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  1. #1
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    Time travel can't change history

    Physicists prove time travel is theoretically ‘possible,’ but changing the past is not
    https://themindunleashed.com/2020/09...st-is-not.html
    based on https://iopscience.iop.org/article/1...61-6382/aba4bc

    Is the report an accurate summary of the article?
    SHARKS (crossed out) MONGEESE (sic) WITH FRICKIN' LASER BEAMS ATTACHED TO THEIR HEADS

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    Don't Hold Your Breath

    Quote Originally Posted by Tom Mazanec View Post
    Physicists prove time travel is theoretically ‘possible,’ but changing the past is not
    https://themindunleashed.com/2020/09...st-is-not.html
    based on https://iopscience.iop.org/article/1...61-6382/aba4bc

    Is the report an accurate summary of the article?
    I don't think this is going to appear in Nature.
    I'm not a hardnosed mainstreamer; I just like the observations, theories, predictions, and results to match.

    "Mainstream isn’t a faith system. It is a verified body of work that must be taken into account if you wish to add to that body of work, or if you want to change the conclusions of that body of work." - korjik

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    I think this is more complex than just a statement that " time travel is theoretically ‘possible,’ but changing the past is not."
    First of all, they haven't proved that time travel to the past is possible, which would require something more substantial than a theoretical paper.
    Secondly, they seem to be saying that you actually 'can' change the past in certain inconsequential ways, if you can travel or signal back into the past; but the consequences of such a change would be so insignificant that they would have no discernable effect on the future. This has a certain significance for the concept of free will, since you do have a certain amount of freedom of action in this model; but the end result is just the same. If History shows that X events have happened, then that will also be true after the formation of a closed timelike curve, even if the exact sequence and nature of those events may have changed slightly. So long as the same X events have occurred (as seen from and recorded by the future), then history is consistent. So if you do somehow kill your grandfather before your father is conceived, you would need to be descended from his identical twin, or from previously frozen sperm.

    Thirdly this paper assumes that alternate timelines are not significant. If causality can be significantly reversed and the past can be changed, then this seems to imply the creation of new timelines. Instead of constraining the changes that can be made to insignificant tweaks, the creation of alternate timelines allows the past to be changed significantly. This paper does not consider that possibility at all, as far as I can see.

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    Agreed.

    Quote Originally Posted by eburacum45 View Post
    I think this is more complex than just a statement that " time travel is theoretically ‘possible,’ but changing the past is not."
    First of all, they haven't proved that time travel to the past is possible, which would require something more substantial than a theoretical paper.
    Secondly, they seem to be saying that you actually 'can' change the past in certain inconsequential ways, if you can travel or signal back into the past; but the consequences of such a change would be so insignificant that they would have no discernable effect on the future. This has a certain significance for the concept of free will, since you do have a certain amount of freedom of action in this model; but the end result is just the same. If History shows that X events have happened, then that will also be true after the formation of a closed timelike curve, even if the exact sequence and nature of those events may have changed slightly. So long as the same X events have occurred (as seen from and recorded by the future), then history is consistent. So if you do somehow kill your grandfather before your father is conceived, you would need to be descended from his identical twin, or from previously frozen sperm.

    Thirdly this paper assumes that alternate timelines are not significant. If causality can be significantly reversed and the past can be changed, then this seems to imply the creation of new timelines. Instead of constraining the changes that can be made to insignificant tweaks, the creation of alternate timelines allows the past to be changed significantly. This paper does not consider that possibility at all, as far as I can see.
    And fourth, not testable.
    Last edited by John Mendenhall; 2020-Oct-10 at 01:04 AM. Reason: typo

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    Until and unless physical time travel actually happens, this is an inapplicable abstraction at best.

    Maybe there's a related effect or spin-off of this hypothesis that has some useful application in models predicting the future, but as far as the past goes? It's already over. Let it go.
    "I'm planning to live forever. So far, that's working perfectly." Steven Wright

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    Quote Originally Posted by Noclevername View Post
    Until and unless physical time travel actually happens, this is an inapplicable abstraction at best.
    Maybe it happens all the time and we are living in the changed history, but because we are part of the changed universe, we don't know it has changed (we changed with it). To paraphrase THHGTTG "There is a theory which states that if anyone ever goes back in time and changes history, the universe will instantly disappear and be replaced by something even more bizarre and inexplicable.

    There is another theory which states that this has already happened.”
    At night the stars put on a show for free (Carole King)

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    I can only see 3 logical scenarios that make any sense to me, and 1 that maybe possible but totally inconceivable.

    1. Your example, where history is being changed constantly and we would/could never be aware
    2. There are infinitely many universes and each event splices into another version, so that all that can happen does happen
    3. Time travel to the past is impossible, therefore historical events cannot be changed
    4. Time is not directional, in that it doesn't really "flow" into the future as we perceive. Therefore changing events: past, present and future would be feasibly possible.

    My current opinion is that number 3 is the most likely scenario. I have yet to see anything to convince me otherwise, though new discoveries in the quantum world may change my view.

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    I don't get what the paper says about changes being inconsequential. If you kill Hitler, someone's going to notice.
    "I'm planning to live forever. So far, that's working perfectly." Steven Wright

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    Whoops, I've Gotten In The Wrong Timeline Again

    Quote Originally Posted by Noclevername View Post
    I don't get what the paper says about changes being inconsequential. If you kill Hitler, someone's going to notice.
    Kill who Who's that?

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    The only way you could kill Hitler in this model is if he is immediately replaced by a look-alike who behaves exactly the same.

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    Quote Originally Posted by eburacum45 View Post
    The only way you could kill Hitler in this model is if he is immediately replaced by a look-alike who behaves exactly the same.
    Makes me wonder how many backup clones the world had in stock in case someone actually did come along and kill Hitler.
    Was the original Hitler named Untermeer?
    This seems like a good way to run into counting problems. If somebody has to stand in for somebody else, who is going to stand in for them?
    Won't we always run one person short?

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    I don't think the the article is that great of a summary of the paper itself. Looking at just the paper, it appears largely concerned with showing that it's possible to have closed timelike curves and still have consistent, stable spacetime. But one of the authors is quoted as suggesting that this means that "Try as you might to create a paradox, the events will always adjust themselves to avoid any inconsistency". I'm not sure the mathematics of the paper particularly support that, though. I'd think a stable CTC in general relativity would be more like the sort of time travel where you go back in time to try to change something, but your actions end up making everything turn out the way you experienced it in the first place (though perhaps you end up with a more complete understanding than you did before). Something like Heinlein's "By His Bootstraps".
    Conserve energy. Commute with the Hamiltonian.

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    Quote Originally Posted by Grey View Post
    I don't think the the article is that great of a summary of the paper itself. Looking at just the paper, it appears largely concerned with showing that it's possible to have closed timelike curves and still have consistent, stable spacetime. But one of the authors is quoted as suggesting that this means that "Try as you might to create a paradox, the events will always adjust themselves to avoid any inconsistency". I'm not sure the mathematics of the paper particularly support that, though. I'd think a stable CTC in general relativity would be more like the sort of time travel where you go back in time to try to change something, but your actions end up making everything turn out the way you experienced it in the first place (though perhaps you end up with a more complete understanding than you did before). Something like Heinlein's "By His Bootstraps".
    The interesting thing to me is that this would seem to suggest FTL travel or communication could be made to be compatible with relativity theory, but wouldn’t completely destroy causality. It would just make it more complex. Of course, a method would be needed.

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    Well that would fit. Occasionally some people on the internet claim things about the world that are so radically different from the world I see that I suspect they actually live on something like the Star Trek mirror world, but have been connected to my Earth’s internet through some oddity of physics. For instance, some will make statements about certain public figures that indicate they are radically different from my world’s. It’s like Kirk vs mirror Kirk.

    "The problem with quotes on the Internet is that it is hard to verify their authenticity." — Abraham Lincoln

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  15. #15
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    two things

    one the current location of the time traveler is not the past location one wishes to be at
    the earth moves as does the sun and the galaxy a LOT over long periods so one would need a space ship with a time machine

    IF THERE EVER WAS TO BE TIME TRAVEL IN THE FUTURE
    THE WOULD BE TIME TRAVELERS NOW
    unless the UFO's are them and not from somewhere else ?

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    Quote Originally Posted by nota View Post
    two things

    one the current location of the time traveler is not the past location one wishes to be at
    the earth moves as does the sun and the galaxy a LOT over long periods so one would need a space ship with a time machine
    If you can choose you point in time you can choose your point in space. "Time travel" is also effectively teleportation. Location, location, location!
    "I'm planning to live forever. So far, that's working perfectly." Steven Wright

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    Quote Originally Posted by nota View Post
    the current location of the time traveler is not the past location one wishes to be at
    the earth moves as does the sun and the galaxy a LOT over long periods so one would need a space ship with a time machine
    This is something I've never considered before, and makes sense. Adams's Heart of Gold seems so much more effective now than Wells's time machine.

    But I'm curious, what about momentum? Is there some chance the machine could be "carried along" on it's trajectory, and land, in time, about where it should?

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    Quote Originally Posted by nota View Post
    two things

    one the current location of the time traveler is not the past location one wishes to be at
    the earth moves as does the sun and the galaxy a LOT over long periods so one would need a space ship with a time machine
    There's no absolute reference frame in space. All motion is relative to something else.

    So you can use the Earth as a reference point for your "time travel", and thus avoid popping out in empty space. You'd just have to make sure your wormhole doesn't open inside the planet.
    "I'm planning to live forever. So far, that's working perfectly." Steven Wright

  19. #19
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    Many concepts of time travel involve travelling through a wormhole to a different location in time and space. The exit point of the wormhole dictates exactly where, and when, you will emerge. If this is not the right location in space you would need to travel from there to your chosen location.

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    Quote Originally Posted by eburacum45 View Post
    Many concepts of time travel involve travelling through a wormhole to a different location in time and space. The exit point of the wormhole dictates exactly where, and when, you will emerge. If this is not the right location in space you would need to travel from there to your chosen location.
    magic thinking
    if your science project need an unproven trick thing [worm holes]
    to make another thing ''work'' [time travel]
    you are not doing science

    just casting magic spells

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    Quote Originally Posted by nota View Post
    magic thinking
    if your science project need an unproven trick thing [worm holes]
    to make another thing ''work'' [time travel]
    you are not doing science
    just casting magic spells
    All methods of travelling into the past require some exotic 'trick' to work. The mathematics and physics of wormholes have been extensively worked out by various theoreticians, including the Nobel laureate Kip Thorne.

    But there are other methods to do the same 'trick'; simply travelling faster than light in certain directions could take you into the absolute past.

    Both of these methods of time travel (wormholes and FLT) have well-defined characteristics, so should allow a careful chrononaut the opportunity to arrive at the correct coordinates in time and space, assuming they work at all. It's not a question of just pressing a button and hoping for the best.

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    btw I would love to see a worm hole or a white hole
    wouldn't a white hole be very very hard to miss ?

    but anything that needs a black hole as one end point
    is not likely user friendly

  23. #23
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    Maybe it's that when you make changes, someone always reverts your edits: Wikihistory.
    Conserve energy. Commute with the Hamiltonian.

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    Just before a time traveler pulls the trigger on Hitler, another time traveler knocks the gun aside because the original timeline is better in the long run.

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    If the past can't be changed, could anything I do while visiting the past be considered to be unethical? If someone was shot dead at noon yesterday and I decide to time travel back and become the shooter, I didn't cut his life short. I'm just discovering that I pulled the trigger. If I decide not to time travel at all it just means that someone else did it. If the past can't be changed, no decision that I make now can have any effect on it and would be irrelevant, not unethical.

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    Quote Originally Posted by Chuck View Post
    If the past can't be changed, could anything I do while visiting the past be considered to be unethical? If someone was shot dead at noon yesterday and I decide to time travel back and become the shooter, I didn't cut his life short. I'm just discovering that I pulled the trigger. If I decide not to time travel at all it just means that someone else did it. If the past can't be changed, no decision that I make now can have any effect on it and would be irrelevant, not unethical.
    Be a heck of a way to find out the theory was wrong! You'd be stuck in the Biff Tannen timeline with a murder rap dogging you.
    "I'm planning to live forever. So far, that's working perfectly." Steven Wright

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    There were possibly 42 assassination plots against Hitler. Maybe the time travelers finally got him in the bunker.

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    Let's say I want to sell some vintage comic books for a fortune but don't happen to have any. I can just time travel back a few decades, sell a calculator or something, and use the money to buy a bunch. I can't just bring them back with me because they won't be properly aged, so I stash them somewhere and go get them when I return to the present. Did I change the past? If not, why don't I just go get the stash without bothering to time travel? But it won't be there if I don't go back in time, so something is different, right?

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    Quote Originally Posted by Chuck View Post
    Let's say I want to sell some vintage comic books for a fortune but don't happen to have any. I can just time travel back a few decades, sell a calculator or something, and use the money to buy a bunch. I can't just bring them back with me because they won't be properly aged, ...
    I'm pretty sure that collectors don't pay extra for aging: they want the condition to be perfect, or as close to it as possible.

    I guess that perfect books that are 60-odd years old would be suspected as counterfeit, but yours would pass every test for authenticity, of course. Well, they might register too young upon carbon-dating. I don't know if such is economically feasible for comic book people.

    I do know that books from the 1940s in excellent condition have been recovered. These were effectively sealed nicely in a dry climate under exceptional conditions.

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    The calculator you took back in time would need batteries. So take a few with you.

    But it it was too advanced, the boffins at DARPA (or its predecessors) would love to get hold of it (and reverse engineer it), and you'd potentially change history anyway. Even the batteries might change the history of technology significantly, and we'd all be driving electric cars by now.

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