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: Ste on 4 Feb 2010 23:22 On 5 Feb, 03:19, mpalenik <markpale...(a)gmail.com> wrote: > On Feb 4, 10:12 pm, Ste <ste_ro...(a)hotmail.com> wrote: > > > On 5 Feb, 00:27, artful <artful...(a)hotmail.com> wrote: > > > > That you cannot see the analgoy just shows you have NO idea what SR > > > says > > > > Both have a longer length object fitting within a shorter spae > > > > Both have a geometrical rotation and projection > > > > The ladder fits thru the doorway just as 'physically' as a pole fits > > > in the barn. > > > How on Earth do you work that one out? > > > For the analogy to work, the ladder must be constrained lengthwise, > > not widthwise.- > > This is what we've been trying to get across to you. It has to do > with coordinates and what one observer calls time and space relative > to another. We'll see. > The ladder is a little bit too long to fit into the barn but you could > rotate it and still fit it into the barn. Say, for example, you lift > the front end so the ladder is now at a 45 degree angle. You could > now fit it into the barn. You've rotated part of it into "height", > from "length". Yes, I'm sure I've employed such a similar wily device when getting a sofa through a doorway before. But we're assuming that the ladder *isn't* rotated. > But there's another way you could rotate it, as well. You can rotate > it into "time". If you run with the ladder, you're "rotating" the > front of it a little bit into the future and the back of it a little > bit into the past. Haha! I've nearly spat my tea out! I've never heard such a ludicrous statement before I came to sci.physics.relativity! Of course, I understand what you're getting at, although I understand the concept differently. What you're getting at is the effect of propagation delays, whereby both doors can appear to close simultaneously when, in physical reality, the distant door has already started to open before the near door closed. But that's why Ken wants a simple answer to the *physical reality*, which is why I devised a setup where the doors are equidistant from an observer (and therefore both must truly close simultaneously), and the question in that case is can the two doors be fully closed while the ladder is inside the barn? > What a moving observer percieves as "time" is the direction of his > motion through 4 dimensional spacetime. What he percieves as "space" > is the 3 dimensional volume perpendicular to that direction. Thus, > "space" in the moving frame is not the same as "space" in the rest > frame. It extends part way into the future along one direction and > part way into the past along the other (in the rest frame). Indeed. What's most worrying for me is that I can understand exactly what you're describing, and yet you can't understand what me and Ken are asking. Incidentally, if you understand the maths of SR (I don't), perhaps you should try testing this one out. Have the observer in the middle of the barn, the doors close simultaneously according to that observer, and the ladder travelling at something just less than the speed of light. Can the observer possibly observe the ladder inside the barn with both doors closed in this situation?
From: Ste on 4 Feb 2010 23:29 On 5 Feb, 02:37, "papar...(a)gmail.com" <papar...(a)gmail.com> wrote: > On 4 feb, 19:14, Ste <ste_ro...(a)hotmail.com> wrote: > > > > > After reading your numerous posts, you seem to be certain that a given > > > reality is always present, which is independent of what is being > > > measured. > > > > I propose you the following thought experiment regarding that: > > > > a) You are in a space ship A in deep space. Your surroundings are such > > > that you experience no gravity at all > > > In other words, "let us imagine that we are not in the real world...". > > Well, this is a gedanken which intend to prove that that absolute > reality does not exist!!! > On the other hand you don't really know what the "real world" is. You > just have some senses which tell you about your immediate surrounding > conditions and that is it!!! I know. As I've said elsewhere, my belief in the real world is axiomatic, not open to falsification. I could just as easily be a brain in a vat - though I don't pretend that I seriously entertain the idea. > > > and, if you look through a > > > window, no stars or galaxies are seen. Basically you are in the middle > > > of nowhere. Of course, you don't know if your space ship is moving and > > > the ship engines appear to be dead. > > > b) Suddenly, you observe another space ship B on your window, which to > > > you appears to be approaching your location (it is becoming larger and > > > larger in size as time goes by). > > > > Now, which of the following conclusions describe the reality of the > > > gedanken: > > > > 1) Space ship B is sitting still and space ship A is moving at speed v > > > towards the location of B. > > > 2) Space ship B is approaching at speed v to your space ship A > > > location, where A sits still. > > > 3) Both space ships are moving in such a way their closing speed is v.. > > > 4) It is impossible to determine if (1), (2) or (3) reflects reality > > > or, in other words, all alternatives (1), (2) and (3) can be true at > > > the same time. > > > Indeed, on the information available, (4) is the only conclusion that > > one can reach. The question is whether this accurately describes > > reality (i.e. whether there really is no way to discern absolute > > velocity), and whether, indeed, there is no way of discerning absolute > > motion. It is a question that I am not yet able to answer. > > > Of course, none of this detracts from the reality that, even if it is > > indiscernible, I still hold that there is an absolute background > > against which either (1), (2), or (3) are true. It is simply that we > > don't know which.- > > In the "real world" there are some that say that the source of the Big > Bang could bechosen as an absolute reference, but it is the same case > of you aboard the space ship, that is we don't know if the Big Bang > was sitting still or moving with respect to something else. At the end > it does not matter at all!!! An absolute frame of reference is not > needed nor an abosulte time is needed. It doesn't matter mathematically, but it does matter physically. As I've said, if an absolute reference frame can be discerned that would allow us to discern the movement of the universe itself, then good. Otherwise, the absolute reference frame can simply encompass the whole universe (which rules out any question of what the big bang - and the universe - is moving relative to).
From: mpalenik on 4 Feb 2010 23:31 On Feb 4, 11:22 pm, Ste <ste_ro...(a)hotmail.com> wrote: > On 5 Feb, 03:19, mpalenik <markpale...(a)gmail.com> wrote: > > > > > > > On Feb 4, 10:12 pm, Ste <ste_ro...(a)hotmail.com> wrote: > > > > On 5 Feb, 00:27, artful <artful...(a)hotmail.com> wrote: > > > > > That you cannot see the analgoy just shows you have NO idea what SR > > > > says > > > > > Both have a longer length object fitting within a shorter spae > > > > > Both have a geometrical rotation and projection > > > > > The ladder fits thru the doorway just as 'physically' as a pole fits > > > > in the barn. > > > > How on Earth do you work that one out? > > > > For the analogy to work, the ladder must be constrained lengthwise, > > > not widthwise.- > > > This is what we've been trying to get across to you. It has to do > > with coordinates and what one observer calls time and space relative > > to another. > > We'll see. > > > The ladder is a little bit too long to fit into the barn but you could > > rotate it and still fit it into the barn. Say, for example, you lift > > the front end so the ladder is now at a 45 degree angle. You could > > now fit it into the barn. You've rotated part of it into "height", > > from "length". > > Yes, I'm sure I've employed such a similar wily device when getting a > sofa through a doorway before. But we're assuming that the ladder > *isn't* rotated. See below. It is rotated not in length and height but in length and time. > > > But there's another way you could rotate it, as well. You can rotate > > it into "time". If you run with the ladder, you're "rotating" the > > front of it a little bit into the future and the back of it a little > > bit into the past. > > Haha! I've nearly spat my tea out! I've never heard such a ludicrous > statement before I came to sci.physics.relativity! > The fact that you find it ridiculous has no bearing on physical reality. All it demonstrates is that you are not familiar with the theory of relativity. > Of course, I understand what you're getting at, although I understand > the concept differently. What you're getting at is the effect of > propagation delays, No, this has nothing to do with propagation delays. > whereby both doors can appear to close > simultaneously when, in physical reality, the distant door has already > started to open before the near door closed. No, the doors close simultaneously in the frame of the barn but NOT in the frame of the moving ladder. This is *after* correcting for any "propagation delays" in the observation of the two doors. > > But that's why Ken wants a simple answer to the *physical reality*, > which is why I devised a setup where the doors are equidistant from an > observer (and therefore both must truly close simultaneously), This is what you don't understand, in this scenario, they are ONLY closing simultaneously in the barn frame. To a moving observer (at ANY location, even a moving observer located at the center of the barn) they do not close simultaneously. *THIS* is what the theory of relativity states. and the > question in that case is can the two doors be fully closed while the > ladder is inside the barn? > > > What a moving observer percieves as "time" is the direction of his > > motion through 4 dimensional spacetime. What he percieves as "space" > > is the 3 dimensional volume perpendicular to that direction. Thus, > > "space" in the moving frame is not the same as "space" in the rest > > frame. It extends part way into the future along one direction and > > part way into the past along the other (in the rest frame). > > Indeed. What's most worrying for me is that I can understand exactly > what you're describing, and yet you can't understand what me and Ken > are asking. > > Incidentally, if you understand the maths of SR (I don't), perhaps you > should try testing this one out. Have the observer in the middle of > the barn, the doors close simultaneously according to that observer, > and the ladder travelling at something just less than the speed of > light. Can the observer possibly observe the ladder inside the barn > with both doors closed in this situation?- Hide quoted text - This is what the original scenario represents. An observer, at rest with respect to the barn, in the center of the barn, sees both doors close at the same time with the ladder inside the barn. The person running with the ladder does not see them close simultaneously, but sees the front and back doors close and open at just the right time for him to run through. This is what the math says and is a very basic problem that might be given to a student who has just started studying relativity.
From: Tom Roberts on 4 Feb 2010 23:47 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
From: Tom Roberts on 5 Feb 2010 00:07
glird wrote: > On Feb 4, 10:57 am, Tom Roberts <tjrob...(a)sbcglobal.net> wrote: >> What's wrong with simply stating the truth: "length contraction" is >purely geometrical in the 4-D spacetime of relativity. > > What's wrong is that length contraction may be something real, but the > 4-d spacetime of relativity is a purely geometrical figment of the > imagination. Again you confuse world and model. When you carry a ladder through a doorway, the orientation of the ladder matters -- i.e. in your words that is "real". But geometry you say "is a purely geometrical figment of the imagination". But the orientation of ladder relative to the doorway is PURELY GEOMETRICAL. And it is PRECISELY the same sort of geometrical relationship as is "length contraction". Bottom line: EVERYTHING you think about the real world is actually thoughts about some MODEL of the world. You attempt to draw a distinction between "a purely geometrical figment of the imagination" and "length contraction being real"; that is a distinction without a difference, and such a distinction is meaningless. BOTH are purely figments of your imagination; it's just that successful models of the world become so ingrained in your manner of thinking that you forget that they are MODELS. Don't do that -- it is impossible to understand physics if you do that. Geometrical relationships have physical consequences. That is, the aspects of the real world that we MODEL as "geometrical relationships" are part and parcel of the way objects behave in the real world. Geometry is no less of a MODEL of the world than is any theory of physics, and every theory of physics has its geometrical aspects. Tom Roberts |