Re: Is space-time real?


"Tom Roberts" <tjroberts137@xxxxxxxxxxxxx> wrote in message
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Pmb wrote:
"Tom Roberts" <tjroberts137@xxxxxxxxxxxxx> wrote in message
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This is not true. In GR such time machines are not possible without
exotic matter that has never been observed and seems highly unlikely (it
must have a negative energy density).

On what do you base this on. Can you provide a reference please. Please
keep in mind that I don't neccesarily disagree with you but wish to
understand the reason why exotic matter must be brought into the problem.

Class.Quant.Grav. 16 (1999) 543-549,
http://www.arxiv.org/abs/gr-qc/9812067


I've heard of time machines which use two cosmic strings passing by each
other. Do the exixtance of exotic matter required for the existance of
cosmic strings?

I doubt very much that could work, without a negative energy density.
Certainly all the "time machine" and "warp drive" manifolds require it.


Tom Roberts

Thank you

Pete


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