Is Length Contraction in SR physical??
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From: Ste on 3 Feb 2010 19:40 On 3 Feb, 21:46, PD <thedraperfam...(a)gmail.com> wrote: > On Feb 2, 8:30 pm, Ste <ste_ro...(a)hotmail.com> wrote: > > > > > > Get your PhD and work the problem for a while and maybe _you_ will be the > > > one to crack quantum gravity and collect the Nobel and come up with answers > > > to questions such as these. > > > Lol. I prefer to leave the mathematical drudgery to someone else. > > I see. So you see your role as being the "big picture thinker", and > the labor of the math and theoretical development should be left to > the equivalent of "physics technicians"? > > Who in physics do you think you'd be emulating by that position? It's because I really only want to understand for personal curiosity - I don't so see myself as a "big thinker" in physics. What I have been clear about before is that I value a "physical" explanation over a mathematical one.
From: Ste on 3 Feb 2010 19:45 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.
From: Ste on 3 Feb 2010 20:04 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. > Moreover it is now very well known that SR does not even need this > to be derived. It has been said to you, you could have checked this, > you didn't, you just refused to believe it. Fine, it's your faith. > > > - 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?
From: PD on 3 Feb 2010 20:12 On Feb 3, 3:58 pm, kenseto <kens...(a)erinet.com> wrote: > On Feb 3, 3:36 pm, PD <thedraperfam...(a)gmail.com> wrote: > > > > > On Feb 3, 12:19 pm, kenseto <kens...(a)erinet.com> wrote: > > > > On Feb 3, 12:11 pm, PD <thedraperfam...(a)gmail.com> wrote: > > > > > On Feb 3, 10:04 am, kenseto <kens...(a)erinet.com> wrote: > > > > > > On Feb 3, 4:41 am, artful <artful...(a)hotmail.com> wrote: > > > > > > > On Feb 3, 4:18 pm, Tom Roberts <tjroberts...(a)sbcglobal.net> wrote: > > > > > > > > Uncle Ben wrote: > > > > > > > > On Feb 1, 11:36 pm, Tom Roberts <tjroberts...(a)sbcglobal.net> wrote: > > > > > > > >> [...] > > > > > > > > > Tom Robrts takes the conservative position on what is "physical." > > > > > > > > Hmmm. I tried not to make any statement about what is or is not "physical", > > > > > > > because that word is too ambiguous. > > > > > > > > To me it is irrelevant whether one considers this or that quantity to be > > > > > > > "physical". What is important is whether or not a given quantity can be an > > > > > > > appropriate model for some physical phenomenon. For that, it's QUITE CLEAR that > > > > > > > no coordinate-dependent quantity can be a valid model of any physical > > > > > > > phenomenon, as arbitrary human choices cannot possibly affect physical > > > > > > > phenomena. Nor can the perspective from which one looks at an object affect the > > > > > > > object itself. Coordinates are, of course, arbitrary human choices that define > > > > > > > the perspective one uses to look at and describe objects and situations. > > > > > > > > > To > > > > > > > > be consistent, he would have to deny physicality to kinetic energy and > > > > > > > > to the magnetic field of a moving charge. Or even motion itself. > > > > > > > > I deny that any of those can be valid models for physical phenomena. I make no > > > > > > > statement about their "physicality" -- arguments over word meanings are > > > > > > > uninteresting (but inappropriate word meanings must be dealt with before the > > > > > > > real discussion can even begin). > > > > > > > > In every case I know of, if you analyze the physical situation sufficiently > > > > > > > well, you will find an appropriate quantity that is a valid model for the > > > > > > > physical phenomena in question. For instance, when considering a collision > > > > > > > between two particles, don't use kinetic energy, use the Mandelstamm s (total > > > > > > > energy squared in their center-of-momentum frame); instead of magnetic field, > > > > > > > use the Maxwell 2-form; instead of motion, use the particles' individual > > > > > > > trajectories. Tensor and geometric analysis provide methods to analyze all > > > > > > > situations of interest in a coordinate-free manner. This is one of the major > > > > > > > lessons of GR (but it took about a half-century to sink in). > > > > > > > > > Or do > > > > > > > > I not understand? > > > > > > > > The issue is more subtle than you seem to think. It is not merely about the > > > > > > > meanings of words, or about what is or is not "physical", it is about what types > > > > > > > of quantities can be used to model physical phenomena. > > > > > > > > Tom Roberts > > > > > > > At first reading you seem to be equating 'physical' with 'frame > > > > > > invariant'. ie. Only things that are not dependent on the observer > > > > > > are physical. > > > > > > > But am I right in my assessment that you are really saying that it is > > > > > > only nature / reality itself that is physical. The measurements and > > > > > > calculations we make are parts of our models of reality .. and so are > > > > > > never really 'physical' themselves. The best models (and > > > > > > measurements) for reality are those that are not observer dependent, > > > > > > because physical reality is not observer dependent (ignoring some > > > > > > interpretations of QM :)). > > > > > > > In the case of length contraction, what we define as length (roughly > > > > > > speaking: the spatial distance between two simultaneous events in a > > > > > > given time) is contracted .. even though the proper interval is > > > > > > invariant. In both cases they are valid (but different) measurements > > > > > > of the same pair of events. > > > > > > > Ken's claim that contraction not being 'physical' means a pole doesn't > > > > > > physically fit between the barn doors at the same time in the barn > > > > > > frame of reference (in the well-known 'paradox'). I guess the issue > > > > > > there is really whether 'between the barn doors at the same time in > > > > > > the barn frame of reference' itself is physical .. as it is observer / > > > > > > frame dependent. > > > > > > 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. > > > Ken, look again. It is stated explicitly in the pole and barn paradox > > that in the pole frame, the pole is LONGER than the barn. This means > > the physical shortening of the rod obviously is not frame-independent. > > If it required the rod to be physically shorter to all observers, then > > it would be claimed to be shorter than the barn in the pole frame, > > too. Since this is not claimed, then it is not required to be observer > > independent. > > You get a clue....in the barn frame you claimed that the doors are > closed simultaneously while the pole is completely inside the barn. In the barn frame, yes. Only in the barn frame. > This means that the pole is physically shortened and physically > shortened pole is not observer dependent. No it certainly does not mean that. Because if it did mean that, then the pole would also have to be shorter than the barn in the pole frame. But this is explicitly said to be NOT the case in the barn and pole puzzle. In the pole frame, the pole is LONGER than the barn. So the shortening cannot possibly observer independent. > That's why your claim of > physical length contraction is bogus. > Physicists with clues (not you apparently)invented the alternate > explanation that length contraction is a geometric effect. This > explanation avoids the bogus assertion that the pole is physically > contracted and BTW geometric porjection is observer dependent. > > Ken Seto > > > Here's a quarter. Buy a clue. > > > > > > Ken Seto > > > > > > > Obviously there is *some* 'physical' relationship between the barn > > > > > > doors and pole that means an observer in that frame would measure the > > > > > > pole as being within the barn. How would you best describe that > > > > > > relationship? Is talking about 'between the barn doors at the same > > > > > > time in the barn frame of reference' something valid and meaningful to > > > > > > say?- Hide quoted text - > > > > > > > - Show quoted text -- Hide quoted text - > > > > > - Show quoted text -- Hide quoted text - > > > > > - Show quoted text -- Hide quoted text - > > > - Show quoted text -- Hide quoted text - > > > - Show quoted text -
From: YBM on 3 Feb 2010 20:23
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? >> Moreover it is now very well known that SR does not even need this >> to be derived. It has been said to you, you could have checked this, >> you didn't, you just refused to believe it. Fine, it's your faith. >> >>> - 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). But for a third moving observer, moving along the y axis, flashes won't be simultaneous. 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. |