Is Length Contraction in SR physical??
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From: Tom Roberts on 5 Feb 2010 00:10 Juan R. González-Álvarez wrote: > Tom Roberts wrote on Wed, 03 Feb 2010 19:52:10 -0600: >> "Proper length" [...] is >> intrinsic to the object, > > It is not. Yes, it is. If you think not, give an argument or counterexample. >> and b) it is invariant. > > Only under certain approximations (i.e. under certain *specific* > class of transformations). No approximation is necessary in SR or GR. The context of this thread is relativity, not some other theory that only you know about. Tom Roberts
From: Tom Roberts on 5 Feb 2010 00:11 mpalenik wrote: > Yes, proper length is only invariant under transforms in the Lorentz > group. Not true. In GR it is invariant under any coordinate transform. Tom Roberts
From: Bruce Richmond on 5 Feb 2010 00:11 On Feb 4, 9:24 pm, "J. Clarke" <jclarke.use...(a)cox.net> wrote: > artful 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. > > Uh, logic says that either the pole fits in the barn or it does not fit in > the barn. It can't do both in relativity.- Hide quoted text - > > - Show quoted text - But it can do both. It all depends on what time it is at both ends of the rod. RoS changes the timing of the entering and exiting events.
From: Tom Roberts on 5 Feb 2010 00:22 artful wrote: > I think it better to say length contraction can be *modeled* by a > geometrical projection. But "length contraction" is itself an aspect of a MODEL. it makes no sense to model some aspect of a model. In SR, "length contraction" _IS_ geometrical projection. That's what it means. > Pure geometry is a purely abstract notion .. No more abstract than SR or any other physical theory. I have just come to realize more fully that geometry is part of every physical theory. Our theories are mathematical, in that they express relationships as mathematical equations; geometry is merely one aspect of that. > and length contraction is more than just that because, as you say, it > has physical consequences. The orientation of a ladder relative to a doorway also has physical consequences, but is just as "purely geometrical" as is "length contraction". ALL geometrical relationships among physical objects have physical consequences; That is, the aspects of objects and their relationships that we model as geometry are part and parcel of how those objects behave. I don't think we differ very much here.... Tom Roberts
From: Peter Webb on 5 Feb 2010 00:21
"mpalenik" <markpalenik(a)gmail.com> wrote in message news:354b0737-033a-4c12-a8dc-4e17f4b00974(a)d27g2000yqn.googlegroups.com... 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). ______________________________________ An excellent explanation for somebody familiar with the concept of Minkowski 4-space. But if Ste was familiar with the concept of Minkowski 4-space, we wouldn't be having this discussion in the first place. But I'm impressed. |