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From: harrow@exodus.DEC (Jeff Harrow NCSE TWO/E92 DTN=247-3134)
Newsgroups: net.space
Subject: Speed Of Light
Message-ID: <2073@decwrl.UUCP>
Date: Wed, 8-May-85 10:11:25 EDT
Article-I.D.: decwrl.2073
Posted: Wed May  8 10:11:25 1985
Date-Received: Fri, 10-May-85 21:53:38 EDT
Sender: daemon@decwrl.UUCP
Organization: DEC Engineering Network
Lines: 51

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)
/-------\


Now, for simplicity we assume that the Earth has zero velocity (I don't
think this will mess us up):  A laser is fired from Earth which
propagates out at 1.0C (relative to Earth!). 

Ship #1 took off first, a LONG time ago, going in the same direction at
0.1C (relative to Earth!). 

Ship #2 now takes off going at .9C (relative to Earth!) and fires a
SECOND laser in the same direction, said second laser traveling at 1.0C
(relative to Ship #2, NOT to Earth!!). 

Hence, BOTH lasers are going at 1.0C, but RELATIVE TO DIFFERENT BASES.
When both lasers get to Ship #1, won't the laser from Earth have an
apparent speed of .9C (C minus the speed of Ship #1 (.1C)), and won't
the laser from Ship #2 have an apparent speed of 1.8C (.9C Ship #2
velocity plus 1.0C speed of laser minus 0.1C velocity of Ship #1)? 

Now I KNOW that I'm missing some basic concept here because it would
seem that in the first case the ship was GOING FASTER THAN LIGHT,
RELATIVE TO EARTH, and that in the second case the speed of the laser
from Ship #2 was FASTER THAN C (as perceived on Ship #1). 

If C IS relative (perhaps a poor word (or the operative problem?) to be
using here) to a base, then it would seem that MANY thing could go
"faster than light", but if it's NOT relative to a base, "how can that
be"? 

Any good ideas to explain this to a layman? 


Jeff 

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