Is Length Contraction in SR physical??
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From: kenseto on 4 Feb 2010 10:04 On Feb 3, 8:12 pm, PD <thedraperfam...(a)gmail.com> wrote: > On Feb 3, 3:58 pm, kenseto <kens...(a)erinet.com> wrote: > > > > > > > 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. > > In the barn frame, yes. Only in the barn frame. No....once it is physically contracted it is contracted to all observers. > > > This means that the pole is physically shortened and physically > > shortened pole is not observer dependent. > > No it certainly does not mean that. Sure it means that. > Because if it did mean that, then > the pole would also have to be shorter than the barn in the pole > frame. The point is: the pole is not physically contracted in the barn frame or the pole frame. In the barn frame the geometric porjection of the pole unto the barn frame is contracted and this projected length is able to fit into the barn with both doors close simultaneously. In the pole frame the geometric projection of the length of the barn is expanded and this expanded length is able to encase the pole completely with both doors close simultaneously. Notice that using the geometric concept both frames reach the same conclusion that the projection of the pole is able to be encased by the projection of the barn completely with the doors close simultaneously. Also this approach satisfies the PoR and it is frame dependent. >But this is explicitly said to be NOT the case in the barn and > pole puzzle. In the pole frame, the pole is LONGER than the barn. So > the shortening cannot possibly observer independent. What you said here violates the PoR.<sgrug> Ken Seto > > > > > 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 -- Hide quoted text - > > - Show quoted text -- Hide quoted text - > > - Show quoted text -
From: PD on 4 Feb 2010 10:19 On Feb 3, 9:32 pm, Ste <ste_ro...(a)hotmail.com> wrote: > On 4 Feb, 02:26, PD <thedraperfam...(a)gmail.com> wrote: > > > > > > > Alright, then for the purposes of illustration, I will switch into > > teaching mode and I will show you where relativity of simultaneity > > comes from. > > This will take the form of a string of short posts in conversational > > fashion, step by step. Are you ready and engaged? > > > I'll presume the answer is yes and we'll start with the first step. > > > Let's take two Events, where "event" in physics means something that > > can be stamped with a particular location and a particular time in any > > reference frame of choice. An event is like the popping of a > > firecracker here, or the contact between a penny and the ground there. > > We'll choose two events so that they leave a mark of their location > > that we can measure the location later. We'll also presume the two > > events are in different locations. > > > Now I want to ask how it is I would ever discern whether those two > > spatially separated events are simultaneous? > > > One way this could be arranged is as follows: > > 1. Have a signal be generated from each event, and have the speed of > > the signal be the same from each event. > > 2. Have a signal detector positioned midway between the locations of > > the two events, so that the distance can be confirmed (at any time) to > > be equal from detector to either event. > > 3. Now under those circumstances, we *know* that the time of > > propagation from either event to the detector must be equal. > > 4. Then there are two possible cases: > > 4a. If the original events are truly simultaneous, then the signals > > will arrive at the detector at the same time. > > 4b. If the original events are truly nonsimultaneous, then the signals > > will arrive at the detector at different times. > > > Inverting this, we can make the following conclusions based purely on > > observations: > > IF: > > 1) The detector receives signals from the two events at the same time, > > and > > 2) The signal speed can be checked to be equal from both events, and > > 3) The distance the signal traveled is the same from either event, > > THEN > > The original events MUST have been TRULY simultaneous. > > > or alternatively > > IF: > > 1) The detector receives signals from the two events not at the same > > time, and > > 2) The signal speed can be checked to be equal from both events, and > > 3) The distance the signal traveled is the same from either event, > > THEN > > The original events MUST have been TRULY nonsimultaneous. > > > Now I ask you whether you agree this is a way to determine > > unambiguously from observation the simultaneity or nonsimultaneity of > > two spatially separated events. Yes or no? > > If it is presumed that the two events are an equal amount of distance > away from the observer, and that the information propagates along that > distance at an equal speed, then yes I would say simultaneity can be > determined this way in the absence of gravity considerations. What I > would make the point though that I recognise a distinction between > "proper distance" (i.e. that of a straight line) and the extended path > that real light might need to take in the presence of a gravity field. We can certainly arrange things such that the effect of gravity is much less than the size of the effects being examined, or that gravity is numerically accounted for, or that the set-up is configured so that gravity does not bend the path of the light. Let's take it as a presumption that in a *real* experiment, one of these three strategems is employed, and thus we'll dispense with gravitational curvature as a consideration, for simplicity. > > > Note here that we have accounted for propagation delay here, and the > > conclusions are not masked by that. > > > OK so far? > > Yep. OK, good. Then here's the plan for the next string of posts, just so you know where we're going: 1. A simple note about experimental observations regarding this situation. 2. The idealized "thought experiment" by Einstein that distills these experimental observations. I may follow this up with a more elaborate example that makes the symmetry more apparent. 3. Explaining why the observations in (1) and (2) are completely consistent with known laws of physics *other* than relativity. 4. Understanding why these results are due to the frame-independence of the speed of light. 5. Review of the experimental data on the frame-independence of the speed of light. So then, now that we have an unambiguous way of determining true simultaneity of a pair of events, an approach that we can agree on, then we'll make an interesting observation in nature. We can set up an experiment that looks at a single pair of spatially separated events (events A and B) with two detectors in two different reference frames X and Y, which are moving relative to each other. Experiments of this type have actually been performed. I've already pointed you to a list of references that includes such experiments (among others). I want to emphasize that there is only ONE pair of events that is looked at from two perspectives at the same time, not an experiment that is repeated first with one detector and then with the other. And here is the result: 1. The detector in X is midway between the events and so the distance traveled by the signals are equal, as confirmed by an independent measurement. 2. The speed of the signals is equal, as confirmed by an independent measurement. 3. The signals arrive at the detector in X at the same time. Therefore the original events are simultaneous. 1'. The detector in Y is midway between the events and so the distance traveled by the signals are equal, as confirmed by an independent measurement. 2'. The speed of the signals is equal, as confirmed by an independent measurement. 3'. The signals arrive at the detector in Y at different times. Therefore the original events are not simultaneous. Thus, depending on X or Y, the events are simultaneous or not simultaneous. And since the same procedure is used in both, there is no obvious way to determine which of these is "true" and why the other is not "true". Before you splutter, "But... how can this BE?" let me remind you that these are the real observations -- that is, as nature ACTUALLY exhibits itself -- and we've already decided the conclusions that would necessarily be drawn from those observations. With me so far?
From: PD on 4 Feb 2010 10:47 On Feb 4, 9:04 am, kenseto <kens...(a)erinet.com> wrote: > On Feb 3, 8:12 pm, PD <thedraperfam...(a)gmail.com> wrote: > > > > > On Feb 3, 3:58 pm, kenseto <kens...(a)erinet.com> wrote: > > > > 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. > > > In the barn frame, yes. Only in the barn frame. > > No....once it is physically contracted it is contracted to all > observers. That's YOUR claim. That is NOT the claim of relativity, and that is NOT what is observed in nature. Remember that "physical" does not mean "material". > > > > > > This means that the pole is physically shortened and physically > > > shortened pole is not observer dependent. > > > No it certainly does not mean that. > > Sure it means that. No, Ken, it does not. It may mean that to YOU, but not to physics. > > > Because if it did mean that, then > > the pole would also have to be shorter than the barn in the pole > > frame. > > The point is: the pole is not physically contracted in the barn frame > or the pole frame. In the barn frame the geometric porjection of the > pole unto the barn frame is contracted and this projected length is > able to fit into the barn with both doors close simultaneously. In the > pole frame the geometric projection of the length of the barn is > expanded and this expanded length is able to encase the pole > completely with both doors close simultaneously. > Notice that using the geometric concept both frames reach the same > conclusion that the projection of the pole is able to be encased by > the projection of the barn completely with the doors close > simultaneously. Also this approach satisfies the PoR and it is frame > dependent. > > >But this is explicitly said to be NOT the case in the barn and > > pole puzzle. In the pole frame, the pole is LONGER than the barn. So > > the shortening cannot possibly observer independent. > > What you said here violates the PoR.<sgrug> No, it certainly does not. The PoR does NOT say that the same results will be seen in different reference frames. It says that the DIFFERENT results are all consistent with the SAME laws of physics in the different reference frames. It would help if you learned things like what the PoR actually says. > > Ken Seto > > > > > > 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 -- Hide quoted text - > > > - Show quoted text -- Hide quoted text - > > > - Show quoted text -
From: Tom Roberts on 4 Feb 2010 10:57 artful wrote: > 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'. As I have said before, you are free to use words however you want. But don't expect others to understand what you mean without explanation. Most people, when they hear "the contraction of the pole is physical" immediately think that the pole itself has been squished; we both know that is wrong, but many of your readers don't. I prefer to use words in ways that foster communication rather than hinder it. While that is difficult at best, at least I try to avoid phrases that are known to confuse people. > 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. As a frame of reference is a purely abstract concept, adding the adjective "physical" to it is both inconsistent and incredibly confusing. I doubt that anybody would know what you mean without extensive explanation. > I don't know if there is a term for something being 'frame dependent > but not observer invariant within the frame'. I think "fictitious" covers it well. As in "centrifugal and Coriolis forces" are fictitious. Given the confusion that surrounds them (especially among people who never took Physics 101), I'm surprised you want to extend it to other situations.... What's wrong with simply stating the truth: "length contraction" is purely geometrical in the 4-D spacetime of relativity. As is common, geometrical relationships can have physical consequences (e.g. the ladder does or does not fit through the doorway). Physics can never be separated from geometry. Tom Roberts
From: glird on 4 Feb 2010 11:23
On Feb 4, 10:57 am, Tom Roberts <tjrob...(a)sbcglobal.net> wrote: > > What's wrong with simply stating the truth: "length contraction" is >purely geometrical in the 4-D spacetime of relativity. What's wrong is that length contraction may be something real, but the 4-d spacetime of relativity is a purely geometrical figment of the imagination. glird |