Is Length Contraction in SR physical??
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From: Tom Roberts on 3 Feb 2010 20:52 PD wrote: > On Feb 2, 11:34 pm, Tom Roberts <tjroberts...(a)sbcglobal.net> wrote: >> Hmmm. For a given object, its proper length can be measured between any pair of >> events at its endpoints that happen at any times, equal or different, as long as >> the measurements are made in its rest frame [#]. > > And this is precisely my quibble. Here you are saying, in essence, > there is a spatial property we'll call a length that is frame- > invariant, as long as you measure it in one particular frame. :>) But the object most definitely has an underlying physical property related to what we call "length". "Length" is part of the model, but the real object has a physical property that we model as a front and a back located in different places with a definite separation. "Proper length" is the appropriate quantity in the model to relate to that physical property. "Proper length" is not the same as "length", because a) it is intrinsic to the object, and b) it is invariant. The fact that it is intrinsic to the object is what makes it useful. This is just one more instance of rather different properties sharing the same English word. Caveat lector. Tom Roberts
From: Ste on 3 Feb 2010 21:01 On 4 Feb, 01:23, YBM <ybm...(a)nooos.fr.invalid> wrote: > Ste a écrit : > > > > > > > On 3 Feb, 23:56, YBM <ybm...(a)nooos.fr.invalid> wrote: > >> Ste a écrit : > > >>> However I am interested to hear how a finite speed of light means > >>> events that are observed as simultaneous in one reference frame cannot > >>> be observed as simultaneous in another > >> This is NOT the case. This is not especially because of *finite* > >> speed of light that simultaneity is relative. Like most cranks of > >> this group you seem to think that SR is just about visual effects > >> when it is quite the opposite. Precisely in the classical derivation > >> of SR this is the *invariance* of light speed NOT its *finiteness* > >> which implies relativity of simultaneity. > > > Indeed, but I already understood that. I wish we'd get on instead of > > discussing the detail, and if I don't understand I'll tell you, > > instead of you telling me. > > If you understand this, why did you write clear false statements about > it? Because I didn't capture all of my understanding in the few words that I chose - my discussions with Paul about sonic booms clearly reveals that I understood the importance of the notion of the invariance of the speed as well as the finiteness. The point is that there is a medium, space, in which (for the purposes of this argument anyway) light always travels at the same speed in all directions. > >>> - because after a few minutes > >>> of thinking, I can definitely conceive of a situation where events do > >>> remain simultaneous in more than one reference frame. > >> It won't work, trying to figure out why *by yourself* would be a > >> very good way to study SR. > > > Well let us suppose that you have two flashing light beacons on an > > axis 'y', and let us suppose that you have an axis 'x' which runs > > through axis 'y' at a point which is equidistant from each beacon and > > which is perpendicular to axis 'y'. Let us suppose that two observers > > (A and B) then set off away from the beacons along axis 'x' at two > > different speeds (and therefore two different frames of reference are > > in play). Are you telling me that the beacons only appear to flash > > simultaneously for *either* A or B, but not both? > > You missed the point. In this specific case beacons would flash > simultaneously for both observer (please don't use "appear" as > it emphasize the usual confusion between SR and visual effects). I will tend to use "appear" any time that I'm referring to what is observed. > But for a third moving observer, moving along the y axis, flashes > won't be simultaneous. So in other words, it was *utter nonsense* to say that simultaneity can never exist in more than one reference frame! > You have a problem with logic: saying that for two specific frames > some kind of events would be simultaneous doesn't mean that it > will be so for all frames. I *never said* it will be for all frames, and in fact I'm intuitively at home with the concept - that's why it took me just a few moments of geometric manipulation in my head to think of a counterexample. Indeed it was *Paul* who argued that it will never be for *any* frames. I quote him: "two events that are simultaneous in one frame CANNOT be simultaneous in another frame, according to the laws of physics."
From: artful on 3 Feb 2010 21:12 On Feb 4, 11:45 am, Ste <ste_ro...(a)hotmail.com> wrote: > On 3 Feb, 23:23, artful <artful...(a)hotmail.com> wrote: > > > On Feb 4, 10:10 am, Ste <ste_ro...(a)hotmail.com> wrote: > > > > That said, if you're satisfied that you understand something well and > > > cannot possibly explain something simpler, then sometimes you've just > > > got to accept the differences. But what I would say is that discussing > > > something and being forced to explain normally hones one's own > > > understanding as well as communication skills (and it also shares > > > knowledge and understanding for the general good), and I often think > > > that part of the problem here is that, actually, people have merely > > > learned a great deal by rote and actually don't understand at all, and > > > know that discussing the subject might reveal those deficiencies. > > > Many more don't understand *at all* but think that they do, so they > > continue to try to discuss the subject without realising that doing so > > reveals those deficiencies. > > Which is why it helps not to be pretentious about one's own > understanding - I'm certainly not, I only understand as much as I > claim to understand, and I freely admit that I don't understand > everything, and that there are a number of questions that I cannot yet > make a judgment on. Note that I wasn't referring to you. I do respect that you admit you don't know everything. Though your antagonism toward any physics that is expressed mathematically is not admirable.
From: PD on 3 Feb 2010 21:26 On Feb 3, 5:10 pm, Ste <ste_ro...(a)hotmail.com> wrote: > On 3 Feb, 20:49, PD <thedraperfam...(a)gmail.com> wrote: > > > > > On Feb 3, 2:11 pm, Ste <ste_ro...(a)hotmail.com> wrote: > > > > On 3 Feb, 18:19, kenseto <kens...(a)erinet.com> wrote: > > > > > On Feb 3, 12:11 pm, PD <thedraperfam...(a)gmail.com> wrote: > > > > > > > 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. > > > > Presumably the discussion can move forward by discussing how you each > > > differ in your understandings. > > > Not really. The pole and barn puzzle is documented in black and white. > > Some people cannot comprehend what they read in a coherent manner. If > > 98% of the people who read the documented puzzle and understand it to > > mean one thing, and the remaining 2% who read the documented puzzle > > understand it to mean a variety of other things, can a discussion > > between a member of the majority and a member of the minority move > > substantially forward without resolving the difference? > > I think you overestimate the coherence of the explanations that are > given out there, and the adherents of various explanations tend to be > quite hard line about it and simply disparaging of any alternative. > > That said, if you're satisfied that you understand something well and > cannot possibly explain something simpler, then sometimes you've just > got to accept the differences. But what I would say is that discussing > something and being forced to explain normally hones one's own > understanding as well as communication skills (and it also shares > knowledge and understanding for the general good), and I often think > that part of the problem here is that, actually, people have merely > learned a great deal by rote and actually don't understand at all, and > know that discussing the subject might reveal those deficiencies. 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? Note here that we have accounted for propagation delay here, and the conclusions are not masked by that. OK so far?
From: PD on 3 Feb 2010 21:29
On Feb 3, 6:05 pm, Ste <ste_ro...(a)hotmail.com> wrote: > On 3 Feb, 21:21, PD <thedraperfam...(a)gmail.com> wrote: > > > On Feb 2, 5:09 pm, Ste <ste_ro...(a)hotmail.com> wrote: > > > > You shouldn't need *any* maths to describe in broad terms what this > > > "length contraction" is all about, and in any event anyone who > > > understands their subject should be able to sum it up in a few > > > sentences. It should not be this complex Paul, and the fact that it is > > > simply demonstrates that science has lapsed into obscuritanism. > > > I'm sorry, but I find this whole expectation that "everything should > > be so simple that it can be explained to anyone in a few sentences" to > > be just crapola. It isn't true in chemistry, it isn't true in > > molecular biology, it isn't true in architecture, it isn't true in > > auto mechanics, it isn't true in musical composition. > > I agree it isn't true that the entire subject should be be able to be > summed up in a few sentences. What I'm saying is that it's most > rundimentary concepts should be sum-uppable in a few sentences. This is ALSO not a realistic expectation. I'm sorry, it just isn't. > If I > were describing the essentials of an internal combustion engine, for > example, I might draw a simple diagram of a bore and a piston, and > point to where the explosion happens. If further questions are asked, > I can pad this out with further detail, without any overhaul of the > fundamental concept that I started with. > > And the same is true of chemistry - assuming someone understands > magnetism, then I can explain ionic bonding in terms of two "magnetic" > atoms. Oh, good heavens. I hope this is not what you took away from your chemistry class! > And then I can move onto biology, and explain the heart in > terms of a simple expansion and contraction of a cavity - and then > briefly explain how a heart valve works so that the expansion and > contraction actually pumps blood in one direction. > > > Furthermore, the attitude that "Well, if you CAN'T explain it in a > > satisfactory way to anyone who asks in just a few sentences, then you > > are guilty of obscuritanism," is similarly crapola. > > No that's not what I'm saying. What I'm saying is that you shouldn't > need a degree in maths and another in physics to broadly answer "does > a ladder moving at speed fit inside a garage". Nor do you. The books I've recommended are suited to those with a high school education. It just can't be done in a few sentences. > > > Not every body of knowledge should be expected to be grasped by anyone > > in just a few sentences. There is a reason why people spend years to > > become reasonably expert in an area, and not just a month. > > I agree. As I say, I'm not saying there is no need for expertise, but > the value of expertise is in accuracy, nuance, etc. > > > > > No, sir. You are not following. There is no mechanical shrinking of > > > > the clock that is going on. > > > > If there is no "mechanical shrinking", then I interpret this to mean > > > the contraction of lengths is *apparent*, not "real". > > > Then again you are using your own private understandings of the term > > "apparent" and "real" and not those shared by physicists. I don't know > > why you would expect physicists to explain things using YOUR > > understandings of these terms. > > I'm not expecting physicists to explain things using my terms. I'm > expecting them to work with me while I translate their terms into my > own. > > > > I'll let you do the maths Paul - that is, after all, the virtue of a > > > *co-operative endeavour*, where different people bring different > > > skills to the table. > > > I'm sorry but that's crapola too. I refer you to an anecdote, archived > > from this newsgroup by a kindly soul:http://users.telenet.be/vdmoortel/dirk/Physics/Gems/WhatItTakes.html > > Indeed, but unlike that chap I haven't yet waltzed into a physics > department claiming to have all the answers. What is more, I know my > strengths and I see no reason not to play to them - one does not > expect a musician to be able to craft an instrument, a dancer to > construct a stage, or a cartoonist to understand the paper-making > process. |