From: KELSEY@SLACVM.SLAC.STANFORD.EDU (KELSEY, Michael H.)
Newsgroups: rec.arts.startrek.tech
Subject: Re: FTL travel/communication
Date: 14 Aug 92 08:46:56 GMT
Organization: Stanford Linear Accelerator Center

In article <1992Aug13.181330.6422@tfs.com>, paigen@tfs.com (David Paigen) says:
>
>hinson@maxwell.physics.purdue.edu (Jason W. Hinson) writes:
>
>>I wrote the following message to someone who asked me to explain why FTL
>>travel/communication impies a causality problem, and I thought it might
>>be of some interest to readers of r.a.s.tech:
>
>Ok.
>
>>senario:
>>Person A is on earth, and person B speeds away from earth at a velocity v.
>>To make things easy, lets say that v is such that for an observer on
>>earth, person B's clock runs slow by a factor of 2.
>
>Right, person B has a slow clock.
>
>[deleted]
>
>>NOW we must switch to B's point of view.  From his point of view, A has
>>been speeding away from him at a velocity v.  So, to B, it is A's clock
>>that has been running slow.
>
>*BZZT* wrong, but that you for playing :-)

"BZZT* wrong" yourself, bucko.  Maybe you should check a good freshman
physics text, like Halliday and Resnick.

The original poster is _correct_.  The equations of special relativity
predict that an observer who considers himself at rest will *always* see
the time in a moving reference frame run slow.  Period.  Observer A sees
the clock on B's ship run slow, and person B sees A's clock _also_run_slow_.
This is the heart of the so-called "Twin Paradox" (which is resolved by the
fact that one of the twins follows a curved path in space-time, while the
other follows a straight path).

>
>If A thinks B has a slow clock, then B thinks A has a fast clock.  From
>this point onwards your scenario is meaningless.  I have seen arguments
>about this causality violation before, but I can't remember how it works.
>Please try again, only with a bit more care.
>
Perhaps you should review a basic college-level physics text before you
suggest that people use "a bit more care."  And if you "can't remember how
it works," why are you attacking someone else's _correct_ analysis?

>I have been unable to construct a scenario that violates causality using
>telephone (instantaneous) communication and horseback (c) travel.  Is it
>required that one party be traveling near the speed of light in order to
>violate causality?

No, it isn't.  What violates causality is if a signal (a message or a
ship), _exceeds_ the speed of light.  You can have two observers, at rest
relative to one another, and an instantaneous communications system will
violate causality.  The violation is just more obvious (you can see the
propagation backward in time) if you do your analysis with one observer
in a moving reference frame.
>
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                                                   -- Mike Kelsey

[  My opinions are not endorsed by SLAC, Caltech, or the US government  ]
What is your _name_?    "kelsey@slacvm.slac.stanford.edu"
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