5 ms·

> Are you implying traveling to the past doesn't change anything? Exactly. The whole timeline already exists and is self-consistent. Whatever you do in the pas

> Are you implying traveling to the past doesn't change anything? Exactly. The whole timeline already exists and is self-consistent. Whatever you do in the past already happened, and it did so in a way that was consistent with the future you travelled back from. Time travel to the past is not inconsistent with known physics (see the various GR spacetimes with CTCs). There are reasons to think it’s probably impossible in our universe, however.

ASalazarMX 8y ago
Don't be silly. If you dedicated your whole life to invent a time machine, succedded at your 80s, met your younger self and gave him your research so he can build it in his 30s, how is that self-consistent?
- deleted 8y ago
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- antidesitter 8y ago
  It’s not. You just described an inconsistent timeline, but this has nothing to do with whether time travel creates such inconsistent timelines. To be more explicit, the grandfather paradox is not an argument against time travel [1], precisely because the past cannot be changed (as you assumed in your example). [1] https://en.wikipedia.org/wiki/Grandfather_paradox#Philosophical_analysis https://en.wikipedia.org/wiki/Grandfather_paradox#Philosophi...
  - ASalazarMX 8y ago
    Time travel _to_the_past_ has all the potential to create inconsistent timelines. You can deliberately cause severe inconsistencies because you have the crucial component: being in the past. The only way for the timeline to defend from such inconsistencies would be to proactively thwart all your attempts, but then, Time/Fate has become a conscious entity.
    - antidesitter 8y ago
      > You can deliberately cause severe inconsistencies... No, you can't. Whatever it turns out you do already happened. You're assuming that being in the past means you can somehow "change" it, which is incoherent nonsense. As David Lewis pointed out [1], the idea that self-consistency in the face of time travel requires some kind of "thwarting" of your actions by "the universe" is just a poor (and unnecessary) plot device employed by some science fiction authors. You should read up on Novikov self-consistency [2]. Read the Lewis paper too, it doesn't require a physics background and will illuminate the heart of the matter. See in particular the discussion on compossibility. [1] https://www.csus.edu/indiv/m/merlinos/paradoxes%20of%20time%20travel.pdf https://www.csus.edu/indiv/m/merlinos/paradoxes%20of%20time%... [2] https://en.wikipedia.org/wiki/Novikov_self-consistency_principle https://en.wikipedia.org/wiki/Novikov_self-consistency_princ...
      - ASalazarMX 8y ago
        Come on, just humor me. Following my first example, why couldn't I visit my younger self and give him my life research? What physical law could possibly prevent us from working together and building a second time machine in a few years, thus guaranteeing a paradox? I understand why I shouldn't be able to accidentally cause a paradox, but if I know how to cause one and have the means to do it, and the universe is mindless, what could stop me?
        Pharmakon 8y ago
        Stephen Hawking posited that the first and only thing a time machine would do is detonate as a direct consequence of virtual particles reaching arbitrarily high energy density through repeated loops. If you don’t like various chronology protection conjecture, fair enough, but building a time machine means something unless you’re inserting magic into it. Our universe appears to lack the necessary extreme curvature or negative mass to sustain CTC’s. In the absence of such natural curvature or means to create it artificially and stabilize it, the whole question is a non-starter.

ASalazarMX 8y ago

Don't be silly. If you dedicated your whole life to invent a time machine, succedded at your 80s, met your younger self and gave him your research so he can build it in his 30s, how is that self-consistent?

deleted 8y ago
[deleted]
antidesitter 8y ago
It’s not. You just described an inconsistent timeline, but this has nothing to do with whether time travel creates such inconsistent timelines. To be more explicit, the grandfather paradox is not an argument against time travel [1], precisely because the past cannot be changed (as you assumed in your example). [1] https://en.wikipedia.org/wiki/Grandfather_paradox#Philosophical_analysis https://en.wikipedia.org/wiki/Grandfather_paradox#Philosophi...
- ASalazarMX 8y ago
  Time travel _to_the_past_ has all the potential to create inconsistent timelines. You can deliberately cause severe inconsistencies because you have the crucial component: being in the past. The only way for the timeline to defend from such inconsistencies would be to proactively thwart all your attempts, but then, Time/Fate has become a conscious entity.
  - antidesitter 8y ago
    > You can deliberately cause severe inconsistencies... No, you can't. Whatever it turns out you do already happened. You're assuming that being in the past means you can somehow "change" it, which is incoherent nonsense. As David Lewis pointed out [1], the idea that self-consistency in the face of time travel requires some kind of "thwarting" of your actions by "the universe" is just a poor (and unnecessary) plot device employed by some science fiction authors. You should read up on Novikov self-consistency [2]. Read the Lewis paper too, it doesn't require a physics background and will illuminate the heart of the matter. See in particular the discussion on compossibility. [1] https://www.csus.edu/indiv/m/merlinos/paradoxes%20of%20time%20travel.pdf https://www.csus.edu/indiv/m/merlinos/paradoxes%20of%20time%... [2] https://en.wikipedia.org/wiki/Novikov_self-consistency_principle https://en.wikipedia.org/wiki/Novikov_self-consistency_princ...
    - ASalazarMX 8y ago
      Come on, just humor me. Following my first example, why couldn't I visit my younger self and give him my life research? What physical law could possibly prevent us from working together and building a second time machine in a few years, thus guaranteeing a paradox? I understand why I shouldn't be able to accidentally cause a paradox, but if I know how to cause one and have the means to do it, and the universe is mindless, what could stop me?
      - Pharmakon 8y ago
        Stephen Hawking posited that the first and only thing a time machine would do is detonate as a direct consequence of virtual particles reaching arbitrarily high energy density through repeated loops. If you don’t like various chronology protection conjecture, fair enough, but building a time machine means something unless you’re inserting magic into it. Our universe appears to lack the necessary extreme curvature or negative mass to sustain CTC’s. In the absence of such natural curvature or means to create it artificially and stabilize it, the whole question is a non-starter.