Is Length Contraction in SR physical??
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From: kenseto on 3 Feb 2010 16:58 On Feb 3, 3:36 pm, PD <thedraperfam...(a)gmail.com> wrote: > On Feb 3, 12:19 pm, kenseto <kens...(a)erinet.com> wrote: > > > > > > > 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, look again. It is stated explicitly in the pole and barn paradox > that in the pole frame, the pole is LONGER than the barn. This means > the physical shortening of the rod obviously is not frame-independent. > If it required the rod to be physically shorter to all observers, then > it would be claimed to be shorter than the barn in the pole frame, > too. Since this is not claimed, then it is not required to be observer > independent. > You get a clue....in the barn frame you claimed that the doors are closed simultaneously while the pole is completely inside the barn. This means that the pole is physically shortened and physically shortened pole is not observer dependent. That's why your claim of physical length contraction is bogus. Physicists with clues (not you apparently)invented the alternate explanation that length contraction is a geometric effect. This explanation avoids the bogus assertion that the pole is physically contracted and BTW geometric porjection is observer dependent. Ken Seto > Here's a quarter. Buy a clue. > > > > > > > > > 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 -- Hide quoted text - > > - Show quoted text -- Hide quoted text - > > - Show quoted text -
From: kenseto on 3 Feb 2010 17:00 On Feb 3, 3:11 pm, Ste <ste_ro...(a)hotmail.com> wrote: > On 3 Feb, 18:19, kenseto <kens...(a)erinet.com> wrote: > > > On Feb 3, 12:11 pm, PD <thedraperfam...(a)gmail.com> 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. > > > > 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. > > Presumably the discussion can move forward by discussing how you each > differ in your understandings. See my reply to PD.
From: artful on 3 Feb 2010 17:16 On Feb 4, 3:31 am, Tom Roberts <tjrob...(a)sbcglobal.net> wrote: > 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 thanks for the response .. glad to see I'm following what you are saying and that (for the most part) we're "on the same page". I would still say that a pole fitting between barn doors at a given time in the barn frame is still a valid description of what happens physically. In that sense length contraction is 'physical'. I know that the label 'physical' is used differently by different people (leading to confusion) .. I wonder what you would think of my notion of 'physical in frame of reference' : a well-defined measurement (OF something physical) that is invariant for all observers at rest WITHIN the inertial frame of reference. For example, you could not then say that a person physically gets shorter when they move away from you because they look short when you compares their size visually with (say) a ruler at a fixed distance from the observers eyes ... the reason that is not 'physical' is that other observer's co-moving with you, using the same method, would not get the same answer. So it is not only frame dependent, it is *observer* dependent within the frame. However, for the pole and barn. all observers at rest in the barn frame agree that the pole fits inside. I don't know if there is a term for something being 'frame dependent but not observer invariant within the frame'.
From: Ste on 3 Feb 2010 18:10 On 3 Feb, 20:49, PD <thedraperfam...(a)gmail.com> wrote: > On Feb 3, 2:11 pm, Ste <ste_ro...(a)hotmail.com> wrote: > > > > > > > On 3 Feb, 18:19, kenseto <kens...(a)erinet.com> wrote: > > > > On Feb 3, 12:11 pm, PD <thedraperfam...(a)gmail.com> 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. > > > > > 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. > > > Presumably the discussion can move forward by discussing how you each > > differ in your understandings. > > Not really. The pole and barn puzzle is documented in black and white. > Some people cannot comprehend what they read in a coherent manner. If > 98% of the people who read the documented puzzle and understand it to > mean one thing, and the remaining 2% who read the documented puzzle > understand it to mean a variety of other things, can a discussion > between a member of the majority and a member of the minority move > substantially forward without resolving the difference? I think you overestimate the coherence of the explanations that are given out there, and the adherents of various explanations tend to be quite hard line about it and simply disparaging of any alternative. That said, if you're satisfied that you understand something well and cannot possibly explain something simpler, then sometimes you've just got to accept the differences. But what I would say is that discussing something and being forced to explain normally hones one's own understanding as well as communication skills (and it also shares knowledge and understanding for the general good), and I often think that part of the problem here is that, actually, people have merely learned a great deal by rote and actually don't understand at all, and know that discussing the subject might reveal those deficiencies.
From: artful on 3 Feb 2010 18:23
On Feb 4, 10:10 am, Ste <ste_ro...(a)hotmail.com> wrote: > On 3 Feb, 20:49, PD <thedraperfam...(a)gmail.com> wrote: > > > > > > > On Feb 3, 2:11 pm, Ste <ste_ro...(a)hotmail.com> wrote: > > > > On 3 Feb, 18:19, kenseto <kens...(a)erinet.com> wrote: > > > > > On Feb 3, 12:11 pm, PD <thedraperfam...(a)gmail.com> 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. > > > > > > 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. > > > > Presumably the discussion can move forward by discussing how you each > > > differ in your understandings. > > > Not really. The pole and barn puzzle is documented in black and white. > > Some people cannot comprehend what they read in a coherent manner. If > > 98% of the people who read the documented puzzle and understand it to > > mean one thing, and the remaining 2% who read the documented puzzle > > understand it to mean a variety of other things, can a discussion > > between a member of the majority and a member of the minority move > > substantially forward without resolving the difference? > > I think you overestimate the coherence of the explanations that are > given out there, and the adherents of various explanations tend to be > quite hard line about it and simply disparaging of any alternative. > > That said, if you're satisfied that you understand something well and > cannot possibly explain something simpler, then sometimes you've just > got to accept the differences. But what I would say is that discussing > something and being forced to explain normally hones one's own > understanding as well as communication skills (and it also shares > knowledge and understanding for the general good), and I often think > that part of the problem here is that, actually, people have merely > learned a great deal by rote and actually don't understand at all, and > know that discussing the subject might reveal those deficiencies. Many more don't understand *at all* but think that they do, so they continue to try to discuss the subject without realising that doing so reveals those deficiencies. |