Is Length Contraction in SR physical??
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From: kenseto on 7 Feb 2010 09:06 On Feb 5, 9:36 pm, mpalenik <markpale...(a)gmail.com> wrote: > On Feb 5, 9:04 pm, Ste <ste_ro...(a)hotmail.com> wrote: > > > > > > > On 5 Feb, 15:55, PD <thedraperfam...(a)gmail.com> wrote: > > > > On Feb 4, 8:34 pm, Ste <ste_ro...(a)hotmail.com> wrote: > > > > > On 5 Feb, 01:12, artful <artful...(a)hotmail.com> wrote: > > > > > > On Feb 5, 11:49 am, kenseto <kens...(a)erinet.com> wrote: > > > > > > > On Feb 4, 6:04 pm, mpalenik <markpale...(a)gmail.com> wrote: > > > > > > > > On Feb 4, 5:59 pm, "kens...(a)erinet.com" <kens...(a)erinet.com> wrote: > > > > > > > > > It it does violate the PoR. You made the contradcictory claims that > > > > > > > > the pole can fit into the barn physically (materially) an at the same > > > > > > > > time you claim that the pole cannot fit into the barn physically > > > > > > > > (materially)......that a violation of the PoR. > > > > > > > > No. The doors are not closed simultaneously in the pole's frame, nor > > > > > > > are the two ends of the pole simultaneously in the barn in the pole's > > > > > > > reference frame. In the barn's frame, the two ends of the pole are in > > > > > > > the barn simultaneously and the doors shut simultaneously. In the > > > > > > > pole's frame, the two ends of the pole are in the barn at different > > > > > > > times and the doors shut at different times. > > > > > > > Sigh..You are making the contradictory claims: > > > > > > 1. The pole can fit into the barn with both doors close > > > > > > simultaneously. > > > > > > In the frame of the barn > > > > > > > 2. The pole cannot fit into the barn with both doors close > > > > > > simultaneously. > > > > > > In the frame of the pole > > > > > > Two different meanings for 'simultaneously'. So they are not > > > > > contradictory > > > > > > You really are not very good at thinking or arguing logically. > > > > > He seems perfectly reasonable to me. > > > > > He's saying that, if one stands in the middle of the barn equidistant > > > > from the doors on each side, and one closes the doors simultaneously > > > > (i.e. what I would call "absolutely simultaneously", but which for > > > > conveience we'll say "simultaneous according to an observer standing > > > > equidistant from each door), then is it possible that both doors can > > > > appear closed while the ladder is observed to be inside the barn. > > > > > It's a simple question, and the answer is "no!". > > > > Your conclusion "no" is in conflict with experimental measurement. > > > When intuition conflicts with experimental measurement, then it is > > > intuition that must give way. > > > I checked Paul before I gave this answer. Length contraction has never > > been experimentally tested. So my intuition does *not* conflict with > > experimental evidence.- Hide quoted text - > > > - Show quoted text - > > Length contraction must follow for logical consitancy based on other > measurements. If the speed of light is constant in every reference > frame, length contraction must necesessarily follow, as we've > described it to you. No there is no need for physical length contraction. The physical length of a meter stick remains the same in all frames of reference. An SR observer assumes that the light path length of his meter stick is the same as the physical length of his meter stick. He uses the SR equation to predict the light path length of a meter stick to be contracted by a factor of 1/gamma. This agree with the newer SR concept that length contraction is a geometric projection effect. Ken Seto >That's how the original derivation of all of > special relativity worked. You apply logic to the two postulates of > relativity and see what it necessitates for consistancy. If the speed > of light is constant, E=mc^2 logically follows (or rather, E^2 = > p^2c^2 + m^2c^4), as does length contraction, time dilation, and the > differences is simultanaity. Can you admit this is the case? So > while length contraction may not have been experimentally measured > directly, what has been measured? > > The consistancy of the speed of light in every frame > Time dilation (in multiple different ways) > The relativstic energy/momentum relationship (repeatedly, in multiple > different ways) > Gravitational lensing (which matches the predictions of GR, which > requires that the predictions of special relativity hold in each local > Lorentz frame) > The dopplar shift of predicted by relativity (which is slightly > different than the non-relativistic dopplar shift that you get for > something like sound waves, for example). > > Ste, could you try doing the exercise that I posted in reply to Ken's > message?- Hide quoted text - > > - Show quoted text -
From: jem on 7 Feb 2010 09:35 Tom Roberts wrote: > jem wrote: >> Tom Roberts wrote: >>> 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. > > First, remember that every measurement by a given apparatus yields a > value that is invariant (under coordinate transforms). That is, no > matter what other coordinates might be used by some other observer, when > that observer transforms her own measurements to the apparatus, she > finds that her transformed value is precisely the value the apparatus > itself gave. But, of course, if the same apparatus were at rest in some > other frame it would in general yield a different value. > > Such values have been called "frame-dependent invariants". I > don't really like that name, but it does capture the essence. > > My point is: for a given object its length might be measured in some > frame as dx, and in another frame as dx'. But any valid physical theory > will not use EITHER dx or dx'; instead it will use invariants, such as > dL, defined as the 4-vector representing the displacement from one end > of the object to the other at a given event (position along its > trajectory in space-time). The reason theories must do this ought to be > obvious: choice of frame cannot possibly affect the physical phenomena > being modeled, and if either dx or dx' were used in the model then there > would be variations in the model that do not correspond to the phenomena > being modeled, thus making the model invalid. > > >> which suggests that in science related contexts, "physical" should >> equate to (real-world) "measurable". > > I think you need to be more careful to separate world and model. > "Physical" implies a quantity in the world. But, of course, we don't > really know what happens in the world without some model, and then all > we can understand is the model (our minds only process thoughts and > cannot possibly access the world directly, and ALWAYS construct a model > and work with that). So any sort of "physical" quantity is necessarily > schizophrenic (oxymoronic) -- the quantity is part of the model but the > adjective claims it is part of the world. That's why I maintain that the > best way to apply "physical" to a quantity is to require that the > quantity be a valid model for some physical phenomenon. Better still, > don't apply "physical" at all to quantities of the model. Note there's > no reason to expect that all such "physical quantities" are measurable. > > Note that I generally avoid using this adjective, except in > threads like this where someone else uses it. Except for the > phrase "physical theory", by which I of course mean a theory > of physics (rather than of mathematics). > > >>> 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. >> >> SR uses coordinate-dependent quantities in its equations > > SR is not really a physical theory. SR can be considered to be a > META-theory that prescribes conditions on physical theories, or SR can > be considered as a geometrical underpinning of physical theories, or SR > can be considered to be the local limit of GR; it is not in itself a > COMPLETE physical theory. In order to test any prediction of SR, one > must add an additional theory, such as classical electrodynamics or QED > or GR. For instance, SR talks about "the speed of light", but says > NOTHING about what "light" is, and you must add a theory of light in > order to proceed. If you don't believe me, just look at Einstein's 1905 > paper -- classical electrodynamics plays a prominent role (he calls it > "Maxwell-Hertz equations"). > > Another aspect of SR not being a complete theory is the difficulties in > its definition. SR depends inherently on the definition of "inertial > frame", which is itself not easily defined in a self-consistent and > non-circular manner (Einstein's "system of coordinates in which Newton's > laws hold good" is not really sufficient). Not to mention basing the > theory on "speed of light" without specifying what "light" is. The best > foundation of SR is as the local limit of GR -- in practice, that is how > it is actually used. > > > Tom Roberts FYI, there are a number of socially acceptable ways to respond to someone who's explained why they disagree with something you've said, e.g., 1. "I stand corrected." 2. "Your explanation falls short because ..." 3. "You misinterpreted what I said." 4. "You have no idea what you're talking about." However, walking off in a huff because somebody had the audacity to question you, isn't one of those ways.
From: kenseto on 7 Feb 2010 09:37 On Feb 6, 3:16 pm, PD <thedraperfam...(a)gmail.com> wrote: > On Feb 6, 12:29 pm, kenseto <kens...(a)erinet.com> wrote: > > > > > > > On Feb 5, 11:05 am, PD <thedraperfam...(a)gmail.com> wrote: > > > > On Feb 4, 5:12 pm, "kens...(a)erinet.com" <kens...(a)erinet.com> wrote: > > > > > On Feb 4, 11:54 am, mpalenik <markpale...(a)gmail.com> wrote: > > > > > > On Feb 4, 10:04 am, kenseto <kens...(a)erinet.com> wrote: > > > > > > > On Feb 3, 8:12 pm, PD <thedraperfam...(a)gmail.com> wrote: > > > > > > > > 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. > > > > > > > No....once it is physically contracted it is contracted to all > > > > > > observers. > > > > > > > > > This means that the pole is physically shortened and physically > > > > > > > > shortened pole is not observer dependent. > > > > > > > > No it certainly does not mean that. > > > > > > > Sure it means that. > > > > > > > > Because if it did mean that, then > > > > > > > the pole would also have to be shorter than the barn in the pole > > > > > > > frame. > > > > > > > The point is: the pole is not physically contracted in the barn frame > > > > > > or the pole frame. In the barn frame the geometric porjection of the > > > > > > pole unto the barn frame is contracted and this projected length is > > > > > > able to fit into the barn with both doors close simultaneously. > > > > > > This is correct so far. > > > > > > > In the > > > > > > pole frame the geometric projection of the length of the barn is > > > > > > expanded and this expanded length is able to encase the pole > > > > > > completely with both doors close simultaneously. > > > > > > This is incorrect. In the pole frame, the length of the barn is > > > > > *also* contracted but the closing of the doors is no longer > > > > > simultaneous. *Time* as well as distance gets "geometrically > > > > > projected" in the pole frame. > > > > > No. What I said is correct. What the pole observer sees must agree > > > > with what the barn observer sees. > > > > NO! This is NOT what the principle of relativity says. They do NOT > > > have to see the same thing. > > > The pole observer have to agree with the barn observer that the pole > > is completely inside the barn with both door close simultnaeously. > > No, they DON'T. That is NOT what the principle of relativity says. > READ what the principle of relativity says, Ken. > The principle of relativity says the LAWS of physics are the same in > both frames, not the accounting of the events in both frames. > And the two different accounts of what happened are both consistent > with the same laws of physics. This is fully consistent with the PoR. > > > Since the pole observer said that the pole cannot be inside the barn > > with the door close simultaneously then it is a violation of the PoR. > > No, Ken, this is NOT a violation of the PoR. Learn what the PoR > actually says, rather than what you THINK it says. > > The barn observer says the pole is completely inside the barn with the > barn doors closed simultaneously. This is why there are no marks on > the doors from the pole ends. > The pole observer says the pole is never completely inside the barn, > and the barn doors don't close simultaneously. This is why there are > no marks on the doors from the pole ends. > > These two statements are CONSISTENT with the PoR, because the laws of > physics are consistent with both. > > It is NOT TRUE that the PoR says the two observers agree on the reason > why there are no marks on the There is only one pole....SR made the following contradictory claims: 1. The pole is physically contracted to fit into the physical length of the barn. 2. The same pole is not physically contracted and thus it is not able to fit into the barn. Ken Seto > > read more »- Hide quoted text - > > - Show quoted text -...
From: kenseto on 7 Feb 2010 09:39 On Feb 6, 3:17 pm, PD <thedraperfam...(a)gmail.com> wrote: > On Feb 5, 8:16 pm, Ste <ste_ro...(a)hotmail.com> wrote: > > > > > > > On 5 Feb, 16:02, PD <thedraperfam...(a)gmail.com> wrote: > > > > On Feb 4, 8:16 pm, Ste <ste_ro...(a)hotmail.com> wrote: > > > > > On 5 Feb, 00:49, kenseto <kens...(a)erinet.com> wrote: > > > > > > On Feb 4, 6:04 pm, mpalenik <markpale...(a)gmail.com> wrote: > > > > > > > On Feb 4, 5:59 pm, "kens...(a)erinet.com" <kens...(a)erinet.com> wrote: > > > > > > > > It it does violate the PoR. You made the contradcictory claims that > > > > > > > the pole can fit into the barn physically (materially) an at the same > > > > > > > time you claim that the pole cannot fit into the barn physically > > > > > > > (materially)......that a violation of the PoR. > > > > > > > No. The doors are not closed simultaneously in the pole's frame, nor > > > > > > are the two ends of the pole simultaneously in the barn in the pole's > > > > > > reference frame. In the barn's frame, the two ends of the pole are in > > > > > > the barn simultaneously and the doors shut simultaneously. In the > > > > > > pole's frame, the two ends of the pole are in the barn at different > > > > > > times and the doors shut at different times. > > > > > > Sigh..You are making the contradictory claims: > > > > > 1. The pole can fit into the barn with both doors close > > > > > simultaneously. > > > > > 2. The pole cannot fit into the barn with both doors close > > > > > simultaneously. > > > > > Take comfort Ken that at least there's someone here who understands > > > > your simple questions. > > > > So far it appears that there are at least two people confused and > > > incredulous. > > > Ken has been confused about this for 15 years. We'll soon see whether > > > you can be unconfused at the rate that most freshman students become > > > unconfused about it. > > > I don't know Ken's past history, but certainly he seems to be asking a > > fairly reasonable questions this time - and judging by the responses, > > it's a question that children get taught not to ask! > > 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.??
From: paparios on 7 Feb 2010 11:03
On 6 feb, 23:30, Ste <ste_ro...(a)hotmail.com> wrote: > On 6 Feb, 10:29, mpalenik <markpale...(a)gmail.com> wrote: > > > On Feb 6, 5:01 am, Ste <ste_ro...(a)hotmail.com> wrote: > > > > On 6 Feb, 09:23, mpalenik <markpale...(a)gmail.com> wrote: > > > > But as I'm saying to you, I think you're confusing an optical effect > > > with a physical effect. That is, you're confusing *appearances* with > > > concrete reality. > > > What part of this picture do you think is optical? It's *geometrical* > > it doesn't have anything to do with what you can visibly see. > > Don't you realise that SR is about the behaviour of *light* - that is, > EMR? And SR describes how *observations* made by way of *light* change > in response to physical circumstances? > This is obviously wrong. First because EMR includes radio, infrared, ultraviolet, x-rays and gamma rays BESIDES visible light. The fact is that what is relevant is what it is called the maximum speed of propagation of interactions (that is when something happens here, after what interval of time the event propagates to the destination). This maximum speed of propagation of interactions is equal to the speed of light in vacuum. > > > > An important point, however, is that neither one of the sets of axes > > > > (either x,t or x',t') is inherantly better than the other. Neither > > > > set is more correct than the other. > > > > Indeed. > > > How can you agree with that and claim that the differences in > > measurement are optical? > > Because *that* is what SR is all about - it is about describing the > behaviour of *light*. > Wrong again!!! > > The differences in measurement are due to > > the different coordinate systems. It has nothing to do with what you > > *see* it has to do with how you make your measurements. > > It has *everything* to do with what you *see*. > It has to do with what you *measure* > > When one > > observer measures length, he measures along the x axis. When another > > observer measures length, he measures along the x' axis. The proper > > length of the object doesn't change, but the measured length has > > nothing to do with optics, visibility, or propagation delays. > > Of course the measured length has *everything* to do with optics. How > do you think we usually carry out measurements? > > And why do you think the ladder appears the correct size for an > observer in the x' frame? Once again wrong. Measurements are performed in all frequencies of the spectrum (remember radiotelescopes?) Miguel Rios |