Can Future Generations Really Travel Back in Time?

[quantum123]

Mon Aug 20, 11:55 AM ET

A new concept for a time machine could possibly enable distant future generations to travel into the past, research now suggests.

Unlike past ideas for time machines, this new concept does not require exotic, theoretical forms of matter. Still, this new idea requires technology far more advanced than anything existing today, and major questions remain as to whether any time machine would ever prove stable enough to enable actual travel back in time.

Note:
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

Also, the peer-reviwed article is at

http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=PRVDAQ000076000004044002000001&idtype=cvips&gifs=yes

 

[pervect]

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.

 

[uiulic]

So interesting a topic which is often in a fiction movie.

 

[G01]

Specifically, I'm not quite sure how the proposal(s) get around the need for exotic matter for a compact time machine.

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.I'm sorry, couldn't resist! This is a pretty interesting stuff. Neat find.

 

[Yum Yum]

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...

 

[pervect]

So this would have to be denser than a black hole?

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.

http://prola.aps.org/abstract/PRD/v9/i8/p2203_1

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:

There are two rather general analyses, by Tipler [13] and by Hawking [11], which put
constraints on the creation of a time machine in a compact region of space without violating the WEC. At first sight each of these analyses might appear to preclude a model like the one presented here. A closer look, however, reveals that there is no inconsistency. This was already demonstrated and explained in Ref. [5] (which, too, presents a time-machine model satisfying the WEC). In short, Hawking’s analysis refers to compactly-generated time machines, which probably is not the case here (see below).

Our model is consistent with Tipler’s analysis because it includes a closed null geodesic (denoted N below) which is future incomplete (though no local irregularity occurs there; See also the discussion in Ref. [5].

I'm having a difficult time interpreting what this actually means.

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...

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.

 

[quantum123]

Can spacetime be non-compact? How would you demonstrate it experimentally?

 

[SpitfireAce]

are you by chance talking about the movie "primer"?

 

[pervect]

Can spacetime be non-compact? How would you demonstrate it experimentally?

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 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.