Is Length Contraction in SR physical??
in [Relativity]
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From: JT on 2 Feb 2010 17:00 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. 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: Ste on 2 Feb 2010 18:09 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. > > > 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. 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". > > > 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.
From: Ste on 2 Feb 2010 18:18 On 2 Feb, 17:38, PD <thedraperfam...(a)gmail.com> wrote: > On Feb 2, 10:30 am, Ste <ste_ro...(a)hotmail.com> wrote: > > > On 2 Feb, 13:55, kenseto <kens...(a)erinet.com> wrote: > > > > On Feb 1, 6:38 pm, artful <artful...(a)hotmail.com> wrote: > > > > > On Feb 2, 1:55 am, "kens...(a)erinet.com" <kens...(a)erinet.com> wrote: > > > > > > Is Length Contraction in SR physical?? > > > > > Define 'physical' > > > > Look up the dictionary. > > > I have a better suggestion. "Physical" is what the world would look > > like if observed by God, where information is conveyed instantaneously > > and is not subject to noise imparted by any other physical process. > > That's an interesting concept. > What we've gleaned from the laws of physics so far is that no > information is conveyed instantaneously, ever. There does not seem to > be a visible exception to this regularity of nature. > So it appears that what you think should be regarded as physical is > how nature is, if the laws of physics that nature appears to respect > were violated. > Or another way of saying it is, physicists should be trying to > understand the regularities and rules that nature operates by, but by > discarding the regularities and rules so far observed. > Do I have this right? No, it's about stepping outside of mere observation and having a hypothesis about what is *really* happening, which is (to a certain degree) not supported by the observations alone. Physicists seem to be stuck in this problem of recognising that what is observed depends on the observer's circumstances (much in the same way that the Sun rises and sets at different times depending on where you are on the Earth), but they don't seem to be able to (or even concerned to) take a leap and actually start talking about reality, instead of just conceiving of infinitely complex mathematical rules that will determine when the Sun rises and sets based on geographic position, without actually saying *anything at all* about the fundamental physical basis of these observational discrepancies.
From: waldofj on 2 Feb 2010 18:59 > 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. AH, h2so4 professor. Don't synthesize anything I wouldn't synthesize. Oh, and the reciprocal of pi to your good wife.
From: J. Clarke on 2 Feb 2010 18:50
Ste 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. Lemme guess--you've got a degree in "education"? >>>> 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. If I then *pretend* that I haven't increased the > distance, the only other variable that can budge is time. This is what happens when you try to understand relativity without the math. Calculate the trajectory, don't wave your arms about spouting bullshit. >>> 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". An observer in one reference frame sees it--it is "real" to him. Another in another reference frame does not. This is why it's called "relativity". >>>> 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. You are the one presenting some radical new theory which if true will set the Physics world on its ear and gain you the Nobel. If you can't do the math then perhaps you shouldn't be pretending to be a physicist. |