Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

A Solution to the Grandfather Paradox

  1. Mar 18, 2003 #1
    [SOLVED] A Solution to the Grandfather Paradox

    For those who don't have a clue of what the grandfather paradox is, here is the definition:

    Okay, so my solution:
    My solution is enitirely based on Richard Feymann's multiple history idea.

    So, you go to the past, kill your grandfather. This automatically creates a new history. (sort of like a branch, so to speak)You would still be in existence in the original history, but not in the new one. This causes me to ask, can you exist twice? If your grandfather's twin becomes your grandfather, would you still be the same person(genetically speaking, and not refering to personality traits, as that gets quite a bit complicated)?
  2. jcsd
  3. Mar 18, 2003 #2
    well, this isn't really your solution; I've seen this (or variations thereof) in other places. Basically, when you go back in time and kill your grandfather it causes a rift whereby in one universe your grandfather is dead and you don't exist and in another universe your grandfather is still alive and so you do still exist (and this would be the universe you'd be in when you travel "back to the future.")

    Well, if you subscribe to the multiple history idea (or the multiverse theory) than you exist a near-infinite amount of times.
  4. Mar 18, 2003 #3
    I agree that a multiple universe model avoids time-travel paradoxes. But 'time-travel' becomes impossible (apart from physical limitations) because it would be pretty hard to select the 'correct' past - a past that is exactly like the one you lived through except that you actually existed earlier than you are 'meant' to.

    Each time you disturb a subatomic particle, the universe 'branches' because of quantum indeterminacy. So it's not just about conscious decisions that create branching, and that the number of possible universes is utterly, mindbogglingly, astronomical.

    So if this is true, then there are an infinite number of universes where 'you' exist - where the different 'yous' have identical genes, but different life experiences. And there are a greater infinite number of universes where 'you' are almost genetically identical to your current self, save for a gene or two. Would that be the 'same' you?

    So when you go to the 'past', it can't be the 'same' past that you actually lived through (because 'you' weren't there with your teenage grandfather!). In that alternative universe, your grandfather's only grandchild may be genetically identical to you, but there is a good chance that it won't be - either because your grandfather may end up having children with someone other than your 'grandmother', or that a different sperm may fertilise the ovum etc. So if you killed your grandfather in that alternative universe, you won't instantly disintegrate because that 'grandfather' isn't your grandfather anyway.
  5. Mar 18, 2003 #4
    yeah, zimbo brings up a good point. So, when you go back in time, do you, in a sense, appear in a "future past" since the universe branches? I know this sounds very confusing, so if anyone can ask me where it's confusing, I'll fill in the spaces!
  6. Mar 18, 2003 #5
    Pretty much so. We would need to evolve a new range of verb tenses to deal with time-travel.:wink: Like when you want to talk about what you 'will' do once you get to the past, or what you 'did' before you 'left' the future, or what would 'had' already happened when you go back 100 years next week.
  7. Mar 18, 2003 #6
    Yeah, I agree with you. So do you understand what I said?
  8. Mar 18, 2003 #7
    If time travel is "going to happen" -- in the future -- then it already has ...

    Of course it would all depend on whether or not you could go backwards. If, the other hand you could only go forward, then you could pretty much create any scenerio you wanted (without worrying about the future), becasue it hasn't happened yet.
  9. Mar 19, 2003 #8
    Wouldn't your sudden appearance in the "past" or the parellel universe (if such QM universes really exist) violate the second law of thermodynamics? Or would you somehow suck up energy from the entire universe to be created?
  10. Mar 19, 2003 #9
    Although many physicists are loath to admit it, QM is a paradoxical magical theory of existence that permits anything and everything. The many worlds theory is just one of the more paradoxical magical interpretations.

    A somewhat less magical theory, if that isn't an oxymoron, that attempts to overthrow the grandfather paradox involves the concept that the past cannot be changed. Thus, instead of requiring the paradoxical infinite universes of the many worlds theory, it involves self-referential time loops.

    For example, because the past cannot be changed it is impossible to go back in time and kill your grandfather or yourself because, obviously, you are here and did not do so. However, it is possible in this theory to go back in time and save your grandfather's life because, obviously, you are here and did so. Similarly, you could not go further back in time than when your time machine was first turned on.

    Again, these are very much magical theories. There is one serious attempt in progress I am aware of to build the first time machine. It uses counterrotating concentric circular laser beams that have been first sent through a Bose-Einstein condensate. The condensate slows the light to a crawl and, in doing so, increases its inertial mass.

    The hope is that inside the rings of light a small area of spacetime can be distorted enough for light and particles to pass through. However, most physicists don't believe it is possible to get enough energy to do the trick or if spacetime is distorted enough to do this the result will be a feedback that will explode in the experimenters faces and destroy the machine.

  11. Mar 19, 2003 #10

    Yeah, but then the universes' history of that time period branches out. So, hypothetically speaking, (and assuming time travel to the past is possible), a history would branch out. You could change the past, and when you do so, you create a parallel universe, so to speak, or rather a new history. The original history would still exist.
  12. Mar 19, 2003 #11

    How would it violate entropy?
  13. Mar 19, 2003 #12
    No, it is not a branching out of history. It is a temporal loop, the temporal version of a wormhole shortcut through space. Einstein's spacetime implies the past, present, and future all coexist in a static state. That is why we can travel into the future and experience different rates of the passage of time.
  14. Mar 19, 2003 #13
    Mainly, this can be solved by saying that the peson going to the past is only an observer, and cannot change anything (don't ask me how )

    I don't see your point.
    If time travel will happen in far future, it is possible that the person that will go to past will reach our near future, therefore 'time travel' wouldn't have happened yet !
  15. Mar 19, 2003 #14
    I beg to differ that time travel is not possible but not time changing.let me see hear and you make your own judgement,but as you will see all you can say is you can prove it wrong and you can prove it right.if telepathy existed and it may,people have precognitive experiences all the time.when they do they believe it was a warning of a possible future,and they saw what would have been.so when the event happens they change what would have been to what was.in time travel paradoxs.this would be explain as the future was already suppose to be what happened and the alternate future never was going to happen.so what happens if you accidently kill someone by making a mistake.you couldnt live with there death on your hands and wanted to change it.see where this is going!you sent a telepathic message to your self in the past.in the past you recieve the signal.to you it was a precognitive experience,so when you came up to when you made the mistake that killed the person you wouldntr do it.thus the paradox is avoided and the universe does blow up.all the one could assume at the moment you changes time.time just keeps going.like i said cant say it can can't say in can't.so what happens next when you try to argue what happens in the time line if this happens and you come up to the first event before it looped around to change time.ahh haa.what do you know.you had a premonition and you kept yourself from killing someone.so how would you know if that was time changing or the real thing.like I said.try to bet that arguement!
    Last edited: Mar 19, 2003
  16. Mar 19, 2003 #15
    I have thought about that too. BUT it is entirely incorrect, at least not in modern theory. If you would be so kind as to look at the first post, you'll see that there are such things as "alternate universes" as you call them(and I like the name). When you started talking about sending telepathic messages to yourself in the past, I wondered if you were making this up as you went along, to fill in the spaces? Can you provide some data to substantiate your telepathy-related statements?
    First off, everything you said is hypothetical, and not an inference. You can't send a message to your past self, because that would require time travel, and not telepathy. Telepathy is still hypothetical(and it doesn't make too much sense to me). In alternate universe, two probable choices are made, separately (one in one universe, another in the second unvierse) and each have there own consequences. Also, what you said sounds a lot like De Ja vu. Your apperent explanation is that you talk to yourself from the past. This is simply not possible....i don't see how it is.
    Anyhow, the fact is that there are myriads of probabilities that don't neccasserily(sp?!) have to do with something that's "bad"(like murder, as you suggested). Whether or not I wear a pink or green t-shirt with blue or bedge pants is a probable situation. If I pair my pink t-shirt with bedge (ahh! fashion disaster!! ) or if I wear a green shirt instead with the bedge pants and so on. These are all different probabilities, and thus multiple histories.
  17. Mar 19, 2003 #16

    yeah, sorta like a branch? I feel like I'm taking this too literally. I mean, I imagine that there really are parallel/alternative universes. This further implies that Hawking's idea of multiple universes may be true.


    so how does that relate with the multiple history theory [?]
  18. Mar 19, 2003 #17
    No, that isn't quite it. This newer theory incorporates a kind of temporal uncertainty principle. The more you learn about your future, the less control or free will you appear to have over it. For example, if I found out from my future self that my father was gonna die in a car accident I could warn him but the universe would still arrange for him to die that way or my future self might have been lying to me or whatever.

    The same kind of thing works in reverse, future and past, have strange meanings when it comes to such discussions. Essentially if I went into the past to try and kill myself the universe would arrange things somehow so I would fail. In addition, when I use the word "universe" in this context it is just a convenience due to the difficulty of talking coherently about such things.

    Sorry if that isn't much clearer. Just remember this is a theory based on Relativistic ideas where the past, present, and future all coexist in some sense. In other words, even causality is a slave to fate.
  19. Mar 19, 2003 #18

    So is death involved with multiple histories? I mean, can the probabilities of preventable deaths differ from history to history? Are all deaths preventable?
  20. Mar 19, 2003 #19
    Again, what I am talking about are not multiple histories, but a single continuous history. It just has loops in it similar to the way wormholes might work. If you go into the past and meet yourself as a child, for example, this is not a new event from your point of view. You remember your future self meeting you as a child but now you are experiencing the event from the other side as the adult.

    Just as when we might travel through a wormhole from one part of the universe to another and the universe is still the same universe, theoretically it might be possible to travel back in time and still remain in the same universe. It just requires a rather healthy stretch of imagination and the laws of causality.
  21. Mar 19, 2003 #20
    Oh, I see now!I have a much better view of it...I think it just might even correspond with the multiple histories theory.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: A Solution to the Grandfather Paradox
  1. Is there a paradox? (Replies: 12)

  2. Andromeda Paradox (Replies: 7)

  3. A postal paradox (Replies: 18)

  4. Moravec's Paradox (Replies: 2)