (let’s say before 2023), and so, since all those events, with respect to time from 2023 and ahead, simply happened BEFORE (in an ordinary causal way) of the future timeline (2023 and ahead), can they be considered as following normal-causality?
In other words, can the “subsequent” events (from 2023 and ahead) be considered simply partially “different” from what would have happened if Bob didn’t mess-up with time, but not an “altered” timeline?
CLARIFICATION.
Another way to “interpret” QUESTION 3 is this (just an example to simplify a situation that is much more complex, especially down to the gazillion situations of the quantum realm):
Let’s say that, even though Bob tried to not to do anything differently from the moment when he received the message from his future self in 2020 up to the moment when he will actually send that message to his past self (Dec. 31, 2022), still, one day in between those two dates, happy because of the “news” from the future of Alice liking him, he decides to celebrate by drinking a glass of champagne. This very event didn’t happen in the “original” timeline. Notice that I wrote in bold the “didn’t happen” part. Why? Because this is an event that, from the point of view of the time-reversed communication (let’s say in its “reality”), had actually happened! (let’s call this a CLASS-A event, “A” as “actually”), because there really was an “happened” (materialized?) timeline in which Bob never met Alice, otherwise there would have been no time-reversed communication! (where/when/how that “reality” is/was materialized? That’s another story).
On the other hand, let’s say that, if Bob never met Alice (and never sent the time-reversed message to himself that December 31, 2022), starting from 2023 he would have kept living in TOWN B instead of going back to TOWN A to meet Alice, and so, let’s say, he would have purchased a sandwich in TOWN B in 2025 (just an example).
Now, from the point of view of Bob’s time-reversed communication, that sandwich purchase in TOWN B did NOT happen. It never happened. Because he never got to that point of being Alice-less in TOWN B in 2025. Yes, we could say that it would have potentially happened (notice I wrote this in bold, again) if Bob never messed-up with time-machines, but never had a chance to actually “physically/storycally” happen!! (Let’s call this a CLASS-P event, “P” as “potentially”).
So, a way to re-formulate QUESTION 3 based on the above clarification could be: To the best of your knowledge and understanding of the matters involved, is there ANY “DIFFERENCE” (physical, moral, ethical, or whatever) between CLASS-A events and CLASS-P events?
Thanks so much to whoever will share opinions.
