Is Length Contraction in SR physical??
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From: kenseto on 4 Feb 2010 18:12 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. The pole observer does not see the barn is contracted....he must see the projected length of the barn is expanded in order to agree with what the barn observer sees. > > In the barn frame, closing the doors simultaneously just long enough > for the man with the pole to run from one end of the barn to the other > corresponds to the front door being open, while the back door is > closed just long enough for the man with the pole to run to it, then > the back door opening and the front door closing after the back end of > the pole enters the barn. The problem with what you said is that you claim both possibilities: 1. The pole can fit into the barn physically with both doors close simultaneously. 2. The pole cannot fit into the barn physically with both doors close simultaneously. That's not physics when you claim all possibilities. Ken Seto > > There is a four dimensional projection going on--you can't project a > vector onto another in a way that makes it appear bigger, only > smaller--for example: > > Say you have two vectors that are parallel to each other. The > projection of vector 2 onto vector 1 has the same magnitude as vector > 2. Now, say you rotate them at 90 degrees to each other. The > projection of vector 2 onto vector 1 has a length of zero. So the > projection of vector 2 onto vector 1 will always have a length between > 0 and the length of vector 2. > > The same is true the other way around. The projection of vector 1 > onto vector 2 doesn't get *bigger* as the angle between the vectors > increases. It *also* gets smaller. This projection ranges in length > between 0 and the length of vector 1. > > In relativity, the same type of thing is going on, except instead of > rotating solely in space, the "rotation" is taking place in space AND > time, and so, the projection must take into account both space and > time.- Hide quoted text - > > - Show quoted text -
From: mpalenik on 4 Feb 2010 18:15 On Feb 4, 6:12 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. The pole observer does not see the > barn is contracted....he must see the projected length of the barn is > expanded in order to agree with what the barn observer sees. > No, it isn't, and even a cursory glance at the most elementary texts discussing relativity will tell you so. > > > > In the barn frame, closing the doors simultaneously just long enough > > for the man with the pole to run from one end of the barn to the other > > corresponds to the front door being open, while the back door is > > closed just long enough for the man with the pole to run to it, then > > the back door opening and the front door closing after the back end of > > the pole enters the barn. > > The problem with what you said is that you claim both possibilities: > 1. The pole can fit into the barn physically with both doors close > simultaneously. From the barn's frame it can. > 2. The pole cannot fit into the barn physically with both doors close > simultaneously. In the pole's frame, it cannot. Simultanaity is frame dependant in relativity. What is simultaneous in one frame is not in the other. The two ends of the pole are never simultaneously within the barn in the pole's frame. > > That's not physics when you claim all possibilities. > I'm not claiming all possibilities. I am claiming a very specific thing happens. You can even calculate the times during which the ends of the pole are within the barn in the two reference frames. You get a different answer in each frame. Time, space, and simultanaity are frame dependant quantities in relativity.
From: Ste on 4 Feb 2010 18:33 On 4 Feb, 22:09, mpalenik <markpale...(a)gmail.com> wrote: > On Feb 4, 5:00 pm, Ste <ste_ro...(a)hotmail.com> wrote: > > > > Yes. > > > Ok, but now you've cut your own throat, because if the axiom was some > > unobservable phenomena, then you've just said it's reasonable to infer > > its existence from the conclusions/predictions that are drawn from it. > > Or have I misunderstood? > > The axiom is not unobservable if it has observable consequences. I > think you're misunderstanding the definition of "observable". No, I merely think you're confused. If it is tolerable to have an axiom that is unobservable, but whose effects are not, then that vindicates my "faith" in the existence of variables that may not be directly observable, but whose existence is inferrered from consequential effects. > > > I disagree. Scientists are concerned with the unobservable only > > > insofar as those have necessary, distinctive, and accessibly > > > observable consequences. > > > But then you've subtly contradicted your earlier statement. Now > > scientists are concerned with the unobservable in certain > > circumstances. > > No. Science is concerned with making observations and then basing a > theory off of those observations. Which is really a non-sequitur in relation to my statement. > If it is not observable, you cannot > base a theory off of it. If an axiom is indirectly observable, it is > still observable--that is, if it is the only explanation for a given > observation, then making that observation is the same as observing the > axiom. But what if observations are unable to discern between two axioms that both explain the same effects? > > > If those do not, then science is simply not > > > concerned with them at all. This is why science has NO SAY on matters > > > of spirits, souls, and deities. > > > I dare say any self-respecting scientist is in competition with those > > concepts. > > Not at all. It doesn't come up in science ever. But I've been told that "science" is "what scientists do". Are you seriously telling me that a scientist has never contemplated spirits, souls, and deities? And are you telling me that no scientist is religious (in the traditional sense)? > > > > > So your notion of "real" variables that > > > > > cannot be measured has the same objective reality as, say, heaven.. > > > > > My "real" variables *are* objective reality, > > > > You don't KNOW they are objective reality, until you can devise a test > > > that shows their distinctive footprints. You have FAITH that they are > > > objective reality. > > > Yes, just as I have faith that there *is* an objective, naturalistic, > > reality. But observation cannot discern between this and any other > > axiomatic belief. > > That's why science doesn't try to comment on reality beyond the way > reality can be observed. But it *does* comment on reality beyond mere observation, *all the time*. What is string theory but a comment on reality beyond what is observed? What are the various interpretations of QM, but a comment on reality beyond what is observed? > > > > > Your inference of "real" properties independent of observed properties > > > > > has the same scientific verifiability as God, which is to say none. > > > > > But the scientific method itself has the same verifiability as God. > > > > You've just never thought about the philosophy of science enough to > > > > see the arbitrary assumptions that underpin it. > > > > The scientific method does not have a priori validity, nor does it > > > claim it. What it is is an *operational* convention that is adopted > > > because it has demonstrated success. > > > So in what way could it be falsified? > > For example, if the laws of physics were to completely change tomorrow > morning, that would indicate that the scientific method is not good > because it relies on past behavior being a predictor of future > behavior. That wouldn't falsify falsificationism, it would merely falsify the existing laws of physics. What you mean is if *determinism* was falsified, that would falsify the scientific method. But I'm a determinst, and an indeterministic world would also falsify my views. So what test distinguishes between the "scientific method", and my method which apparently isn't scientific?
From: Ste on 4 Feb 2010 18:47 On 4 Feb, 22:24, mpalenik <markpale...(a)gmail.com> wrote: > In relativity, velocity is simply a choice of coordinates. In other > words, what we call "time" and "space" are only defined for a given > observer in a given frame of reference. Another observer who calls > slightly different directions through the universe "time" and "space" > will see your forward motion through what you call "time" as forward > motion partly through "time" and partly through "space", which is why > you appear to have a velocity to him, and vice versa. And I ridicule this interpretation. > Since you seem to believe there is an absolute set of coordinates in > the universe that define time and space (thereby defining velocity), > maybe the question could be posed a slightly different way. There is certainly an absolute time (I do not recognise it as a "real" dimension in any event, but a device we use to express change and make predictions). As for space, it is difficult to say, but at the very least I would refer to the "absolute reference frame" as that frame which encompasses the whole universe within one frame. > Lets say that I have a spherical ball with no markings on it > whatsoever and there's an ant sitting on the surface. To define the > location of the ant, I draw a system of coordinates on the ball. I > choose a point for the north pole and use spherical coordinates (like > those you see on a globe). I describe the location of the ant in > these coordinates. > > Now, you decide to do the same thing, except you choose a different > point as the north pole and draw your spherical coordinates that way. > You describe the location of the ant in your coordinates. So in other words, you're describing exactly the same thing with different maths? > We both give different numbers to describe the location of the ant, > although we are both referring to the same point. Which of our sets > of numbers is correct? Both, within their own terms. > Saying that an object has a velocity with respect to you is no > different than defining your own set of coordinates in the universe. Indeed. The question still remains whether there is "absolute velocity". Certainly this defining of coordinates relative to one's self doesn't explain why an accelerometer on a rocket measures 10g (or whatever), while the Earth registers no change.
From: kenseto on 4 Feb 2010 19:16
On Feb 4, 4:11 pm, artful <artful...(a)hotmail.com> wrote: > On Feb 5, 2:57 am, Tom Roberts <tjrob...(a)sbcglobal.net> wrote: > [snip] > > > I think "fictitious" covers it well. > > Yeup .. that's the word that had eluded me :) Thanks. > > > What's wrong with simply stating the truth: "length contraction" is purely > > geometrical in the 4-D spacetime of relativity. As is common, geometrical > > relationships can have physical consequences (e.g. the ladder does or does not > > fit through the doorway). Physics can never be separated from geometry. > > I think it better to say length contraction can be *modeled* by a > geometrical projection. Pure geometry is a purely abstract notion .. > and length contraction is more than just that because, as you say, it > has physical consequences. Of course, one can then argue that if the > consequences of length contraction (including measurement of it by > whether things fit in barns) are physical, then doesn't that mean the > length contraction is physical in the same sense. ie Can something > non-physical have a physical consequent? Surely the geometry is > modelling something physical .. that's pretty much the point of > physics.. in which case is it misleading to call it *purely* > geometrical? > > Further .. I think perhaps the confusion would be relieved by changing > your working somewhat: > > Physical reality is modeled by the geometry of the 4-D spacetime of > relativity. As is common, geometrical relationships can have physical > consequences (e.g. the ladder does or does not fit through the > doorway). "length contraction" is one such physical consequent Fitting a ladder through a narrow door way is not the same as fitting a long pole into a shorter barn with both doors close simultaneously. |