Is Length Contraction in SR physical??
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From: PD on 5 Feb 2010 10:55 On Feb 4, 8:34 pm, Ste <ste_ro...(a)hotmail.com> wrote: > 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!". Your conclusion "no" is in conflict with experimental measurement. When intuition conflicts with experimental measurement, then it is intuition that must give way.
From: PD on 5 Feb 2010 11:00 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. 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.
From: PD on 5 Feb 2010 11:02 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.
From: PD on 5 Feb 2010 11:05 On Feb 4, 5:12 pm, "kens...(a)erinet.com" <kens...(a)erinet.com> wrote: > On Feb 4, 11:54 am, mpalenik <markpale...(a)gmail.com> wrote: > > > > > On Feb 4, 10:04 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. > > > > > > 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. > > > This is correct so far. > > > > 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. > > > This is incorrect. In the pole frame, the length of the barn is > > *also* contracted but the closing of the doors is no longer > > simultaneous. *Time* as well as distance gets "geometrically > > projected" in the pole frame. > > No. What I said is correct. What the pole observer sees must agree > with what the barn observer sees. NO! This is NOT what the principle of relativity says. They do NOT have to see the same thing. Ken, even Galileo understood this over 400 years ago. If someone at the top of a ship's mast drops a ball, that person will see the ball fall in a straight line to the bottom of the mast. Someone watching the same ball from the shore as the ship goes by will see the ball fall NOT in a straight line but in a parabola. It is simply NOT true that observers in different reference frames must see the same things, and this has been understood for 400 years. If you don't understand it, then you need to back up considerably and start again. > The pole observer does not see the > barn is contracted....he must see the projected length of the barn is > expanded in order to agree with what the barn observer sees. > > > > > In the barn frame, closing the doors simultaneously just long enough > > for the man with the pole to run from one end of the barn to the other > > corresponds to the front door being open, while the back door is > > closed just long enough for the man with the pole to run to it, then > > the back door opening and the front door closing after the back end of > > the pole enters the barn. > > The problem with what you said is that you claim both possibilities: > 1. The pole can fit into the barn physically with both doors close > simultaneously. > 2. The pole cannot fit into the barn physically with both doors close > simultaneously. > > That's not physics when you claim all possibilities. > > Ken Seto > > > > > There is a four dimensional projection going on--you can't project a > > vector onto another in a way that makes it appear bigger, only > > smaller--for example: > > > Say you have two vectors that are parallel to each other. The > > projection of vector 2 onto vector 1 has the same magnitude as vector > > 2. Now, say you rotate them at 90 degrees to each other. The > > projection of vector 2 onto vector 1 has a length of zero. So the > > projection of vector 2 onto vector 1 will always have a length between > > 0 and the length of vector 2. > > > The same is true the other way around. The projection of vector 1 > > onto vector 2 doesn't get *bigger* as the angle between the vectors > > increases. It *also* gets smaller. This projection ranges in length > > between 0 and the length of vector 1. > > > In relativity, the same type of thing is going on, except instead of > > rotating solely in space, the "rotation" is taking place in space AND > > time, and so, the projection must take into account both space and > > time.- Hide quoted text - > > > - Show quoted text -
From: PD on 5 Feb 2010 11:17
On Feb 5, 2:59 am, Ste <ste_ro...(a)hotmail.com> wrote: > On 5 Feb, 07:56, "Peter Webb" <webbfam...(a)DIESPAMDIEoptusnet.com.au> > wrote: > > > > > "Ste" <ste_ro...(a)hotmail.com> wrote in message > > >news:ba85d7b7-90c7-47b9-a9a1-6e04278969c1(a)k19g2000yqc.googlegroups.com.... > > On 5 Feb, 04:31, mpalenik <markpale...(a)gmail.com> wrote: > > > > On Feb 4, 11:22 pm, Ste <ste_ro...(a)hotmail.com> wrote: > > > > > Yes, I'm sure I've employed such a similar wily device when getting a > > > > sofa through a doorway before. But we're assuming that the ladder > > > > *isn't* rotated. > > > > See below. It is rotated not in length and height but in length and > > > time. > > > It is not "rotated" in any physical sense. > > > ____________________________________________ > > Perhaps if you defined what you mean by "physical sense" this statement > > would have some meaning. > > Speaking of "physical", I'm starting to wonder whether I've actually > died and gone to hell for my sins, where the Devil has decided that I > am to be tormented by having to explain the meaning of "physical" for > all eternity. Let's be careful about semantics here. You certainly aren't expected to explain THE meaning of "physical" because YOUR understanding of the meaning of that term is not the definitive one. You are certainly free to explain YOUR meaning of "physical". Then, physicists can explain THEIR meaning of the word "physical". Then, as you have attested is your intent, you can map THEIR meaning of that word to YOUR meaning of that word. |