Questions on world-line
in [Relativity]
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From: kenseto on 24 Aug 2006 16:50 "Timo A. Nieminen" <timo(a)physics.uq.edu.au> wrote in message news:Pine.WNT.4.64.0608250424530.1512(a)serene.st... > On Wed, 23 Aug 2006, kenseto wrote: > > > "Timo A. Nieminen" <timo(a)physics.uq.edu.au> wrote: > >> On Tue, 22 Aug 2006, kenseto wrote: > >> > >>> In SR the world line is the path of an object in space with the passage > > of > >>> time. Each object has its own world-line. > >>> Questions: > >>> 1. Does this mean that the world-line of an object is the result of the > >>> individual motion of the object? > >>> 2. SR says that there is no such thing as individual motion. There is > > only > >>> relative motion then how does an individual object have world-line? > >> > >> Consider an object, with position in a particular inertial coordinate > >> system (aka inertial reference frame) given by r(t). It can only be in one > >> place at a time, and is always somewhere at any time; thus r(t) is a > >> function. The problem is that not only does r depend on choice of > >> coordinate system, but so does t. > >> > >> So, what to do? Introduce a parameter that is independent of choice of > >> coordinate system, such as the "proper time" T, and we can write > >> R(T) = (r,t) > >> as a 4-vector valued function. How can any individual object _not_ have a > >> world line? > > [cut] > > > > You did not answer my question: What is the motion of an object that cause > > it to have a world-line??? > > If that was your question, you should have asked it in the first place. > You asked: "how does an individual object have world-line?" The answer is > that motion really has nothing to do with it; an object has a world line > if it is somewhere at any given time. An object somewhere at any given time and somewhere else at different given time is velocity. So your bogus assertion is a bunch of baloney. Ken Seto > > -- > Timo Nieminen - Home page: http://www.physics.uq.edu.au/people/nieminen/ > E-prints: http://eprint.uq.edu.au/view/person/Nieminen,_Timo_A..html > Shrine to Spirits: http://www.users.bigpond.com/timo_nieminen/spirits.html >
From: Igor on 24 Aug 2006 16:58 kenseto wrote: > "Timo A. Nieminen" <timo(a)physics.uq.edu.au> wrote in message > news:Pine.WNT.4.64.0608250424530.1512(a)serene.st... > > On Wed, 23 Aug 2006, kenseto wrote: > > > > > "Timo A. Nieminen" <timo(a)physics.uq.edu.au> wrote: > > >> On Tue, 22 Aug 2006, kenseto wrote: > > >> > > >>> In SR the world line is the path of an object in space with the > passage > > > of > > >>> time. Each object has its own world-line. > > >>> Questions: > > >>> 1. Does this mean that the world-line of an object is the result of > the > > >>> individual motion of the object? > > >>> 2. SR says that there is no such thing as individual motion. There is > > > only > > >>> relative motion then how does an individual object have world-line? > > >> > > >> Consider an object, with position in a particular inertial coordinate > > >> system (aka inertial reference frame) given by r(t). It can only be in > one > > >> place at a time, and is always somewhere at any time; thus r(t) is a > > >> function. The problem is that not only does r depend on choice of > > >> coordinate system, but so does t. > > >> > > >> So, what to do? Introduce a parameter that is independent of choice of > > >> coordinate system, such as the "proper time" T, and we can write > > >> R(T) = (r,t) > > >> as a 4-vector valued function. How can any individual object _not_ have > a > > >> world line? > > > > [cut] > > > > > > You did not answer my question: What is the motion of an object that > cause > > > it to have a world-line??? > > > > If that was your question, you should have asked it in the first place. > > You asked: "how does an individual object have world-line?" The answer is > > that motion really has nothing to do with it; an object has a world line > > if it is somewhere at any given time. > > An object somewhere at any given time and somewhere else at different given > time is velocity. So your bogus assertion is a bunch of baloney. And if that's your definition of velocity, no wonder you're having trouble understanding all this.
From: Barry on 24 Aug 2006 17:46 Timo A. Nieminen wrote: > On Wed, 23 Aug 2006, kenseto wrote: >> You did not answer my question: What is the motion of an object that >> cause >> it to have a world-line??? > The answer > is that motion really has nothing to do with it; an object has a world > line if it is somewhere at any given time. I don't think so. An object has a world line if it is somewhere or somewheres throughout a period of time. Motion does have something to do with it, the object must move through spacetime. What you describe might be more accurately called an event (or a cotemperaneous cluster of events). Barry
From: kenseto on 25 Aug 2006 09:34 "Igor" <thoovler(a)excite.com> wrote in message news:1156453115.500831.55120(a)i3g2000cwc.googlegroups.com... > > kenseto wrote: > > "Timo A. Nieminen" <timo(a)physics.uq.edu.au> wrote in message > > news:Pine.WNT.4.64.0608250424530.1512(a)serene.st... > > > On Wed, 23 Aug 2006, kenseto wrote: > > > > > > > "Timo A. Nieminen" <timo(a)physics.uq.edu.au> wrote: > > > >> On Tue, 22 Aug 2006, kenseto wrote: > > > >> > > > >>> In SR the world line is the path of an object in space with the > > passage > > > > of > > > >>> time. Each object has its own world-line. > > > >>> Questions: > > > >>> 1. Does this mean that the world-line of an object is the result of > > the > > > >>> individual motion of the object? > > > >>> 2. SR says that there is no such thing as individual motion. There is > > > > only > > > >>> relative motion then how does an individual object have world-line? > > > >> > > > >> Consider an object, with position in a particular inertial coordinate > > > >> system (aka inertial reference frame) given by r(t). It can only be in > > one > > > >> place at a time, and is always somewhere at any time; thus r(t) is a > > > >> function. The problem is that not only does r depend on choice of > > > >> coordinate system, but so does t. > > > >> > > > >> So, what to do? Introduce a parameter that is independent of choice of > > > >> coordinate system, such as the "proper time" T, and we can write > > > >> R(T) = (r,t) > > > >> as a 4-vector valued function. How can any individual object _not_ have > > a > > > >> world line? > > > > > > [cut] > > > > > > > > You did not answer my question: What is the motion of an object that > > cause > > > > it to have a world-line??? > > > > > > If that was your question, you should have asked it in the first place. > > > You asked: "how does an individual object have world-line?" The answer is > > > that motion really has nothing to do with it; an object has a world line > > > if it is somewhere at any given time. > > > > An object somewhere at any given time and somewhere else at different given > > time is velocity. So your bogus assertion is a bunch of baloney. > > And if that's your definition of velocity, no wonder you're having > trouble understanding all this. > That's my definition of motion. If an object is at different location at different time then he is in a state of motion.
From: kenseto on 25 Aug 2006 09:46
"YBM" <ybmess(a)nooos.fr> wrote in message news:44eddc45$0$19777$636a55ce(a)news.free.fr... > kenseto a ?crit : > > "YBM" <ybmess(a)nooos.fr> wrote in message > > news:44ece480$0$19781$636a55ce(a)news.free.fr... > > > >>kenseto a ?crit : > >> > >>>"YBM" <ybmess(a)nooos.fr> wrote in message > >>>news:44ec8a99$0$19782$636a55ce(a)news.free.fr... > >>> > >>> > >>>>kenseto a ?crit : > >>>> > >>>> > >>>>>The point is: worldline or trajectory in spacetime requires some kind > > > > of > > > >>>>>motion to happen. In SR there is only relative motion.....does that > > > > mean > > > >>>>>that worldline of an object is the result of relative motion?? > >>>> > >>>>I should be dreaming... You are just about to get it ! > >>> > >>> > >>>RFOTFLOL this idiot falls into the trap.....an object doesn't need any > >>>reference to have a worldline. > >> > >>Well, I'm right not to have bet on that... not that I did believe it > >>actualy. > >> > >>Back to basics : the worldline of Ken Seto is defined in a specific > >>frame F as the set of the t-uples (x,y,z,t) tagging in F the events > >>"at time t, Ken Seto is at coordinates (x,y,z)". > > > > > > This is not basic you are talking about relative motion between me and > > frame F. In my frame of reference my coordinates are (0,0,0) at all time. > > Why? Because SR and I assumed that I am in a state of rest and all the > > objects around me are doing the moving. Even though that I assume that I am > > in a state of rest I still have a worldline. This applies to all the object > > that are at rest wrt me. > > So what ? In that case (Seto at rest in F), my definition works as well > and gives { (x_s,y_s,z_s,t) / t \in R } where (x_s,y_s,z_s) is your > constant spacial coordinates... You keep on using F as a reference for me. I can have a world line without any reference. F can see that I have a different world line after I accelerated. But that's because I changed my state of absolute motion. > > BTW, you seem (in another post of this thread) to suggest that a > worldline could be defined without refering to a frame. Please do so. |