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From: gwyn@brl-vld@sri-unix.UUCP
Newsgroups: net.physics
Subject: Re:  Traveling at speed of light
Message-ID: <12576@sri-arpa.UUCP>
Date: Thu, 13-Oct-83 20:20:22 EDT
Article-I.D.: sri-arpa.12576
Posted: Thu Oct 13 20:20:22 1983
Date-Received: Tue, 18-Oct-83 08:40:51 EDT
Lines: 16

From:      Doug Gwyn (VLD/VMB) <gwyn@brl-vld>

This business about increased mass has nothing to do with how the
object got to be moving.  The principle is,
	If an object would appear to have mass m0 to an observer
	at rest with respect to the object, then the object appears
	to have mass m(v) = m0 * c / sqrt( c^2 - v^2 ) to an observer
	moving with velocity v with respect to the object.
You can turn the description around and say that the object is moving
with velocity v with respect to the observer, if you wish.

"mass" as used here is not an absolute property, since it involves the
state of motion of the observer.  Some of us would prefer to reserve
the word "mass" for m0, usually known as the "rest mass" of the object.
The reason the concept is used this way is because this definition of
"mass" obeys many of the laws of Newtonian physics.