Is Length Contraction in SR physical??
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From: "Juan R." González-Álvarez on 3 Feb 2010 07:30 Tom Roberts wrote on Tue, 02 Feb 2010 23:39:18 -0600: > PD wrote: >> I >> find frame-dependent quantities like kinetic energy and magnetic field >> to be arguably physical, even if frame-dependent and coordinate- >> dependent. > > And I argue that you are mistaken -- you confuse such > coordinate-dependent quantities with the more fundamental, underlying > quantities that are invariants: > kinetic energy => s > magnetic field => Maxwell 2-form > ... etc. > > > Tom Roberts There is very little arguing in yours... -- http://www.canonicalscience.org/ BLOG: http://www.canonicalscience.org/en/publicationzone/canonicalsciencetoday/canonicalsciencetoday.html
From: Peter Webb on 3 Feb 2010 08:26 "Ste" <ste_rose0(a)hotmail.com> wrote in message news:1b629a06-65d7-4a00-9cab-1317b473f5c4(a)r6g2000yqn.googlegroups.com... > On 3 Feb, 04:08, "Peter Webb" <webbfam...(a)DIESPAMDIEoptusnet.com.au> > wrote: >> > You are the one presenting some radical new theory which if true will >> > set >> > the Physics world on its ear and gain you the Nobel. If you can't do >> > the >> > math then perhaps you shouldn't be pretending to be a physicist. >> >> I'm not pretending to be a physicist - >> >> __________________________ >> And yet you claim that all physicists are wrong about SR. > > My cheek is astonishing, is it not? Incidentally I'm not claiming SR > is "wrong", what I'm saying is that the physical basis of it appears > to be poorly understood and inadequately described. It may appear to you that its physical basis is poorly understood, but that is only because you probably aren't comfortable with mathematical constructs like Minkowski's space time. And you also seem to have a problem with what physics says in general, but you are focusing this on relativity. Physics give you the mathematical equations, but it really has very little to say beyond those equations. Newton operated in a world of 3D space, and gave equations for how the planets moved, without ever saying what "space" was, what its "physical basis" is. Similarly, the 4D equations of Minkowski space (ie the geometric interpretation of SR) provide an even better estimate of the movements of the planets, but as to what the "physical basis" of Minkowski space, I have as little idea as the "physical basis" of the 3D world I perceive and Newton modelled. > Indeed, there seem > to be as many interpretations of SR as there are observers of the > theory. > "Interpretations" ? Yes, there are lots of completely different but mathematically equivalent ways of formulating and describing Special Relativity. They all produce exactly the same equations, obviously. But if you mean "interpretations" as in the various "interpretations" of Quantum Mechanics, no, pretty much every physicist agrees on how to "interpret" SR. The same is not true of GR, which has cosmological implications, but SR is bread and butter technology used every day in particle accelerators and numerous fields of science. > > >> and I find it comical that you >> think "reality" is a "radical new theory". I know Einstein basically >> made many of the arguments that I'm making. >> >> ______________________________ >> I would be amazed if you and Einstein agreed on anything to do with >> physics. >> However, this is easily tested. Produce a quote you have made in this >> thread, and a quote from Einstein, which show the same "basic argument". > > Perhaps we should start with "If you can't explain it simply, you > don't understand it well enough" - I forget whether I said that above, > or on a different thread. Also "it is the theory that decides what you > can observe" - I've been trying to hammer that one home with Paul for > the past few weeks. > Gee, and I bet you and Einstein both said at some point "Nobody agrees with my theories, but I am sure I am right". > And to describe both me and Einstein: "I have no particular talent - I > am merely inquisitive" or "It's not that I'm so smart, it's just that > I stay with problems longer". > Well, Einstein may have said "its not that I'm smart ...", but in fact he was very, very smart. > The fact is, I'm able to conduct a reasonable debate with experts > because, for the past month, I've gone to bed thinking about physics, > and I've woken up thinking about physics, and I've spent most spare > hours thinking or reading about physics. You have been studying physics for a whole month?
From: jem on 3 Feb 2010 08:35 Tom Roberts wrote: > PD wrote: >> I >> find frame-dependent quantities like kinetic energy and magnetic field >> to be arguably physical, even if frame-dependent and coordinate- >> dependent. > > And I argue that you are mistaken -- you confuse such > coordinate-dependent quantities with the more fundamental, underlying > quantities that are invariants: > kinetic energy => s > magnetic field => Maxwell 2-form > ... etc. > Who's confused? You insultingly tell an expert (PD) that only invariant quantities can be considered physical, then turn around and concede to a kook (Ste) that it doesn't matter to Physics what's considered physical. Would you agree that a mathematical theory is not a physical theory, and that a scientific theory is a physical theory? If so, can you describe the essential difference(s) between the two? Does your description mention invariance?
From: kenseto on 3 Feb 2010 11:04 On Feb 3, 4:41 am, artful <artful...(a)hotmail.com> wrote: > On Feb 3, 4:18 pm, Tom Roberts <tjroberts...(a)sbcglobal.net> wrote: > > > > > > > Uncle Ben wrote: > > > On Feb 1, 11:36 pm, Tom Roberts <tjroberts...(a)sbcglobal.net> wrote: > > >> [...] > > > > Tom Robrts takes the conservative position on what is "physical." > > > Hmmm. I tried not to make any statement about what is or is not "physical", > > because that word is too ambiguous. > > > To me it is irrelevant whether one considers this or that quantity to be > > "physical". What is important is whether or not a given quantity can be an > > appropriate model for some physical phenomenon. For that, it's QUITE CLEAR that > > no coordinate-dependent quantity can be a valid model of any physical > > phenomenon, as arbitrary human choices cannot possibly affect physical > > phenomena. Nor can the perspective from which one looks at an object affect the > > object itself. Coordinates are, of course, arbitrary human choices that define > > the perspective one uses to look at and describe objects and situations.. > > > > To > > > be consistent, he would have to deny physicality to kinetic energy and > > > to the magnetic field of a moving charge. Or even motion itself. > > > I deny that any of those can be valid models for physical phenomena. I make no > > statement about their "physicality" -- arguments over word meanings are > > uninteresting (but inappropriate word meanings must be dealt with before the > > real discussion can even begin). > > > In every case I know of, if you analyze the physical situation sufficiently > > well, you will find an appropriate quantity that is a valid model for the > > physical phenomena in question. For instance, when considering a collision > > between two particles, don't use kinetic energy, use the Mandelstamm s (total > > energy squared in their center-of-momentum frame); instead of magnetic field, > > use the Maxwell 2-form; instead of motion, use the particles' individual > > trajectories. Tensor and geometric analysis provide methods to analyze all > > situations of interest in a coordinate-free manner. This is one of the major > > lessons of GR (but it took about a half-century to sink in). > > > > Or do > > > I not understand? > > > The issue is more subtle than you seem to think. It is not merely about the > > meanings of words, or about what is or is not "physical", it is about what types > > of quantities can be used to model physical phenomena. > > > Tom Roberts > > At first reading you seem to be equating 'physical' with 'frame > invariant'. ie. Only things that are not dependent on the observer > are physical. > > But am I right in my assessment that you are really saying that it is > only nature / reality itself that is physical. The measurements and > calculations we make are parts of our models of reality .. and so are > never really 'physical' themselves. The best models (and > measurements) for reality are those that are not observer dependent, > because physical reality is not observer dependent (ignoring some > interpretations of QM :)). > > In the case of length contraction, what we define as length (roughly > speaking: the spatial distance between two simultaneous events in a > given time) is contracted .. even though the proper interval is > invariant. In both cases they are valid (but different) measurements > of the same pair of events. > > Ken's claim that contraction not being 'physical' means a pole doesn't > physically fit between the barn doors at the same time in the barn > frame of reference (in the well-known 'paradox'). I guess the issue > there is really whether 'between the barn doors at the same time in > the barn frame of reference' itself is physical .. as it is observer / > frame dependent. No it was specified that, in the barn frame, the barn doors close simultaneously for a very brief period while the pole is completely inside the barn. This requires real physical contraction and not observer dependent. Ken Seto > > Obviously there is *some* 'physical' relationship between the barn > doors and pole that means an observer in that frame would measure the > pole as being within the barn. How would you best describe that > relationship? Is talking about 'between the barn doors at the same > time in the barn frame of reference' something valid and meaningful to > say?- Hide quoted text - > > - Show quoted text -
From: Tom Roberts on 3 Feb 2010 11:05
JT wrote: > Tom what prevents the timedilation to be a geometrical interpretation > of time, afterall you seem to say that the length is. Nothing. "Time dilation" _IS_ geometrical. The common theme here is projecting some quantity onto a specific coordinate system. In the case of the building, its physical structure is independent of one's perspective, but the width one ascribes to it depends on perspective -- one has PROJECTED the locations of the building's corners onto one's visual field. In the case of "length contraction", one PROJECTS the endpoints of the moving object onto one's own coordinates. In the case of the "time dilation" of a moving clock, one PROJECTS the events of successive ticks of the clock onto one's own coordinates. > One even could say that doppler is a geometrical projection of time > ticks in a Euclidian space or would that be wrong? It depends on what you mean. In SR the Doppler shift of the light from a moving source is determined purely by the geometrical relationship between source and detector. Tom Roberts |