Is Length Contraction in SR physical??
in [Relativity]
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From: Ste on 4 Feb 2010 22:10 On 5 Feb, 02:16, mpalenik <markpale...(a)gmail.com> wrote: > On Feb 4, 8:56 pm, Ste <ste_ro...(a)hotmail.com> wrote: > > > On 5 Feb, 00:19, artful <artful...(a)hotmail.com> wrote: > > > > Don't you realize that EVERY reference frame encompasses the whole > > > universe? > > > No, an reference frame only encompasses that which is in the reference > > frame. By definition, if there is more than one reference frame, then > > you can't be using all of them at once. As I say, in my "universal > > reference frame", an observer does not undergo a change in reference > > frame when accelerating. > > This is wrong on so many levels. > > First of all, in the framework of GR, you actually can describe an > accelerating observer with a single reference frame in the geometry of > curved space time. I know, which is why I keep saying "I don't dispute the maths of relativity", although it seems I may as well bang my head against the wall. Indeed, I wish I was able to credibly threaten to shoot dead the next person who says "but all of this is mathematically compatible with GR". > But we don't need to get into that right now, we > can have a meaningful discussion simply within the framework of SR. Indeed. > Second of all, replace the term "reference frame" with "point of > view"--meaning your viewpoint of the velocities and distances to > objects around you. You're essentially saying that an observer does > not undergo a change of "point of view" when accelerating. Yes, I understand the concept of "point of view". Surely that was proven elsewhere in my rebuttal to Paul who suggested that there were no two reference frames in which two events can occur simultaneously. > Third of all, a single reference frame DOES encompass all objects in > the universe. From your reference frame, you can assign 6 coordinates > to every object in the universe--3 for position and 3 for velocity. Indeed. > This is how every object is encompassed by a single reference frame. > Every object has coordinates within every reference frame. It is > simply a matter of different reference frames assigning different > coordinates to each object. But all frames are equally valid. Indeed, because "a change of reference frame" doesn't involve anything physical, and so you can use whatever terms you like. After all, I can point to my computer keyboard and say "keyboard", or I can point to it and say "hammer", but it doesn't change physical reality.
From: Ste on 4 Feb 2010 22:12 On 5 Feb, 00:27, artful <artful...(a)hotmail.com> wrote: > On Feb 5, 11:16 am, kenseto <kens...(a)erinet.com> wrote: > > > > > > > On Feb 4, 4:11 pm, artful <artful...(a)hotmail.com> wrote: > > > > On Feb 5, 2:57 am, Tom Roberts <tjrob...(a)sbcglobal.net> wrote: > > > [snip] > > > > > I think "fictitious" covers it well. > > > > Yeup .. that's the word that had eluded me :) Thanks. > > > > > 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. > > > > I think it better to say length contraction can be *modeled* by a > > > geometrical projection. Pure geometry is a purely abstract notion ... > > > and length contraction is more than just that because, as you say, it > > > has physical consequences. Of course, one can then argue that if the > > > consequences of length contraction (including measurement of it by > > > whether things fit in barns) are physical, then doesn't that mean the > > > length contraction is physical in the same sense. ie Can something > > > non-physical have a physical consequent? Surely the geometry is > > > modelling something physical .. that's pretty much the point of > > > physics.. in which case is it misleading to call it *purely* > > > geometrical? > > > > Further .. I think perhaps the confusion would be relieved by changing > > > your working somewhat: > > > > Physical reality is modeled by the geometry of 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). "length contraction" is one such physical consequent > > > Fitting a ladder through a narrow door way is not the same as fitting > > a long pole into a shorter barn with both doors close simultaneously. > > That you cannot see the analgoy just shows you have NO idea what SR > says > > Both have a longer length object fitting within a shorter spae > > Both have a geometrical rotation and projection > > The ladder fits thru the doorway just as 'physically' as a pole fits > in the barn. How on Earth do you work that one out? For the analogy to work, the ladder must be constrained lengthwise, not widthwise.
From: mpalenik on 4 Feb 2010 22:19 On Feb 4, 10:12 pm, Ste <ste_ro...(a)hotmail.com> wrote: > On 5 Feb, 00:27, artful <artful...(a)hotmail.com> wrote: > > > That you cannot see the analgoy just shows you have NO idea what SR > > says > > > Both have a longer length object fitting within a shorter spae > > > Both have a geometrical rotation and projection > > > The ladder fits thru the doorway just as 'physically' as a pole fits > > in the barn. > > How on Earth do you work that one out? > > For the analogy to work, the ladder must be constrained lengthwise, > not widthwise.- Hide quoted text - > > - Show quoted text - This is what we've been trying to get across to you. It has to do with coordinates and what one observer calls time and space relative to another. The ladder is a little bit too long to fit into the barn but you could rotate it and still fit it into the barn. Say, for example, you lift the front end so the ladder is now at a 45 degree angle. You could now fit it into the barn. You've rotated part of it into "height", from "length". But there's another way you could rotate it, as well. You can rotate it into "time". If you run with the ladder, you're "rotating" the front of it a little bit into the future and the back of it a little bit into the past. What a moving observer percieves as "time" is the direction of his motion through 4 dimensional spacetime. What he percieves as "space" is the 3 dimensional volume perpendicular to that direction. Thus, "space" in the moving frame is not the same as "space" in the rest frame. It extends part way into the future along one direction and part way into the past along the other (in the rest frame).
From: mpalenik on 4 Feb 2010 22:26 On Feb 4, 10:10 pm, Ste <ste_ro...(a)hotmail.com> wrote: > > Indeed, because "a change of reference frame" doesn't involve anything > physical, and so you can use whatever terms you like. After all, I can > point to my computer keyboard and say "keyboard", or I can point to it > and say "hammer", but it doesn't change physical reality.- Hide quoted text - > You can put an observer into a reference frame and from that frame, the observer has natural ways of measuring space and time. He can measure the entire universe from his reference frame but those measurements are only valid within his system of space and time coordinates. However, there are invariant quantities that you can build out of these measurements that will not change no matter what frame you make your measurements from. You can measure the entire universe from your own reference frame. But your measurements of the universe will change depending on the frame you're in.
From: Ste on 4 Feb 2010 22:57
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". |