5 ms·

> But if traveling back to the past, and changing something Therein lies the problem. What makes you think the past can be changed? If the past can’t be change

> But if traveling back to the past, and changing something Therein lies the problem. What makes you think the past can be changed? If the past can’t be changed, there’s no problem with time travel (at least on these grounds). Moreover, if you adopt the block universe view, as general relativity seems to demand, there’s no such thing as change at all. There’s no becoming, only being. This was the conclusion reached by Parmenides in ancient times. Julian Barbour wrote a book on the subject called The End of Time, for those who are interested.

ASalazarMX 8y ago
Are you implying traveling to the past doesn't change anything? You could spread antibiotic resistant bacteria, you could walk over a bee that was going to pollinate a flower with a significant mutation, you could breathe their seasonal flu and carry it over to your time before it manifests. The only way time travel consistent with known physics is if past past travel is impossible. You can visit the future, but if you do, you can't travel back. Besides paradoxes, time travel to the past would make perpetual motion machines possible.
- antidesitter 8y ago
  > 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
      [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...
        8y ago

ASalazarMX 8y ago

Are you implying traveling to the past doesn't change anything? You could spread antibiotic resistant bacteria, you could walk over a bee that was going to pollinate a flower with a significant mutation, you could breathe their seasonal flu and carry it over to your time before it manifests. The only way time travel consistent with known physics is if past past travel is impossible. You can visit the future, but if you do, you can't travel back. Besides paradoxes, time travel to the past would make perpetual motion machines possible.

antidesitter 8y ago
> 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
    [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...
        8y ago