Is Length Contraction in SR physical??
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From: Tom Roberts on 3 Feb 2010 00:34 PD wrote: > On Feb 1, 10:36 pm, Tom Roberts <tjroberts...(a)sbcglobal.net> wrote: >> 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). > > Just a small quibble here. "Proper length" is a synonym for "spacetime > interval" for space-like-separated events, Hmmm. For a given object, its proper length can be measured between any pair of events at its endpoints that happen at any times, equal or different, as long as the measurements are made in its rest frame [#]. This is another ambiguity in "length" that has not been touched on (and which applies only in the object's rest frame). The proper length between such pairs is constant; not so the invariant interval between such pairs. That's because we are projecting the interval between those pairs onto the object's rest frame, which is the frame in which its "proper length" is defined. [#] That is, if I hold a ruler up to an object and ensure there is no relative movement between them, I do not need to mark the object's endpoints simultaneously to obtain a correct measurement of its proper length. > [...] For an arbitrary pair of spacelike-separated events I agree with all you say, but for the proper length OF AN OBJECT I do not. We learned in Kindergarten that to measure the length of something you must apply the ruler to it without slipping. Tom Roberts
From: Tom Roberts on 3 Feb 2010 00:39 PD wrote: > I > find frame-dependent quantities like kinetic energy and magnetic field > to be arguably physical, even if frame-dependent and coordinate- > dependent. And I argue that you are mistaken -- you confuse such coordinate-dependent quantities with the more fundamental, underlying quantities that are invariants: kinetic energy => s magnetic field => Maxwell 2-form ... etc. Tom Roberts
From: Tom Roberts on 3 Feb 2010 00:44 Ste wrote: > "physicists" should be using > a theory for which there is no definition of "physical", and moreover > there is no attempt being made to discern it's "physical" basis. Define "physical" however you wish, and use it that way. NOTHING about any theory of physics will change. Nor will the validity of any of their predictions. All that will change is that you PERSONALLY will feel better. But that is not science. Tom Roberts
From: Ste on 3 Feb 2010 02:55 On 3 Feb, 04:08, "Peter Webb" <webbfam...(a)DIESPAMDIEoptusnet.com.au> wrote: > > 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. > > I'm not pretending to be a physicist - > > __________________________ > And yet you claim that all physicists are wrong about SR. 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. > 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. 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 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". 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.
From: Ste on 3 Feb 2010 03:06
On 3 Feb, 04:19, YBM <ybm...(a)nooos.fr.invalid> wrote: > Ste a écrit : > > > > > > > On 3 Feb, 02:51, artful <artful...(a)hotmail.com> wrote: > >> On Feb 3, 12:02 pm, Ste <ste_ro...(a)hotmail.com> wrote: > > >>> On 2 Feb, 20:10, PD <thedraperfam...(a)gmail.com> wrote: > >>>> On Feb 2, 9:24 am, Ste <ste_ro...(a)hotmail.com> 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. It should be a question taken seriously in > >>>>> physics. > >>>> And it is. > >>>> I will give you just a sketch, meant to inspire further investigation, > >>>> not to teach. > >>>> The physical cause is traced to the *meaning* of the physical property > >>>> length, which is the difference in the locations of the endpoints of > >>>> something, where those locations are measured *simultaneously*. The > >>>> importance of the simultaneity becomes apparent if you try to measure > >>>> the locations of the endpoints of, say, a moving car and attribute the > >>>> difference to the length. Then in turn, it becomes clear that length > >>>> is frame-dependent if simultaneity is frame-dependent -- the frame- > >>>> dependence of simultaneity is the physical cause of the frame- > >>>> dependence of length. > >>> Ah, now I know this is the difference between us. I hold that > >>> "simultaneity" is *not* frame dependent. > >>>> So now the question becomes, what is the > >>>> physical reason for the frame-dependence of simultaneity? This in turn > >>>> is traced to the *meaning* of simultaneity, and how we would know that > >>>> two spatially separated events are simultaneous; this in turn hinges > >>>> on the synchronous arrival of equal-speed signals from the events at > >>>> the midpoint between the events. From here it is relatively simple to > >>>> show that the frame-independence of the speed of light *demands* that > >>>> for a given pair of events, that definition of simultaneity will be > >>>> satisfied in one reference frame and not in another reference frame. > >>>> That is, the frame-independence of the speed of light is the physical > >>>> cause of the frame-dependence of simultaneity, which in turn is the > >>>> physical cause of the frame-dependence of length. So then the question > >>>> is, what is the physical cause of the frame-independence of the speed > >>>> of light? The answer to this question has to do with the hyperbolic > >>>> structure of space-time and the fact that causal relationships are > >>>> limited to a region of spacetime called the past- and future-light > >>>> cones. This hyperbolic structure of space and time is thus the > >>>> physical cause of the frame-independence of the speed of light, which > >>>> is in turn the physical cause of the frame-dependence of simultaneity, > >>>> which in turn is the physical cause of the frame-dependence of length. > >>>> To expand this in a way that would be compellingly easy to follow > >>>> would take more pages than I'm willing to invest. Fortunately, this is > >>>> done in numerous books designed to explain these things in a > >>>> compellingly easy-to-follow fashion. Would you like some > >>>> recommendations? > >>> You have in fact answered the whole of my question here Paul. The > >>> difference lies in our concepts of "simultaneity". As I said, my > >>> concept of "simultaneous" is "those events which occur at the same > >>> instant, and would be observed to do so universally, *if* information > >>> about the occurence of those events could be conveyed instantaneously". > >> Simultaneity issues in SR are nothing to do with instantly conveying > >> information. > > >> According to SR your concept of simultaneity being universal does not > >> hold in reality (as nice and simple and idea as it is). > > > I disagree. > > Then you could be interesteed by this (espacially bibiography) :http://plato.stanford.edu/entries/spacetime-convensimul/ The article seems overly concerned with establishing and measuring simultaneity, which is a different question from whether some "absolute" simultaneity exists. > > You seem to be saying that if a tree falls in the woods > > and no one is around to hear it > > It's not what he is saying. Please pay attention. The point is that the tree either falls when an observer becomes aware of it (and therefore potentially the tree never falls), or it falls at some other, earlier, time, which is independent of observation. I hold the latter position to be true. |