Is Length Contraction in SR physical??
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From: Ste on 6 Feb 2010 21:52 On 6 Feb, 17:14, PD <thedraperfam...(a)gmail.com> wrote: > On Feb 5, 8:03 pm, Ste <ste_ro...(a)hotmail.com> wrote: > > > > Suppose events A and B are 10,000 m apart. The detectors X and Y are > > > obviously 5,000 m from each event. Detectors X and Y are in motion > > > relative to each other at a constant speed of 100,000 km/sec, along > > > the line that includes A, B, X, and Y. > > > > Now that you have some numbers, are you ready to proceed with the > > > discussion as I've outlined? > > > So you have A, B, and X, all on a common axis, and Y moving along this > > axis relative to X at 100,000km/s? > > Yes. Ok. > > > > > 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". > > > > > The answer to this will have to turn on a specific discussion of the > > > > evidence. > > > > Actually it doesn't depend on the details of the evidence. We have > > > already determined that the procedure determines simultaneity or > > > nonsimultaneity unambiguously. > > > No we haven't! > > > If X and Y are moving in such a way as to be approaching one event and > > receding from the other, and if they are moving in *opposite* > > directions (relative to each other - they may in fact be both > > approaching one event and both receding from the other), then there > > will be no simultaneity at all, because there has to be a time > > interval for the measurement to take place, > > Please look at the procedure again. I have. I can picture the situation in my head. > The only thing that is required is to note at the detector X or the > detector Y whether the signals from the events arrive at the same time > or at different times. This is a point decision. It is a yes or no > question. "Signal from A just arrived at X. Did signal from B arrive > at X at the same time? Yes or no." If detection is instantaneous (i.e. if a photon is absorbed instantaneously), then it is possible for A and B to be simultaneous according to both X and Y. However, if detection is not instantaneous, then it is *not* possible. I'm working on the assumption that detection in the real world is *not* instantaneous, but is a process that requires a time interval. > If the answer is yes, and if we ALSO know that the distance from X to > A is the same as the distance from X to B (which we can check later if > we wish), and if we ALSO know that the signal speeds from A and B are > the same (which we can check later if we wish), then we KNOW the > events A and B were simultaneous, even though they happened some time > ago. Likewise, if the answer is no, then we KNOW the events A and B > were not simultaneous, even though they happened some time ago. As I say, there is a third way here: the answer is "no", and we know that the events were indeed simultaneous. > Do you agree that those are the right conclusions, based on the yes or > no question above, and given that the other conditions can be > established? No. I think your mistake is in assuming that both the photon and detector have an absolutely zero diameter (and therefore detection occurs as soon as the surface of the zero-diameter objects touch). In reality, nothing in space will have a diameter of zero. > > and if they are moving in > > opposite directions along an axis in common with A and B, then there > > is *no way* they can maintain equidistance for the whole of that time > > interval. > > Nor do they have to. But if the events leave a mark, then it is > certainly possible for anyone to go measure with a tape measure any > time later the distance from the marks to the detector and DIRECTLY > MEASURE that distance, right? Yes, and if you do that according to my assumptions above, you'll find that the detection *process* takes place over a spatial distance, and you'll find that those distances are *not* the same for both X and Y. > > The fault, therefore, lies in saying that X and Y are "midway between > > the events", when in fact they are not. > > Let's not jump ahead. I'll show how this works in Einstein's > distillation of this kind of experiment in a bit. > > I just want you to understand the conditions that we have to > establish, and what consequences would necessarily follow from that. Ok. > > > If the procedure is trustworthy in > > > frame X, then it is trustworthy in frame Y. If the two frames are > > > otherwise equivalent, there is no reason to say, "But the answer > > > arrived at in X is right and the answer in Y is wrong." We've already > > > agreed this procedure works. > > > The procedure is not trustworthy. The two frames are not equivalent, > > because X and Y do *not* remain midway between the events for the > > entirely of the time interval (and, more importantly, the deviations > > from the midpoints are not symmetric). > > If the events leave a mark, and we can later measure the distance from > the marks to the detector and we find those distances are equal, would > you then agree the conclusions are the necessary ones? Yes I would agree with that, although I would warn you that I already have the picture drawn in my head.
From: YBM on 6 Feb 2010 21:56 Ste a �crit : > On 6 Feb, 10:29, mpalenik <markpale...(a)gmail.com> wrote: .... > Because *that* is what SR is all about - it is about describing the > behaviour of *light*. No. >> The differences in measurement are due to >> the different coordinate systems. It has nothing to do with what you >> *see* it has to do with how you make your measurements. > > It has *everything* to do with what you *see*. No. >> When one >> observer measures length, he measures along the x axis. When another >> observer measures length, he measures along the x' axis. The proper >> length of the object doesn't change, but the measured length has >> nothing to do with optics, visibility, or propagation delays. > > Of course the measured length has *everything* to do with optics. How > do you think we usually carry out measurements? > > And why do you think the ladder appears the correct size for an > observer in the x' frame? Wrong again. Stop pretending talking abour SR when you're not, you are talking about a fantasy out of you mind.
From: Ste on 6 Feb 2010 22:02 On 6 Feb, 17:16, PD <thedraperfam...(a)gmail.com> wrote: > On Feb 5, 8:14 pm, Ste <ste_ro...(a)hotmail.com> wrote: > > > > > > > On 5 Feb, 16:00, PD <thedraperfam...(a)gmail.com> wrote: > > > > On Feb 4, 9:57 pm, Ste <ste_ro...(a)hotmail.com> wrote: > > > > > On 5 Feb, 02:48, "Peter Webb" <webbfam...(a)DIESPAMDIEoptusnet.com.au> > > > > wrote: > > > > > > "Ste" <ste_ro...(a)hotmail.com> wrote in message > > > > > >news:3979bca1-a35e-4399-bc71-75d2fc680757(a)21g2000yqj.googlegroups.com... > > > > > On 5 Feb, 01:12, artful <artful...(a)hotmail.com> wrote: > > > > > > > On Feb 5, 11:49 am, kenseto <kens...(a)erinet.com> wrote: > > > > > > > > On Feb 4, 6:04 pm, mpalenik <markpale...(a)gmail.com> wrote: > > > > > > > > > On Feb 4, 5:59 pm, "kens...(a)erinet.com" <kens...(a)erinet.com> wrote: > > > > > > > > > > It it does violate the PoR. You made the contradcictory claims that > > > > > > > > > the pole can fit into the barn physically (materially) an at the > > > > > > > > > same > > > > > > > > > time you claim that the pole cannot fit into the barn physically > > > > > > > > > (materially)......that a violation of the PoR. > > > > > > > > > No. The doors are not closed simultaneously in the pole's frame, nor > > > > > > > > are the two ends of the pole simultaneously in the barn in the pole's > > > > > > > > reference frame. In the barn's frame, the two ends of the pole are in > > > > > > > > the barn simultaneously and the doors shut simultaneously. In the > > > > > > > > pole's frame, the two ends of the pole are in the barn at different > > > > > > > > times and the doors shut at different times. > > > > > > > > Sigh..You are making the contradictory claims: > > > > > > > 1. The pole can fit into the barn with both doors close > > > > > > > simultaneously. > > > > > > > In the frame of the barn > > > > > > > > 2. The pole cannot fit into the barn with both doors close > > > > > > > simultaneously. > > > > > > > In the frame of the pole > > > > > > > Two different meanings for 'simultaneously'. So they are not > > > > > > contradictory > > > > > > > You really are not very good at thinking or arguing logically. > > > > > > He seems perfectly reasonable to me. > > > > > > He's saying that, if one stands in the middle of the barn equidistant > > > > > from the doors on each side, and one closes the doors simultaneously > > > > > (i.e. what I would call "absolutely simultaneously", but which for > > > > > conveience we'll say "simultaneous according to an observer standing > > > > > equidistant from each door), then is it possible that both doors can > > > > > appear closed while the ladder is observed to be inside the barn. > > > > > > It's a simple question, and the answer is "no!". > > > > > > ______________________________ > > > > > Its a simple question, and the answer is "yes". It is quite easy to devise > > > > > an inertial frame in which the doors both appear closed and the ladder is > > > > > fully contained in the barn. > > > > > The question wasn't whether "an inertial frame can be devised...". > > > > Even I can think of situations where it would *appear* that the ladder > > > > was inside and both doors closed. The question is whether this is an > > > > artefact of observation, or whether the ladder is actually inside with > > > > both doors closed. The answer remains "no, the ladder does not fit > > > > inside with both doors closed". > > > > There is a simple test. > > > We can determine whether the doors closed simultaneously. We do this > > > using the test of simultaneity I indicated to you above: > > > 1. Have the door-closings generate signals that travel at identical > > > speeds from either event. > > > 2. Have a detector positioned at equal distance from the barn doors. > > > 3. If the detector receives the signals from the barn door-closings at > > > the same time, then the two doors closed simultaneously unambiguously.. > > > You and I have agreed on this procedure. > > > If the detector is stationary, then yes we agree that this determines > > simultaneity. > > > > Secondly, we can determine whether any part of the pole was outside > > > the barn when those doors were closed by looking for the marks the > > > ends of the pole would have made on the door. In the absence of those > > > marks, we can safely conclude that the entire pole was inside the barn > > > at the time the doors were closed. > > > Agreed. I would expect this test is definitive, were it possible to > > carry out. > > And this has been tested in equivalent experiment. Documented. Has it really? Which experiment was that, because my sources say that length contraction has *never* been experimentally observed. > Seto does not read experimental papers, because he finds them > confusing. So instead he believes what makes sense to him, rather than > letting experimental results tell him something different. You have to concede at least the possibility Paul that the reason Ken finds them confusing, and so do I, is because *we* can see that they rest on certain assumptions. That is, assumptions that you've internalised, and assumptions which me and Ken reject.
From: Ste on 6 Feb 2010 22:19 On 6 Feb, 20:17, PD <thedraperfam...(a)gmail.com> wrote: > On Feb 5, 8:16 pm, Ste <ste_ro...(a)hotmail.com> wrote: > > > > > > > On 5 Feb, 16:02, PD <thedraperfam...(a)gmail.com> wrote: > > > > On Feb 4, 8:16 pm, Ste <ste_ro...(a)hotmail.com> wrote: > > > > > On 5 Feb, 00:49, kenseto <kens...(a)erinet.com> wrote: > > > > > > On Feb 4, 6:04 pm, mpalenik <markpale...(a)gmail.com> wrote: > > > > > > > On Feb 4, 5:59 pm, "kens...(a)erinet.com" <kens...(a)erinet.com> wrote: > > > > > > > > It it does violate the PoR. You made the contradcictory claims that > > > > > > > the pole can fit into the barn physically (materially) an at the same > > > > > > > time you claim that the pole cannot fit into the barn physically > > > > > > > (materially)......that a violation of the PoR. > > > > > > > No. The doors are not closed simultaneously in the pole's frame, nor > > > > > > are the two ends of the pole simultaneously in the barn in the pole's > > > > > > reference frame. In the barn's frame, the two ends of the pole are in > > > > > > the barn simultaneously and the doors shut simultaneously. In the > > > > > > pole's frame, the two ends of the pole are in the barn at different > > > > > > times and the doors shut at different times. > > > > > > Sigh..You are making the contradictory claims: > > > > > 1. The pole can fit into the barn with both doors close > > > > > simultaneously. > > > > > 2. The pole cannot fit into the barn with both doors close > > > > > simultaneously. > > > > > Take comfort Ken that at least there's someone here who understands > > > > your simple questions. > > > > So far it appears that there are at least two people confused and > > > incredulous. > > > Ken has been confused about this for 15 years. We'll soon see whether > > > you can be unconfused at the rate that most freshman students become > > > unconfused about it. > > > I don't know Ken's past history, but certainly he seems to be asking a > > fairly reasonable questions this time - and judging by the responses, > > it's a question that children get taught not to ask! > > When Ken started asking these questions 15 years ago, they were > reasonable questions. When after a couple of years it was clear he was > not listening to the answers given to his questions, the tone of the > responses became a little different. Well I can assure you I wouldn't tolerate this crowd for 15 years, but I'm hoping that perhaps I can articulate myself better than Ken and make some progress. If not, then I'll comfort myself with Einstein's words that "the only thing that is infinite is human stupidity".
From: PD on 6 Feb 2010 22:20
On Feb 6, 8:30 pm, Ste <ste_ro...(a)hotmail.com> wrote: > On 6 Feb, 10:29, mpalenik <markpale...(a)gmail.com> wrote: > > > On Feb 6, 5:01 am, Ste <ste_ro...(a)hotmail.com> wrote: > > > > On 6 Feb, 09:23, mpalenik <markpale...(a)gmail.com> wrote: > > > > But as I'm saying to you, I think you're confusing an optical effect > > > with a physical effect. That is, you're confusing *appearances* with > > > concrete reality. > > > What part of this picture do you think is optical? It's *geometrical* > > it doesn't have anything to do with what you can visibly see. > > Don't you realise that SR is about the behaviour of *light* - that is, > EMR? And SR describes how *observations* made by way of *light* change > in response to physical circumstances? > > > > > An important point, however, is that neither one of the sets of axes > > > > (either x,t or x',t') is inherantly better than the other. Neither > > > > set is more correct than the other. > > > > Indeed. > > > How can you agree with that and claim that the differences in > > measurement are optical? > > Because *that* is what SR is all about - it is about describing the > behaviour of *light*. No, it is not. Maxwell's equations and QED describe light. SR is about the structure of spacetime, which is why it also applies to, say, the strong nuclear interaction, which is described by QCD. > > > The differences in measurement are due to > > the different coordinate systems. It has nothing to do with what you > > *see* it has to do with how you make your measurements. > > It has *everything* to do with what you *see*. It has to do with what is measured. > > > When one > > observer measures length, he measures along the x axis. When another > > observer measures length, he measures along the x' axis. The proper > > length of the object doesn't change, but the measured length has > > nothing to do with optics, visibility, or propagation delays. > > Of course the measured length has *everything* to do with optics. How > do you think we usually carry out measurements? > > And why do you think the ladder appears the correct size for an > observer in the x' frame? |