Is Length Contraction in SR physical??
in [Relativity]
|
Prev: Simultaneous events and Einstein's absolute time
Next: New Theory --- The Theory of Quantum Wave Sources
From: PD on 5 Feb 2010 13:43 On Feb 5, 12:59 am, Ste <ste_ro...(a)hotmail.com> wrote: > > It's laughable that two people who already claim to understand are > patting themselves on the back for coming up with an allegedly > "impressive" explantion, and yet the people to whom the concept was to > be explained are still not convinced. I continue to be mystified by your expectation that it is the objective of physicists to compellingly explain physics to nonphysicists. As though, if there are those who yet do not understand and are yet not convinced, then somehow physics has failed, and furthermore this probably indicates that the physical models have something wrong with them. 70% of people do not believe in evolution. Does this mean that biologists have failed somehow, or that evolution is therefore suspect? Where does this expectation come from?
From: mpalenik on 5 Feb 2010 13:44 On Feb 5, 10:50 am, kenseto <kens...(a)erinet.com> wrote: > On Feb 4, 6:15 pm, mpalenik <markpale...(a)gmail.com> wrote: > > > > > > > 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. > > Do you realize that this requires that the pole is physically > (materially) contracted? IOW it is a geometric projection effect. > > > > > > 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. > > This is a geometric projection effect. So in the barn frame you > choose real physical contraction and in the pole frame you choose > geometric contraction....Right? > > Ken Seto > No. I think you're confusing "geometric projection" with something like "optical illusion". It is not an optical illusion in either frame but it is a geometric projection in both. When we measure distance, we're measuring the distance something takes up in 3 dimensional space. If we rotate something in four dimensions and then slide it forward along its time axis, it actually takes up less of our 3 dimensional space, although it takes up the same amount of space as before along its own spacial axis. I understand the wording is a bit confusing there, but you can visualize it like this: This will actually produce the opposite result of relativity--length expansion in both frames (instead of length contraction in both frames). This is because orthogonality works differently in Minkowski space than it does in the euclidean space that we're used to. It is mathematically well defined, and I could explain to you how to draw it physically, but for now I think it would just confuse you. So, if we can start out with this, and you understand this, then I can explain to you how to modify it to work in Minkowski spacetime. Does that sound ok so far? Get a piece of paper, draw a horizontal axis labeled x and a vertical axis labeled t. These represent space and time axis. An object "at rest" will move straight forward through t. Let's call the origin, where the 2 lines intersec t=0, or "the present". Now, lets draw the world line of a moving object. This line should be at an angle with respect to the t axis. Label this line t'. The angle with the t axis should be less than 45 degrees because a 45 degree angle represents something moving at the speed of light (equal distances through time and space). Draw the line through the origin. Extend it in both directions (into the past and future). This represents an object that starts out in the past moving at some speed and continues moving horizontally as time goes on. With me so far? Now lets take a look at the "present", the origin. This is where all 3 lines intersect. At the origin, draw another line that is perpendicular to t'. Label it x'. This represents "space" as seen by the moving observer. The moving observer sees his own world line as "time" and this perpendicular axis as "space". Now, pick two points along the x' axis. This represents the ladder. Extend some dotted lines from them in the direction of t' (forward and backward). This represents the two ends of the ladder as they move. Look at the points where they intersect the original x axis. You'll notice you had to extend one point "forward" along t' and point "backward" along t'. This represents the fact that one end of the ladder is from the "future" of the t' frame, and the other end is from the "past". Also notice that the length of the ladder appears *greater* in the x frame than the x' frame. Two quick comments, though -- which end of the ladder is in the future and which is in the past is the *opposite* of how it would be in Minkowski spacetime. Also, in this diagram, you will see length *expansions*, whereas in Minkowski spacetime, you get length *contractions*. Please try to actually draw this out. If you get confused, let me know. Once you completely understand this, we can move on to modify it to work in Minkowski spacetime.
From: PD on 5 Feb 2010 14:48 On Feb 4, 4:00 pm, Ste <ste_ro...(a)hotmail.com> wrote: > On 4 Feb, 15:37, PD <thedraperfam...(a)gmail.com> wrote: > > > On Feb 3, 9:23 pm, Ste <ste_ro...(a)hotmail.com> wrote: > > > > > Science is in the business of testing propositions by confronting the > > > > consequences of those propositions against *observable* measurement.. > > > > This is the spine of the scientific method. > > > > Indeed. But what if I have axioms that are not testable, but from > > > which testable hypotheses can be drawn? If the hypotheses that are > > > drawn from the axioms appear to be true, then is it reasonable to say > > > that the axioms are true? > > > 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? It's ok to infer the existence of unobservables if it can be shown that those unobservables lead to necessary and observable consequences that are distinct from the necessary and observable consequences of other models, with which the consequences can be tested in experimental observation. > > > If the hypotheses that are drawn from the axioms, that are NECESSARY > > consequences of those axioms, appear to be false, then it is > > reasonable to say that the axioms are false. > > > In formal logic, this is the equivalence between "If P is true, then Q > > is true" and "If Q is false, then P is false". That is, if Q follows > > necessarily from the assumption of P, then you can disprove P by > > showing that Q is false. > > Indeed. > > > > > Thus you are saying that > > > > if science, by virtue of its methodology, confronts these propositions > > > > against observable measurement and finds the propositions therefore > > > > suspect, then there is something wrong with the scientific > > > > methodology. > > > > No, I'm saying that scientists are failing to apply their own > > > methodology to itself. > > > Not so. I told you what the methodology is. Science is in the > > business of testing propositions by confronting the consequences of > > those propositions against *observable* measurement. It is NOT in the > > business of making certain uncontestable propositions. > > I have a simple question, namely how is this scientific method > falsifiable? And of course that means you have to start talking about > *why* you follow this method - more on this below. The method itself is not falsifiable. The method is not presented as an objective truth of nature, and so it does not warrant the same test as its subject matter. More on this below. > > > > > The difficulty, you see, is that there becomes an awfully soft line > > > > between those propositions which need to be kept sacred and those that > > > > don't. Indeed, that list of sacred propositions might vary from person > > > > to person. And then, you see, it becomes rather difficult to declare > > > > whether ANY of them are *truly* sacred, or whether science is in the > > > > position to desanctify ANY of them. > > > > "The inherent contradictions..." > > > What contradictions? > > I was alluding to the contradictions inherent in the philosophy of > science. Well, that's still isn't very illuminating, now, is it? But at this point, we have drifted off into questioning whether science as an activity is worth anything, or whether what it produces is worth anything. That's fine. There are people who don't believe that fine art does anything for mankind, and there are others for whom politics (not to mention "political science") is a wholly and intrinsically worthless venture. If you don't care for science or take no stock in its method, then don't participate in it. > > > > > And that is not right. Science deals with things both material and > > > > immaterial, where I am using the *physics* meaning of "material". > > > > I'll remind you that the *physics* meaning of "material" is > > > > "consisting of matter", where matter is defined as "substance which > > > > has mass and occupies volume". (Now, you may have some other, rather > > > > more vague definition of "material", but then your statement of > > > > science being the investigation of the material world ends up lacking > > > > any semantic content.) > > > > So is physics not the study of space as well as matter? > > > Yes, it is! And of fields, and of a number of things that are not > > material by the definition of material used by physics. That's why I > > say that physics is NOT the study of the material world. > > Sorry I think I misread what you wrote. All I will say, then, is that > the word "material" has a meaning outside of physics, and in > particular in philosophy, that is somewhat less specific than the > definition you hold. I understand that. I have a degree in philosophy. But we are not talking about "material" as it is used in philosophy. We are talking about "material" as it is used in physics, because we are discussing what it is that physics studies. Likewise, if you said that music is the art of playing notes, you might find that musicians would quibble and say that there is more to music than notes. If you then turned around and said that you understand "note" to mean something more general than musical notes, and you also include meanings of tone (as in "a note of suspense") or other sensory perception (as in "this wine has notes of oak and cinnamon"), you will still have musicians telling you you're full of it. > I'll make a note of your preferred nomenclature, > but it's probably worth bearing in mind that many people (including, > I'm sure, physicists) will often use the phrase "material world" to > mean something much broader, and will generally encompass both matter, > space, and everything in between. That's fine. You'll note that the broader, more colloquial use has therefore vaguer and weaker semantic content, where the physical term is quite precise and distinguishable from other concepts. You expressed an intent earlier to translate what *physicists* mean by words into what *you* mean be the same words. This should not incite you to berate physicists for not sharing the colloquial, vague usage. > > > > > > > > Incidentally, even this axiom of yours that the external world > > > > > consists only of what is observed is, itself, an unfalsifiable > > > > > position, in that there is no concievable evidence that could disprove > > > > > the contention. > > > > > Nor did I say that the external world consists ONLY of that which is > > > > observable. What I said is that *science* concerns itself with the > > > > portion that is observable. The part that is unobservable, such as > > > > souls, spirits, deities, simply is not a concern of science. > > > > But theoretical scientists are informed by their beliefs about the > > > fundamental nature of the real world. It is absurd to say that > > > science, by which we mean scientists, are not concerned with the > > > unobservable. > > > 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. Insofar as those have necessary, distinctive, and accessibly observable consequences. Something proposed as unobservable that does not meet that criterion is of no value in science. > > > 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. I heartily disagree. If that were true, then no self-respecting scientist could have religious faith. You'll find this is not the case. > > > > > 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. Then it is of no value in science, because the only unobservables that are of interest in science are those that ... well, do I really have to repeat this AGAIN? > > > > > 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? And how can its success be > discerned from other explanations, for example herd mentality? After > all, it's not very "objective" if scientists, and *only* scientists, > agree that their own method is the best. I just said that it does not have a priority validity over other methods, nor does it claim to be able to demonstrate objective superiority as a strategem. There is no proof that the scientific method is the best method for acquisition of truth, nor does it claim it without proof. The scientific method's endorsement comes from productivity in practice, compared to other methods tried. It has done a better job than other methods for determining how nature works. This is not to say that it can be PROVEN to be the best method possible. And in fact, there are those who simply do not like the scientific method and do not believe anything that scientists say, and there is no way to convince them that they should. Nor is such an attempt made. Those who are interested in pursuing science implicitly endorse the scientific method because the method is the definition of science as an activity, which is why it is called THE scientific method. Those who want to use other methods for knowledge acquisition regarding the universe are certainly welcome to do so. It just is no longer science. That's what theologians and philosophers and mathematicians do, for example. Just because you are talking about natural things does not mean that what you are doing is science. > > > It does not claim to be the best > > or most universal method for knowledge acquisition. It is, however, a > > method that has been adopted by a certain group of people (scientists) > > under the operating assumption that it yields results. > > Yes, so this "operating assumption" is axiomatic - scientists "have > faith" that it has produced the best results so far, and will continue > to do so. No, not even that. It is just the method that someone *chooses* when one decides to be a scientist. This is not necessarily contingent on believing that it is the only or best way to understand nature. It is just the way that is *chosen*.
From: Tom Roberts on 5 Feb 2010 17:06 Juan R. González-Álvarez wrote: > Tom Roberts wrote on Thu, 04 Feb 2010 22:54:29 -0600: >> No. How could arbitrary human choices of coordinates possibly affect the >> physical phenomena that underlie the things one measures? > > ^^^ Here is the origin of your misunderstanding :-D How so? That is, please explain how choice of coordinates does affect physical phenomena. In the process you'll have to explain how and why every physical theory we have and every physics textbook is wrong.... Tom Roberts
From: Tom Roberts on 5 Feb 2010 17:09
Juan R. González-Álvarez wrote: > Tom Roberts wrote on Thu, 04 Feb 2010 22:47:11 -0600: >> 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). > > That is not true. So give an argument or counterexample. When you act like a crackpot, don't be surprised when people treat you like one. Tom Roberts |