Is Length Contraction in SR physical??
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From: kenseto on 11 Feb 2010 09:39 On Feb 10, 10:32 pm, "Inertial" <relativ...(a)rest.com> wrote: > "Tom Roberts" <tjroberts...(a)sbcglobal.net> wrote in message > > news:kvSdnQR4g8u56O7W4p2dnAA(a)giganews.com... > > > Nor does the rod get "physically shorter" by any measure we take in any > > other frame, either. Making a measurement of its length does NOT affect > > the length of the rod itself, regardless of how the measurement is made.. > > Just using 'length' is also a linguistic problem (similar to that of using > 'physical'). > > Does it mean 'proper length' / 'intrinsic length' / 'rest length' ... or > does it mean the 'measured length' (ie the distance between the coordinates > of two points at a given time in a given inertial frame) (or is there a > better term for that 'length' that I can't think of atm :)) ??? There is no such thing as measured length of a moving rod. There is a predicted length or geometric projected length of a moving rod. I don't understand why you physicists keep on using the word *measured* instead of *predicted*. Is it to give your SR theory more credence? Ken Seto > > If we refer to its 'length' 'in some inertial frame', then that would seem > to imply one means the 'measured length', because the 'proper' / 'intrinsic' > / 'rest' length does not depend on the frame of reference/ So when we say > 'the length of the rod is shorter in the frame of the barn', that would seem > to imply that length as measured in that frame, and not the rest length. > > Though a tilted ladder doesn't get 'physically shorter' it is also not as > 'tall' (it has a lower 'height'). Can one say it is 'physically' not as > tall? A 6 foot ladder lying on the ground is still a 6 foot ladder, but it > is no longer 6 foot tall. > > It all comes down to the ambiguities of the English language (and I suspect > the same or similar ambiguities in other spoken languages). That being one > of the reasons why relationships and statements in physics are often made > using the less ambiguous language of mathematics. > > Now. . the question is .. does Ken understand the linguistic issues here .... > and is he of the opinion that the measured length of a rod (((ie the > distance between its endpoints at a given time in a given frame of > reference))) is predicted to be shorter in a frame in which it is is motion > in a direction parallel to the line between those endpoints (eg in the pole > and barn paradox). Ie if it was possible to devise an experiment where one > could accurately (enough) measure that length, would that measurement be > shorter than the proper/intrinsic/rest length of the rod?
From: PD on 11 Feb 2010 09:41 On Feb 11, 2:27 am, Ste <ste_ro...(a)hotmail.com> wrote: > On 11 Feb, 07:04, Paul Stowe <theaether...(a)gmail.com> wrote: > > > > > On Feb 10, 11:36 am, Ste <ste_ro...(a)hotmail.com> wrote: > > > {Snip...} > > > > The question therefore remains, how can the speed of propagation > > > possibly be measured to be constant in all frames. > > > The answer to your question is actually simple and 'intuitive' if you > > think > > about what must happen in a medium. The propagation of any type of > > disturbance travels by 'conduction' from one entity to the next. This > > is > > set by the mean speed and spacing. If the medium is 'incompressible' > > the > > entities are all touching (spacing is zero) and the entities > > 'infinitely > > hard' In that case, the speed of propagation is infinite, and no > > delta > > 'pressures' are possible 'within the medium. OTOH, in any > > compressible > > medium there is spacing, and the entities have momentum and energy. > > This > > results a distinctive independent set speed by which any disturbances > > (like wave propagation) will occur. This is designated as c for ANY! > > medium > > > Now it should be obvious that in the case of a medium it is this > > process > > that always dominates... The speed of sources must, by that > > constraint, > > alter there emission/field profiles to conform to this limitation. > > > So now, start with a source of a omni-directional wave generator 'at > > rest' > > with respect to the medium. The resulting waves propagate outward 'at > > c' > > in all directions, resulting in a perfectly spherical field form. > > Next, > > give this source some speed v, obviously c hasn't changed so, in the > > direction of motion the source is displacing forward at v so each > > wave > > front must be separating 'from the source' at c - v. In the > > perpendicular > > (transverse) direction the wave fronts are still separating from the > > source at c. Thus, for the hemisphere in front of the moving source > > the > > wave field form is no longer spherical, but flatten into an > > ellipsoid. > > Now what happens to the back half??? Intuitively you would think > > that > > the wave front would be separating 'from the source' at c + v. > > > However, remember that a wave is an oscillation (a back & forth > > motion) so, > > one cycle is c - v and c + v.. It turn, the cycle must remain in- > > phase > > with the rest of the field (it's one contiguous field). Thus the > > actual > > distance of the wave form must contracted by Sqrt(1 - [v/c]^2) on the > > axis > > of motion. The end result, the field flattens (distorts) into an > > ellipsoid who's radius is defined as a function v multiplied by the > > cosine > > of angle (w) relative to the axis of motion. > > > R(w) = R[Sqrt(1 - [vCos w/c]^2)] > > > Now, look at a situation where we have two interacting fields in > > equilibrium. > > If at rest the centerline distances are, say, x. Then, if both are > > in > > uniform motion each field so x -> x' = xSqrt(1 [v/c]^2). Now, > > consider > > the QM interpretation of matter. If matter consists of waves it makes > > no difference what particular type, all wave must behave this way. > > > Now, let's thake the classic MMX and evaluate it in the context of a > > QM > > wavicle system. The time it take a photon to to complete a complete > > transit is L/c and, because the system is moving, in axis > > perpendicular > > axis L = 2d/Sqrt(1 - [v/c]^2)... Along the axis of motion the matter > > contracts and d -> d' = dSqrt(1 - [v/c]^2). The round trip time is > > L = 2d'/(1 - [v/c]^2) = 2d/Sqrt(1 - [v/c]^2). > > > The result is obvious, the reason is likewise obvious. > > > Last I checked any number divided by itself is unity, thus, > > > Sqrt(1 - [v/c]^2) / Sqrt(1 - [v/c]^2) = 1 > > > making all inertial measurements of wave speed invariant. But!, there > > are real physical consequences. These are, real field distortions, > > and > > a measureable alteration of rate in physical processes. However, > > since > > all physical processes are affected equally it is also clear that the > > actual perception/observations/measurements in moving systems are all > > equally affected, making the perception in all such systems appear > > the > > same. However, it is physically noticable by relative motion. > > > Paul Stowe > > I'm afraid I didn't quite recognise this explanation as "intuitive". Nor should you.
From: PD on 11 Feb 2010 09:55 On Feb 11, 1:34 am, Ste <ste_ro...(a)hotmail.com> wrote: > On 11 Feb, 01:57, PD <thedraperfam...(a)gmail.com> wrote: > > > > > On Feb 10, 8:20 am, PD <thedraperfam...(a)gmail.com> wrote: > > > > On Feb 10, 8:05 am, PD <thedraperfam...(a)gmail.com> wrote: > > > > > On Feb 10, 12:21 am, Ste <ste_ro...(a)hotmail.com> wrote: > > > > > > On 9 Feb, 21:02, PD <thedraperfam...(a)gmail.com> wrote: > > > > > > > On Feb 9, 12:26 pm, Ste <ste_ro...(a)hotmail.com> wrote: > > > > > > > > > Two trains are on adjacent tracks, going in opposite directions, > > > > > > > > though I say that only to deliberately reinforce an ambiguity here. It > > > > > > > > doesn't matter whether the trains are going at different speeds, and > > > > > > > > in fact it isn't even important if one of the trains is stopped, or in > > > > > > > > fact whether they are going in the same direction but one faster than > > > > > > > > the other. All that matters is that there is a relative velocity > > > > > > > > between them. > > > > > > > > > Two lightning strikes occur, drawn to the trains because of the > > > > > > > > friction of the air between the trains. In fact, one lightning strike > > > > > > > > leaves a scorch mark (a 1 cm spot, if you want to be precise) on > > > > > > > > *both* trains as it hits. The other strike leaves a scorch mark > > > > > > > > somewhere else on *both* trains. > > > > > > > > > The question now is, were the strikes simultaneous or not? > > > > > > > > > There is an observer on train A, and an observer on train B, and they > > > > > > > > are both looking out the window when the strikes occur. > > > > > > > > > They make the following observations: > > > > > > > > 1. The observer on train A sees the two lightning flashes > > > > > > > > simultaneously. > > > > > > > > 2. The observer on train B sees the flash from the front of his train > > > > > > > > before he sees the flash from the rear of his train. > > > > > > > > > Now, it is not yet possible to determine whether the strikes were > > > > > > > > simultaneous originally. We have more work to do. But I want to see if > > > > > > > > you have a picture in your head of what has transpired. > > > > > > > > I have a basic picture, yes. > > > > > > > OK, then. > > > > > > Let's now follow up these two observations above and couple them with > > > > > > more observations. > > > > > > 3. After the strikes, the observer on train A runs a tape measure from > > > > > > his location to the scorch mark of one strike and makes note of the > > > > > > number. Then he runs a tape measure from his location to the scorch > > > > > > mark of the other strike and makes note of the number. These numbers > > > > > > are equal. Note the scorch marks are on his train, but that's an > > > > > > undeniable marker of where the event WAS when the signal propagation > > > > > > began. > > > > > > Not really. If his train is moving, then the scorch marks will have > > > > > actually moved from the location of the event. > > > > > With respect to what is the train moving? In this reference frame, the > > > > train is not moving at all, though the other one is. I remind you that > > > > it is not stated, nor is it clear, whether both trains are moving or > > > > only one is. Nor does it matter, because even if the train is moving > > > > relative to the track doesn't guarantee that the train is moving in > > > > any absolute sense. For example, if the track itself were moving (say > > > > because the surface of the earth is moving) and the train is moving in > > > > the opposite direction, one could easily visualize that the train is > > > > not moving at all, even if the train is moving relative to the track. > > > > > This is a crucial point about reference frames. We are making > > > > statements about observations made IN THIS REFERENCE FRAME, and in > > > > this reference frame, the train is not moving, the scorch marks are > > > > not moving, and we can measure the speed of light in this reference > > > > frame. > > > > > > > 4. After the strikes, the observer on train B runs a tape measure from > > > > > > his location to the scorch mark of one strike and makes note of the > > > > > > number. Then he runs a tape measure from his location to the scorch > > > > > > mark of the other strike and makes note of the number. These numbers > > > > > > are equal. Note the scorch marks are on his train, but that's an > > > > > > undeniable marker of where the event WAS when the signal propagation > > > > > > began. > > > > > > I'm not sure I agree with this. > > > > > It is exactly the symmetric situation with train A. Since the strikes > > > > left marks on both trains, there is no reason to rule it out here if > > > > it was permissible on A. > > > > > > > 5. The observer on A runs some experiments to measure the speed of > > > > > > light and the isotropy of the speed of light (that it is the same in > > > > > > either direction), and finds that the signal speed is the same. (Note > > > > > > this isotropy would NOT hold if the signal were sound, for example.) > > > > > > And *how* does he measure this? > > > > > A variety of ways. You could, for example, follow the procedures used > > > > by experimenters as documented in the papers referenced on the first > > > > Google search return on "experimental basis for relativity". > > > > > > > 6. The observer on B runs some experiments to measure the speed of > > > > > > light and the isotropy of the speed of light (that it is the same in > > > > > > either direction), and finds that the signal speed is the same. (Note > > > > > > this isotropy would NOT hold if the signal were sound, for example.) > > > > > > > Given these *observations* 1, 3, and 5, what would the observer on > > > > > > train A conclude about the simultaneity of the original strikes? > > > > > > I must admit I don't have a clear enough picture of what is happening. > > > > > This gedanken seems to presuppose the very thing in question, that is, > > > > > relativity. > > > > > No, it doesn't presuppose anything other than what is *actually > > > > observed* in experiment. I cannot underscore this enough. For example, > > > > the claims that both (5) and (6) are both true may seem > > > > counterintuitive. How can both trains measure the speed of light to be > > > > the same from both directions, if the trains are moving relative to > > > > each other? Certainly an aether-based theory would not hold this is > > > > true. Does this mean we are *assuming* relativity is true so that > > > > these statements are both true? No. Statements (5) and (6) are the > > > > results of *experimental observation*. Nature really does behave this > > > > way, even if we find it counterintuitive. > > > > > > Let's refine it a bit by stipulating that the Earth is stationary, the > > > > > track is stationary, and the clouds are stationary, > > > > > On what basis would you make such an arbitrary stipulation, when you > > > > KNOW that this is not the case? > > > > You may be tempted to say, "Because we have to have an absolute > > > > reference for stationary *someplace*, and we might as well make it > > > > Earth because we live here." A moment's thought will tell you this is > > > > foolish. Physical laws don't care where we live. Then, in the search > > > > for finding an absolute reference for rest, you may eventually ask > > > > yourself why such an absolute reference would be needed at all, > > > > especially if there is nothing you can clearly identify that would fit > > > > the bill... > > > > > > and we'll also > > > > > stipulate that the lightning strike happens in an instant (even though > > > > > it doesn't), and marks all locations at that instant. > > > > > > Now, where are the trains on the tracks when the lightning strikes, > > > > > and are they moving? > > > > > You see? You are trying to establish an absolute reference frame, even > > > > if it means doing so completely arbitrarily, JUST SO you can say > > > > whether the trains are absolutely moving or not. > > > > As a side note, let me just offer the word of encouragement that you > > > are asking all the right questions and wrestling with all the right > > > issues. In other words, this is what students do when they actually > > > learn something. You are on your way to really understanding what > > > relativity is saying, and also on your way to learning how to check > > > whether the claims that are made do in fact match experimental > > > observation. > > > > But as a cautionary note, let me also remind you that we are ONLY > > > trying to put together an understanding of where the frame-dependence > > > of simultaneity comes from, which is only one small stepping stone in > > > the exploration of special relativity, which in turn is itself a small > > > stepping stone in the exploration of general relativity. As you can > > > see, this takes work, and extended thinking, and asking lots of > > > serious questions. This is why many of the basic ideas in physics > > > cannot be explained compellingly in a few sentences to interested and > > > intelligent hobbyists. Physics students would be expected to discuss > > > this in class for about an hour, then think about it and work through > > > issues for about four or five hours outside of class (with other > > > students or with the teacher for some of that), before moving on to > > > the next stepping stone. > > > I hope you're not losing interest in this exercise. I would hope that > > you would find the articulation of where some of the ideas of > > relativity come from to be both valuable and intriguing. > > Are you not finding the instruction helpful? > > I haven't lost interest, but I must admit I still haven't really got a > clear picture of where the trains are or how they are moving. If > indeed it is unspecified whether they are moving relative to the track > or the clouds, then we may as well just do away with tracks and > lightning, and just talk about two trains that are moving relative to > each other, suspended in the air against a blank background. The > involvement of tracks and clouds just complicates the matter. Exactly! And this is a profound insight. The clouds and the track do not provide any reference for "stationary" that has any physical meaning other than a *completely* capricious choice (and in fact one we know is ill-suited), and so worrying about the trains' movement relative to them is indeed an unnecessary distraction. (In fact, in some books designed to teach, the setting is changed to spaceships passing each other to remove this extraneous distraction. But this often leads readers to the conclusion that special relativity only applies to esoteric phenomena and has no relevance in earthly applications, which is also an unintended perception.) And indeed, Einstein wasn't the first to notice that physics doesn't really respect a notion of absolute motion, in the sense that there is some reference against which one can say objects are "truly" moving or "truly" stationary. Galileo and Newton both came to this conclusion in the 1600's, and that is very much central to *classical* physics. What is interesting is that ANY inertial reference frame is equivalent for examining the physics of what is going on in a series of events, and the laws of physics are EXACTLY THE SAME in describing those events, regardless of choice of reference frame. This is true even though some of the statements about the properties of the things involved may be different. For example, in one reference frame, a falling ball might be initially at rest, while in another it is moving horizontally at some speed. Regardless, the same laws of two-dimensional projectile motion describe what happens to that same object from either reference frame. What Einstein ran up against was looking at the (relatively new) laws of electrodynamics, which gave the appearance of implying some kind of absolute speed, and therefore some absolute reference, which had been unheard of in all the classical physics up to that point. So he asked a rather straightforward question: What if this, too, is just a relative velocity like all other velocities in classical physics, so that there isn't any new absolute reference required at all? Novices often get this completely backwards, thinking that classical physics DOES imply some absolute reference frame against which things can be said to be "truly" moving or "truly stationary", and that relativity forced the abandonment of this classical physics assumption. Quite the opposite. Einstein simply suggested that electrodynamics is just as relative as classical physics is, and followed the implications of that statement. PD
From: kenseto on 11 Feb 2010 10:07 On Feb 10, 8:49 pm, PD <thedraperfam...(a)gmail.com> wrote: > On Feb 10, 4:57 pm, kenseto <kens...(a)erinet.com> wrote: > > > > > > > On Feb 10, 9:39 am, PD <thedraperfam...(a)gmail.com> wrote: > > > > On Feb 10, 8:06 am, kenseto <kens...(a)erinet.com> wrote: > > > > > On Feb 9, 5:56 pm, PD <thedraperfam...(a)gmail.com> wrote: > > > > > > On Feb 9, 4:29 pm, kenseto <kens...(a)erinet.com> wrote: > > > > > > > On Feb 9, 11:32 am, PD <thedraperfam...(a)gmail.com> wrote: > > > > > > > > On Feb 8, 2:58 pm, kenseto <kens...(a)erinet.com> wrote: > > > > > > > > > On Feb 8, 3:25 pm, PD <thedraperfam...(a)gmail.com> wrote: > > > > > > > > > > On Feb 7, 8:39 am, kenseto <kens...(a)erinet.com> wrote: > > > > > > > > > > > > When Ken started asking these questions 15 years ago, they were > > > > > > > > > > > reasonable questions. When after a couple of years it was clear he was > > > > > > > > > > > not listening to the answers given to his questions, the tone of the > > > > > > > > > > > responses became a little different. > > > > > > > > > > > Why should I listen to what you said when you did made sense.?? > > > > > > > > > > Ken, it isn't wise to stop listening to people if you do not > > > > > > > > > understand what they are saying. It would be important in that case to > > > > > > > > > say, "I don't understand what you are saying. Can you explain it > > > > > > > > > differently so I can understand it?" > > > > > > > > > I don't listen to you because you keep on making contradictory claims. > > > > > > > > For example the pole is physically contracted to fit the barn and at > > > > > > > > the same time the pole does not fit into the barn because it is not > > > > > > > > physically contracted. > > > > > > > > Those are not contradictory claims. > > > > > > > Yes they are. > > > > > > > >Those are different accounts made > > > > > > > in different reference frames. There is no requirement that the > > > > > > > accounts be identical in different reference frames, and therefore > > > > > > > they are not contradictory. > > > > > > > They have to agree whether the physical length of the pole can fit > > > > > > into the shorter barn. > > > > > > No, they do not. There is no such requirement. > > > > > There is the requirement that if you claim physical contraction then > > > > the pole is really contracted > > > > I have no idea what you mean by "really contracted". It is physically > > > contracted in one frame because it actually fits between the doors > > > when the doors are closed. How can that be called anything other than > > > physical? It does NOT mean *materially* contracted though -- as in > > > squeezing or cooling. > > > Contraction by cooling is "really physically contracted", or > > "materially contracted", and it is NOT a geometrically contracted > > effect. > > Indeed, but you act as though this is the ONLY way something can be > physically contracted. Not so at all. > > > IOW, when a meter stick is "really physically contracted" or > > "materially contracted" its physical length or material length is > > "physically" or "materially" shorter than the observer's meter stick. > > "Physically" does not mean "materially". Never has, Ken. Get that out > of your head. It's wrong. > > > > > I don't understand why you insist on hijacking the word "physical" and > > give it a different meaning than the standard meaning. > > The standard meaning is the meaning given by physicists, since that > which is physical is what is studied by physics. > Electric field is physical, but it is not material. This should be > enough to tell you that what you think is the "standard meaning" has > something wrong with it, Ken. > > So much of your difficulty in understanding physics, Ken, is that you > insist that words mean what you want them to mean and you never ask > what they really do mean. If you only asked what some words meant in > physics, so much of your misunderstandings would be quickly resolved. > > > I don't > > understand why you don't except the correct phrase that length > > conraction in SR is a "geometric projection effect". > > > BTW the physical length or material length of the pole DOES NOT > > actually fits between the doors when the doors are closed. The > > *geometric projection* of the pole can fit into the barn with both > > doors are closed.....the reason is that geometric projection is not > > physical or material. > > Oh, Ken, Ken, Ken. The doors are both closed at the same time, and the > ends of the pole do not touch the doors. The pole is completely inside > the barn. > Yet you want to insist that the pole is only geometrically inside the > barn and not physically inside the barn? Is it physically sticking out > of the barn? How does it physically do that without physically making > marks on the barn doors where the pole physically hits them? Here's the problem: What you wrote here means that there is material or physical contraction. Why? Because the only way that the material pole can be completely inside the barn with both doors closed simultaneously is that it is physically or materially contracted. If this is true then why do you need the explanation that length contraction in SR is a geomrtical projection effect? > > Do you see what kind of nonsense your word games get you into? ROTFLOL....it is you who is playing word games. Tom Roberts said that length contraction in SR is a geometric projection effect and you said that length contraction in SR is a physical or material effect. So who should I trust? You or Tom Roberts? I trust Tom Roberts becasue he seem to be more knowledgeable than you. Ken Seto > > > > > > > Ken Seto > > > > You get so confused about terms like "physically contracted" and > > > "materially contracted" and "really contracted", as though they all > > > mean the same things. They do not. The sooner you learn the > > > distinctions, the better. > > > > >.....IOW, not just a geometric projection > > > > effect. > > > > > > > The physical length cannot fit into the barn is > > > > > > an absolute concept and it is not observer dependent. > > > > > > I'm sorry, Ken, that is just wrong. > > > > > It is not wrong....also assertion is not a valid arguement. > > > > Factual matters are decided by documented facts, not argument, Ken. > > > The point is not to *convince* you that you are wrong. I'm only > > > pointing out when you ARE wrong, and I'd be happy to direct you to > > > where you can look up the documented facts. However, there is no point > > > in trying to convince you that you are wrong by making a compelling > > > argument. I might as well be arguing with a stone pig. > > > > > > > > Nor is it contradictory to say that a falling ball has a straight-line > > > > > > > trajectory AND a parabolic trajectory in the same fall. Galileo knew > > > > > > > that. I don't see why you don't understand that. > > > > > > Do you understand what I wrote in this paragraph? Do you see why this > > > > > is also not a contradiction? > > > > > What you are describing here is a geometric projection of a falling > > > > ball in the ship from the shore observer's point of view > > > > No, it is a PHYSICAL shape of a trajectory. That is the point. The > > > straight line path is a PHYSICAL trajectory. The parabolic path is a > > > PHYSICAL trajectory. The falling ball has BOTH a straight line > > > physical trajectory AND a parabolic physical trajectory, as seen in > > > different frames. What is *measured* is physical. > > > > >....this is > > > > not the same as in the barn and the pole paradox where you claimed > > > > that the pole is physically contracted.- 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: PD on 11 Feb 2010 10:09
On Feb 11, 12:59 am, Ste <ste_ro...(a)hotmail.com> wrote: > On 11 Feb, 01:36, PD <thedraperfam...(a)gmail.com> wrote: > > > On Feb 10, 1:36 pm, Ste <ste_ro...(a)hotmail.com> wrote: > > > > The question therefore remains, how can the speed of propagation > > > possibly be measured to be constant in all frames. > > > Yes, indeed, and we'll get there eventually, if you like. For now, we > > were just trying to figure out where this little observational fact > > gives rise to frame-dependent simultaneity. > > Indeed. What I will say is that it appears the only remaining > explanation is to make reference to the circumstances of the source > *and* recipient in explaining the speed of light, but that would > require a radically new way of looking at things. I'm not sure what you mean. Can you explain? |