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From: gwyn@Brl-Vld.ARPA
Newsgroups: net.physics
Subject: Re:  Relativity questions
Message-ID: <107@sri-arpa.UUCP>
Date: Wed, 25-Apr-84 09:39:34 EDT
Article-I.D.: sri-arpa.107
Posted: Wed Apr 25 09:39:34 1984
Date-Received: Mon, 30-Apr-84 05:43:51 EDT
Lines: 18

From:      Doug Gwyn (VLD/VMB) <gwyn@Brl-Vld.ARPA>

Only RELATIVE speeds matter.  If the Earth were moving away from the Sun,
then yes, there would be a red-shift of sunlight detected on the Earth.

The proper way to put the constancy of the speed of light is "The speed
of light in a vacuum is the same for all observers no matter what their
state of motion".  The funny mathematics of special relativity ensures
that this condition is met; in particular, one cannot just add two
velocities the simple way to get the net effect of combined velocity.

The twin paradox is harder to explain in simple terms since more is
involved than just relative velocities.

These matters are fully explained in any good introduction to special
relativity, such as those mentioned on this mailing list a few months
back.  I highly recommend studying such material rather than trying
to reason out these matters intuitively.