Shows about time travel

Discussion in 'Physics & Math' started by arfa brane, Apr 9, 2018.

  1. Yazata Valued Senior Member

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    I don't think of it as a literal logical paradox. Perhaps it's more of a causal anomaly.

    Imagine that you go back in time and kill your grandfather before your father is conceived. You are arrested by the police in that day and tell them some cock-and-bull story about being a time traveler from the future, the dead man's grandson in fact. The court declares you insane and you are committed. Things continue to unfold in a natural manner from their point of view - although different from how you "remember", you are never born etc.

    So if you do change how the future unfolds, who are you and where did you come from?

    In one interpretation, you just popped out of nowhere with (false?) "memories" of a future that never happens. That's a causal anomaly, not a logical paradox. Something happening (you appearing) with no cause.

    On another interpetation, one that posits a much more abundant 'many-worlds' ontology, at each and every moment the future consists of many different possibilities, each one associated with a timeline, a possible world in which that possibility actually occurred. (And each of those timelines multiplying into its own tree, as new possibilities continually emerge at each moment.)

    So in this ontology, you would have originated in one possible timeline (timeline A, where your grandfather wasn't murdered). You went back down your branch to a point where his living of dying were real possibilities and caused yourself to move into the future along another timeline (timeline B, where he was murdered).

    Where did you come from? Not 'out of nowhere' as in the causal anomaly model, but from timeline A. Even though from the perspective of timeline B, timeline A never happens. Equally, from the perspective of timeline A, timeline B never happened (it's the "grandfather paradox" in fact).

    In other words, time travel might be a lot more complicated than travel backwards and forwards along a single one-dimensional past -> future line. There might be an effectively infinite number of possible futures (and presents, since all this was presumably happening in the past too), each real in its own way and where time travelers are capable of influencing which direction they take from the point of interference, provided that they are equipped with time-machines.

    It's an old science-fiction plot device.
     
    Last edited: Aug 2, 2018
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  3. Q-reeus Valued Senior Member

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    Someone just popping into existence might be 'explicable' as a temporal anomaly. Recall though there is supposed to be an actual cause - that same individual existing in the future, traveling back in time so as to prevent his/her's own existence in that future. Can't have both that causative future event occurring, and not occurring. The traveling somewhen else (not somewhere else ala MWI) paradox remains.
    Sure MWI notionally allows an in-principle time & space travel to some other forking time-line. However statistically unlikely it may be, what fundamentally then prevents a hopping back into an earlier, common to both branchings time-line, and there preventing one's own future existence in any of the two subsequent branchings? There must afaik be uniqueness to the branching events in MWI (although there are many flavors of MWI(s) - so I won't bet my life(s) on that). Sci-fi plot indeed.
     
    Last edited: Aug 3, 2018
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  5. Lizard Registered Member

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    The Grandfather Paradox: the moment you kill your Grandfather you would cease to exist. Your time-travelling becomes null and void. This is as I've explained: the moment you make that decision; when that WILL is enforced, is the moment your travelling days are over. ☺
     
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  7. Dinosaur Rational Skeptic Valued Senior Member

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    From Janus58 Post 70
    The above is based on the lack of a frame of reference for motion in space.

    The inability to define the spacial movement does not do away with the requirement for movement in space to accompany the typical SciFi version of time travel.


    The absence of a clear definition of Earth’s movement does not do away with the requirement for movement in space to accompany the typical SciFi version of time travel.

    The Earth is not stationary as was once believed. A stationary Earth would defy logic. It would require a weird view of the motion of other objects in the Solar system and support the notion of Earth as the center of the universe.

    Once the possibility of time travel is accepted paradoxes & anomalies arise, which strongly negates the possibility of the typical SciFi version of time travel.
     
  8. iceaura Valued Senior Member

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    There's an obvious way out of the Grandfather Paradox. Let's rename it the Grandmother Paradox, to narrow the field of explanations.

    You already didn't go back in time and kill your Grandmother, so it isn't going to have happened. The only question is what goes/went wrong if/when you try/ied. And here it gets wild - because any improbability, including quantum delocalization effects on a macroscopic scale, is infinitely more likely than an event of probability zero.
     
  9. Dinosaur Rational Skeptic Valued Senior Member

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    Assuming the possibility of Tine Travel allows for some very interesting SciFi. I enjoy SciFi stories. I just do not believe in the ones allowing for paradoxes

    Note that fi in the above stands for fiction.

    I believe that some version of Special or General Relativity allows for a type of Time Travel into the future. Actually I think it is a way to get there faster than your twin brother or other contemporaries.

    However, the past is done & over with. At least (as mentioned in various Threads) time travel to the past in the SciFi sense is not possible.
     
  10. DaveC426913 Valued Senior Member

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    Unfortunately, this explanation is merely a rewording of the "Cosmic Censorship" solution to time travel paradoxen. "You can try, but something will prevent you."

    It is, essentially, no more than an appeal to a supernatural force.

    Were I to build a time machine and travel backwards to meet my grandmother, the natural physics of the universe is incapable of taking action to stop it. There is nothing that could be certain to stop me.

    If one believes in supernatural entities, then all is well, but I don't expect to see a scientific paper about time travel - that appeals to a supernatural cosmic Fixer - being published anytime soon.
     
  11. Neddy Bate Valued Senior Member

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    That is a good point, because the "supernatural cosmic Fixer" would also have to ensure that every other 'cause+effect' remains intact. The time traveler would not even be free to spill a glass of milk, no matter how hard they tried.
     
  12. iceaura Valued Senior Member

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    My wording would be "You can try, but something has prevented you". I believe the difference is significant, no?
    The laws of probability are not supernatural, surely? You are attempting an event of probability zero.
    The certainty is that something did stop you.
     
  13. Yazata Valued Senior Member

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    From the time traveler's own point of view. But as anyone living subsequently in the world in which the grandfather was killed can testify, that future never happens. The time traveler is never born and he never leaves on his time journey.

    So... either the time traveler comes from a different alternative version of the future in which the grandfather wasn't killed and guides the world in which he kills that gentleman onto a different course, or else he just pops out of nowhere with what prove to be false memories of a future that is proven by unfolding events to never happen.

    The difference between the two interpretations seems to revolve around whether we imagine time as a single one-dimensional line, or something more like a branching tree. (And that in turn might reflect where we stand on physical determinism and the ontological status of possible worlds.)

    My argument is that in neither interpretation must there be a actual logical paradox. Any implicit paradox seems to me to arise from how the scenario is imagined and interpreted.
     
  14. Q-reeus Valued Senior Member

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    Let's remove the homicidal aspect. Our into the past time traveler carries back an extensive records of world events - newspaper clippings, USB memory stick recordings of TV newscasts, lottery winning numbers etc. - things that his/her traveling back would have no effect on. Let's say the travel is 10 years into the past. The sealed package of records of yet-future events are time-stamped and deposited in a Swiss vault, duly witnessed by respected authorities. With instructions to have the package opened 10 years hence, and contents publicly revealed. This scenario would entirely circumvent the 'false memories' case. Branching time-lines 'out' will not remove the above 'sensational proof of time travel that rocks the world(s)'.
    Moral - enjoy the time-travel SciFi movies but realize one is definitely suspending disbelief.
     
  15. Throbber Registered Senior Member

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    What if one reversed time and killed, "Adam?"
     
  16. Throbber Registered Senior Member

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    Should I believe a traveller and invite someone to join me in my travels, if they do something to negate my travels, would this negate their invitation? I beleive it's called biting the hand that feeds you.
     
  17. Michael 345 Looking for Bali in Nov Valued Senior Member

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    Been reading this thread but no one seems to have mentioned the massive problem about time travel

    Forget about paradox and time lines and the movement of the proposed destination or even how to build power and calibrated such a machine which would travel in time

    TIME does not exist

    Past did exist but no longer- future will exist but not yet

    As mentioned in other threads a period between one arbitrary NOW and another arbitrary NOW is AGE not a measurement of TIME

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  18. Throbber Registered Senior Member

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    Very interesting argument R2D345. ☺ Liking it. ☺
     
  19. Michael 345 Looking for Bali in Nov Valued Senior Member

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    To follow up try to obtain a book called The Invention of Time and Space by Patrice F. Dassonville which goes in great detail of why time does not exist

    Comprehensive and well worth a read

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  20. Throbber Registered Senior Member

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    Thank you Michael! I will try to give it a read. ☺
     
  21. Q-reeus Valued Senior Member

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    3,052
    By the same author of The Invention of Time and Space: https://www.ecronicon.com/ecpp/pdf/ECPP-07-00262.pdf
    A few choice snippets:

    "The revolution of the Earth around the Sun is an observable phenomenon, but the “year”, which has no physical existence, is not. We
    found this interesting verse of the Greek poet Homer in Song II of “ODYSSEY” (8th BC): “when the forth year came and when seasons start
    again” [2]. The return of seasons every four years indicates that during the 8th century BC in Greece, the year had three months.
    Erratic values of the year prove that the year is a concept."

    Nonsense. So there may have been one or two odd usages in antiquity. Nowadays it has a quite precise and universal definition and meaning - as a unit of time(span).

    "The length is a concept. Therefore, it is not measurable. One does not measure the length of an object ; instead, one measures what separates the two ends of the object: the result is called “length of the object”."

    Silly definitions made to appear profound and meaningful. Takeaway message - rulers don't measure length. Clocks don't measure time. Really? Try converting a carpenter, or engineer, or fighter pilot. Or just about anyone, except a relative handful of 'deep thinkers' who add nothing of practical value to the world. Except for the 'practical monetary benefit' of book sales for niche authors of such maybe.
     
  22. DaveC426913 Valued Senior Member

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    The same effect as if one killed Superman or Sherlock Holmes.
     
  23. Throbber Registered Senior Member

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    Are you denying the existence of other humans before you and the reality of there being a first, "man?"
     

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