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Time Travel: No worry about paradoxes

  1. Jun 29, 2008 #1
    I'm no theoretical physicist or physicist of any kind, but I have some basic grasp of how the universe works and after thinking long and hard about time travel, I don't think we have to worry about the paradoxes which are inherent to time travel when time traveling into the past. If we are to believe that time is relative like Einstein has stated in his equations, such as the twin brother paradox (where one brother stays on Earth while his twin brother goes off in a rocket and comes back to find his brother on Earth has aged into an old man, but he has remained the same as when he left), then I would imagine that something fairly similar would occur when a time traveler time travels into the past. For instance, let's say that a time traveler from the far future goes back in time via starship traveling faster than light with the Alcubierre warp drive bubble to prevent his birth from occuring by killing his grandfather and succeeds. His grandfather is now dead, but the time traveler continues to exist, he does not literally disappear like Marty McFly in Back To The Future because that would violate the laws of energy conservation, so that's out. Just like the twin brother scenerio, the paradox is a real one, but the time traveler's existence is never compromised (unless of course he's caught, arrested and brought on charges for murder). Since Einstein believed that the past, present and future is happening all at the same time, one can say that there is only one timeline but once a time traveler time travels either into the past or future, they are basically parallel timelines/universes that do not affect one another, they are all relative. Hence, if you're in the past you can change history without compromising the present or future. By the same token, if you're in the present you can change history without compromising the future and vice-versa. This is somewhat different than the parallel timeline/universes that we are accustomed to seeing on shows like Star Trek and Stargate SG-1 where a time traveler changes the past and in that instant a parallel timeline/universe is created. This is not possible because just like the disappearance of Marty McFly, it violates the laws of energy conservation. The fact is, mass like energy cannot be created or destroyed. Therefore, if parallel timelines/universes do exist, they must have been created at the exact same time as when our universe was created.

    Thoughts anyone?

    See the attached diagram I created in Illustrator to see what I mean.

    Attached Files:

  2. jcsd
  3. Jun 29, 2008 #2
    I recall that the twin brother paradox doesn't work......in that everything is 'undone' again when you decelerate from light speed back to normal speed. So whatever you gained from preserving your age, it becomes neutralised when you decelerate when you reach earth again.

    The other thing is.....if you assume that you could travel in time, and then you insert yourself into a 'past' environment.........then how does the energy and mass tally up? Like, when you enter another time, then do you introduce 'more' energy/mass to the whole universe? Or does energy/mass disappear from somewhere when you do your jump in time/space? And here, we just keep in mind the current theory of conservation of mass/energy.
  4. Jun 30, 2008 #3
    If you were to travel back in time, you merely rewind into your own past. This would not get you anywhere besides the moments and places in time you had already been.
    Unless you assume there is some other independent direction of time. However, science has not discovered this second time direction, and I would think that such an extra dimension of time is not a physical possibility.
    In fact it would require a total new physics, which has to account for there being two time directions, instead of one.
  5. Jun 30, 2008 #4
    The moment you kill your grandfather, the future that is in the future changes, not the future in your past.
  6. Jul 1, 2008 #5
    In this fascinating article about the quantum eraser experiment, physicist John Cramer is putting retrocasualty to the test and the results were quite interesting. He supposedly erased the past! But you don't have to take my word on it. Here is a direct quote from Cramer himself:

    Tests of quantum phenomena that were previously featured in this column have recently been popping up in the popular press. Early in 1997, physicists in Zurich demonstrated the extent of nonlocality of Einstein-Podolsky-Rosen experiments (see my column "Einstein's Spooks and Bell's Theorem", Analog, January, 1990) using fiber optic cables belonging to the Swiss Telephone System to show the nonlocal correlations of two detectors separated by more than a hundred kilometers. Last December, the first demonstration of the quantum teleportation of photons (see "The Quantum Physics of Teleportation", Analog, December, 1993) was performed in Austria. In this column I want to describe another test of quantum mechanics, the quantum eraser, which brings some of the most peculiar and counter-intuitive aspects of the theory into sharp focus.


    See link: http://www.npl.washington.edu/AV/altvw90.html

    Any thoughts on this? Anyone? Anyone? LOL
    Last edited by a moderator: Jul 1, 2008
  7. Jul 1, 2008 #6


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    That's not true. I'm no expert but my understanding is that, with careful attention to the "acceleration" and "decelleration" you can show that the twin who stays home does age faster than the one who moved. The relativistic effects of acceleration and deceleration do not cancel.

    And, of course, the "twin paradox" has nothing to do with time travel.

  8. Jul 1, 2008 #7


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    No, you would get to the same moments and places only if you moved "back" in space as well as time. You do not have to "assume there is some other independent direction of time", only that there are directions of space independent of time- and there are.
  9. Jul 5, 2008 #8
    That's a different concept, I was mere thinking in the line of "unrolling the timeline" (which would effectively mean, setting the whole universe back to the past instances of time).

    As to my knowledge, and apart from the effects of relativity on time (and things like drugs on perceived time) there is no way of changing time itself.
  10. Jul 7, 2008 #9
    Wouldn't this be implying that there is no such thing as free will. That whatever you do or choose has already been chosen.
  11. Jul 7, 2008 #10
    There are many formulations of the Twin "paradox". Most of them are solved by recongnizing that the situation is not symmetric. The effect is real.

    No matter. We need to repeat that all over again and again. There is a book called "black holes and time wraps" by Thorne. The book is old but the argument is still valid. I don't want to spoil, but the conclusion is that any time you switch a time machine on, it will self destruct.

    The concept of closed time-like loop : It is a path in space-time that links an event to another other event in the past-cone. That means light and you as well can travel from an event to the future and back to the same event again. The vacuum is full of fluctuations of light of quantum nature. Light will interfere constructively along this time-like loop, pretty much like in a laser, and without need for pumping energy into it. It will use the energy you needed to construct the loop to destruct your device.

    Read the book, serious physicists as you can imagine have thought about this for a long time now. There is no known way out of this for now.
  12. Jul 21, 2008 #11
    if you time travel into the past, does it become the present if youre there and living in it?this time travel stuff is oh so confusing.
  13. Jul 21, 2008 #12


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    I imagine changing something in the past does not end the timeline, it simply branches to a new one.

    The original universe goes on as it always had (grandfather is alive, Tim the TimeTraveller is born, grows up, leaves in his time machine).

    Tim appears in this new universe where he kills a man that would have been his grandfather. This man of course never had children. Tim himself was not born in this universe anyway, so it's not like he's about to disappear. The new universe continues on in a new direction, no Time Traveler is born in it. No paradox.
  14. Jul 21, 2008 #13
    thats a sweet way to get around those pesky paradoxes.
  15. Jul 21, 2008 #14


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    Yeah, it's just those pesky extra universes that pop into existence every time you time travel.
  16. Jul 21, 2008 #15
    so the solution to one problem really does nothing but give rise to another problem..didnt see that one coming..hah.
  17. Jul 21, 2008 #16
    Yeah, the many-worlds interpretation is the only possible candidate for "time travel" in any sense of the word, and it's a candidate for sending information into the past, not even a person.

    But consider this: let's say hypothetically you could get around Heisenberg uncertainty and use a double slit sort of device with entangled photons to send a message into the past. Because of MWI, new worlds are created in the act of send the message, and those new worlds will be the ones affected by your message. Because most of the macroscopic world is still largely deterministic, you will likely be able to send information about the future which will largely turn out to be accurate.

    Unfortunately this doesn't help you much, but if your future counterparts in other branches sent you messages, you'd benefit from them. Because your future "you" is still "you", in all likelyhood you'd get messages from your future benevolent self just as your past self is getting messages from you. As long as everyone's working together you can game the system and win the lottery, etc. All with no paradoxes.

    Unfortunately, if there's a single world, and relativity is right, you're screwed.
  18. Jul 21, 2008 #17
    sounds kinda iffy.
  19. Jul 21, 2008 #18
    Hello, we're talking about time travel. Did you want a scientific proof!? :)
  20. Jul 21, 2008 #19
    haha. true. i meant nothing by it, peter..../
  21. Jul 21, 2008 #20
    whats wrong with there being an infinite amount of universes? or is that the stupidest question ever?
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