NetNews Usenet Archive 1992 #19/NN_1992_19.iso/spool/sci/physics/13696

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From: [e-mail] (Cameron Randale Bass)
Newsgroups: sci.physics
Subject: Re: FTL Microwave Signal
Message-ID: <[e-mail]>
Date: 27 Aug 92 18:26:28 GMT
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Organization: University of Virginia
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In article <[e-mail]> [e-mail] (Brent Buckner) writes:
>In article <[e-mail]> [e-mail] (Cameron Randale Bass) writes:
>>In article <[e-mail]> [e-mail] (Brent Buckner) writes:
>>>
>>>1.  We can usefully transform the FTL data transfer into backward-
>>>    in-time data transfer (essentially, given that the transfer is from
>>>    A to B, then we can put A and B into sufficiently
>>>    different frames of reference).
>>
>>     This assumption makes absolutely no sense if we are in possession
>>     of a signal that goes faster than light.   Under those circumstances,
>>     the assumptions that underlie the theory that gives rise to
>>     backward in time behavior are incorrect.
>>
>
>Which assumptions would those be?  We can still do the math for
>SR (and GR) using c, whether or not there is FTL signalling.
>Certainly, we would have to refine our notion of causality;
>this is embedded in assumption 2.

    The assumption that there is a limiting signal velocity.

    If we find a causal FTL signal the math will be completely
    incorrect using c since the math relies on there being nothing
    causal greater than c.  You'd get odd answers like FTL signals
    violating causality.

>>>2.  We only get closed timelike loops.
>>
>>     Similar problem.  Timelike makes sense in a SR (or GR) formulation.
>>     It does not make much sense if one can exchange signals faster
>>     than the assumed limiting speed.  Assuming the 'new' limiting
>>     speed is infinite, we could just coordinatize using the 'new'
>>     assumption and arrive at a Galilean relativistic frame in which
>>     there is no such thing as a 'closed timelike loop'.
>>
>
>We would still be using c in our coordinate transforms.

     That would be silly since you clearly get nonphysical results for
     signals that travel faster than c.

>               [munch]
>>
>>     The point is, that the assumptions by which we arrived at
>>     the 'backward in time' behavior of FTL signals are invalidated
>>     by the existence of causal FTL signals and thus the theory
>>     would be incorrect.
>>
>
>In an appeal to authority, I'll point out that in the wormhole
>work by Thorne et al, they were quite willing to have FTL
>signals with G.R..

     Let's just stick to SR.  It is very difficult for me to determine
     what the results would be for GR since the entire concept of
     Lorentz covariance might be called into question by the hypothetical
     existence of FTL signalling.  As far as FTL in current (or old) GR work
     goes, I suspect that is debatable.

                               dale bass

--
C. R. Bass                                           [e-mail]
Department of Mechanical and Aerospace Engineering
University of Virginia
Charlottesville, Virginia                            [phone]