NetNews Usenet Archive 1992 #16/NN_1992_16.iso/spool/sci/physics/12111

Internet Message Format | 1992-07-31 | 3.4 KB

Path: sparky!uunet!gatech!destroyer!sol.ctr.columbia.edu!zaphod.mps.ohio-state.edu!cs.utexas.edu!ut-emx!johncobb
From: [e-mail] (John W. Cobb)
Newsgroups: sci.physics
Subject: Re: Twins Paradox Resolved
Message-ID: <[e-mail]>
Date: 31 Jul 92 16:23:51 GMT
References: <[e-mail]> <[e-mail]>
Sender: [e-mail]
Reply-To: [e-mail] (John W. Cobb)
Organization: The University of Texas at Austin
Lines: 53

In article <[e-mail]>, [e-mail] (Wayne Throop) writes:
|>> From: [e-mail] (Tom Van Flandern)
|>> Message-ID: <[e-mail]>
|>> In any frame, the apparent rate of progress of time (and distance) for
|>> a fast-moving traveler is quite different when the traveler is moving
|>> away, from what it is for an approaching traveler.  Naturally, neither
|>> the E-frame nor the T-frame runs systematically slower or faster than
|>> the other, since they are equivalent.  If we can agree on the
|>> resolution of the paradox, then perhaps we can go on to discuss its
|>> significance.
|>
|>This is so clearly wrong that I suspect I'm missing Tom's point
|>altogether.  The "rate of progress of time (and distance)" of
|>a fast-moving traveler is identical whether "moving away" or
|>"approaching".  This is simple to see, because both are altered
|>by the factor of (1-v^2)^.5, which is always the same factor
|>regardless of the position of the traveler along a trajectory,
|>and regardless of the direction of motion along that trajectory.
|>
|>It is true that neither the E-frame or the T-frame runs systematically
|>slower or faster.... they BOTH run slower or faster, depending on
|>who is measuring.
|>
|>So, no, we (at least I) can't agree on a "resolution" of the "paradox"
|>that involves differing factors for approaching vs receeding objects.

In my mind the problem here is that the question: "what is the apparent
rate of progress of time" is an ill defined question unless one also specifies
who is doing the measuring. All clocks run at the rate of 1 second per second
(gosh that's deep). The problem is that some posters are implicitly
assuming that the observer is in a frame such that one clock is moving in
one direction and the other is moving in another direction and both have
the same rate of speed aware from this observer (the home planet). Now
each traveler will view events differently in their frame. Now some posters
are implicitly understanding this to be so and others are missing it, and
when the second and third round of conversation occurs both parties may
be missing the fact that they are not on the same wavelength.

In my mind, this issue is not well suited to a net discussion. The core
idea is a little tricky and certainly non-intuitive. Moreover, one can
develop new intuitions that are also wrong. My suggestion would be that
for those who are truly troubled by this "paradox" to spend some time
and get in deep into a well written textbook. The actual physics can be
comprehended at a freshman physics level, but its significance is clearly
important.  A superficial, 5 minute net pep talk, replete with jargon and
catechism, is not going to clear up the confusion caused by sloppy language
and mis-communication. Perhaps George Phillies (or others) could
recommend some texts that treat this issue with pedagogical grace. Just
my 2 cents worth
provided at 100% discount.

john w. cobb
[e-mail]