Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.1 6/24/83; site sdcsla.UUCP Path: utzoo!linus!decvax!ittvax!dcdwest!sdcsvax!sdcsla!west From: west@sdcsla.UUCP (Larry West) Newsgroups: net.physics Subject: Why FTL is illegal, in many small words. Message-ID: <713@sdcsla.UUCP> Date: Mon, 5-Nov-84 00:02:21 EST Article-I.D.: sdcsla.713 Posted: Mon Nov 5 00:02:21 1984 Date-Received: Thu, 8-Nov-84 03:41:43 EST Organization: UC San Diego: Institute for Cognitive Science Lines: 91 mhuxt!js2j asked: "Could someone explain why FTL is illegal? In small words?" I sent a mail response, but it occurs to me that others may be interested; also, I'd be interested in criticism of my reply: is it clear? convincing? correct? And if not, why not? [Reply by mail, please.] (I know this is a little long-winded, but my intention is to make it easy to follow.) References: <327@mhuxt.UUCP> <8130@watarts.UUCP> <333@mhuxt.UUCP> Einstein's Special Relativity (1905) was founded on two postulates: 1) Absolute uniform motion cannot be detected; 2) The speed of light is independent of the motion of the source. The postulates were inspired by the fact that light waves propagate in a vacuum (as predicted by Maxwell's equations), which is really a fairly strange thing for waves to do -- normally waves are simply disturbances (compression/expansion) of the media through which they propagate. This lack of a medium means there's no absolute reference to compare the speed of light against (as verified by Michelson & Morley (1887)). Contrast this with sound waves, which can always be compared to the medium in which they travel. The second postulate is the most direct blow against Faster Than Light motion, so let's do a little thought experiment to see why it is reasonable. So (without relativity), if you have two observers moving relative to each other, and they measure different speeds of light, which would be correct? For example, if observer "A" (female) is moving toward Earth at 100,000 km/sec, and observer "B" (male) is on Earth, and they each (simultaneously) shoot a 1-second laser pulse at the other: A--> <--B (approaching at 100,000 km/sec) [Presumably they have synchronized their watches and agreed beforehand on when to fire their lasers, and they time the interval between when they fire and when they receive the other's light pulse.] A would (in non-relativistic Universes) see the light from B moving at a speed greater than the speed of the light which she sent to B. A: "My light pulse left at 300,000 km/sec, but I measure B's pulse as approaching me at 400,000 km/sec." B: (ditto) So A would expect her pulse to reach B after B's pulse reached her. And likewise, B would see A's light moving faster than B's own pulse. A: "Since B's pulse left at the same time as mine, and his pulse is traveling faster, he must have received my pulse after I received his." B: (ditto, change gender of pronouns) When they meet, each believes the other's light was travelling faster, and thus that the other one will have recorded a longer time-interval between firing and detection. Thus we arrive at a contradiction, and hence one (or more) of our assumptions is incorrect. And the best way of resolving this (and other problems which arise from similar thought experiments) is Special Relativity, which posits those two postulates given above. Note that this contradiction would not occur with sound waves, because the medium (air, water, whatever) enters into the matter [so to speak], and essentially provides an absolute frame of reference (and the easiest one to calculate in). {Of course, a relativistic explanation of the above would not be nearly as simple, since simultaneity and synchronization are out, and so forth.} So, no faster than light travel, unless you can find some other assumption (besides "speed of light is relative to observer/source") to give up -- and if you do, no need to broadcast it to the net -- let's talk it over, and I'll make the plane reservations to Stockholm. Hope this was worth your time in reading through it. -- Larry West, UC San Diego, Institute for Cognitive Science -- UUCP: {decvax!ucbvax,ihnp4}!sdcsvax!sdcsla!west -- ARPA: west@NPRDC {{ NOT: }} -- -- Larry West, UC San Diego, Institute for Cognitive Science -- UUCP: {decvax!ucbvax,ihnp4}!sdcsvax!sdcsla!west -- ARPA: west@NPRDC {{ NOT: }}