A constant speed of light in all reference frames? Surely you can't be serious.
in [Relativity]
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From: Bruce Richmond on 28 Feb 2010 11:33 On Feb 28, 1:54 am, Ste <ste_ro...(a)hotmail.com> wrote: > On 27 Feb, 15:54, PD <thedraperfam...(a)gmail.com> wrote: > > > > > > > On Feb 26, 6:54 pm, Ste <ste_ro...(a)hotmail.com> wrote: > > > > On 26 Feb, 17:34, PD <thedraperfam...(a)gmail.com> wrote: > > > > > On Feb 25, 9:05 pm, Ste <ste_ro...(a)hotmail.com> wrote: > > > > > > I also doubt one exists at the moment, but I see that as a problem. > > > > > It's utterly irreconcilable, within any conceptual framework that I > > > > > know of, to have situations where, for example, a large ladder can end > > > > > up in a smaller barn according to an observer stationary in the barn, > > > > > but not according to an observer riding the ladder. > > > > > Why? And here we can systematically trace back to the assumptions you > > > > are making and then question them. In this case, you have a firm > > > > belief that length is definable in such a way that it is intrinsic to > > > > the object and frame-independent, and that physical "fitting" is a > > > > function of the *intrinsic* lengths of two objects (or an object and a > > > > container). > > > > My only contention is that it is *not realistic* to say that from the > > > barn frame frame the ladder contracts and fits inside, while saying > > > that from the ladder frame it is the barn that contracts and the doors > > > actually never shut simultaneously. It is simply not realistic. > > > I don't know what basis you have for judging whether something is > > "realistic". > > I must admit I can't quite put my finger on it myself. > > > I'm guessing that it means that it is consistent with > > your intuition, and that your intuition tells you that something > > cannot fit in one frame and not fit in another, or that two events are > > simultaneous in one frame and not simultaneous in another. If this is > > accurate, then I would ask on what basis you trust your intuition. Or, > > even more aptly, why do you trust your intuition so much that you rule > > out other possibilities as real if they conflict with your intuition? > > Because on the one hand my physical (i.e. practical-mechanical) > intuitions are well-developed and highly consistent with my experience > of reality, and secondly it is not readily apparent that SR is > inconsistent with these intuitions. In the face of people who suggest > the two are inconsistent, obviously I've got to first consider whether > there's a language difference between me and the opponent (i.e. no > common apprehension of verbal meaning), and secondly whether the > opponent is confused or simply wrong (i.e. no common apprehension of > the evidence). > > The ultimate resolution of this question seems to be confounded > firstly by the degree to which there is no shared language, secondly > the degree to which opponents seem to be unclear about the conceptual/ > qualitative basis of SR, and thirdly the preconceptions and > psychological style of many posters. > > > > If > > > such a thing appears to happen, then it is obviously an artefact of > > > subjective observation. > > > I disagree. In science, if there is a conflict between experimental > > observation and intuition, then it is *intuition* that becomes > > suspect, not the experimental result, especially if the latter is > > confirmed independently and by complementary means. > > I'm afraid there is no room for a discrepancy between intuition and > observation. Intuition is supposed to account for observation, and > there is no question of observation taking a back seat to intuition. > So that in that way we agree. > > But this is not the same as crude observationalism. > > > > > > > > > > > Your disbelief of SR stems from the fact that you don't understand it. > > > > > > My disbelief, really, stems from the blatant lack of conceptual > > > > > understanding of the theory. I mean, as I repeatedly point out, I > > > > > don't know a single equation of relativity, and yet I can root out the > > > > > conceptual contradictions immediately when people here have a crack at > > > > > making meaningful qualitative statements in SR. The classic example, > > > > > of course, was Paul's contention that "what is simultaneous in one > > > > > frame can never be simultaneous in another", which of course isn't > > > > > true according to SR. > > > > > I'm sorry? It is very much true in SR that two spatially separated > > > > events that are simultaneous in one frame are not simultaneous in > > > > another frame moving relative to the first. > > > > But I contradicted that when I pointed out that two observers can be > > > moving relative to each other, and yet undoubtedly events can be > > > simultaneous for both. > > > Not spatially separated ones, no. > > I'm confused, because I thought we previously agreed that two > observers travelling along the same axis, maintaining equidistance > from both events at all times, would both report each event as > simultaneous with the other event. And moreover, if they not only both > maintained equidistance from both events, but if they maintained a > separation which was equal for both observers (which, if both > observers are moving relative to each other, requires either a > collision course between observers, or travel in diametrically > opposite directions), then there is no question that the signals are > received simultaneously. > > Illustration: > > E1 > > -------- > > E2 > > The line represents the line between events E1 and E2, along which the > observers may move while always reporting both events to be > simultaneous. You are correct about E1 and E2 being simultaneous to all observers on the line despite their motion relative to other observers on the line. The statement about spatially seperated events is about seperation along the axis of travel. In the train experient A and B are on the tracks, which we call the x axis. You have E1 and E2 off to the sides of the tracks on the y axis, which isn't normally considered in the train experiment. See if this makes sense to you. We were given the definition of simultaneous, that if an observer at the midpoint between two strikes sees the flashes at the same instant the strikes were simultaneous. We have four events given to us. 1. A strike hits A and A' when they are together. 2. A strike hits B and B' when they are together. 3. M and M' pass each other before observing either strike. 4. M observes the strikes at A and B at the same instant. It is not given that the strikes were simultaneous. It is up to M and M' to figure that out on their own, based on measurements made in their own coordinate system. M knows that A and B are the same distance from him. Since both flashes traveled at the same speed, the same time elapsed as they were traveling to him. And since the flashes arrived at the same instant, the strikes must have happen at the same instant *as measured on clocks in his coordinate system*. Now let's look at things after the strikes but before the flashes have arrived. A A' F1 M M' F2 B B' A and A' were together when F1 was emitted. B and B' were together when F2 was emitted. There is one and only one wave front moving outward frome each strike. It was given that both flashes will reach M at the same instant. From the diagram you can see that at the instant M sees the two flashes F2 has already been observed passing M', while F1 has not yet reached M'. So the flashes do not reach M' at the same instant. M' knows that A' and B' are the same distance from him. Since both flashes traveled at the same speed, the same time elapsed as they were traveling to him. And since the flashes arrived at different times, the strikes must have happen at different times *as measured on clocks in his coordinate system*. From the tracks it looks like the clocks on the train were set out of sync to force the transit time of the flashes to be the same, but on the train they were just following the clock sync procedure. The important thing is that according to the clocks on the train the flashes happen at different times. In the train frame M just happen to be at the right spot for the two flashes that happen at different times to reach him at the same instant. Hope this helps. Some here don't seem to understand that it's not the math that is difficult to grasp, it is what the math is modeling that isn't clear. For me it was easier to see this in terms of LET. Neither frame is at rest wrt the ether, but both must sync their clocks in such a way that the same flash is measured to travel at c in their own frame. I figured out how to do that using LET, and since SR and LET use the same math, it must work for SR. Given that math you can then see that things are symetric. If you pick a third frame to view things from, such that the first two are moving in opposit directions at the same speed, then the first two will be equally contracted and their clocks will tick at the same rate. Try drawing some space,time diagrams. That is using the x and t axis. It's not hard to do and it shows the geometry you keep hearing about. Again, knowing how to draw the lines doesn't explain what is happening, but it does make it easier to see relationships between frames after you figure out what's going on. Bruce - Hide quoted text - > > - Show quoted text -- Hide quoted text - > > - Show quoted text -
From: Bruce Richmond on 28 Feb 2010 12:20 On Feb 27, 8:42 am, Jerry <Cephalobus_alie...(a)comcast.net> wrote: > On Feb 27, 7:00 am, "Peter Webb" > > > > > > <webbfam...(a)DIESPAMDIEoptusnet.com.au> wrote: > > "Ste" <ste_ro...(a)hotmail.com> wrote in message > > > You can teach the child maths. But you can't tell him anything about > > > the nature of the universe (because otherwise you're providing the > > > conceptual model by the back door, when it is your argument that this > > > is not necessary to understand physics, only the maths is required). > > > That was not my argument. > > > But I still don't get what your problem is with the conceptual model of SR > > provided by Minkowski space-time? It is isomorphic to SR, uses only simple > > geometry, and all the key equations of SR become simple geometric > > constructs. Minkowski contributed nothing to SR except a superb conceptual > > model, apparently exactly the thing you want. > > > So why not learn it? > > You should give Ste a specific book recommendation: > > Spacetime Physics, by Taylor and Wheeler > > The second edition can generally be picked up used for about $25. > The first edition is frequently found on eBay with a "Buy it now" > price of $5 to $10. > > To Ste: > I actually prefer the first edition. It is a relatively thin, > large format paperback that is deceptively easy to skim through > without understanding. It is not a book for skimming. It has lots > of problems with solutions, and the only way to truly learn the > subject is to WORK THE PROBLEMS!!! None of the problems uses > advanced math. If you can work the problems and get the correct > answers, only then will you really understand what relativity is > all about. > > As Peter pointed out, Minkowski spacetime is a superb conceptual > model, and not at all difficult to understand provided that you > take the time to learn it properly. > > There is no shortcut to learning how to do the math. > But the math is simple! > > Jerry- Hide quoted text - > > - Show quoted text - IMO that book stinks for explaining SR. It presents the math but doesn't provide the underlying reason for the math. To make matters worse it sometimes assumes the reader knows things without stating them. For example, in one of the first problems Billy (or whoever) *sees* a rocket fly by. He sees a spark as it passes a door frame at such and such a time. We then do some calculations. It was never mentioned that the times Billy uses are coordinate times read from clocks at the point where the event takes place, as opposed to the time on Billy's watch, including travel time. Another thing I don't like is their constant repetition of how things aren't what we expect because we aren't used to dealing with the speeds involved. That's BS. Things aren't always what we expect because we aren't used to switching pespectives. When you are driving down the road the rain drops that fall straight down for the observer on the side of the road fall diagonally. There is nothing strange about that, it is just a different perspective. The goal here should be to make sense of what you see, not to show how weird they are. A good book for that is "Relativity And Common Sense" by Bondi. Bruce
From: Tom Roberts on 28 Feb 2010 12:39 Paul Stowe wrote: > BTW, are unobservables science? It depends on what you mean. For instance, many/most current scientific theories contain unobservable quantities. The presence of unobservable quantities does not disbar a theory from the realm of science, as long as the theory itself is testable, and satisfies the tests. It is in the nature of human endeavors that not all aspects of a theory will be tested. It requires judgment and experience to evaluate whether a given theory as been tested sufficiently well. At present, SR, GR, and the standard model meet the criteria of the physics community at large; string theory and loop quantum gravity do not. Note that every theory has a domain of applicability, and at present for the standard model that extends only to 2 TEV or so, which is why the LHC results are so eagerly anticipated. Science is the process of formulating theories and testing them experimentally. A theory consisting only of unobservable quantities would not belong to science, as it could not be tested (e.g. most theologies). Theories with inherently unobservable processes or quantities at their core are unlikely to be accepted by the community as scientific (e.g. "creation science", "intelligent design", Lorentz Ether Theory). With regard to that last example, our knowledge and experience have grown significantly since 1904. What could credibly be presented as science in 1904 might not meet the standards of today. It's pretty clear that applies to LET. Indeed, Lorentz himself said something similar in ~1921. Tom Roberts
From: Paul Stowe on 28 Feb 2010 13:54 On Feb 28, 9:39 am, Tom Roberts <tjroberts...(a)sbcglobal.net> wrote: > PaulStowewrote: > > BTW, are unobservables science? > > It depends on what you mean. For instance, many/most current scientific theories > contain unobservable quantities. The presence of unobservable quantities does > not disbar a theory from the realm of science, as long as the theory itself is > testable, and satisfies the tests. > > It is in the nature of human endeavors that not all aspects of > a theory will be tested. It requires judgment and experience to > evaluate whether a given theory as been tested sufficiently > well. At present, SR, GR, and the standard model meet the criteria > of the physics community at large; string theory and loop quantum > gravity do not. Note that every theory has a domain of > applicability, and at present for the standard model that extends > only to 2 TEV or so, which is why the LHC results are so eagerly > anticipated. > > Science is the process of formulating theories and testing them experimentally. > A theory consisting only of unobservable quantities would not belong to science, > as it could not be tested (e.g. most theologies). Theories with inherently > unobservable processes or quantities at their core are unlikely to be accepted > by the community as scientific (e.g. "creation science", "intelligent design", > Lorentz Ether Theory). > > With regard to that last example, our knowledge and experience > have grown significantly since 1904. What could credibly be > presented as science in 1904 might not meet the standards of > today. It's pretty clear that applies to LET. Indeed, Lorentz > himself said something similar in ~1921. > > Tom Roberts And that knowledge and experience applies equally well to Lorentz's and Pioncare's works as to any other. When presented SR was applied only to phenomena known 'at that time'. As new discoveries are made it is natural to apply the same rules unless observations suggest otherwise. Thus your comment about observables... In Lorentz's model the local invariance is just as much universal as it is in SR. In fact, his model's physical basis demands it. Moreover, his model naturally accepts the fact that light speed can vary from one region to another since it is the resultant of density and compressibility. If either varies c does to. Paul Stowe
From: Paul Stowe on 28 Feb 2010 14:19
On Feb 28, 8:33 am, Bruce Richmond <bsr3...(a)my-deja.com> wrote: > On Feb 28, 1:54 am, Ste <ste_ro...(a)hotmail.com> wrote: > > > > > > > On 27 Feb, 15:54, PD <thedraperfam...(a)gmail.com> wrote: > > > > On Feb 26, 6:54 pm, Ste <ste_ro...(a)hotmail.com> wrote: > > > > > On 26 Feb, 17:34, PD <thedraperfam...(a)gmail.com> wrote: > > > > > > On Feb 25, 9:05 pm, Ste <ste_ro...(a)hotmail.com> wrote: > > > > > > > I also doubt one exists at the moment, but I see that as a problem. > > > > > > It's utterly irreconcilable, within any conceptual framework that I > > > > > > know of, to have situations where, for example, a large ladder can end > > > > > > up in a smaller barn according to an observer stationary in the barn, > > > > > > but not according to an observer riding the ladder. > > > > > > Why? And here we can systematically trace back to the assumptions you > > > > > are making and then question them. In this case, you have a firm > > > > > belief that length is definable in such a way that it is intrinsic to > > > > > the object and frame-independent, and that physical "fitting" is a > > > > > function of the *intrinsic* lengths of two objects (or an object and a > > > > > container). > > > > > My only contention is that it is *not realistic* to say that from the > > > > barn frame frame the ladder contracts and fits inside, while saying > > > > that from the ladder frame it is the barn that contracts and the doors > > > > actually never shut simultaneously. It is simply not realistic. > > > > I don't know what basis you have for judging whether something is > > > "realistic". > > > I must admit I can't quite put my finger on it myself. > > > > I'm guessing that it means that it is consistent with > > > your intuition, and that your intuition tells you that something > > > cannot fit in one frame and not fit in another, or that two events are > > > simultaneous in one frame and not simultaneous in another. If this is > > > accurate, then I would ask on what basis you trust your intuition. Or, > > > even more aptly, why do you trust your intuition so much that you rule > > > out other possibilities as real if they conflict with your intuition? > > > Because on the one hand my physical (i.e. practical-mechanical) > > intuitions are well-developed and highly consistent with my experience > > of reality, and secondly it is not readily apparent that SR is > > inconsistent with these intuitions. In the face of people who suggest > > the two are inconsistent, obviously I've got to first consider whether > > there's a language difference between me and the opponent (i.e. no > > common apprehension of verbal meaning), and secondly whether the > > opponent is confused or simply wrong (i.e. no common apprehension of > > the evidence). > > > The ultimate resolution of this question seems to be confounded > > firstly by the degree to which there is no shared language, secondly > > the degree to which opponents seem to be unclear about the conceptual/ > > qualitative basis of SR, and thirdly the preconceptions and > > psychological style of many posters. > > > > > If > > > > such a thing appears to happen, then it is obviously an artefact of > > > > subjective observation. > > > > I disagree. In science, if there is a conflict between experimental > > > observation and intuition, then it is *intuition* that becomes > > > suspect, not the experimental result, especially if the latter is > > > confirmed independently and by complementary means. > > > I'm afraid there is no room for a discrepancy between intuition and > > observation. Intuition is supposed to account for observation, and > > there is no question of observation taking a back seat to intuition. > > So that in that way we agree. > > > But this is not the same as crude observationalism. > > > > > > > > Your disbelief of SR stems from the fact that you don't understand it. > > > > > > > My disbelief, really, stems from the blatant lack of conceptual > > > > > > understanding of the theory. I mean, as I repeatedly point out, I > > > > > > don't know a single equation of relativity, and yet I can root out the > > > > > > conceptual contradictions immediately when people here have a crack at > > > > > > making meaningful qualitative statements in SR. The classic example, > > > > > > of course, was Paul's contention that "what is simultaneous in one > > > > > > frame can never be simultaneous in another", which of course isn't > > > > > > true according to SR. > > > > > > I'm sorry? It is very much true in SR that two spatially separated > > > > > events that are simultaneous in one frame are not simultaneous in > > > > > another frame moving relative to the first. > > > > > But I contradicted that when I pointed out that two observers can be > > > > moving relative to each other, and yet undoubtedly events can be > > > > simultaneous for both. > > > > Not spatially separated ones, no. > > > I'm confused, because I thought we previously agreed that two > > observers travelling along the same axis, maintaining equidistance > > from both events at all times, would both report each event as > > simultaneous with the other event. And moreover, if they not only both > > maintained equidistance from both events, but if they maintained a > > separation which was equal for both observers (which, if both > > observers are moving relative to each other, requires either a > > collision course between observers, or travel in diametrically > > opposite directions), then there is no question that the signals are > > received simultaneously. > > > Illustration: > > > E1 > > > -------- > > > E2 > > > The line represents the line between events E1 and E2, along which the > > observers may move while always reporting both events to be > > simultaneous. > > You are correct about E1 and E2 being simultaneous to all observers on > the line despite their motion relative to other observers on the > line. The statement about spatially seperated events is about > seperation along the axis of travel. In the train experient A and B > are on the tracks, which we call the x axis. You have E1 and E2 off > to the sides of the tracks on the y axis, which isn't normally > considered in the train experiment. > > See if this makes sense to you. We were given the definition of > simultaneous, that if an observer at the midpoint between two strikes > sees the flashes at the same instant the strikes were simultaneous. > We have four events given to us. > > 1. A strike hits A and A' when they are together. > 2. A strike hits B and B' when they are together. > 3. M and M' pass each other before observing either strike. > 4. M observes the strikes at A and B at the same instant. > > It is not given that the strikes were simultaneous. It is up to M and > M' to figure that out on their own, based on measurements made in > their own coordinate system. > > M knows that A and B are the same distance from him. Since both > flashes traveled at the same speed, the same time elapsed as they were > traveling to him. And since the flashes arrived at the same instant, > the strikes must have happen at the same instant *as measured on > clocks in his coordinate system*. > > Now let's look at things after the strikes but before the flashes have > arrived. > > A A' F1 M M' F2 B B' > > A and A' were together when F1 was emitted. B and B' were together > when F2 was emitted. There is one and only one wave front moving > outward frome each strike. It was given that both flashes will reach > M at the same instant. From the diagram you can see that at the > instant M sees the two flashes F2 has already been observed passing > M', while F1 has not yet reached M'. So the flashes do not reach M' > at the same instant. > > M' knows that A' and B' are the same distance from him. Since both > flashes traveled at the same speed, the same time elapsed as they were > traveling to him. And since the flashes arrived at different times, > the strikes must have happen at different times *as measured on clocks > in his coordinate system*. > > From the tracks it looks like the clocks on the train were set out of > sync to force the transit time of the flashes to be the same, but on > the train they were just following the clock sync procedure. The > important thing is that according to the clocks on the train the > flashes happen at different times. In the train frame M just happen > to be at the right spot for the two flashes that happen at different > times to reach him at the same instant. > > Hope this helps. Some here don't seem to understand that it's not the > math that is difficult to grasp, it is what the math is modeling that > isn't clear. For me it was easier to see this in terms of LET. > Neither frame is at rest wrt the ether, but both must sync their > clocks in such a way that the same flash is measured to travel at c in > their own frame. I figured out how to do that using LET, and since SR > and LET use the same math, it must work for SR. > > Given that math you can then see that things are symetric. If you > pick a third frame to view things from, such that the first two are > moving in opposit directions at the same speed, then the first two > will be equally contracted and their clocks will tick at the same > rate. > > Try drawing some space,time diagrams. That is using the x and t > axis. It's not hard to do and it shows the geometry you keep hearing > about. Again, knowing how to draw the lines doesn't explain what is > happening, but it does make it easier to see relationships between > frames after you figure out what's going on. > > Bruce > > - Hide quoted text - > > > > > > > - Show quoted text -- Hide quoted text - > > > - Show quoted text - INDEED! Very good explanation, especially the comment about there is only one wavefront per flash. How one perceives the order is due to propagation transit times (including so-called time dilation and the physical differential contraction along the axis of motion). The ordering of observations in a universe with a finite upper limit on the speed at which information can be propagated will invariably lead to variations in the order of observations. Couple this with the fact that clock synchronization inherently includes any systemic velocity effects leads to relative simultaneity perception. Again this is neither mysterious or 'special'. Paul Stowe |