Is Length Contraction in SR physical??
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From: PD on 3 Feb 2010 15:53 On Feb 2, 4:00 pm, JT <jonas.thornv...(a)hotmail.com> wrote: > On 2 Feb, 22:52, Tom Roberts <tjrob...(a)sbcglobal.net> wrote: > > > > > Ste wrote: > > > But the question remains, what is the *physical* cause of the > > > discrepancy when the object to be measured is moving differently from > > > the measuring equipment. > > > The answer is that this is not "physical", this is GEOMETRICAL. > > > Look at a building from directly in front, and it has a given > > width. Look at it from a front corner and it has a smaller > > width. There is no "physical cause" of this change, it is purely > > GEOMETRICAL. In SR, "time dilation" and "length contraction" > > are likewise purely geometrical -- they are geometrical > > projections just like the width of that building. > > > Continuing the analogy: there are physical consequences of that > > change (e.g. the building's width will or won't fit on the film > > of a given camera). But nothing about the building itself has > > changed, only the geometrical relationship between building and > > observer has changed. Similar remarks apply to "time dilation" > > and "length contraction" -- they can be "physical" for some > > meanings of the word, and aren't "physical" for other meanings. > > > Extending the analogy: if you want to "model" the building, it's > > clear that the relevant width is the one marked on the building's > > plans -- measured parallel to the building's front. It's clear that > > the possibility of measuring many different "widths" from other > > points of perspectives is completely irrelevant. Ditto in SR, where > > proper length and proper time are relevant to modeling phenomena > > related to a given object, and it is irrelevant that that one can > > measure other values for "length" and "time" in other frames. > > > > It should be a question taken seriously in > > > physics. > > > The appropriately related question is taken seriously. And answered easily. But > > by including a false premise as part of YOUR question (that there is a "physical > > cause"), you make YOUR question impossible to take seriously. > > > A famous example: When did you stop beating your wife? > > > Tom Roberts > > Tom what prevents the timedilation to be a geometrical interpretation > of time, afterall you seem to say that the length is. It IS the same thing. > > Now you may say that time have no geometrical interpretation but in > spacetime it certainly have. > > One even could say that doppler is a geometrical projection of time > ticks in a Euclidian space or would that be wrong? > > JT
From: PD on 3 Feb 2010 16:11 On Feb 2, 9:02 pm, Ste <ste_ro...(a)hotmail.com> wrote: > On 2 Feb, 21:52, Tom Roberts <tjrob...(a)sbcglobal.net> wrote: > > > Ste wrote: > > > But the question remains, what is the *physical* cause of the > > > discrepancy when the object to be measured is moving differently from > > > the measuring equipment. > > > The answer is that this is not "physical", this is GEOMETRICAL. > > > Look at a building from directly in front, and it has a given > > width. Look at it from a front corner and it has a smaller > > width. There is no "physical cause" of this change, it is purely > > GEOMETRICAL. In SR, "time dilation" and "length contraction" > > are likewise purely geometrical -- they are geometrical > > projections just like the width of that building. > > I assume you mean the *apparent width* of *the side you were facing* > becomes smaller. The width of the whole building against the > background actually increases if the building is square. > > > Continuing the analogy: there are physical consequences of that > > change (e.g. the building's width will or won't fit on the film > > of a given camera). But nothing about the building itself has > > changed, only the geometrical relationship between building and > > observer has changed. Similar remarks apply to "time dilation" > > and "length contraction" -- they can be "physical" for some > > meanings of the word, and aren't "physical" for other meanings. > > My view is that they are decidedly *not* physical, they are > *apparent*. Let's be clear about measurements and reality. Lay a stick down on the floor near the corner between two walls. Now shine lights toward both walls so that the stick's shadow is cast along the two walls. This generates two quantities that have the dimensions of length: the X-length (along one wall) and the Y-length (along another wall). Those are perfectly legitimate and real lengths, and they are measurable. They obviously also have values that will change if you rotate the floor and the walls underneath the stick. There is *another* length you can ask for: Is there a length that is independent of the rotation of the walls and the floor? Yes. This can be measured a couple of ways. One is to measure with a ruler that is always laid parallel to the stick, regardless of the walls and the floor. Another way is to take the real X-length and the real Y-length and combine them this way: sqrt((X-length)^2 + (Y-length)^2). When you do this, even though the values of X-length and Y-length will change when you rotate the walls, the combination will not. If however, you allow the stick to be tipped up from the floor, then you find that sqrt((X-length)^2 + (Y-length)^2) no longer works as an invariant. Now what you have to do is include a third and again real length Z-length like so: sqrt((X-length)^2+(Y-length)^2+(Z-length)^2). Now comes a separate question: Taking the more general combination sqrt ((X-length)^2+(Y-length)^2+(Z-length)^2), is this combination always invariant regardless of the *motion* of the two walls and the floor? Common sense has told us for centuries that the answer to that is yes. Experiments in the last century have told us the answer is no. This doesn't mean that there isn't an invariant property of the stick. It's just not the one we thought it was. However, this really shouldn't be much of a shock, at least no more than discovering that sqrt((X-length) ^2 + (Y-length)^2) isn't invariant when the stick is tipped from the floor. Now what is invariant is this: sqrt((X-length)^2+(Y-length)^2+(Z- length)^2- (t-duration/c)^2). But you see, this is no longer really appropriately called a length, because it involves a measurement of time as well, and length is a spatial quantity. If you ask, is there any *strictly* spatial quantity that is invariant regardless of the motion of the two walls and the floor? No. The one that involves just the spatial quantities is sqrt((X-length)^2+(Y- length)^2+(Z-length)^2), and that is experimentally not invariant. The one that involves more than spatial quantities is sqrt((X-length)^2+(Y- length)^2+(Z-length)^2- (t-duration/c)^2), and that one is experimentally invariant. Hopefully this makes things clearer. > > > Extending the analogy: if you want to "model" the building, it's > > clear that the relevant width is the one marked on the building's > > plans -- measured parallel to the building's front. It's clear that > > the possibility of measuring many different "widths" from other > > points of perspectives is completely irrelevant. Ditto in SR, where > > proper length and proper time are relevant to modeling phenomena > > related to a given object, and it is irrelevant that that one can > > measure other values for "length" and "time" in other frames. > > > > It should be a question taken seriously in > > > physics. > > > The appropriately related question is taken seriously. And answered easily. But > > by including a false premise as part of YOUR question (that there is a "physical > > cause"), you make YOUR question impossible to take seriously. > > No I actually didn't think that the cause was "physical", but others > have suggested it is - for example, another poster suggested that > tests have been done with laser beams that prove an object's length > has physically shrunk. Whether that is wrong, or whether it is due to > some other effect, I don't yet know. > > > A famous example: When did you stop beating your wife? > > Lol.
From: PD on 3 Feb 2010 16:13 On Feb 2, 2:35 pm, JT <jonas.thornv...(a)hotmail.com> wrote: > On 2 Feb, 20:14, PD <thedraperfam...(a)gmail.com> wrote: > > > > > On Feb 2, 11:54 am, "kens...(a)erinet.com" <kens...(a)erinet.com> wrote: > > > > On Feb 2, 10:19 am, PD <thedraperfam...(a)gmail.com> wrote: > > > > > On Feb 2, 8:45 am, kenseto <kens...(a)erinet.com> wrote: > > > > > > On Feb 1, 11:36 pm, Tom Roberts <tjroberts...(a)sbcglobal.net> wrote: > > > > > > > kenseto wrote: > > > > > > > On Feb 1, 6:42 pm, Tom Roberts <tjroberts...(a)sbcglobal.net> wrote: > > > > > > >> This is a LINGUISTIC issue. When someone asks "Is length contraction in SR > > > > > > >> physical?", they invariably don't have a definite meaning of "physical" in mind > > > > > > >> (if they did, they could answer the question themselves). Generally, they would > > > > > > >> consider a "physical length contraction" to mean that the object ITSELF gets > > > > > > >> physically shorter. This is manifestly not so in SR. But it is indeed what > > > > > > >> kenseto has repeatedly failed to understand. > > > > > > > > The general public know what physical length means.....the length of a > > > > > > > ruler is physical. > > > > > > > You clearly haven't a clue. In SR, length is most definitely not "physical" in > > > > > > the usual sense. In the USUAL sense, physical attributes of an object do not > > > > > > change with the way someone looks at it. > > > > > > Right the physical length (or material length) of a ruler does not > > > > > change no matter who is look at it. > > > > > You are not listening. Tom *just* got through telling you that > > > > "physical length" and "material length" are not necessarily > > > > synonymous, and yet you just repeated your assumption that they are.. > > > > > > >But the length of an object DOES change > > > > > > with the way one looks at the object (i.e. which inertial frame one uses to > > > > > > measure its length). > > > > > > This length change is not physical (or material) length change.....it > > > > > is a projection effect or a rotational effect. > > > > > It is not a material effect, but it IS a physical effect. Material and > > > > physical do not mean the same thing. > > > > > > Much like I see you to > > > > > be shorter from a distance. BTW no measurement of length contraction > > > > > ever been made so you do you keep on using the word measure? > > > > > > >The related PHYSICAL attribute is proper length -- that is > > > > > > an attribute of an object that is intrinsic to the object (and does not change > > > > > > with how one looks at it -- it is an invariant). > > > > > > Right....the proper length of a ruler is the physical or material > > > > > length of the ruler. It is invarient. > > > > > > > > If you don't like the word physical how about > > > > > > > "material" length contraction? In any case you stated above that > > > > > > > material length contraction does not occur in SR and I agree with that > > > > > > > fully. So I don't know why you accused me of repeatly failed to > > > > > > > understand!!!!!! > > > > > > > Indeed, you clearly do not understand why I say you fail to understand this, > > > > > > because you STILL don't understand it. Your statement three sentences earlier > > > > > > ("the length of a ruler is physical") CONTRADICTS your claim here. > > > > > > I didn't fail to understand anything. You said that in SR length > > > > > contraction is not physical and I agreed. > > > > > > > The word "physical" has at least two different meanings that could be applied here: > > > > > > So why bother to invent a new meaning for the word physical that > > > > > contradicts the dictionary meaning for the word physical??? > > > > > Words used in physics have more precise meanings than the ones found > > > > in the dictionary. If you want to communicate with physicists about > > > > physics, then it is extremely important to DROP the meanings of words > > > > as listed in the dictionary and LEARN the meanings of those same words > > > > as they are used by physicists. > > > > > > Why not > > > > > just stick to the correct meeaning of length contraction in SR: that > > > > > length contraction is a geometric projection effect??? > > > > > > > A) the object itself contracts > > > > > > B) the object itself does not change, but the measurements have > > > > > > physical consequences (such as a long pole fitting in a short barn > > > > > > when moving rapidly enough wrt the barn). > > > > > > > (A) is not correct in SR; > > > > > > I agree. > > > > > > > (B) is correct in SR. Both COULD be called "physical" > > > > > > so one MUST specify what they mean when they use that word in this context. > > > > > > No both cannot be called physical or material.... > > > > > They are both physical. Only (A) is material, but (B) is also > > > > physical. "Physical" does not mean "material". The electric field of a > > > > charged object is not material but it is very physical. > > > > > > remember physical > > > > > length or material length is invariant as you said in A. You invented > > > > > a new meaning for the word physical that is contradictory to the > > > > > dictionary meaning for for the word physical or material. B is correct > > > > > if you said that the geometric projection of a moving rod is > > > > > contracted. > > > > > It is not contradictory. It's just DIFFERENT. Physics uses certain > > > > words to have very precise and special meanings that are DIFFERENT > > > > than the definitions you will find in the dictionary. > > > > Sure it is contradictory. > > > 1. Physical length is invariant as per the dictionary. > > > Really? You'll have to cite a dictionary definition for "physical > > length" that includes the word "invariant". > > As usual, you are *assuming* connections that ought not to be made. > > I'll remind you that "physical" does NOT mean "material". > > Yes you are correct how could anyone be stupid enough to expect > physical to have any connection to a material unit. Why should it be LIMITED to a material unit? > No here we must > embrace the aether, and to really come to the core of subject we have > to go into the paranormal and quantum mechanic. > > There is no spoon........ > > No, a physical unit could there could be no such thing, such existance > would be a ludicrous assertion. > > Embrace the spoon Ken, it can be heavier then you think..... > > JT > > > > > > 2. Physical length is observer dependent as per SR . > > > Yes, indeed. > > > > These are two contradictory definition for Physical Length. > > > SR is right. Statement (2) is right. Statement (1) is not right, and > > there is no reference you can point to that makes statement (1). It > > came out of your head. > > > > Ken Seto > > > > > > > When you use the word "physical" without saying what you mean by it, people get > > > > > > confused. ESPECIALLY YOURSELF. > > > > > > There is no confusion on my part. There is only one correct meaning > > > > > for the word "physical" and I agree with you that physical length or > > > > > material length is invariant. > > > > > > Ken Seto > > > > > > > Ditto for "material"-- changing the word does not change the problem. > > > > > > > > BTW your SR brothers PD and other disagree with you. They insisted > > > > > > > that a moving meter stick itself gets physically (materially) > > > > > > > shorter. > > > > > > > Again you demonstrate your inability to read or understand. PD most definitely > > > > > > did NOT say that. I'm not sure what "others" you mean. In any case, you _REALLY_ > > > > > > _REALLY_ _REALLY_ need to learn how to read. You have no hope whatsoever of > > > > > > understanding subtle concepts like SR until you learn how to read. > > > > > > > [I give up. It's hopeless until you LEARN how to read.] > > > > > > > Tom Roberts- Hide quoted text - > > > > > - Show quoted text -- Hide quoted text - > > > > > - Show quoted text -- Hide quoted text - > > > > > - Show quoted text -- Hide quoted text - > > > > > - Show quoted text -
From: PD on 3 Feb 2010 16:21 On Feb 2, 5:09 pm, Ste <ste_ro...(a)hotmail.com> wrote: > On 2 Feb, 17:35, PD <thedraperfam...(a)gmail.com> wrote: > > > > > On Feb 2, 10:08 am, Ste <ste_ro...(a)hotmail.com> wrote: > > > > On 1 Feb, 21:33, kenseto <kens...(a)erinet.com> wrote: > > > > > What I am enquiring about are as follows: > > > > 1. Tom Roberts said that length contraction is a geometric projection > > > > effect....that means that length contraction is not physical > > > > contraction. This means that length contraction is only apparent and > > > > not physical. The problem with Tom's approach is that what is > > > > "geometric projection" mean physically??? > > > > I agree this vague language needs boiling down into something > > > comprehensible. > > > I'd recommend you start with Spacetime Physics, by Taylor and Wheeler, > > or with General Relativity from A to B, by Geroch. Both use a minimum > > of math, are accessible to high-school-level readers, and do take the > > time to make these things comprehensible. > > You shouldn't need *any* maths to describe in broad terms what this > "length contraction" is all about, and in any event anyone who > understands their subject should be able to sum it up in a few > sentences. It should not be this complex Paul, and the fact that it is > simply demonstrates that science has lapsed into obscuritanism. I'm sorry, but I find this whole expectation that "everything should be so simple that it can be explained to anyone in a few sentences" to be just crapola. It isn't true in chemistry, it isn't true in molecular biology, it isn't true in architecture, it isn't true in auto mechanics, it isn't true in musical composition. Furthermore, the attitude that "Well, if you CAN'T explain it in a satisfactory way to anyone who asks in just a few sentences, then you are guilty of obscuritanism," is similarly crapola. Not every body of knowledge should be expected to be grasped by anyone in just a few sentences. There is a reason why people spend years to become reasonably expert in an area, and not just a month. > > > > > 2. OTOH runts of the SRians such as PD asserts that length contraction > > > > is physical....a 80 meter long pole can fit into a 40 meter long barn > > > > with both doors close simultaneously for a very brief period. This > > > > assertion of length contraction is physical. > > > > I think personally this view is inconsistent with SR itself, not to > > > mention seeming like a more outlandish hypothesis than is necessary. > > > It's a necessary consequence of SR. The barn and pole puzzle is a > > TEACHING puzzle. > > It's not a necessary consequence at all. If I double the distance > between two points, it immediately takes twice as long to communicate > between the points. But that has NOTHING to do with the pole and barn puzzle. > If I then *pretend* that I haven't increased the > distance, the only other variable that can budge is time. > > > > and it would not explain > > > physically why a clock would slow down (when, by rights, one would > > > expect a clock based on any physical process to speed up as it became > > > smaller, or at least remain constant). > > > No, sir. You are not following. There is no mechanical shrinking of > > the clock that is going on. > > If there is no "mechanical shrinking", then I interpret this to mean > the contraction of lengths is *apparent*, not "real". Then again you are using your own private understandings of the term "apparent" and "real" and not those shared by physicists. I don't know why you would expect physicists to explain things using YOUR understandings of these terms. > > > > > I agree with Tom's premise with a twist. I said that the observer > > > > assumes that the light path length of his meter stick is assumed to be > > > > the physical length of his meter stick. The length contraction formula > > > > is used to determine the light path length of a meter stick moving wrt > > > > the observer. This explanation eliminates all the paradoxes encounter > > > > by the SR assertion that length contraction is physical. > > > > I think the whole thing is physically explained if you simply picture > > > an atom at rest with photons ejected in all directions, and then add a > > > forward bias to every trajectory, and you immediately see why things > > > would start to appear to slow down. > > > By all means, put such a scheme together. Be sure to show where the > > factor of 1/sqrt(1-v^2/c^2) comes from in such an explanation, because > > that part is quantitatively checked. > > I'll let you do the maths Paul - that is, after all, the virtue of a > *co-operative endeavour*, where different people bring different > skills to the table. I'm sorry but that's crapola too. I refer you to an anecdote, archived from this newsgroup by a kindly soul: http://users.telenet.be/vdmoortel/dirk/Physics/Gems/WhatItTakes.html
From: PD on 3 Feb 2010 16:30
On Feb 3, 1:55 am, Ste <ste_ro...(a)hotmail.com> wrote: > > My cheek is astonishing, is it not? Incidentally I'm not claiming SR > is "wrong", what I'm saying is that the physical basis of it appears > to be poorly understood and inadequately described. Indeed, there seem > to be as many interpretations of SR as there are observers of the > theory. This is of course a function of the material you choose to peruse on the subject. If you have the expectation that any arbitrary sample of material on a subject should generate the same quality of understanding, and that discussion with any arbitrarily populated panel should similarly provide you with the same quality of understanding, then I question your general wisdom. > > > and I find it comical that you > > think "reality" is a "radical new theory". I know Einstein basically > > made many of the arguments that I'm making. > > > ______________________________ > > I would be amazed if you and Einstein agreed on anything to do with physics. > > However, this is easily tested. Produce a quote you have made in this > > thread, and a quote from Einstein, which show the same "basic argument".. > > Perhaps we should start with "If you can't explain it simply, you > don't understand it well enough" - I forget whether I said that above, > or on a different thread. It IS possible to explain it simply. Just not in a few sentences. This is why books have been recommended to you. Good, cheap, short: Choose one. What you want is something that is easy to understand AND in a few sentences AND delivered to you on the free forum of your choice. You are not entitled to that, and no dare about "betcha can't do it" is going to change that fact. > Also "it is the theory that decides what you > can observe" - I've been trying to hammer that one home with Paul for > the past few weeks. And that is also an unwise comment that is not thought highly of by physicists. And yes, I know who said it. Not everything he said is right. > > And to describe both me and Einstein: "I have no particular talent - I > am merely inquisitive" or "It's not that I'm so smart, it's just that > I stay with problems longer". Please do not underestimate the work that Einstein put in or the skills that he mastered along the way. Please do not overestimate yourself, especially in light of how little work and how few skills you are willing to apply. > > The fact is, I'm able to conduct a reasonable debate with experts > because, for the past month, I've gone to bed thinking about physics, > and I've woken up thinking about physics, and I've spent most spare > hours thinking or reading about physics. No, I'm sorry. The world is RIFE with people who believe that doing physics involves just having a rough intuitive grasp of things, a reasonably sharp wit, and the willingness to ponder for a while. They are deluded. You have been able to engage in reasonable debate here because, and solely because, there are people here who are willing to try to inform you of some things you are sorely misinformed about. |