Is Length Contraction in SR physical??
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From: "Juan R." González-Álvarez on 5 Feb 2010 07:03 Tom Roberts wrote on Thu, 04 Feb 2010 23:11:51 -0600: > mpalenik wrote: ^^^ Tom's snipping of his original claims goes here. >> Yes, proper length is only invariant under transforms in the Lorentz >> group. > > Not true. In GR it is invariant under any coordinate transform. Proper lenght is invariant under any coordinate transforms *within* the framework of GR. That is not the same than saying it is always "invariant" under "any coordinate transforms" and thus an intrinsic property of the object (which was your misguided *original* claim, now sniped by you). -- http://www.canonicalscience.org/ BLOG: http://www.canonicalscience.org/en/publicationzone/canonicalsciencetoday/canonicalsciencetoday.html
From: "Juan R." González-Álvarez on 5 Feb 2010 07:23 Tom Roberts wrote on Thu, 04 Feb 2010 23:10:48 -0600: > Juan R. González-Álvarez wrote: >> Tom Roberts wrote on Wed, 03 Feb 2010 19:52:10 -0600: >>> "Proper length" [...] is >>> intrinsic to the object, >> >> It is not. > > Yes, it is. If you think not, give an argument or counterexample. Do you mean arguments as yours?: "it is intrinsic to the object", "Yes, it is"... >>> and b) it is invariant. >> >> Only under certain approximations (i.e. under certain *specific* class >> of transformations). > > No approximation is necessary in SR or GR. Both SR and GR are based in well-defined approximations and only valid when the approximations hold. This is one of reasons which your claim that proper length is an "intrinsic property of an object" was invalid. > The context of this thread is > relativity, not some other theory that only you know about. Don't be ridiculous Tom... . Do you mean, for instance, that only I in the world know the relativistic theory of Stuckelberg, Horwitz, and Piron? http://order.ph.utexas.edu/mtrump/manybody/ http://en.wikipedia.org/wiki/Relativistic_dynamics How many people live in your petit universe Tom? One? -- http://www.canonicalscience.org/ BLOG: http://www.canonicalscience.org/en/publicationzone/canonicalsciencetoday/canonicalsciencetoday.html
From: J. Clarke on 5 Feb 2010 08:03 Juan R. González-Álvarez wrote: > Tom Roberts wrote on Thu, 04 Feb 2010 23:11:51 -0600: > >> mpalenik wrote: > > ^^^ Tom's snipping of his original claims goes here. > >>> Yes, proper length is only invariant under transforms in the Lorentz >>> group. >> >> Not true. In GR it is invariant under any coordinate transform. > > Proper lenght is invariant under any coordinate transforms > *within* the framework of GR. > > That is not the same than saying it is always "invariant" under > "any coordinate transforms" and thus an intrinsic property of the > object (which was your misguided *original* claim, now sniped by you). OK, give us an example of a validated physical model in which it is not invariant.
From: jem on 5 Feb 2010 09:16 Tom Roberts wrote: > jem wrote: >> Tom Roberts wrote: >>> But if you want to construct models of natural phenomena, in a >>> process we call science, then the choices of quantities used to form >>> the model are important. Some quantities, such as >>> coordinate-dependent ones, simply cannot be used in a valid model >>> because they have aspects that are inconsistent with the world we >>> inhabit. >> >> If a coordinate-dependent quantity that represents a measurement >> (e.g., length in SR), is inconsistent with the world we inhabit, it >> calls the underlying theory into question just as surely as would a >> coordinate-independent inconsistency. > > First, remember that every measurement by a given apparatus yields a > value that is invariant (under coordinate transforms). That is, no > matter what other coordinates might be used by some other observer, when > that observer transforms her own measurements to the apparatus, she > finds that her transformed value is precisely the value the apparatus > itself gave. As I've pointed out to you before, this is a completely trivial observation, and of near-zero significance since everybody takes it for granted. But, of course, if the same apparatus were at rest in some > other frame it would in general yield a different value. > > Such values have been called "frame-dependent invariants". I > don't really like that name, but it does capture the essence. > > My point is: for a given object its length might be measured in some > frame as dx, and in another frame as dx'. But any valid physical theory > will not use EITHER dx or dx'; instead it will use invariants, such as > dL, defined as the 4-vector representing the displacement from one end > of the object to the other at a given event (position along its > trajectory in space-time). The reason theories must do this ought to be > obvious: choice of frame cannot possibly affect the physical phenomena > being modeled, and if either dx or dx' were used in the model then there > would be variations in the model that do not correspond to the phenomena > being modeled, thus making the model invalid. Your point is invalid, since the only aspects of models that are of interest are those that correspond to measurements of the phenomena being modeled. What a model may happen to say in other situations is irrelevant. > > >> which suggests that in science related contexts, "physical" should >> equate to (real-world) "measurable". > > I think you need to be more careful to separate world and model. You've got to be kidding. You strip all but the conclusion from a sentence and complain that what remains lacks preciseness. Reread the whole sentence - I think you'll find that adequate care was taken to support its conclusion. > "Physical" implies a quantity in the world. But, of course, we don't > really know what happens in the world without some model, and then all > we can understand is the model (our minds only process thoughts and > cannot possibly access the world directly, and ALWAYS construct a model > and work with that). So any sort of "physical" quantity is necessarily > schizophrenic (oxymoronic) -- the quantity is part of the model but the > adjective claims it is part of the world. Models generate quantities via logic and the world generates them via measurement. The latter is the physical part. That's why I maintain that the > best way to apply "physical" to a quantity is to require that the > quantity be a valid model for some physical phenomenon. Better still, > don't apply "physical" at all to quantities of the model. Note there's > no reason to expect that all such "physical quantities" are measurable. > > Note that I generally avoid using this adjective, except in > threads like this where someone else uses it. Except for the > phrase "physical theory", by which I of course mean a theory > of physics (rather than of mathematics). Ok, and exactly what distinguishes a physics theory (i.e. a physical theory) from a math theory (i.e. a non-physical theory)? Answer that question and you'll have a good idea how the word "physical" can be consistently used in this context. Of course I did that in my last post. >>> Any model >>> that uses coordinate-dependent quantities in its equations [#] will >>> not correspond accurately to phenomena, because the arbitrary human >>> choices involved in constructing coordinates do not affect the >>> phenomena being modeled. >> >> SR uses coordinate-dependent quantities in its equations > > SR is not really a physical theory. Yeah, right. SR obviously makes testable predictions without having to add other theories to it (e.g., differentially aging twins, long poles fitting in short barns, etc.). Please don't come back with "you need additional theories to know how to identify poles, barns and twins". And "inertial frame" is certainly well-defined in SR (as every term used in every model must be), i.e., those reference frames where spatial measurements are homogeneous and isotropic and temporal measurements are homogeneous. SR can be considered to be a > META-theory that prescribes conditions on physical theories, or SR can > be considered as a geometrical underpinning of physical theories, or SR > can be considered to be the local limit of GR; it is not in itself a > COMPLETE physical theory. In order to test any prediction of SR, one > must add an additional theory, such as classical electrodynamics or QED > or GR. For instance, SR talks about "the speed of light", but says > NOTHING about what "light" is, and you must add a theory of light in > order to proceed. If you don't believe me, I invariably believe you, except when the subject involves the importance of invariance. just look at Einstein's 1905 > paper -- classical electrodynamics plays a prominent role (he calls it > "Maxwell-Hertz equations"). > > Another aspect of SR not being a complete theory is the difficulties in > its definition. SR depends inherently on the definition of "inertial > frame", which is itself not easily defined in a self-consistent and > non-circular manner (Einstein's "system of coordinates in which Newton's > laws hold good" is not really sufficient). Not to mention basing the > theory on "speed of light" without specifying what "light" is. The best > foundation of SR is as the local limit of GR -- in practice, that is how > it is actually used. > > > Tom Roberts
From: kenseto on 5 Feb 2010 09:24
On Feb 4, 8:12 pm, artful <artful...(a)hotmail.com> wrote: > On Feb 5, 11:49 am, kenseto <kens...(a)erinet.com> wrote: > > > > > > > On Feb 4, 6:04 pm, mpalenik <markpale...(a)gmail.com> wrote: > > > > On Feb 4, 5:59 pm, "kens...(a)erinet.com" <kens...(a)erinet.com> wrote: > > > > > It it does violate the PoR. You made the contradcictory claims that > > > > the pole can fit into the barn physically (materially) an at the same > > > > time you claim that the pole cannot fit into the barn physically > > > > (materially)......that a violation of the PoR. > > > > No. The doors are not closed simultaneously in the pole's frame, nor > > > are the two ends of the pole simultaneously in the barn in the pole's > > > reference frame. In the barn's frame, the two ends of the pole are in > > > the barn simultaneously and the doors shut simultaneously. In the > > > pole's frame, the two ends of the pole are in the barn at different > > > times and the doors shut at different times. > > > Sigh..You are making the contradictory claims: > > 1. The pole can fit into the barn with both doors close > > simultaneously. > > In the frame of the barn > > > 2. The pole cannot fit into the barn with both doors close > > simultaneously. > > In the frame of the pole > > Two different meanings for 'simultaneously'. So they are not > contradictory > > You really are not very good at thinking or arguing logically.- Hide quoted text - > > - Show quoted text - Hey idiot there is only one barn and one pole. The pole can fit into the barn physically or the pole cannot fit into the barn physically. You can't claim both possibilities. The concept of relativity of Simultaneity violates the the SR postulates. Ken Seto |