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Time Travel Machine Outlined

  1. Aug 21, 2007 #1
    For copyright reasons, I'm trimming the original post, which appears to be a direct quote from

    http://news.yahoo.com/s/livescience/20070820/sc_livescience/timetravelmachineoutlined [Broken]

    Also, the peer-reviwed article is at

    http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=PRVDAQ000076000004044002000001&idtype=cvips&gifs=yes [Broken]
    Last edited by a moderator: May 3, 2017
  2. jcsd
  3. Aug 21, 2007 #2


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    Unfortunately, I don't really have a lot to say about this article, other than that it appears to be a refinement of previous work by the author, i.e. http://arxiv.org/abs/gr-qc/0503077.

    Specifically, I'm not quite sure how the proposal(s) get around the need for exotic matter for a compact time machine. Perhaps someone else will have a comment.
  4. Aug 21, 2007 #3
    So interesting a topic which is often in a fiction movie.
  5. Aug 21, 2007 #4


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    The trick is to take a big time machine and shove it into a small space, such as a 1950's Police Public Call Box.:biggrin:

    I'm sorry, couldn't resist! This is a pretty interesting stuff. Neat find.
  6. Aug 22, 2007 #5
    So this would have to be denser than a black hole?

    Yeah, this is overly similar to something I seen on TV a while ago... The "we can only travel back the the point where we turned the machine on" was the give away...
  7. Aug 23, 2007 #6


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    No. "Compactness" is a topological property. See for instance http://en.wikipedia.org/wiki/Compact_space

    An example of a non-compact time machine would be Tippler's infinitely long rotating cylinder.


    Of course it's not possible to actually build such an infinite cylinder, so Tippler's time machine is only a mathematical curiosity, not a time machine in the sense of something that one could actual construct.

    As far as Ori's time machine goes, in one of his earlier papers he has this to say about the compactness of his time machine:

    I'm having a difficult time interpreting what this actually means.
    Ronald Mallett's time machine http://en.wikipedia.org/wiki/Ronald_Mallett perhaps? That one has been discussed here previously. My personal opinion is that Mallett has not satisfactorily addresed how he purports to have gotten around Hawking's proof that a compact time machine must violate the WEC. Ori states that his time machine is in some sense not compact, but I don't really follow what Ori is saying here.
    Last edited: Aug 23, 2007
  8. Aug 24, 2007 #7
    Can spacetime be non-compact? How would you demonstrate it experimentally?
  9. Aug 24, 2007 #8
    are you by chance talking about the movie "primer"?
  10. Aug 25, 2007 #9


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    Sure, an infinite FRW universe would not be compact. (I'd better explain the jargon: a FRW universe is a very standard cosmology, see http://en.wikipedia.org/wiki/Friedmann-Lemaître-Robertson-Walker_metric ).

    I think that results such as "circles in the sky" would imply that our universe is compact, i.e. see for instance:

    http://www.sciencenews.org/pages/pdfs/data/1998/153-08/15308-13.pdf [Broken] or http://www.iop.org/EJ/abstract/0264-9381/15/9/013

    I'm not sure if there is any way to directly demonstrate that the universe is not compact, but since we can show the converse (in at least some cases) the question is addressable by science to some extent.

    Part of the issue is related to the fact that we can only observe part of the universe, the so-called "observable universe", so we can't necessarily directly test all the topological properties of the universe as a whole.
    Last edited by a moderator: May 3, 2017
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