Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.2 9/5/84; site mordor.UUCP Path: utzoo!linus!philabs!cmcl2!seismo!ut-sally!mordor!@S1-A.ARPA:host.MIT-MC.ARPA From: @S1-A.ARPA:host.MIT-MC.ARPA Newsgroups: net.space Subject: Re: Speed Of Light Message-ID: <1723@mordor.UUCP> Date: Thu, 9-May-85 14:23:17 EDT Article-I.D.: mordor.1723 Posted: Thu May 9 14:23:17 1985 Date-Received: Sun, 12-May-85 00:02:55 EDT Sender: daemon@mordor.UUCP Lines: 21 From: Rick McGeer (on an aaa-60-s) Many, many, many physics undergraduates have thought of the same thought experiment, having fallen in to the same trap you did; namely, light travels at only one velocity (c) and two observers in different inertial reference frames will measure the same velocity. Or, to put it better: Two ships, A and B, are launched from earth, A travelling at .1 c and B travelling at .2 c. A laser on earth is then fired after the ships. Observers in A report that the beam appeared to travel at c (not .9c) and observers in B report that the beam appeared to travel at c (not .8c). Ah, you say, but A and B are travelling at different velocities wrt the light source. Surely this velocity difference, which is real enough (that is, observers in all three frames would agree with it) must manifest itself somehow in their observation of the laser beam, and you are correct. An observer on earth would report that the beam had a wavelength w, an observer on A would say w + w0 (w0 > 0), and and observer on B would say w + w0 + w1 (w1 > 0). Rick.