Is Length Contraction in SR physical??
in [Relativity]
|
Prev: Simultaneous events and Einstein's absolute time
Next: New Theory --- The Theory of Quantum Wave Sources
From: "Juan R." González-Álvarez on 4 Feb 2010 07:17 jem wrote on Wed, 03 Feb 2010 08:35:53 -0500: > Tom Roberts wrote: >> PD wrote: >>> I >>> find frame-dependent quantities like kinetic energy and magnetic field >>> to be arguably physical, even if frame-dependent and coordinate- >>> dependent. >> >> And I argue that you are mistaken -- you confuse such >> coordinate-dependent quantities with the more fundamental, underlying >> quantities that are invariants: >> kinetic energy => s >> magnetic field => Maxwell 2-form >> ... etc. >> >> > Who's confused? You insultingly tell an expert (PD) that only invariant > quantities can be considered physical, then turn around and concede to a > kook (Ste) that it doesn't matter to Physics what's considered physical. The problem is not that Tom is seriously confused about all those points, but he is ignoring the USUAL (standarized) definitions used in both science and engineering. I would recomend everyone a try to http://physics.nist.gov/cuu/Units/introduction.html specially the DEFINITION of *PHYSICAL QUANTITY*, the definition of *value of a physical quantity*, and the example of the height of the Washington Monument (h_W = 169 m) as physical quantity. It is more; IUPAC, IUPAP, and other scientific bodies even give recomendations about the symbols that would be used to denote the physical quantities that Tom dislike. For instance the recommended symbols for kinetic energy are E_k, K, and T. Goldstein, in his book in mechanics give the expression for the kinetic energy in special relativity. Goldstein uses the symbol T for it... -- http://www.canonicalscience.org/ BLOG: http://www.canonicalscience.org/en/publicationzone/canonicalsciencetoday/canonicalsciencetoday.html
From: "Juan R." González-Álvarez on 4 Feb 2010 07:25 Tom Roberts wrote on Wed, 03 Feb 2010 10:50:34 -0600: > jem wrote: >> Tom Roberts wrote: >>> PD wrote: >>>> I >>>> find frame-dependent quantities like kinetic energy and magnetic >>>> field to be arguably physical, even if frame-dependent and >>>> coordinate- dependent. >>> >>> And I argue that you are mistaken -- you confuse such >>> coordinate-dependent quantities with the more fundamental, underlying >>> quantities that are invariants: >>> kinetic energy => s >>> magnetic field => Maxwell 2-form >>> ... etc. >> >> Who's confused? You insultingly tell an expert (PD) that only >> invariant quantities can be considered physical, then turn around and >> concede to a kook (Ste) that it doesn't matter to Physics what's >> considered physical. > > It all depends on what you are trying to do. > > If you just want to apply labels like "physical", then how you do that > is up to you -- no method of applying labels has any significance > (except in the usage of such labels). > > But if you want to construct models of natural phenomena, in a process > we call science, then the choices of quantities used to form the model > are important. Some quantities, such as coordinate-dependent ones, > simply cannot be used in a valid model because they have aspects that > are inconsistent with the world we inhabit. > > >> Would you agree that a mathematical theory is not a physical theory, >> and that a scientific theory is a physical theory? If so, can you >> describe the essential difference(s) between the two? Does your >> description mention invariance? > > Science in general, and physics in particular, is the process of > constructing models of the world we inhabit, and testing those models > experimentally. With the improved understanding we have gained over the > past few centuries, all models of modern physics are necessarily > mathematical. That is, they present relationships in mathematical > formulas, and prescribe a correspondence between symbols in those > formulas and phenomena (or measurements) in the real world. Any model > that uses coordinate-dependent quantities in its equations [#] will not > correspond accurately to phenomena, because the arbitrary human choices > involved in constructing coordinates do not affect the phenomena being > modeled. So the quantities involved in valid models will all be > invariant under coordinate changes. > > [#] unless they happen to be coordinate-dependent representations of > coordinate-INdependent quantities; replace the former by the latter. > > > Tom Roberts Either you live in your own universe totally disconected from what physicists do each day or you are deliberately lying, just how when you acccused PD from confusing 3 and 4-quantities... -- http://www.canonicalscience.org/ BLOG: http://www.canonicalscience.org/en/publicationzone/canonicalsciencetoday/canonicalsciencetoday.html
From: mpalenik on 4 Feb 2010 07:53 On Feb 3, 10:32 pm, Ste <ste_ro...(a)hotmail.com> wrote: > On 4 Feb, 02:26, PD <thedraperfam...(a)gmail.com> wrote: > > > > > > > > > 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? > > If it is presumed that the two events are an equal amount of distance > away from the observer, and that the information propagates along that > distance at an equal speed, then yes I would say simultaneity can be > determined this way in the absence of gravity considerations. What I > would make the point though that I recognise a distinction between > "proper distance" (i.e. that of a straight line) and the extended path > that real light might need to take in the presence of a gravity field. > You don't even need to assume that the two events are an equal distance away or that information is "propagates at an equal speed" from both of them. If you know the distance to the two objects and you know the speed at which the information is propagating, you can calculate when the two events occured. Relativity does not have a problem with this. Where we *do* have a problem with is when one or more of those objects is moving at a different speed than the others. In this case, what appears simultaneous in one of the frames will not be in any of the others, even *after* you correctly compensate for the distances between objects and speed of information propagation. Also, you've misdefined "proper distance". Proper distance is the total path length in 4 dimensional space. This length is computed using the metric tensor, which also defines the gravitational field. The proper distance that an object travels in a gravitational field is the total 4-dimensional distance through spacetime that it has moved, not the distance that it would have moved through flat space. However, there's no reason whatsoever to bring gravity into this discussion.
From: jem on 4 Feb 2010 08:55 Tom Roberts wrote: > jem wrote: >> Tom Roberts wrote: >>> PD wrote: >>>> I >>>> find frame-dependent quantities like kinetic energy and magnetic field >>>> to be arguably physical, even if frame-dependent and coordinate- >>>> dependent. >>> >>> And I argue that you are mistaken -- you confuse such >>> coordinate-dependent quantities with the more fundamental, underlying >>> quantities that are invariants: >>> kinetic energy => s >>> magnetic field => Maxwell 2-form >>> ... etc. >> >> Who's confused? You insultingly tell an expert (PD) that only >> invariant quantities can be considered physical, then turn around and >> concede to a kook (Ste) that it doesn't matter to Physics what's >> considered physical. > > It all depends on what you are trying to do. > > If you just want to apply labels like "physical", then how you do that > is up to you -- no method of applying labels has any significance > (except in the usage of such labels). When well-defined, such labels can enhance communication. Unless well-defined, such labels will hinder communication. > > But if you want to construct models of natural phenomena, in a process > we call science, then the choices of quantities used to form the model > are important. Some quantities, such as coordinate-dependent ones, > simply cannot be used in a valid model because they have aspects that > are inconsistent with the world we inhabit. If a coordinate-dependent quantity that represents a measurement (e.g., length in SR), is inconsistent with the world we inhabit, it calls the underlying theory into question just as surely as would a coordinate-independent inconsistency. > >> Would you agree that a mathematical theory is not a physical theory, >> and that a scientific theory is a physical theory? If so, can you >> describe the essential difference(s) between the two? Does your >> description mention invariance? > > Science in general, and physics in particular, is the process of > constructing models of the world we inhabit, and testing those models > experimentally. With the improved understanding we have gained over the > past few centuries, all models of modern physics are necessarily > mathematical. That is, they present relationships in mathematical > formulas, and prescribe a correspondence between symbols in those > formulas and phenomena (or measurements) in the real world. Right. The essential difference between non-physical (i.e. mathematical) and physical (i.e. scientific) theories is a prescribed correspondence between the symbols of the former and the measurements of the latter - which suggests that in science related contexts, "physical" should equate to (real-world) "measurable". Any model > that uses coordinate-dependent quantities in its equations [#] will not > correspond accurately to phenomena, because the arbitrary human choices > involved in constructing coordinates do not affect the phenomena being > modeled. This addendum seems to be a flat-out non-sequitur. SR uses coordinate-dependent quantities in its equations - where's the problem? So the quantities involved in valid models will all be > invariant under coordinate changes. > > [#] unless they happen to be coordinate-dependent representations > of coordinate-INdependent quantities; replace the former by the > latter. >
From: jem on 4 Feb 2010 09:36
> On Feb 3, 1:55 am, Ste <ste_ro...(a)hotmail.com> wrote: >> The fact is, I'm able to conduct a reasonable debate with experts >> because, for the past month, I've gone to bed thinking about physics, >> and I've woken up thinking about physics, and I've spent most spare >> hours thinking or reading about physics. Actually, Ste, you've spent the month arrogantly parading your ignorance in this newsgroup, and egotistically misconstruing PD's kid-glove attempts to point that out to you, as "debate". You, pal, are loony-tunes delusional - you're gonna fit right in at SPR. |