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From: freeman@spar.UUCP (Jay Freeman)
Newsgroups: net.space
Subject: Re: Speed Of Light
Message-ID: <236@spar.UUCP>
Date: Wed, 8-May-85 16:20:55 EDT
Article-I.D.: spar.236
Posted: Wed May  8 16:20:55 1985
Date-Received: Sat, 11-May-85 08:19:18 EDT
References: <2073@decwrl.UUCP>
Reply-To: freeman@max.UUCP (Jay Freeman)
Organization: Schlumberger Palo Alto Research, CA
Lines: 44
Keywords: relativity

/* libation to line-eater */

In article <2073@decwrl.UUCP> harrow@exodus.DE
(Jeff Harrow NCSE TWO/E92 DTN=247-3134) writes:

>There's something I've never quite understood:
>
>Consider the following setup:
>
>\-------/   (Ship #2 going at .9C) (Laser, at 1.0C)       (Ship #1 at 0.1C)
 |       |           >--> ------------------------
>| Earth |----------------------------------------------------    >-->
>|       |  (Laser, going at 1.0C)
>/-------\
>
>
> ...

Theory and experiment indicate that both laser beams will be  perceived
as moving at 1.0 C both by observers on Earth and on by observers on
each space ship.

Note that the original conclusion, that one observer will see light
moving at 1.8 C and another at 0.1 C, stems from the implicit assumption
that the "right way" to compare speeds is by simple addition and
subtraction:  But this amounts to making an hypothesis about the
physical world, which can be tested by experiment, and so forth.

And the experiments and reasoning associated with the development of
the special theory of relativity have indicated that mere addition and
subtraction are NOT the "right way" to compare speeds. The rule is more
complicated, and it leads to the conclusion that all light beams moving
in vacuum have the same speed as seen by all observers.

When relative speeds are very small, the rule suggested by relativity
reduces to simple addition and subtraction, so that our common-sense
notions are vindicated.  However, common sense is derived from common
experience, and there is no reason to assume that it should apply to
situations and conditions far beyond the everyday.

Flamers will note that there is a lot more that one could say about
these matters :-) ...
-- 
-- Jay Reynolds Freeman (Schlumberger Palo Alto Research)