Is Length Contraction in SR physical??
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From: Ste on 3 Feb 2010 15:11 On 3 Feb, 18:19, kenseto <kens...(a)erinet.com> wrote: > On Feb 3, 12:11 pm, PD <thedraperfam...(a)gmail.com> wrote: > > > > No it was specified that, in the barn frame, the barn doors close > > > simultaneously for a very brief period while the pole is completely > > > inside the barn. This requires real physical contraction and not > > > observer dependent. > > > No, it doesn't. You obviously don't understand the pole and barn > > puzzle at all. > > No it is you who don't understand the pole and the barn paradox. Presumably the discussion can move forward by discussing how you each differ in your understandings.
From: PD on 3 Feb 2010 15:36 On Feb 3, 12:19 pm, kenseto <kens...(a)erinet.com> wrote: > On Feb 3, 12:11 pm, PD <thedraperfam...(a)gmail.com> wrote: > > > > > On Feb 3, 10:04 am, kenseto <kens...(a)erinet.com> wrote: > > > > On Feb 3, 4:41 am, artful <artful...(a)hotmail.com> wrote: > > > > > On Feb 3, 4:18 pm, Tom Roberts <tjroberts...(a)sbcglobal.net> wrote: > > > > > > Uncle Ben wrote: > > > > > > On Feb 1, 11:36 pm, Tom Roberts <tjroberts...(a)sbcglobal.net> wrote: > > > > > >> [...] > > > > > > > Tom Robrts takes the conservative position on what is "physical." > > > > > > Hmmm. I tried not to make any statement about what is or is not "physical", > > > > > because that word is too ambiguous. > > > > > > To me it is irrelevant whether one considers this or that quantity to be > > > > > "physical". What is important is whether or not a given quantity can be an > > > > > appropriate model for some physical phenomenon. For that, it's QUITE CLEAR that > > > > > no coordinate-dependent quantity can be a valid model of any physical > > > > > phenomenon, as arbitrary human choices cannot possibly affect physical > > > > > phenomena. Nor can the perspective from which one looks at an object affect the > > > > > object itself. Coordinates are, of course, arbitrary human choices that define > > > > > the perspective one uses to look at and describe objects and situations. > > > > > > > To > > > > > > be consistent, he would have to deny physicality to kinetic energy and > > > > > > to the magnetic field of a moving charge. Or even motion itself. > > > > > > I deny that any of those can be valid models for physical phenomena. I make no > > > > > statement about their "physicality" -- arguments over word meanings are > > > > > uninteresting (but inappropriate word meanings must be dealt with before the > > > > > real discussion can even begin). > > > > > > In every case I know of, if you analyze the physical situation sufficiently > > > > > well, you will find an appropriate quantity that is a valid model for the > > > > > physical phenomena in question. For instance, when considering a collision > > > > > between two particles, don't use kinetic energy, use the Mandelstamm s (total > > > > > energy squared in their center-of-momentum frame); instead of magnetic field, > > > > > use the Maxwell 2-form; instead of motion, use the particles' individual > > > > > trajectories. Tensor and geometric analysis provide methods to analyze all > > > > > situations of interest in a coordinate-free manner. This is one of the major > > > > > lessons of GR (but it took about a half-century to sink in). > > > > > > > Or do > > > > > > I not understand? > > > > > > The issue is more subtle than you seem to think. It is not merely about the > > > > > meanings of words, or about what is or is not "physical", it is about what types > > > > > of quantities can be used to model physical phenomena. > > > > > > Tom Roberts > > > > > At first reading you seem to be equating 'physical' with 'frame > > > > invariant'. ie. Only things that are not dependent on the observer > > > > are physical. > > > > > But am I right in my assessment that you are really saying that it is > > > > only nature / reality itself that is physical. The measurements and > > > > calculations we make are parts of our models of reality .. and so are > > > > never really 'physical' themselves. The best models (and > > > > measurements) for reality are those that are not observer dependent, > > > > because physical reality is not observer dependent (ignoring some > > > > interpretations of QM :)). > > > > > In the case of length contraction, what we define as length (roughly > > > > speaking: the spatial distance between two simultaneous events in a > > > > given time) is contracted .. even though the proper interval is > > > > invariant. In both cases they are valid (but different) measurements > > > > of the same pair of events. > > > > > Ken's claim that contraction not being 'physical' means a pole doesn't > > > > physically fit between the barn doors at the same time in the barn > > > > frame of reference (in the well-known 'paradox'). I guess the issue > > > > there is really whether 'between the barn doors at the same time in > > > > the barn frame of reference' itself is physical .. as it is observer / > > > > frame dependent. > > > > No it was specified that, in the barn frame, the barn doors close > > > simultaneously for a very brief period while the pole is completely > > > inside the barn. This requires real physical contraction and not > > > observer dependent. > > > No, it doesn't. You obviously don't understand the pole and barn > > puzzle at all. > > No it is you who don't understand the pole and the barn paradox. Ken, look again. It is stated explicitly in the pole and barn paradox that in the pole frame, the pole is LONGER than the barn. This means the physical shortening of the rod obviously is not frame-independent. If it required the rod to be physically shorter to all observers, then it would be claimed to be shorter than the barn in the pole frame, too. Since this is not claimed, then it is not required to be observer independent. Here's a quarter. Buy a clue. > > > > > > Ken Seto > > > > > Obviously there is *some* 'physical' relationship between the barn > > > > doors and pole that means an observer in that frame would measure the > > > > pole as being within the barn. How would you best describe that > > > > relationship? Is talking about 'between the barn doors at the same > > > > time in the barn frame of reference' something valid and meaningful to > > > > say?- Hide quoted text - > > > > > - Show quoted text -- Hide quoted text - > > > - Show quoted text -- Hide quoted text - > > > - Show quoted text - > >
From: PD on 3 Feb 2010 15:49 On Feb 3, 2:11 pm, Ste <ste_ro...(a)hotmail.com> wrote: > On 3 Feb, 18:19, kenseto <kens...(a)erinet.com> wrote: > > > On Feb 3, 12:11 pm, PD <thedraperfam...(a)gmail.com> wrote: > > > > > No it was specified that, in the barn frame, the barn doors close > > > > simultaneously for a very brief period while the pole is completely > > > > inside the barn. This requires real physical contraction and not > > > > observer dependent. > > > > No, it doesn't. You obviously don't understand the pole and barn > > > puzzle at all. > > > No it is you who don't understand the pole and the barn paradox. > > Presumably the discussion can move forward by discussing how you each > differ in your understandings. Not really. The pole and barn puzzle is documented in black and white. Some people cannot comprehend what they read in a coherent manner. If 98% of the people who read the documented puzzle and understand it to mean one thing, and the remaining 2% who read the documented puzzle understand it to mean a variety of other things, can a discussion between a member of the majority and a member of the minority move substantially forward without resolving the difference?
From: PD on 3 Feb 2010 15:51 On Feb 2, 7:02 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. But it is, according to the *definition* of simultaneity, and experimental evidence. Insisting that it is *not* frame dependent is to either declare that there is something wrong with the tested laws of physics or that there is something wrong with the experimental evidence. In which case, one would be naturally tempted to ask how it is you KNOW that simultaneity is not frame-dependent, other than it being an article of faith? If you would like, we can discuss both the unambiguous definition of simultaneity and the experimental evidence that shows that your belief is not matched by observation. This evidence fully accounts for propagation time. It's apparent to me that you've never been educated on this. > > > > > 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". Two comments: - Propagation time can be accounted for, and it IS in the analysis alluded to above, and it is STILL true that after this accounting, two events that are simultaneous in one frame CANNOT be simultaneous in another frame, according to the laws of physics. - Aside from this, it is the laws of nature that say that information cannot be passed instantaneously. Thus, it is a smallish bit of foolishness to attempt to describe how nature works if only the laws of nature were not in effect.
From: PD on 3 Feb 2010 15:52
On Feb 3, 10:32 am, kenseto <kens...(a)erinet.com> wrote: > On Feb 2, 10:46 pm, artful <artful...(a)hotmail.com> wrote: > > > > > On Feb 3, 2:29 pm, Ste <ste_ro...(a)hotmail.com> wrote: > > > > 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. You seem to be saying that if a tree falls in the woods > > > and no one is around to hear it, then it didn't happen. > > > How could you possibly draw that conclusion from what I wrote? It > > appears you do not know what you are disagreeing with. > > > > And moreover, > > > the tree falls earlier for a person near to the tree than for a person > > > further away. But I reject these arguments. > > > Noone is making such arguments. > > > > The bottom line is that > > > the tree falls, and it falls at the same time for all people, > > > regardless of whether some may observe the event later than others. > > > SR also says if an event happens it happens for everyone, regardless > > of when it is observered > > So if two events happened simultaneously they happened simultaneously > for everyone regardless of when they are observed......right? That's what STE says. But then again, STE is talking out of his hat, without any knowledge of what experimental observations show. > > Ken Seto > > > > > You seem to be arguing against things that people here are NOT saying- Hide quoted text - > > > - Show quoted text - |