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From: dgary@ecsvax.UUCP (D Gary Grady)
Newsgroups: net.physics
Subject: Re: speed
Message-ID: <1428@ecsvax.UUCP>
Date: Thu, 6-Jun-85 10:58:04 EDT
Article-I.D.: ecsvax.1428
Posted: Thu Jun  6 10:58:04 1985
Date-Received: Mon, 10-Jun-85 06:46:53 EDT
References: <359@osiris.UUCP> <56@rtp47.UUCP>, <11222@brl-tgr.ARPA>
Organization: Duke U Comp Ctr
Lines: 33

Sometimes simplifying a notion leads to a misunderstanding (how's that
for a generic opening line?).  For instance, we've found that telling
people that a floppy disk is like a phonograph record causes no end of
confusion.  When you pull the disk out of the drive without stopping
the program, they are as astonished as they would be if ZZTop kept
caterwauling even after you removed the record from the turntable.

In relativity people are often told that "all motion is relative" and
they wonder then why we can't view the traveling twin as being at rest
and the other twin as taking the round-trip journey.  In fact, asking
that question shows good physics intuition; they are looking for
symmetries.

The problem is that NOT all motion is relative.  Inertial reference
frames are equally valid, which says something quite different.  The
twin at rest is at rest in one inertial reference frame the whole time.
The traveling twin has to use her rocket motor to change directions, so
that twin is NOT at rest in an inertial frame for the whole time.
That's the non-symmetry in the Twin Paradox.

APS used to sell an excellent book of reprints on Special Relativity
that contained papers on the Twin Paradox (including the famous Dingle
argument in Nature) and other fascinating things.  Of interest to
science fiction buffs is one on how far one could go accelerating
continuously at 1g.  Turns out you could get to another galaxy in a
little over two decades of ship time, or get anywhere in the universe in
(I think) thirty-some years.  Of course, no such journey could be
round-trip in any real sense of the word...
-- 
D Gary Grady
Duke U Comp Center, Durham, NC  27706
(919) 684-3695
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