Is Length Contraction in SR physical??
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From: artful on 4 Feb 2010 16:11 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
From: "Juan R." González-Álvarez on 4 Feb 2010 16:18 eric gisse wrote on Thu, 04 Feb 2010 12:16:54 -0800: > Juan R. González-Álvarez wrote: > >> Tom Roberts wrote on Wed, 03 Feb 2010 19:52:10 -0600: >> >> (...) >> >>> "Proper length" is not the same as "length", because a) it is >>> intrinsic to the object, >> >> It is not. > > Actually, it is. You know how I know this to be true? Because that is > how proper length is _defined_. No, it is not. >>> and b) it is invariant. >> >> Only under certain approximations (i.e. under certain *specific* class >> of transformations). > > Coordinate transformations and boosts are not "approximations", nor is > it very specific. Is *reading* a message before replying a too difficult requirement for you? P.S: Re-added news sniped by Eric -- http://www.canonicalscience.org/ BLOG: http://www.canonicalscience.org/en/publicationzone/canonicalsciencetoday/canonicalsciencetoday.html
From: mpalenik on 4 Feb 2010 16:20 On Feb 4, 4:05 pm, Ste <ste_ro...(a)hotmail.com> wrote: > On 4 Feb, 13:09, mpalenik <markpale...(a)gmail.com> wrote: > > > > > > > On Feb 3, 10:23 pm, Ste <ste_ro...(a)hotmail.com> wrote: > > > > > > Indeed. A belief in the material world is axiomatic, not open to > > > > > falsification. > > > > > Well, here you see we run into a small problem. > > > > > 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? > > > If disproving the testable hypothesis disproves the axiom, then the > > axiom is testable. > > But this relies on falsificationism, which is itself axiomatic. You > have yet to explain *why* falsificationism in science is desirable. > > > > As I say, without instantaneous communication, the hypothesis of > > > absolute time is unfalsifiable, but it is an inference that I find > > > *necessary* for my coherent understanding. If I did not *presume* that > > > events happen independent of their observation, I would not be able to > > > apply my intuitions to SR at all. > > > First of all, it sounds like you're saying that since you can't > > understand relativity, you've made up some of your own ideas that you > > like better, which isn't how science works. > > No, I'm saying that I *do* understand relativity. And can you provide > a definition of "science" (the definition remains contentious, you > see)? > > > However, nobody is saying that science says events do not occur > > outside of their observation, only that science is only concerned with > > describing the observable universe. Science works through observation > > and thus can only describe the universe by how it appears to work. > > This is just rhetoric. No one is talking about describing the universe > other than in the way it appears to work - there was never a religion > that didn't describe the universe in terms of how it appears to work. > Except for you--who thinks that scientists should be concerned with something deeper than the apparent reality. > > > > > > > > > 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. > > > Of course they're not. For example, it's possible that gravity > > doesn't exist and there are actually tiny invisible pink elephants > > that we can't measure in any way--one for each particle in the > > universe--that push every particle to make it look like gravity is > > acting on them. You certainly can't disprove this theory. There's no > > observable prediction that it makes. Science is only concerned with > > the fact that there's an observable thing that appears to be gravity. > > Whether it's really elephants or the hand of God, or some sort of > > spaghetti monster isn't a scientific question, although there's > > certainly no way to rule any of those things out. > > I'm afraid I simply don't accept this reflects reality. Most > scientists do not even remotely entertain the existence of tiny pink > elephants, and to reiterate, scientists are informed by their beliefs > about the fundamental nature of the real world.- Hide quoted text - > You just don't get it, do you? What I'm talking about is trying to create a theory that defines some kind of "absolute reality" vs. creating a theory that models the visible universe. The point is, there could be some unknowable, unmeasurable reason behind the laws of physics. You cannot disprove these unknowable, untestable, unmeasurable "theories". However, in science, we are not concerned with them whatsoever. We only seek to model observable reality.
From: Ste on 4 Feb 2010 16:21 On 4 Feb, 15:19, PD <thedraperfam...(a)gmail.com> wrote: > On Feb 3, 9:32 pm, Ste <ste_ro...(a)hotmail.com> wrote: > > > If it is presumed that the two events are an equal amount of distance > > away from the observer, and that the information propagates along that > > distance at an equal speed, then yes I would say simultaneity can be > > determined this way in the absence of gravity considerations. What I > > would make the point though that I recognise a distinction between > > "proper distance" (i.e. that of a straight line) and the extended path > > that real light might need to take in the presence of a gravity field. > > We can certainly arrange things such that the effect of gravity is > much less than the size of the effects being examined, or that gravity > is numerically accounted for, or that the set-up is configured so that > gravity does not bend the path of the light. Let's take it as a > presumption that in a *real* experiment, one of these three strategems > is employed, and thus we'll dispense with gravitational curvature as a > consideration, for simplicity. Ok. Obviously with everyone vigorously anticipating a slip-up and their opportunity to go "aha!", I'm having to be more conscious of covering all eventualities, lest any omission be interpreted as "a lack of understanding". > > > Note here that we have accounted for propagation delay here, and the > > > conclusions are not masked by that. > > > > OK so far? > > > Yep. > > OK, good. Then here's the plan for the next string of posts, just so > you know where we're going: > 1. A simple note about experimental observations regarding this > situation. > 2. The idealized "thought experiment" by Einstein that distills these > experimental observations. I may follow this up with a more elaborate > example that makes the symmetry more apparent. > 3. Explaining why the observations in (1) and (2) are completely > consistent with known laws of physics *other* than relativity. > 4. Understanding why these results are due to the frame-independence > of the speed of light. > 5. Review of the experimental data on the frame-independence of the > speed of light. > > So then, now that we have an unambiguous way of determining true > simultaneity of a pair of events, an approach that we can agree on, > then we'll make an interesting observation in nature. > > We can set up an experiment that looks at a single pair of spatially > separated events (events A and B) with two detectors in two different > reference frames X and Y, which are moving relative to each other. > Experiments of this type have actually been performed. I've already > pointed you to a list of references that includes such experiments > (among others). I want to emphasize that there is only ONE pair of > events that is looked at from two perspectives at the same time, not > an experiment that is repeated first with one detector and then with > the other. I just want to be clear that I don't really have a mental picture of this setup (starting distances and velocities remain unspecified), so I might need more information. > And here is the result: > 1. The detector in X is midway between the events and so the distance > traveled by the signals are equal, as confirmed by an independent > measurement. > 2. The speed of the signals is equal, as confirmed by an independent > measurement. > 3. The signals arrive at the detector in X at the same time. > > Therefore the original events are simultaneous. Ok, I think I can agree so far. > 1'. The detector in Y is midway between the events and so the distance > traveled by the signals are equal, as confirmed by an independent > measurement. > 2'. The speed of the signals is equal, as confirmed by an independent > measurement. > 3'. The signals arrive at the detector in Y at different times. > Therefore the original events are not simultaneous. I can conceive of a situation where this will be true, but I need you to specify the velocities. > Thus, depending on X or Y, the events are simultaneous or not > simultaneous. And since the same procedure is used in both, there is > no obvious way to determine which of these is "true" and why the other > is not "true". The answer to this will have to turn on a specific discussion of the evidence. > Before you splutter, "But... how can this BE?" let me remind you that > these are the real observations -- that is, as nature ACTUALLY > exhibits itself -- and we've already decided the conclusions that > would necessarily be drawn from those observations. > > With me so far? No. I want more clarification. I think I know what the issue is, but first I'll let you be more concrete about the *velocities* of the two detectors.
From: Ste on 4 Feb 2010 17:00
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? > 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 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. > > > 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'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. > > > > 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. > 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. > > > 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. > > > 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. > 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. |