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A laser time machine

  1. Feb 2, 2008 #1
    I've just finished reading the book "Time Traveler - A scientist's personal mission to make time travel a reality", by Dr. Ronald L. Mallett, professor of physics at the University of Connecticut. In this book Mallett explains a new theory to travel back in time using a laser ring, which creates frame dragging and closed loops in time, according with the Einstein's gravitational field equations. Did anyone of you read this book, too??? Or does someone know something more about this theory, and what do you think of it???


    And....another thing...Don't you think travelling back to the past would go against the law of thermodynamic, especially the law of entropy??? What do you think...???
     
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  3. Feb 2, 2008 #2

    Mentz114

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    I haven't read the book, but I suspect the good prof's machine is based on a solution of the Einstein field equations which admit closed time-like trajectories. The source of such a space-time is no doubt a rapidly rotating ring of matter or energy.

    There are many ways to get mathematical solutions that appear to admit time travel, but these are just mathematical speculations. It is unlikely that the energy could be found to create any of them. Also, we have no guarantee that all the predictions of GR are correct.

    As you say, time travel seems to violate other laws of physics, even causality, so it doesn't seem to be possible.

    And, if it were possible, where are the time travelers ?
     
  4. Feb 2, 2008 #3

    DaveC426913

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    There's nothing wrong with locally altering entropy - Earth life powered by the sun is a good example.
     
  5. Feb 2, 2008 #4
    But it would violate the conservation of energy, would it not? because if someone travelled to our time from the future, the traveller would represent an injection of energy into the current spacetime and a removal of energy from the future that the traveller leaves?
     
  6. Feb 2, 2008 #5

    JesseM

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    It wouldn't locally violate conservation of energy in any small region of spacetime (locally GR always reduces to SR), and energy is not always conserved globally in GR anyway.

    But there are a lot of questionable aspects of Mallett's particular scheme for time travel--see the "objections" section of the wikipedia article, or the discussion in the previous thread here.
     
  7. Feb 3, 2008 #6
    As for "where are the time travellers now?", Mallett sais: "My type of time-machine could only carry a time treaveller back to the moment the machine was turned on, and not one second before".
    So it's easy to understand why there aren't time travellers visiting us now: because the machine is yet to be made...

    Sorry for my english, but I am 17, from Italy....
     
    Last edited: Feb 3, 2008
  8. Feb 3, 2008 #7

    Mentz114

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    Your english is fine.

    "My type of time-machine could only carry a time treaveller back to the moment the machine was turned on, and not one second before".

    Sounds like it can only be used to go forward, then. I'll wait for the bi-directional family saloon with the automatic calendar.
     
  9. Feb 3, 2008 #8

    JesseM

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    Why do you say it can only be used to go forward? He said it could take the traveler back to the time the machine turned on. The same would be true of the hypothetical wormhole time machine, which could only take the time traveler back to the time when the time difference between the two wormhole mouths was created. All time travel solutions in general relativity work this way, I think.
     
  10. Feb 3, 2008 #9

    Mentz114

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    JesseM,
    If I wanted a machine to go back in time ( say to bet on a horse race) it would be useless to me. The machine would need to have been turned on before the race.

    Don't tenses get difficult when talking about 'time-travel' ?
     
  11. Feb 3, 2008 #10

    JesseM

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    Sure, you can't go back to the 19th century or the dinosaur age or whatever with a GR time machine, but you can win on as many horse races as you want as long as they happen after you get the damn thing working! And once it is working, from your perspective it will definitely be "going back in time", even if the date you go back to is ahead of 2008 A.D. (a trip from 3000 A.D. back to 2999 A.D., say)
     
  12. Feb 3, 2008 #11
    But when I turn on my newly constructed time machine everybody (everybody who uses it from now on to the end of times to go back to the moment it first was turned on) shall have to instantly pop up right next to the machine, right? Otherwise history/time shall have to run over and over with new characters whenever somebody uses the machine in the future because they want to be part of the action in their past?
     
  13. Feb 3, 2008 #12

    JesseM

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    It's not true that everyone who uses a GR time machine always goes back to the date it was created, just that this is the furthest back you can go. With the wormhole time machine, you accelerate one mouth and then bring it back close to the other mouth, and the acceleration has now introduced some set time difference between the two mouths. For example, say this time difference is 3 years, and the two mouths were first brought together with this time difference in the year 2500. Then if someone travels through the right mouth in 2509, they'll exit out the left mouth in 2506. Then if they want to go back further they can just take a short trip through normal space to the right mouth in 2506, and then travel through it, exiting the left mouth in 2503. Finally, if they want to go back even further, they can again take a trip through normal space to the right mouth in 2503, travel through it, and exit the left mouth in 2500...but then that's it, they can't go back any further.
     
  14. Feb 3, 2008 #13
    I see. But if anybody in the future decides to take the machine as far back as possible they will have to suddenly appear when the machine is first turned on?
     
  15. Feb 3, 2008 #14

    JesseM

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    Yes, presuming the wormhole is wide enough to accommodate all these future travelers who want to go back as far as possible...if not, then they'll have to work out some system (or fight for it) to decide who gets the golden ticket!
     
  16. Feb 3, 2008 #15
    So if this one guy called John winns a timetravel in a gameshow and he decides it is cool and goes, then he will pop up in his own past and live alongside his first life??? And then when "the original" John "again" reaches the time of the gameshow he'll win again won't he? And now there will be three of this guy.. But that is just the beginning, isn't it because this thing will go on for ever every three years? And so when the machine is turned on at first, an endless number of John will turn up? And he'll probably elect himself for president! Or did I go wrong somewhere?
     
  17. Feb 3, 2008 #16

    JesseM

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    Unless you believe in parallel universes or some notion that it's possible for a time traveler to "change the past", there'll just be two Johns in the past, not more. For example, if in the year 2500 John is 30 and he sees his 60-year-old self emerge from the future, and then they live alongside each other for 30 years, so in the year 2530 we have a 60-year-old John and a 90-year-old John, then the 60-year-old John will win the gameshow and go back through the time machine to 2500 and be the exact same 60-year-old future self that he remembers meeting when he was 30, living out the rest of his years normally until he's 90 in 2530.
     
    Last edited: Feb 4, 2008
  18. Feb 4, 2008 #17
    But, for example, what may happen, if the "old John", after winning the timetravel, goes back in the past, meet the "young John" and prevent him from winning at the gameshow....???

    In my opinion there are too many paradoxes...:confused:I think time travel won't become a reality..:cry:
     
  19. Feb 4, 2008 #18

    JesseM

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    Well, the most popular theory is that he wouldn't be able to, the timeline is just a fixed 4-dimensional structure that doesn't change. See the Novikov self-consistency principle hypothesis for example.
     
  20. Feb 4, 2008 #19
    Maybe they have a Temporal Prime Directive or something. Sheesh.
     
  21. Feb 4, 2008 #20
    So, this means that the "old John", who goes back to past, hasn't got free will, doesn't he?....Otherwise if he made decisions, he would change the course of the events...


    I like the comparison with the Prime Directive....very good :wink:!!!
     
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