Is Length Contraction in SR physical??
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From: eric gisse on 3 Feb 2010 11:14 kenseto wrote: [...] > 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. ....unless length itself is a frame dependent quantity! 15 years running and every explanation is a new one for Ken, because he doesn't remember it from the last time! > > 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: kenseto on 3 Feb 2010 11:21 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. 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: Tom Roberts on 3 Feb 2010 11:31 artful wrote: > 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. Yes! > 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 :)). Yes! Rather than "the best models" I would say "the only valid models". It's not clear how to make observer-independent measurements, so projecting onto observer-specific coordinates is quite common when comparing model (theory) to experiments (measurements). Historically, the error was made in thinking that such projections were the appropriate quantities to use in the model. This is one of the important lessons from GR, and we now know better. BTW no interpretation of QM violates this, once one realizes that what was historically called "an observer" is really a measurement apparatus. The value displayed by any measurement apparatus is necessarily an invariant [#]. [#] That is, every observer using any coordinates will agree that the value displayed by the apparatus should be the value displayed (i.e. the observers take their own measurements and transform appropriately to the apparatus in question). The value an apparatus measures does, in general, depend on the frame in which the apparatus is at rest (and on other environmental aspects). > 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. As I said, whether or not one calls this "physical" is irrelevant. The question is: is it possible for both ends of the pole to be between the barn doors simultaneously in the barn frame? the answer is clearly: yes. > 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? Yes. At base this is no different from carrying a long ladder through a narrow doorway -- in some orientations it fits, and in some orientations it doesn't. But in SR, relative motion is equivalent to the rotation in 3-space of that ladder. With the proper orientation IN SPACETIME the pole fits between the barn doors in the barn frame. Just like with the proper orientation in space the ladder fits through the doorway in the doorway "frame" (pardon the pun). Tom Roberts
From: kenseto on 3 Feb 2010 11:32 On Feb 2, 10:46 pm, artful <artful...(a)hotmail.com> wrote: > On Feb 3, 2:29 pm, Ste <ste_ro...(a)hotmail.com> wrote: > > > > > > > On 3 Feb, 02:51, artful <artful...(a)hotmail.com> wrote: > > > > On Feb 3, 12:02 pm, Ste <ste_ro...(a)hotmail.com> wrote: > > > > > On 2 Feb, 20:10, PD <thedraperfam...(a)gmail.com> wrote: > > > > > > On Feb 2, 9:24 am, Ste <ste_ro...(a)hotmail.com> wrote: > > > > > > > But the question remains, what is the *physical* cause of the > > > > > > discrepancy when the object to be measured is moving differently from > > > > > > the measuring equipment. It should be a question taken seriously in > > > > > > physics. > > > > > > And it is. > > > > > I will give you just a sketch, meant to inspire further investigation, > > > > > not to teach. > > > > > The physical cause is traced to the *meaning* of the physical property > > > > > length, which is the difference in the locations of the endpoints of > > > > > something, where those locations are measured *simultaneously*. The > > > > > importance of the simultaneity becomes apparent if you try to measure > > > > > the locations of the endpoints of, say, a moving car and attribute the > > > > > difference to the length. Then in turn, it becomes clear that length > > > > > is frame-dependent if simultaneity is frame-dependent -- the frame- > > > > > dependence of simultaneity is the physical cause of the frame- > > > > > dependence of length. > > > > > Ah, now I know this is the difference between us. I hold that > > > > "simultaneity" is *not* frame dependent. > > > > > > So now the question becomes, what is the > > > > > physical reason for the frame-dependence of simultaneity? This in turn > > > > > is traced to the *meaning* of simultaneity, and how we would know that > > > > > two spatially separated events are simultaneous; this in turn hinges > > > > > on the synchronous arrival of equal-speed signals from the events at > > > > > the midpoint between the events. From here it is relatively simple to > > > > > show that the frame-independence of the speed of light *demands* that > > > > > for a given pair of events, that definition of simultaneity will be > > > > > satisfied in one reference frame and not in another reference frame. > > > > > That is, the frame-independence of the speed of light is the physical > > > > > cause of the frame-dependence of simultaneity, which in turn is the > > > > > physical cause of the frame-dependence of length. So then the question > > > > > is, what is the physical cause of the frame-independence of the speed > > > > > of light? The answer to this question has to do with the hyperbolic > > > > > structure of space-time and the fact that causal relationships are > > > > > limited to a region of spacetime called the past- and future-light > > > > > cones. This hyperbolic structure of space and time is thus the > > > > > physical cause of the frame-independence of the speed of light, which > > > > > is in turn the physical cause of the frame-dependence of simultaneity, > > > > > which in turn is the physical cause of the frame-dependence of length. > > > > > > To expand this in a way that would be compellingly easy to follow > > > > > would take more pages than I'm willing to invest. Fortunately, this is > > > > > done in numerous books designed to explain these things in a > > > > > compellingly easy-to-follow fashion. Would you like some > > > > > recommendations? > > > > > You have in fact answered the whole of my question here Paul. The > > > > difference lies in our concepts of "simultaneity". As I said, my > > > > concept of "simultaneous" is "those events which occur at the same > > > > instant, and would be observed to do so universally, *if* information > > > > about the occurence of those events could be conveyed instantaneously". > > > > Simultaneity issues in SR are nothing to do with instantly conveying > > > information. > > > > According to SR your concept of simultaneity being universal does not > > > hold in reality (as nice and simple and idea as it is). > > > I disagree. You seem to be saying that if a tree falls in the woods > > and no one is around to hear it, then it didn't happen. > > How could you possibly draw that conclusion from what I wrote? It > appears you do not know what you are disagreeing with. > > > And moreover, > > the tree falls earlier for a person near to the tree than for a person > > further away. But I reject these arguments. > > Noone is making such arguments. > > > The bottom line is that > > the tree falls, and it falls at the same time for all people, > > regardless of whether some may observe the event later than others. > > SR also says if an event happens it happens for everyone, regardless > of when it is observered So if two events happened simultaneously they happened simultaneously for everyone regardless of when they are observed......right? Ken Seto > > You seem to be arguing against things that people here are NOT saying- Hide quoted text - > > - Show quoted text -
From: kenseto on 3 Feb 2010 11:43
On Feb 2, 12:05 pm, rotchm <rot...(a)gmail.com> wrote: > > > SR predicts that if you measure the length of a moving rod, the > > > *value* obtained will be less than its rest length. That value is > > > "real" in the sense that that value is a real number; is part of the > > > set of the Reals. > > > No measurement of length contraction ever been made. > > > Ken Seto > > Irrelevant to the discussion at hand. As I said, " SR predicts that > *if* ... " > Direct length measurements have not been perfomed. Direct time, > energy, momentum measurements have been performed... but this is not > the topic of this thread. The topic is about the words "real" and > "physical". > > > > So, my advice to you is DO NOT WORRY ABOUT THE CONCEPTS OF "REAL" > > > OR "PHYSICAL". These words are not part of the theory of SR, only part > > > of some different dialects. > > Have you understood/took my advice on this quote above? No I have not taken your advice. |