From owner-freebsd-stable@FreeBSD.ORG Tue Nov 23 00:30:32 2004 Return-Path: Delivered-To: freebsd-stable@freebsd.org Received: from mx1.FreeBSD.org (mx1.freebsd.org [216.136.204.125]) by hub.freebsd.org (Postfix) with ESMTP id 7ED6216A4D7 for ; Tue, 23 Nov 2004 00:30:32 +0000 (GMT) Received: from harmony.village.org (rover.village.org [168.103.84.182]) by mx1.FreeBSD.org (Postfix) with ESMTP id 5DE4D43D6B for ; Tue, 23 Nov 2004 00:30:09 +0000 (GMT) (envelope-from imp@bsdimp.com) Received: from localhost (harmony.village.org [10.0.0.6]) by harmony.village.org (8.13.1/8.13.1) with ESMTP id iAN0Tm9P027612; Mon, 22 Nov 2004 17:29:49 -0700 (MST) (envelope-from imp@bsdimp.com) Date: Mon, 22 Nov 2004 17:30:24 -0700 (MST) Message-Id: <20041122.173024.69158626.imp@bsdimp.com> To: gnb@itga.com.au From: "M. Warner Losh" In-Reply-To: <41A1684E.1020302@itga.com.au> References: <20041116172445.GA14385@kayjay.xs4all.nl> <419A6ED9.9030301@pythonemproject.com> <41A1684E.1020302@itga.com.au> X-Mailer: Mew version 3.3 on Emacs 21.3 / Mule 5.0 (SAKAKI) Mime-Version: 1.0 Content-Type: Text/Plain; charset=us-ascii Content-Transfer-Encoding: 7bit cc: rob@pythonemproject.com cc: freebsd-stable@freebsd.org Subject: Re: What OS are you? fun X-BeenThere: freebsd-stable@freebsd.org X-Mailman-Version: 2.1.1 Precedence: list List-Id: Production branch of FreeBSD source code List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Tue, 23 Nov 2004 00:30:32 -0000 In message: <41A1684E.1020302@itga.com.au> Gregory Bond writes: : Rob wrote: : : >>> You'd better cite your source and / or reasoning, as ~3*10^8m/s =is= : >>> the : >>> accepted constant speed of light in vacuum. : >> : It's deeper than that. The "second" and the "meter" are both defined in : terms of wavelengths of light, which (as a consequence) fixes the speed : of light _by definition_, at _exactly_ *299 792 458 m s^-1. The second is not defined in terms of the speed of light. It is defined in terms of the number of hyperfine transitions of cesium: http://www.bipm.fr/en/si/si_brochure/chapter2/2-1/second.html The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom. This definition refers to a caesium atom at rest at a temperature of 0 K. The meter used to be defined in terms the wavelength of Krypton-86 radiation, but that was changed in 1983. It is nowdefined in terms of how far light travels in a given time interval. See http://physics.nist.gov/cuu/Units/meter.html for a good historical perspective. So the definition of the meter is dependent on the second, but the second is independent. However, the definition implicitly assumes that today's fundamental constants of the universe are indeed constant. There's been some evidence that suggests, but is so far inconclusive, that some or all of the fundamental constants of the universe may vary on the order of a few parts in 10^15 over the last few billion years or so. The definition of the meter was changed before this evidence was known. And this is indeed, very off topic. Warner