Is Length Contraction in SR physical??
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From: PD on 2 Feb 2010 10:52 On Feb 2, 9:40 am, Ste <ste_ro...(a)hotmail.com> wrote: > On 1 Feb, 20:52, PD <thedraperfam...(a)gmail.com> wrote: > > > On Feb 1, 2:49 pm, kenseto <kens...(a)erinet.com> wrote: > > > > So does that mean that length contraction is not physical? > > > No, it does not mean that. > > Why do you think that "observer dependent" means "not physical"? > > Lots of physical properties are observer dependent. > > Because for anyone who believes in material reality, physical > properties should not be "observer dependent", Then I would ask the following questions of you. Is velocity a physical property? It is *clearly* observer-dependent. Having answered that, is kinetic energy a physical property, given that (at least at low speeds) the kinetic energy is proportional to the square of the velocity, and so is also clearly observer-dependent. Having answered that, is the law of conservation of energy, which includes kinetic energy as one of the contributions to the conserved sum, a law about physical properties? If not, then what is it a law about? > and if physical > properties do indeed seem to change depending on how they are > observed, then there should be some coherent explanation for it rather > than just a statement that things are "observer dependent", which in > most people's minds rather leaves the glaring question "why". I think there may just be a clash regarding an unfounded expectation that physical properties "should" be observer-independent. Why would this be an expectation? The reality is that physics is littered with physical properties, about which there are many laws that seem to hold remarkably well. Some of those properties are observer-dependent, and some are observer- independent. There is no rationale required for "why" they are not all one way or the other, any more than there has to be a reason cited for "why" all animals are not mammals. PD
From: Ste on 2 Feb 2010 11:08 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. > 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. 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), 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). > 3. There seem to be contradictory claims amoung the SRians. As always. > 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.
From: eric gisse on 2 Feb 2010 11:16 JT wrote: > On 2 Feb, 07:46, 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. > > That certainly would mean that distance is observer dependent, two > objects at same distance but with relative measured distance, no it do > not make sense. > > But who cares SR is a silly theory to start with. Do you find that you often believe things professionals accept to be 'silly'? Have you gone through, say, what electrical engineers 'believe' and find that you don't understand any of it? > > JT
From: Ste on 2 Feb 2010 11:30 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.
From: J. Clarke on 2 Feb 2010 11:09
Ste wrote: > On 1 Feb, 20:52, PD <thedraperfam...(a)gmail.com> wrote: >> On Feb 1, 2:49 pm, kenseto <kens...(a)erinet.com> wrote: >> >>> So does that mean that length contraction is not physical? >> >> No, it does not mean that. >> Why do you think that "observer dependent" means "not physical"? >> Lots of physical properties are observer dependent. > > Because for anyone who believes in material reality, physical > properties should not be "observer dependent", and if physical > properties do indeed seem to change depending on how they are > observed, then there should be some coherent explanation for it rather > than just a statement that things are "observer dependent", which in > most people's minds rather leaves the glaring question "why". The "why" is because it is in the nature of spacetime to be that way. Physics is decriptive. |