Is Length Contraction in SR physical??
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From: Ste on 3 Feb 2010 12:14 On 3 Feb, 13:26, "Peter Webb" <webbfam...(a)DIESPAMDIEoptusnet.com.au> wrote: > "Ste" <ste_ro...(a)hotmail.com> wrote in message > > > 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. Indeed, because Minkowski's spacetime is a geometrical construct, not a "physical" thing. > 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. No, because the physical basis was fairly obvious to people. As I say, if you understand the "physical basis", you can basically draw a picture and show even a child how things move and how they interact. > 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. If you're not interested in the physical basis, then that is fine. but you shouldn't devalue the need for a physical basis, or complain when others demand such an explanation. > > 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. Well as I say, I've seen a number of interpretations, and even in this place a variety of (mis)understandings abound. > >> 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". Probably. > > 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. Yes, and by many accounts the same applies to me, but for my part I tend to take a similar view about myself as Einstein took about himself, namely that what sets me apart from most people is not "extra intelligence", but simply a great and sustained interest in various subjects which, frankly, most people are not very interested in. > > 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? For a *whole* month.
From: PD on 3 Feb 2010 12:16 On Feb 2, 11:34 pm, Tom Roberts <tjroberts...(a)sbcglobal.net> wrote: > PD wrote: > > On Feb 1, 10:36 pm, Tom Roberts <tjroberts...(a)sbcglobal.net> wrote: > >> The related PHYSICAL attribute is proper length -- that is > >> an attribute of an object that is intrinsic to the object (and does not change > >> with how one looks at it -- it is an invariant). > > > Just a small quibble here. "Proper length" is a synonym for "spacetime > > interval" for space-like-separated events, > > Hmmm. For a given object, its proper length can be measured between any pair of > events at its endpoints that happen at any times, equal or different, as long as > the measurements are made in its rest frame [#]. And this is precisely my quibble. Here you are saying, in essence, there is a spatial property we'll call a length that is frame- invariant, as long as you measure it in one particular frame. :>) > This is another ambiguity in > "length" that has not been touched on (and which applies only in the object's > rest frame). The proper length between such pairs is constant; not so the > invariant interval between such pairs. That's because we are projecting the > interval between those pairs onto the object's rest frame, which is the frame in > which its "proper length" is defined. > > [#] That is, if I hold a ruler up to an object and ensure > there is no relative movement between them, I do not need > to mark the object's endpoints simultaneously to obtain a > correct measurement of its proper length. > > > [...] > > For an arbitrary pair of spacelike-separated events I agree with all you say, > but for the proper length OF AN OBJECT I do not. We learned in Kindergarten that > to measure the length of something you must apply the ruler to it without slipping. > > Tom Roberts
From: PD on 3 Feb 2010 12:17 On Feb 3, 10:21 am, kenseto <kens...(a)erinet.com> wrote: > On Feb 2, 10:10 am, PD <thedraperfam...(a)gmail.com> wrote: > > > > > On Feb 1, 10:36 pm, Tom Roberts <tjroberts...(a)sbcglobal.net> wrote: > > > > kenseto wrote: > > > > On Feb 1, 6:42 pm, Tom Roberts <tjroberts...(a)sbcglobal.net> wrote: > > > >> This is a LINGUISTIC issue. When someone asks "Is length contraction in SR > > > >> physical?", they invariably don't have a definite meaning of "physical" in mind > > > >> (if they did, they could answer the question themselves). Generally, they would > > > >> consider a "physical length contraction" to mean that the object ITSELF gets > > > >> physically shorter. This is manifestly not so in SR. But it is indeed what > > > >> kenseto has repeatedly failed to understand. > > > > > The general public know what physical length means.....the length of a > > > > ruler is physical. > > > > You clearly haven't a clue. In SR, length is most definitely not "physical" in > > > the usual sense. In the USUAL sense, physical attributes of an object do not > > > change with the way someone looks at it. But the length of an object DOES change > > > with the way one looks at the object (i.e. which inertial frame one uses to > > > measure its length). The related PHYSICAL attribute is proper length -- that is > > > an attribute of an object that is intrinsic to the object (and does not change > > > with how one looks at it -- it is an invariant). > > > Just a small quibble here. "Proper length" is a synonym for "spacetime > > interval" for space-like-separated events, and the interval is indeed > > invariant. But the interval is a not strictly a *spatial* quantity, as > > it involves both space and time coordinates, which is why (in my > > opinion) interval is a better term than length, proper or otherwise. > > The purely spatial concept of "length" does not have an invariant > > form. The interval coincides with a spatial length in one frame only, > > the rest frame of the object. But there is no invariant *length* > > (spatial) that is measurable in all frames. There is only a frame- > > dependent length. > > No....spatial length (or physical length) of a meter stick remains the > same in all frames...observer independent. Not according to the definition of physical length that physicists associate with that term. But then again, you are prone to making up your own definitions of terms if you aren't sure what they mean exactly. Then when it is pointed out that you've made up the definition, you sniff that your definition is better. > The only length that is > observer dependent is the light path length of an observed meter > stick. The observer assumes that the light path length of his meter > stick is the spatial or physical length of his meter stick. Using this > assumed standard the observer uses SR/GR or IRT to deftermine the > light path length of an observed meter stick and that's why light path > length is observer dependent. > > Ken Seto > > > > > This terminological fudwickery is what confuses amateurs like Seto who > > are looking for a *spatial* quantity (that is, a length in the > > traditional meaning of that word) that is also "physical" (in the > > sense of being invariant among inertial frames). There is no such > > beast. The fact that there is no such beast is what Seto and some > > others find to be a simply unacceptable aspect of nature. > > > > > If you don't like the word physical how about > > > > "material" length contraction? In any case you stated above that > > > > material length contraction does not occur in SR and I agree with that > > > > fully. So I don't know why you accused me of repeatly failed to > > > > understand!!!!!! > > > > Indeed, you clearly do not understand why I say you fail to understand this, > > > because you STILL don't understand it. Your statement three sentences earlier > > > ("the length of a ruler is physical") CONTRADICTS your claim here. > > > > The word "physical" has at least two different meanings that could be applied here: > > > A) the object itself contracts > > > B) the object itself does not change, but the measurements have > > > physical consequences (such as a long pole fitting in a short barn > > > when moving rapidly enough wrt the barn). > > > > (A) is not correct in SR; (B) is correct in SR. Both COULD be called "physical" > > > so one MUST specify what they mean when they use that word in this context. > > > Exactly. Seto thinks they MUST mean the same thing. They don't. > > > > When you use the word "physical" without saying what you mean by it, people get > > > confused. ESPECIALLY YOURSELF. > > > > Ditto for "material"-- changing the word does not change the problem. > > > > > BTW your SR brothers PD and other disagree with you. They insisted > > > > that a moving meter stick itself gets physically (materially) > > > > shorter. > > > > Again you demonstrate your inability to read or understand. PD most definitely > > > did NOT say that. I'm not sure what "others" you mean. In any case, you _REALLY_ > > > _REALLY_ _REALLY_ need to learn how to read. You have no hope whatsoever of > > > understanding subtle concepts like SR until you learn how to read. > > > > [I give up. It's hopeless until you LEARN how to read..] > > > > Tom Roberts- Hide quoted text - > > > - Show quoted text -- Hide quoted text - > > > - Show quoted text -- Hide quoted text - > > > - Show quoted text -
From: Androcles on 3 Feb 2010 13:04 "PD" <thedraperfamily(a)gmail.com> wrote in message news:34112064-9e46-4892-8d35-1c68ac2f91dd(a)g29g2000yqe.googlegroups.com... On Feb 3, 10:04 am, kenseto <kens...(a)erinet.com> wrote: > 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. No, it doesn't. You obviously don't understand the pole and barn puzzle at all. ============================================= Yes it does. You obviously don't understand the pole and barn paradox at all. You are obviously a lucky idiot: http://www.youtube.com/watch?v=UF9Koljd0RA
From: kenseto on 3 Feb 2010 13:19
On Feb 3, 12:11 pm, PD <thedraperfam...(a)gmail.com> wrote: > On Feb 3, 10:04 am, kenseto <kens...(a)erinet.com> wrote: > > > > > > > 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. > > No, it doesn't. You obviously don't understand the pole and barn > puzzle at all. No it is you who don't understand the pole and the barn paradox. > > > > > > > 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 -- Hide quoted text - > > - Show quoted text -- Hide quoted text - > > - Show quoted text - |