The Grandfather Paradox

Discussion in 'Physics & Math' started by Nogard, Oct 24, 2009.

  1. RJBeery Natural Philosopher Valued Senior Member

    BobG: QM vs SR is appropriate because the apparent contradiction that I discuss in the thread is introduced with the concept of simultaneity being relative, which is a consequence of SR. GR contains the same concept, of course, and I mentioned that only to segue from Professor Cramer's quote about providing "a path to finally merge GRT (General relativity) with QM (Quantum Mechanics)".
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  3. ScaryMonster I’m the whispered word. Valued Senior Member

    The implications of this experiment seem to be to be profound, although Block Time is kind of frightening Dr. Cramer's retrocausal experiment if proved doesn’t automatically prove Block Time to be true but it does imply that fate exsists, one interesting test would be on receiving a signal from the future to consciously not send the signal that caused the retrocausal reaction. Then what would that mean? That the signal from the future came from another dimension?
    But as for it validating the Woodward effect, that would make interstellar flight possible and woodward also thinks that it would make possible a device that is robust enough to handle sufficient amounts of power to be able to generate wormholes with flat enough throats to resemble stargates seen in science fiction, enabling instant transit between points in space-time.

    It sounds like science fiction but if it works it will be coolness personified.
    Last edited: Nov 22, 2009
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  5. Enmos Valued Senior Member

    I don't see what the mechanism would be to prevent a man that traveled to past from killing his younger self.
    I do know of one mechanism that would prevent anyone from killing his younger self though.. time travel isn't possible.
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  7. RJBeery Natural Philosopher Valued Senior Member

    We agree that Dr. Cramer's experiment would have profound implications if successful but you don't need retrocausality to accept block time - SR is enough. Please look for my new thread because I feel we are digressing from the OP...
  8. rakovsky Registered Member

    Let me give a way of thinking about the grandfather paradox that might help to at least address it, by laying out the events in terms of a grandfather and the Lord, an all powerful, all merciful being outside of time constraints.
    Let's say that a person hears that a relative, a grandfather, had a car accident but there is no news yet on whether the relative died in the crash. The person, on hearing the news, then prays to the Lord "Lord, don't let him have died in this car crash, he is a God-loving man, let him live for a long time." The Lord has mercy and decides to make it so that the person didn't die - even though the possible death or survival was a past event before the prayer.

    Now, consider the fact that a person is not God and that his knowledge is limited. The person hears that the relative died and believes it, so they pray to the Lord to take away this trauma and make the person to have survived. The Lord hears the prayer, has mercy, changes the past, and it turns out that the information that the relative was given was false and their belief in the death was mistaken. The grandfather paradox has been solved- the causal link between hearing the news, making the prayer, the Lord's intervention, and the relative's survival despite the information that he had died that caused the prayer.

    Now a Skeptic can go back and ask: What caused the mistaken information? Was it a real crash or a news reporter's confusion? Being limited in time and knowledge, the prayerful person does not know the source of the mistake, he only got the information that he trusted and based his prayer on. The answer to the causal question "What caused the mistaken information?" is not a necessary step in the chain of:
    Information about the grandfather's death - the faithful man's prayer - The Lord's intervention - the survival of the grandfather

    Now let's say that the faithful person directly witnessed the grandfather's death. Does that violate the principle of a causal chain? No, because you must remember that a person is limited in information, and that limit includes the information perceived through the 5 senses. So a person can see his grandfather's death, believe that the grandfather died and pray, then the Lord can intervene, and it turns out that the grandfather survived. "But.... but... I saw him die", the person might think. The answer to the objection is that the person's senses, including the visual image, is itself limited and not absolutely correct.

    So what caused the image to appear in the brain- maybe the grandfather did appear to die but very unusually survived, or maybe it was a hallucination or dream, or maybe it was just a stunt dummy or hologram, or maybe there was some other reason for the mental vision. We don't know the answer because we aren't omniscient.


    To answer the question of the Grandfather paradox, what this suggests to me is that while a person could find a way to destroy the sperm that made him/her before his/her conception, the basic thesis of the Grandrather paradox I think is correct - a person or event cannot remove their own directly linked cause for existence. I do think that a person or event could alter modify their own direct causal links though. The sperm could be destroyed, but it would have to be reassembled through Advanced Technology and then made to function again, since the direct sperm-conception-human causal link would have to remain.
    Last edited: Apr 20, 2017
  9. TheFrogger Banned Valued Senior Member

    It's already happened.
  10. Dinosaur Rational Skeptic Valued Senior Member

    The typical SciFi time machine plot ignores any consideration of where you would be after traveling in time, with most assuming that you would still be in your current location on Earth .

    Our galaxy rotates & moves relative to other galaxies. The Earth is in orbit around the sun, traveling circa 67,000 mile per hour (if my not infallible memory is correct).

    A 5-10 year time travel seems to require moving faster than light in space if it occurs instantaneously as is the case in the Scifi stories I have read.

    Perhaps if time travel is possible, paradoxes are prevented due to your not being still on Earth at your location prior to traveling in time.

    To me the typical time travel stories are fantasy rather than SciFi.

    BTW: To me the Star Gate is SciFi, while the Star Trek transporter is fantasy due to there being no equipment at the destination.
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  11. RJBeery Natural Philosopher Valued Senior Member

    This is so insightful. I realized long ago that any spatial transporter is also a time machine because, although there's a presumption that items are being transported "now", simultaneity has no absolute meaning. I would say that if time travel were possibly in any way a minimum requirement would be to have equipment at both ends of the journey.
  12. Yazata Valued Senior Member

    When contemplating this "paradox", it's probably helpful to have read lots of science fiction. Almost every take on time-travel has appeared in some science-fiction story.

    I'm not convinced that there's any logical reason why it's impossible. I believe that people like Hawking have tried to make that kind of logical argument, that reality doesn't admit contradictions, and the time-travel paradoxes create contradictions. But I don't think that the time-travel paradoxes have to be interpreted as logical contradictions.

    If you travel into the past and kill your father before you were born, then it seems to me that what you get isn't a logical paradox so much as a causal anomaly. What purports to be a time-machine pops out of nowhere containing somebody that purports to be you. He commits a murder, killing a man who he claims is his father. (DNA tests might confirm it.) Then a history unfolds in which he is never born, the time-machine is never sent and his appearance in the past has to be considered an anomalous uncaused event.

    There doesn't seem to me to be anything illogical about that. (Except contradicting our a-priori metaphysical principle that every event has to have a cause. But how sound is that principle?)

    That seems to depend on some idea of eternal recurrence, which I find inherently unbelievable. (Sorry Nietzsche.) But sure, it would seem to evade the logical contradiction problem. The time-traveler may have skipped his own proximate past and landed in a more distant past in which the murder did take place. (Maybe the murder and no-murder pasts alternate on the time-line or something.)

    That's what one would arguably expect in a many-worlds multiverse where anything that can happen does happen.

    So your traveling into the past and doing something that never occurred in your own past would just shift the evolution of events onto another timeline leading to another possible future. It would be like traveling from a leaf on a tree back down its branch to a branching point, then up a different branch. But if reality consists of the entire time-tree, with an effectively infinite number of possibilities branching off at each instant, everything might arguably remain consistent.

    A science fiction novel that explores this is James Hogan's The Proteus Operation. The protagonists live in a world in which Hitler was never stopped, and where the Germans control the entire world except North America. The war has reached a temporary stalemate since both the US and the Canadians (the British monarchy in exile since the fall of Britain) along with the Germans now have nuclear weapons, ICBMs and mutually assured destruction. But the Americans realize that they are doomed long-term, since the Germans control the lion's share of the world's industry and resources. So the Americans invent an untried experimental time-machine. They travel back in time with the plans for an atomic bomb and go to talk to Einstein with hopes of nuking the Germans before they get out of hand. Einstein is amazed at the time machine of course. He quickly recruits some of the other iconic physicists and they contact Roosevelt.

    But problems develop since the time-machine seems incapable of reestablishing contact with the future that sent it. Then it becomes apparent that other time travelers are also active, from a variety of possible futures. Eventually (with the help of Einstein) they figure out that the time-machine can only travel into the past, since there is one unique path from their origin-point down the branches of the time-tree. But they can't use it to travel into the future because there are an infinite number of futures branching off at each instant. There are lots of confrontations with the early 1940's Nazis (who have also received time travelers from yet another future) and between the time travelers themselves. They learn that you can't really change the future, since all possible futures are equally real. But our protagonists' activities do shift their own time-trajectories and they live out the rest of their lives in a universe that is our own, or close enough.

    It seems to me that your line and ray solutions are actually very similar and somewhat equivalent. Both imagine the possibility of different possible futures, one in a tree-like multiverse, the other strung successively in a line.
    Last edited: Apr 22, 2017
  13. C C Consular Corps - "the backbone of diplomacy" Valued Senior Member

    Isaac Asimov's novel "The End of Eternity" was one of the earliest time-travel tales to actually take that into account, wherein its society of temporal engineers (the Eternals) makes the necessary adjustments.

    But as you perhaps suggest, that negligent physical fallacy arose in stories that employed the sci-fi gimmick of an "instant transition" or leap across the slices of time rather than accelerating through all those changing states of the Earth, as in Well's original Time Machine scenario.[1] Not that the latter is credible itself, but at least the machine is thereby hugging the changes in spatial location as much as the changes in temporal location.

    A potential, internal inconsistency in Well's story is where the Time Traveler initially speaks of everyday time-travel as the flux of consciousness moving from one series of brain-states to the next along the 4th-dimension. (Thereby pre-dating Hermann Weyl's similar statement decades later in the context of spacetime and the worldline of a human body embedded in such.[2]). But afterwards the Time Traveler then introduces that small, model time machine which materially speeds through a past or future direction. Rather than such a journey being limited merely to the claim of cognition / subjective experience doing its asymmetric and vastly slower version of that, as aforementioned by him.

    • H.G. WELLS: "Really this is what is meant by the Fourth Dimension, though some people who talk about the Fourth Dimension do not know they mean it. It is only another way of looking at Time. There is no difference between time and any of the three dimensions of space except that our consciousness moves along it. [...] We are always getting away from the present movement. Our mental existences, which are immaterial and have no dimensions, are passing along the Time-Dimension with a uniform velocity from the cradle to the grave. Just as we should travel down if we began our existence fifty miles above the earth's surface.” --The Time Machine

    - - - - - - - -

    [1] H.G. WELLS: "You mean to say that that machine has travelled into the future?” said Filby.

    “Into the future or the past—I don't, for certain, know which.”

    After an interval the Psychologist had an inspiration. “It must have gone into the past if it has gone anywhere,” he said.

    “Why?” said the Time Traveller.

    “Because I presume that it has not moved in space, and if it travelled into the future it would still be here all this time, since it must have travelled through this time.”

    “But,” I said, “If it travelled into the past it would have been visible when we came first into this room; and last Thursday when we were here; and the Thursday before that; and so forth!”

    “Serious objections,” remarked the Provincial Mayor, with an air of impartiality, turning towards the Time Traveller.

    “Not a bit,” said the Time Traveller, and, to the Psychologist: “You think. You can explain that. It's presentation below the threshold, you know, diluted presentation.”

    “Of course,” said the Psychologist, and reassured us. “That's a simple point of psychology. I should have thought of it. It's plain enough, and helps the paradox delightfully. We cannot see it, nor can we appreciate this machine, any more than we can the spoke of a wheel spinning, or a bullet flying through the air. If it is travelling through time fifty times or a hundred times faster than we are, if it gets through a minute while we get through a second, the impression it creates will of course be only one-fiftieth or one-hundredth of what it would make if it were not travelling in time. That's plain enough.” He passed his hand through the space in which the machine had been. “You see?” he said, laughing.
    --The Time Machine

    [2] HERMANN WEYL: "The objective world simply is, it does not happen. Only to the gaze of my consciousness, crawling upward along the life line [worldline] of my body, does a section of this world come to life as a fleeting image in space which continuously changes in time." --Philosophy of Mathematics and Natural Science.
  14. Michael 345 New year. PRESENT is 72 years oldl Valued Senior Member


    You ride the equipment there and back

    But it will never happen

    You can't go back in time because the past


    You can't go forward in time because the future


    That makes sense because otherwise existence would be a whole big block of


    No need for any sort of machine

    You would be able to see yourself in massive conga line going back and forward

    All you need to do is to step out of the line and depending on your fancy walk back to when you were born or forward to when you die


    So if you are going to travel in TIME another prerequisite would be to have a destination

    No problem having a good imagination though

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  15. RJBeery Natural Philosopher Valued Senior Member

    Does "now" exist? What about "now" in another location, like a distant galaxy?
  16. Michael 345 New year. PRESENT is 72 years oldl Valued Senior Member


    Not only that you can make a argument

    (although I prefer discussion)

    that the SAME NOW EXIST on Earth at the exact SAME NOW TIME is happening at the furthest reaches of the Universe

    Of course viewing this all encompassing NOW only applies to the Super Duper entertie who is far enough away from our Universe they are able to observe our Universe from edge to edge

    This extremely distant observer can see the light arriving from all points in our Universe into its eye at the same time which equates to it observing the ONE NOWness of our Universe

    However we are restricted to being on Earth

    While we EXPERIENCE our NOW while the same NOW is happening at the edge of the Universe

    we do not get to EXPERIENCE the edge of the Universe NOW at the same time as our NOW

    we only get to OBSERVE the edge of the Universe's NOW when their light arrives after a 13.3 billion light-year journey

    Certainly looking at the edge of the Universe we are looking into their past (as they would be looking into ours)

    Unfortunately again while we can view the edge of the Universe past we can't visit their PAST

    for exactly the same reason we can't visit our matching PAST

    Neither PAST EXIST in the current NOW

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    Last edited: Apr 23, 2017
  17. Beus Registered Member

    I want to go back in time to 1994 June 12th. That will be the new now. There is no past so there is no time traveling. I just want the whole world revert back to the way it was on that day. Everyone's memory gets deleted except for mine and my Mom's. The 90's were much better, so let us go back.
  18. Dinosaur Rational Skeptic Valued Senior Member

    The typical SciFi time travel stories strongly imply paradoxes, which (to me) indicate that no such technology is possible.

    The Posters who remarked that the Past no longer exists & the future does no yet exist had some cogent thoughts. How do you travel to non-existent (x, y, z, t) locations?
  19. Michael 345 New year. PRESENT is 72 years oldl Valued Senior Member

    One of those would be me

    Check another thread about time for more details

    But is you require my thoughts about time like a movie I can dig it up and post here

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  20. TheFrogger Banned Valued Senior Member

    Why travel to the past to kill your Father? Why not do it now? or in the future?! Even if you killed your Father, you have already been born! What now...?

    You cannot change the past, however you can alter the future, which will eventually BECOME the past.
  21. DaveC426913 Valued Senior Member

    Aaaaaand Counter completely misses the entire point of the Grandfather paradox....
  22. TheFrogger Banned Valued Senior Member

    Actually not. If the PURPOSE of the Grandfather paradox, it's USE is to promote a questioning of time-travel, then the opposite of this is...?
  23. DaveC426913 Valued Senior Member

    ...the topic of a new thread?

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