Is Length Contraction in SR physical??
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From: PD on 3 Feb 2010 21:30 On Feb 3, 6:38 pm, Ste <ste_ro...(a)hotmail.com> wrote: > On 3 Feb, 21:44, PD <thedraperfam...(a)gmail.com> wrote: > > > > > On Feb 2, 7:13 pm, Ste <ste_ro...(a)hotmail.com> wrote: > > > > > Then let's start with the basics. > > > > Let's suppose there is, as you claim, an observer-independent value of > > > > the speed of your coffee cup right now. > > > > For a different observer, that coffee cup will be measured to have a > > > > value of velocity that is different than the observer-independent > > > > value. > > > > What is happening in the physical system of the coffee cup and that > > > > observer to yield a different measured value of velocity? > > > > Nothing (i.e. nothing of relevance here) is physically happening to > > > the coffee cup. The difference is in the circumstances of the > > > observer. > > > The measured velocity OF THE coffee cup is observer-dependent. So what > > is physically happening in the system containing the coffee cup and > > the observer to alter the real value of the velocity to the measured > > velocity? Answer precisely, please. > > I'm not quite sure what you mean. Clearly, the measured velocity can > change if *either* the cup or the observer changes velocity. You can > infer which (i.e. one or both) has changed velocity by the effects of > inertia (the coffee with slop over the edge, for example, if it is the > cup that physically changes velocity). > > > I'm choosing as simple a case as possible, so that you can explain > > what is going on physically simply and in a few sentences. To use your > > own challenge, if you cannot do this, then perhaps your understanding > > of this very simple situation is not as good as you think. > > Indeed. > > > However, I'm willing to even relax the "in a few sentences" criterion > > that you have insisted on. Explain what is going on physically, > > elaborately and using as much detail as necessary. > > You know, clarification questions don't count. ;) I'm going to put this discussion aside while we work on relativity of simultaneity.
From: YBM on 3 Feb 2010 21:35 Ste a �crit : > On 4 Feb, 01:23, YBM <ybm...(a)nooos.fr.invalid> wrote: >> Ste a �crit : >> >> >> >> >> >>> On 3 Feb, 23:56, YBM <ybm...(a)nooos.fr.invalid> wrote: >>>> Ste a �crit : >>>>> 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! Of course not, even cranks here understand that you have to take frames which are significantly different. >> 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." Come on Ste, don't be stupid. Next time you'll choose a frame of reference which is different only by the origin and will play the same silly game.
From: Ste on 3 Feb 2010 22:23 On 4 Feb, 02:12, artful <artful...(a)hotmail.com> wrote: > On Feb 4, 11:45 am, Ste <ste_ro...(a)hotmail.com> wrote: > > > On 3 Feb, 23:23, artful <artful...(a)hotmail.com> wrote: > > > > 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. It's not that I don't like maths, it's that I think some people understand *only* the maths which they have learned carefully by rote, and they don't understand how it relates to the real world (if indeeed the current maths does relate to the real world).
From: Ste on 3 Feb 2010 22:32 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. > Note here that we have accounted for propagation delay here, and the > conclusions are not masked by that. > > OK so far? Yep.
From: Ste on 3 Feb 2010 22:53
On 4 Feb, 02:35, YBM <ybm...(a)nooos.fr.invalid> wrote: > Ste a écrit : > > > > > > > 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! > > Of course not, even cranks here understand that you have to take > frames which are significantly different. > > >> 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." > > Come on Ste, don't be stupid. Next time you'll choose a frame of > reference which is different only by the origin and will play the > same silly game. How was it a silly game? I was told simultaneity can never apply to more than one frame, and I've just given you two frames where that is clearly untrue. I suppose if anything, this proves that I *do* understand. |