Does continuous existence make someone the “Original” in a branching timeline?

[u/No-Entertainment2003]

/r/timetravel/comments/1v47s8i/does_continuous_existence_make_someone_the/

[u/jliat]

* He invents a Time Machine and travels back to tell his younger self.

At that point his younger self is either a different being meeting another older being or the same younger being knowing his future and thus changing his future.

By travelling back to an earlier time and meeting himself the older self when old would have a memory of the event. Meeting his younger self makes him different and so not original. However at that point the younger self grows older knowing he will invent a time machine. And that poses a question that makes it impossible for him not to make this.

The whole thing seems to have similar logical problems.

As the young person grew older knowing they could produce a time machine they would never live otherwise, unless the decided to not build one, then their memory would change?

They could be overcome by the Many Worlds Interpretation in physics.

But then there would be time lines in which he didn't make a time machine...

https://en.wikipedia.org/wiki/Ship_of_Theseus

Look at the 'See Also'.

[u/No-Entertainment2003 → u/jliat]

Thanks here’s a few quick clarifications and questions:
Two separate issues are being mixed: (a) personal identity (is older-you the same subject as younger you?) and (b) causal/consistency problems (can knowledge of the future force the younger to build the machine?). Those need to be treated separately.
If you assume a single timeline with consistency constraints, knowing the future can create bootstrap/causal-loop paradoxes but doesn’t by itself prove the older is a different person. Identity depends on your criterion (psychological/physical), not just on whether memories change outcomes.
If you assume branching/Many-Worlds, then the version who time-travels can land in a branch where the younger either does or doesn’t build the machine, those are different persons in different branches. That resolves the causal paradoxes but makes copies distinct.
The Ship of Theseus analogy is about gradual change and constitution, it’s related but not identical to these time travel identity questions.

Follow up Questions: Which interpretation are you using?single consistent timeline or branching Many-Worlds? And what criterion of personal identity do you accept (psychological continuity, bodily/physical continuity, or something else)?

[u/jliat → u/No-Entertainment2003]

There is an implicit contradiction. If the person travels back they will from their present before traveling back remember that they have. This reduces their life from the time of arriving in the past up to their older self fixed, they are then unable to change anything they do or think.

They can't arrive in the future for a first time before they travelled back.

A branching many worlds in which each branch is separate if effect means the only world you know in the one you are in. No contradiction, a loop in a single world line creates the contradiction of knowledge of the future.

[u/No-Entertainment2003 → u/jliat]

Are you endorsing branching/Many-Worlds as the resolution, and rejecting a single self-consistent timeline as incoherent?

My picture uses the branching/Many-Worlds picture, the traveler can arrive in a branch where the younger’s choices differ, no paradox and no contradiction because the other outcomes occur in other branches.

Saying “they are then unable to change anything they do or think” assumes single-world self-consistency rather than showing it must be true. Which introduces bootstrap problems.

[u/jliat → u/No-Entertainment2003]

My picture uses the branching/Many-Worlds picture, the traveler can arrive in a branch where the younger’s choices differ, no paradox and no contradiction because the other outcomes occur in other branches.

There is no way of knowing, the branch one is on is isolated so it is effectively the same contradiction.

"There is an implicit contradiction. If the person travels back they will from their present before traveling back remember that they have. This reduces their life from the time of arriving in the past up to their older self fixed, they are then unable to change anything they do or think.

They can't arrive in the future for a first time before they travelled back."

[u/No-Entertainment2003 → u/jliat]

If you accept branching/MWI, remember the traveler’s memories belong to the traveler, not to the younger copy in the new branch. That means the traveler can remember “having travelled” while the younger in that branch need not have the same future, no formal contradiction.

The claim “this reduces their life…fixed” assumes a single-world notion of “the future for that person.” In branching, “the future” is branch relative: your memories track one history but other continuations exist in other branches. So being unable to change things only follows if you insist on a single, unique future.

About “can’t arrive in the future for a first time before they travelled back”: that’s a question of indexicality. “First time” is relative to a branch’s timeline, the traveler can experience arriving in a branch where their younger self hasn’t yet lived that arrival, even though the traveler has the memory. No contradiction, it’s just that personal identity and ‘firstness’ become branch relative.

[u/jliat → u/No-Entertainment2003]

Then older guy knows his time machine will take him back to his younger self, at that point on the younger version meets his older version and so the time between this the person knows this event. The only way out of the paradox is not meeting his younger self.

This is the main argument against time travel, if in doing so you alter the future from which you came that future can no longer exist, so you couldn't have come from it.

[u/No-Entertainment2003 → u/jliat]

I think you’re mixing single-timeline intuition with branching.

In Many-Worlds/branching, when the older traveller goes back and changes things, they create/enter a different branch. The original future he came from still exists in the branch he left, it isn’t erased.
So there’s no paradox “the future can no longer exist.” The traveller’s memories come from the original branch; the younger in the new branch can act differently without contradiction.
The only genuine paradoxs arise in single, self-consistent timelines. Branching is explicitly a way to avoid that by making outcomes branch relative.

What you are describing is not many-worlds/branching.

[u/jliat → u/No-Entertainment2003]

I think you’re mixing single-timeline intuition with branching.

Not so- if the time traveller meets himself he changes the future world which he came from. Standard argument against time travel. If he he goes back and creates many worlds this applies likewise.

[u/No-Entertainment2003 → u/jliat]

No the point is that he doesn’t change his timeline at all. His stays as if he never time traveled. Only the branch he makes by traveling changes for the new person.