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Fascinating Time-Travel Paradox

  1. Dec 16, 2004 #1
    I love this time-travel paradox

    Suppose you get visited by a man claiming to be the future you. he tells you the plans to build a time machine, and tells you that you must build it, and use it to tell your younger self about the invention (like he is doing now)

    You then grow up, and use the man's plans to build a time machine, and visit your younger self.

    Where did the actual "idea" of the time machine come from?? Who or what generated that idea, and set those series of events into motion? :surprised

    I have heard that time travel BACK in time is permitted by physics so that makes this scenario is possible.

  2. jcsd
  3. Dec 16, 2004 #2
    AFAIK, time travel is "permitted in physics" in that if you were to travel faster than light, you could stop and look back and see what you were doing a few seconds ago because the light would need time to catch up to you. However, this obviously does not allow you to go back and change anything. What you have done cannot be undone. According to relativity, this is not possible.

    Such theoretical concepts are far beyond me, so I usually don't meddle in this kind of stuff, but I'm sure others will comment and correct any flaws in reasoning I may have made.

    Interesting idea, though.
  4. Dec 16, 2004 #3
    Where in the world do you people get the idea that physics even suggests time travel is possible?
  5. Dec 16, 2004 #4
    But I suddenly have a notion that what if the universe is parallel?And quantum mechanics allow possibilities and one thing have many states mixing together. I mean there can be a time machine which would carry you to another "universe"-state in which you are indeed dead to prevent that contradiction you kill your younger self. But that's good scientific novel material rather than a physics theory.
  6. Dec 16, 2004 #5
    Probably from General Relativity. As far as I know, there isn't a thing in the laws of physics that expressly prohibits it. However it may be very improbable.
  7. Dec 16, 2004 #6


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    What you describe is probably the ONLY time travel permitted. The act of time travel reinforces itself in the scenario. How you would actually get into this loop is perplexing though. I believe Hawing has described something like this. Travel back in time to kill yourself and you will NEVER succeed. The fact that you exist at all to travel back guarantees you will NOT be able to do so. Something will always happen. Time machine break down, get hit by a bus on the way, crazy things of this nature are guaranteed to happen. Otherwise, you would not exist to travel back in time. The loop you described reinforces itself. Going back to kill someone who has caused you grief cannot succeed because once you change history you no longer have a reason to go back. Your plans will always be foiled by physics.
  8. Dec 17, 2004 #7
    I always get a kick out of the time travel movies. Some people go back in time, talk to about 50 people, get in fights, steal cars, etc, and then caution one another 'be careful not to change anything.'

    I tend to subscribe more to a version of Chaozen's parallel universes theory, i.e., an infinite number of possible universes for any given moment in time. Go back in time and kill yourself - No problem. You're now stranded on an alternate 'worldline' in which 'you' (think of him like the evil Captain Kirk :devil: ) die at an early age. Meanwhile, the old universe (worldline) keeps clicking along normally, although short one mad scientist and one time machine.
  9. Dec 17, 2004 #8
    No offense you guys, but if this is going to turn into a Sci-Fi discussion, it should go in the general Discussion board. And grogs, if you are trying to describe that even small actions make huge differences, I believe that's called the butterfly effect. Yes, the movie applies to the theory very well.
  10. Dec 17, 2004 #9


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    Actually there is a law in physics that prohibits this. There are even probabilities and formulas. Until you figure out which law it is, I will remain in the skeptical group.
  11. Dec 17, 2004 #10


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    BINGO! My Thoughts exactly!
  12. Dec 17, 2004 #11


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    If I ever invent a time machine, i will come back and visit myself right now!



    Try it. You never know, it might work.
  13. Dec 17, 2004 #12
    "Permitted" is a bit of a strong word here. There is consensus on travelling to the past.

    Travel to future however, is part of elementary special relativity, and is measurable with particles. The obstacle is having to go near light speed to make it observable.
  14. Dec 17, 2004 #13
    How can future time travel b e part of elementary special relativity, when it seems quite impossible. Can't you observe the future, and then decide to change it? For example, you see youself eating cereal tomorrow, but when tomorrow comes you decide to eat oranges instead
  15. Dec 17, 2004 #14
    Don't know. Don't care. Just as long as I can get the horse track results :biggrin:
  16. Dec 17, 2004 #15


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    There's a very good Isaac Asimov story about a group responsible for managing time travel (who gets to travel through time, what changes should be made, etc), including how these guys ensure their time machine gets invented, in the first place. It's called "End of Eternity".

    But what if someone from your older self's future looked back and realized the damage caused by time travel and decided to come back and try to prevent you from telling yourself how to invent the time machine in the first place? In other words, they were intentionally trying to create a situation in which 'their' present didn't exist. Or is that plot line too similar to Terminator?
  17. Dec 17, 2004 #16


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    Well, gee, it's all so simple really. It's that big time sphere that encircles the 4 corners of the time cube. Just be careful not to get off on a tangent while traveling, or you could fall right off the end of the universe! :rofl:
  18. Dec 17, 2004 #17


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    Time travel is permitted so long as you relinquish the ability to interact with the outside world - meaning that you can never know you've time traveled. Interaction would violate causality.
  19. Dec 17, 2004 #18


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    It wouldn't matter if you were aware you were time traveling or not, just your presence is going to throw things off. You are right that no interaction could be allowed, that means you could not go near people.

    For instance, you decide to buy a pack of gum and walk up to the cashier. A woman gets in line behind you. Your presence has delayed her by 83 seconds. (she would not have had to wait if you had not been there). She gets into her car and is killed when a drunk runs a red light. Before you showed up, she would have not been in a wreck and eventually given birth to the person that finds the cure to cancer. Now that child is never born.

    Another scenario, you're driving a car. The man in the car behind you has to stop for a red light. The light turns green, he continues, nothing happens, he goes on to live for years. Before you appeared, he made the light (your presence caused him to miss it), he was killed a few moments later in a car wreck. Because of you he is alive, he ends up becoming a mass murderer.

    Because you are there, people are continually affected by your presence. You don't have to knowingly do anything in order to cause drastic changes to the future.
  20. Dec 17, 2004 #19

    Chi Meson

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    I'm running on somewhat dated material here, but I bielive the work of Kip Thorne has provided a model for time-travel using the spaces described as "quantum foam." These are the micro-nano-scopic spaces between the "virtual particles" that fill up normal "space." These foam holes do not exist in the spacetime of our everyday world. If these foam holes could be expanded (which requires a huge negative energy density) then a portal could be created BUT...

    ... this portal would allow time travel back only to the moment of the creation of the portal.

    This theory is improbable but, last I heard, Kip Thorne is not a crackpot. Anyone heard an update on this idea?
  21. Dec 17, 2004 #20
    Evo you seem quite pessemistic about it, is it not also possible that your presence may have indirectly saved the lives of millions of people and eventually caused the invention of the cure to AIDES and periodically prevented a nuclear holocaust?

    but onto more theory, if by some slim chance your going back in time some how creates the exact same results as when you were originally there, then your still in the past, but it's the same past you've already experienced and you are now stuck in an indefinite loop of reoccuring time.
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