eklhad@ihnet.UUCP (K. A. Dahlke) (03/14/86)
1. In an article I posted about 27 years ago (it seems like it), I made the statement: > Motion is relative *only* when considering inertial reference frames. > It is not a matter of mathematical complexity, > the earth really does rotate. Was the statement correct? The answer seems to be a resounding *no*. By incorporating artificial constraints (e.g. constant acceleration), my severely emasculated model of general relativity lacked the intrinsic beauty that is rightfully ascribed to Einstein's work. I guess I should have taken that general relativity course in graduate school. Michael McNeil assessed the situation accurately: > I imagine it must > be the memories of all those undergraduate physics classes, where > inertial reference frames were ground into our flesh, never to be > forgotten, while terms like "special" (applied to what one knew) and > "general" (applied to what one did not know) gradually slip away. > I *highly* recommend that people learn *more* about relativity! If I ever get the opportunity, I will take you up on this. 2. I also made the statement (referring to net.origins): > The amount of bogus physics in this newsgroup is astonishing. Was the statement correct? The answer seems to be a resounding *yes*. I need not defend this statement. If you have read the newsgroup, you know what I mean. I am not talking about erroneous implications of general relativity, I am talking about conservation of angular momentum, entropy, Newtonian mechanics, and other first year undergraduate topics. My mistake was to place both statements in the same article. Well, I never said I was immune to bogosity. 3. The format of the article was arguably inappropriate. I violated my own rule, posting the article directly, instead of waiting 24 hours. Every time I post an immediate response, I invariably regret it, to some degree. My frustration at the amount of bogus science in what could be an interesting newsgroup was not aimed at Ken Arrombee, who, along with being right, simply deserves better. My apologies to Ken. Credibility requires months to establish, and minutes to distroy. 4. I never expected such an uproar over a misspelled name! Of course, this is a common net tactic. Any grammatical, typographical, or spelling error implies ignorance (when convenient). > David Fry > Refering to Lorentz > transformations as "Lawrence transformations" shows that your knowledge > in this field is also "bogus". By the way Dave, you misspelled referring :-). Clayton Cramer (someone who usually posts reasonable articles) even tried to rationalize this anti-spelling position: > Misspelling "Lorentz" as "Lorintz", or "Larentz" would suggest that Mr. > Dahlke has at least read something about "Lorentz transformations" but > couldn't remember the spelling. Turning it into "Lawrence" suggests he > hasn't ever read anything on the subject -- just relying on what he heard > on the radio or at a party. Sounds like a pretty dull party to me :-). Actually, I have "read" books on special relativity, though it is not reading in the traditional sense. People (and voice synthesizers) pronounce the various Lorentz spellings identically, making them indistinguishable to me. I make every effort to construct syntactically correct articles by using Unix word processing tools, but homonyms and proper nouns are a perpetual problem. Since /usr/src/cmd/spell/list does not contain famous names, the spelling error is quite understandable. You need not post apologies, mail will do (though I posted mine). -- The moon is more important than the sun, because the moon gives us light at night; when we really need it! Karl Dahlke ihnp4!ihnet!eklhad
weemba@brahms.BERKELEY.EDU (Matthew P. Wiener) (03/18/86)
I am directing all follow ups to net.physics. This discussion has gone on long enough in net.origins. In article <388@ihnet.UUCP> eklhad@ihnet.UUCP (K. A. Dahlke) writes: >1. In an article I posted about 27 years ago (it seems like it), >I made the statement: >> Motion is relative *only* when considering inertial reference frames. >> It is not a matter of mathematical complexity, >> the earth really does rotate. > >Was the statement correct? The answer seems to be a resounding *no*. The statement above about when is motion relative is obviously false from the meaning of the word 'relative', not from any knowledge of physics. The question of does the earth really rotate is that YES, the earth really DOES rotate. "Rotate" in today's modern physics language means that the local space-time geometry has approximately an exterior Kerr metric. (A Kerr metric is just the the mathematical way of describing rotation about a mass in general relativity. I am being circular here, but the abstract description of a Kerr metric makes no reference to rotation. Exterior means I am not considering black holes.) This property is something intrinsic to the very space-time geometry we live in, not to any particular reference frames. Of course, sometimes the easiest way to understand an intrinsic fact is to take a frame-particular point of view. Passing to a frame with the earth at rest does NOT mean the earth has suddenly stopped rotating! (One can of course adopt the meaning that rotation means rotation in your current reference frame. Some people did that in various postings, but the current view thinks that is a pretty useless definition. One wants to study the physics of the situation, not of the frame.) The real beauty of Einstein's theory of general relativity is his vision of intrinsic geometry of space-time as being the essence of gravitation. Passing to coordinate frames is usually required for doing calculations, but from the theoretical point of view, coordinate frames are a blemish. From a practical point of view, coordinate frames are the number one source of confusion about relativity. Rotation seems to be the current winner in the confusion game. How easy it is to view the earth as not moving and concluding that as measured in that frame stars and galaxies are moving megadistances in one day, whizzing at speeds ready to tear the universe apart. And the speed of light is only one light-day per day! This thinking is very tempting, but if one returns to the intrinsic geometric view, one knows that moving slower/faster than light is an intrinsic property of objects under consideration, and so the tachyonic vision can be dismissed as absurd. The paradox can only be seen by insisting on a coordinate frame point of view, and then by pulling a fast one on yourself: the number one gets for the velocity of light in one frame need not have any relation to the number one gets for the velocity of light in another frame. Indeed, in a rotating frame, the number one gets for the velocity of light depends on where the light your timing is located! The further away you measure, the larger the number you get. This sort of confusion is easy to do, and has caught people who should know better before. Even Einstein nodded. As a further example, I consider the very phrase "Lorentz contraction" to mean ouch, nothing more and nothing less. The contraction refers to the difference between two numbers obtained in different reference frames, and is at first a source of astonishment, but from the intrinsic point of view, somebody's measurement of somebody else's space ship is a piece of trivia, not of physics. The physics lies in the what and where and when of the space ship, not in the frame measurements. An even more notorious example is the "twin paradox", which also means ouch in modern day physics. The notion of proper time is well-defined in special relativity, namely as the integral of ds = sqrt(c^2*dt^2-dx^2-dy^2-dz^2) along the path in question. Given a particular path, accelerated or not, one sticks in the induced relations between t,x,y,z into the ds equation, and integrates. Numerically, of all the paths connecting (0,0,0,0) to (T,0,0,0) one gets the largest value for the straight line path x=y=z=0 for all time t. Anyone who wants to drag in general relativity as an explanation here doesn't know what he is talking about: the space-time is flat == uncurved, so if some gullible space traveller wants to believe the travel brochures about earth gravity all the way, that's his problem. Whee! Curvature == gravitation shows up when it is impossible to find ANY frame where ds has the above simple formula, not just the frame one happens to be looking at. Part of the beauty of differential geometry is its ability to detect and measure this phenomena directly. Before I finish, let me point out that I keep referring to the geometry and curvature of space-time, not of space. There's a big difference between the two notions, which is almost never made in the popular literature. For an undergraduate level text on relativity, the beautiful little book by W Rindler _Essential Relativity_ is highly recommended. A little physics, a little calculus, and you're ready to go. (Also recommended to the experts.) ucbvax!brahms!weemba Matthew P Wiener/UCB Math Dept/Berkeley CA 94720