Is Length Contraction in SR physical??
in [Relativity]
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From: Ste on 2 Feb 2010 21:30 On 2 Feb, 20:24, "J. Clarke" <jclarke.use...(a)cox.net> wrote: > Ste wrote: > > On 2 Feb, 16:09, "J. Clarke" <jclarke.use...(a)cox.net> wrote: > >> 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. > > > Not according to Paul Draper. Apparently it is necessary that a > > "scientific theory" be predictive (and therefore prescriptive), not > > just "explanatory" (i.e. descriptive). > > It still describes, it does not address causes. Before I exclaim that "I can't believe my ears", are you saying that this failure to address causes is an undesirable deficiency of current physics, or are you saying that physics should not be concerned with it. If it is the former, then I think we might be in agreement, but not so if it is the latter. > > In any event, the question of "why" has perhaps been misinterpreted. I > > really meant "how". That is, what is happening in the physical system > > to cause a change in measurements which are "observer dependent". > > Get your PhD and work the problem for a while and maybe _you_ will be the > one to crack quantum gravity and collect the Nobel and come up with answers > to questions such as these. Lol. I prefer to leave the mathematical drudgery to someone else.
From: Ste on 2 Feb 2010 21:49 On 2 Feb, 20:32, "J. Clarke" <jclarke.use...(a)cox.net> wrote: > Ste wrote: > > On 2 Feb, 15:52, PD <thedraperfam...(a)gmail.com> wrote: > >> 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. > > > Velocity is a physical property that does not vary with observer- > > perspective, yes. The measurement of velocity is observer-dependent, > > yes. > > I see. So you're the only object in an empty universe. What is your > velocity? How do you know that it has that value? That depends on whether the variable can be measured in an observer- independent fashion. > > The discrepancy between those statements is accounted for by the fact > > that I believe in *material reality*, where the measurement of > > variables does not necessarily reflect the value of the variable > > itself. > > And how may your hypothesis of "material reality" be falsified? If it can't > then you're in the wrong shop. It can't be falsified. That's why I've been spending the last few weeks talking to Paul about how any productive endeavour involves axioms that are basically irrefutable. And before you leap in above your pay grade and accuse me of any sort of subjectivity, let's talk about falsification. Why do you hold that falsification is necessary? > >> 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? > > > Oh I know that is the source of the clash. The reason I expect > > observer-independence is because I believe in material reality, where > > variables exist independent of observation or measurement. > > So measurement is meaningless? The how do you know that those "variables" > exist at all? Variables are inferred to exist from variance in observations. > >> 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. > > > There are no properties of the physical world that are "observer- > > dependent", although the measured value of those properties may depend > > on the circumstances of the observer. > > So you're advocating an absolute frame of reference. That's fine--if you > can sell that you have disproven General Relativity and you will surely win > the Nobel. Good luck with that. No, I'm advocating material reality. To reiterate with emphasis, there are no properties of the physical world that are observer-dependent, although the *measured value* of those properties may depend on the circumstances of the observer.
From: artful on 2 Feb 2010 21:51 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).
From: Ste on 2 Feb 2010 22:02 On 2 Feb, 21:52, Tom Roberts <tjrob...(a)sbcglobal.net> wrote: > Ste 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. > > The answer is that this is not "physical", this is GEOMETRICAL. > > Look at a building from directly in front, and it has a given > width. Look at it from a front corner and it has a smaller > width. There is no "physical cause" of this change, it is purely > GEOMETRICAL. In SR, "time dilation" and "length contraction" > are likewise purely geometrical -- they are geometrical > projections just like the width of that building. I assume you mean the *apparent width* of *the side you were facing* becomes smaller. The width of the whole building against the background actually increases if the building is square. > Continuing the analogy: there are physical consequences of that > change (e.g. the building's width will or won't fit on the film > of a given camera). But nothing about the building itself has > changed, only the geometrical relationship between building and > observer has changed. Similar remarks apply to "time dilation" > and "length contraction" -- they can be "physical" for some > meanings of the word, and aren't "physical" for other meanings. My view is that they are decidedly *not* physical, they are *apparent*. > Extending the analogy: if you want to "model" the building, it's > clear that the relevant width is the one marked on the building's > plans -- measured parallel to the building's front. It's clear that > the possibility of measuring many different "widths" from other > points of perspectives is completely irrelevant. Ditto in SR, where > proper length and proper time are relevant to modeling phenomena > related to a given object, and it is irrelevant that that one can > measure other values for "length" and "time" in other frames. > > > It should be a question taken seriously in > > physics. > > The appropriately related question is taken seriously. And answered easily. But > by including a false premise as part of YOUR question (that there is a "physical > cause"), you make YOUR question impossible to take seriously. No I actually didn't think that the cause was "physical", but others have suggested it is - for example, another poster suggested that tests have been done with laser beams that prove an object's length has physically shrunk. Whether that is wrong, or whether it is due to some other effect, I don't yet know. > A famous example: When did you stop beating your wife? Lol.
From: Ste on 2 Feb 2010 22:06
On 2 Feb, 23:52, "J. Clarke" <jclarke.use...(a)cox.net> wrote: > > Physicists deal with what is observed. If you want to "take a leap and > start talking about reality" as if "reality" is something other than what is > observable, then you're in the wrong shop. I'm sure I would want to go as far as saying that there are aspects of reality that are not observable, but I would say there are aspects of reality that are not easily measurable, or not measurable at all. |