Is Length Contraction in SR physical??
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From: Ste on 2 Feb 2010 12:20 On 2 Feb, 06:46, Sebastian Garth <sebastianga...(a)gmail.com> wrote: > On Feb 1, 8:03 pm, kenseto <kens...(a)erinet.com> wrote: > > > I suggest that you go argue with your SR brother Tom Roberts. He said: > > "Generally, they would consider a "physical length contraction" to > > mean that the object ITSELF gets > > physically shorter. This is manifestly not so in SR". > > Not true. Length contraction is a truly physical phenomena which is > absolutely measurable. Fire two laser beams perpendicularly across the > front and rear of the moving object; the change in length is not an > illusion - within the observer's frame of reference, the object indeed > occupies less space. And what is observed when one does this?
From: PD on 2 Feb 2010 12:35 On Feb 2, 10:08 am, Ste <ste_ro...(a)hotmail.com> wrote: > On 1 Feb, 21:33, kenseto <kens...(a)erinet.com> wrote: > > > > > On Feb 1, 2:03 pm, Ste <ste_ro...(a)hotmail.com> wrote: > > > > On 1 Feb, 17:25, PD <thedraperfam...(a)gmail.com> wrote: > > > > > 2. Secondly, he did NOT say that the length contraction is not > > > > physical. What he did say is that the word "physical" has an ambiguous > > > > meaning at best and does not mean what you think it means. What he did > > > > say is that physics deals with what is MEASURED, and what is measured > > > > is therefore physical in that sense. It was you and ONLY you that said > > > > that if it is a geometric effect (which it is), then it CANNOT be > > > > called a physical effect. Tom did not make that statement. You did. > > > > I think others will contend that "what is measured" does not > > > necessarily relate to a physical reality. After all, if I have an > > > elastic meter ruler, and apply various amounts of tension to it, then > > > the measurement of an object can be changed without any "physical" > > > change in that object. On the other hand, if the object to be measured > > > is also elastic, and I apply the same amount of tension to the object > > > as well as to the meter ruler, then there are "physical" changes even > > > though there is no change in the measurement. > > > > It is this discrepancy that I believe Ken is enquiring about.. > > > 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. > > > 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. > > That is, if the speed of light remains constant, then a physical > "length contraction" would cause an *increase* in the measured speed > of light within reference frame (because distances, and therefore > propagation times, would be reduced), No sir. You are not following. > 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. > > > 3. There seem to be contradictory claims amoung the SRians. > > As always. "Seems" is the operative word. The purpose of puzzles like the twin puzzle or the barn and pole puzzle are to illustrate that a *seeming* paradox is really due to a superficial understanding of what SR actually says. Simple sounding statements like "moving clocks run slow" are deceptive to the point where they can yield nonsensical conclusions, of exactly the type these puzzles elicit. > > > 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.
From: PD on 2 Feb 2010 12:38 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?
From: Sam Wormley on 2 Feb 2010 12:43 On 2/1/10 8:03 PM, kenseto wrote: > > I suggest that you go argue with your SR brother Tom Roberts. He said: > "Generally, they would consider a "physical length contraction" to > mean that the object ITSELF gets physically shorter. This is manifestly not so in SR". > > Ken Seto > Depends on who's measuring, Seto!
From: Sam Wormley on 2 Feb 2010 12:47
On 2/2/10 7:11 AM, JT wrote: > > But who cares SR is a silly theory to start with. > > JT Practical application of SR http://en.wikipedia.org/wiki/Special_relativity#Application_in_cyclotrons |