Questions on world-line
in [Relativity]
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From: Igor on 25 Aug 2006 13:04 kenseto wrote: > "Igor" <thoovler(a)excite.com> wrote in message > news:1156435588.287603.157960(a)p79g2000cwp.googlegroups.com... > > > > kenseto wrote: > > > "Igor" <thoovler(a)excite.com> wrote in message > > > news:1156353371.333406.159360(a)i42g2000cwa.googlegroups.com... > > > > > > > > kenseto wrote: > > > > > "Igor" <thoovler(a)excite.com> wrote in message > > > > > news:1156349832.759078.175940(a)74g2000cwt.googlegroups.com... > > > > > > > > > > > > kenseto wrote: > > > > > > > "Igor" <thoovler(a)excite.com> wrote in message > > > > > > > news:1156265582.026355.324580(a)p79g2000cwp.googlegroups.com... > > > > > > > > The worldline is invariant. Motion is relative. That's all there is > > > > to it. Beyond that, I don't really understand what you're even > asking. > > > > > > > What I am asking is: what motion of an object that gives rise to it > > > worldline? Is it the object's absolute motion (individual motion)? If > not > > > why not? > > > > > > Ken Seto > > > > Ask Galileo. This is not just an issue with SR. You might want to > > reprase your question as "How can motion in space be relative and still > > have a well-defined trajectory?" > > > Of course it is an issue with SR. SR claims that there is only relative > motion and the fact that each object has its own world line refute this > claim. The world line of an object is due to the individual motion of the > object in space. But Galileo claimed the exact same thing, except with invariant time. So it's not strictly a SR thing. Thus, the notion of relative motion has been around since the 17th century. So how can you have relative motion and still have a well-defined trajectory? It's simple. How the motion is described is entirely dependent on the coordinate system, hence relative motion, but all inertial observers must see the same trajectory.
From: PD on 25 Aug 2006 13:10 Igor wrote: > kenseto wrote: > > "Igor" <thoovler(a)excite.com> wrote in message > > news:1156435588.287603.157960(a)p79g2000cwp.googlegroups.com... > > > > > > kenseto wrote: > > > > "Igor" <thoovler(a)excite.com> wrote in message > > > > news:1156353371.333406.159360(a)i42g2000cwa.googlegroups.com... > > > > > > > > > > kenseto wrote: > > > > > > "Igor" <thoovler(a)excite.com> wrote in message > > > > > > news:1156349832.759078.175940(a)74g2000cwt.googlegroups.com... > > > > > > > > > > > > > > kenseto wrote: > > > > > > > > "Igor" <thoovler(a)excite.com> wrote in message > > > > > > > > news:1156265582.026355.324580(a)p79g2000cwp.googlegroups.com... > > > > > > > > > > The worldline is invariant. Motion is relative. That's all there is > > > > > to it. Beyond that, I don't really understand what you're even > > asking. > > > > > > > > > What I am asking is: what motion of an object that gives rise to it > > > > worldline? Is it the object's absolute motion (individual motion)? If > > not > > > > why not? > > > > > > > > Ken Seto > > > > > > Ask Galileo. This is not just an issue with SR. You might want to > > > reprase your question as "How can motion in space be relative and still > > > have a well-defined trajectory?" > > > > > Of course it is an issue with SR. SR claims that there is only relative > > motion and the fact that each object has its own world line refute this > > claim. The world line of an object is due to the individual motion of the > > object in space. > > But Galileo claimed the exact same thing, except with invariant time. > So it's not strictly a SR thing. Thus, the notion of relative motion > has been around since the 17th century. So how can you have relative > motion and still have a well-defined trajectory? It's simple. How the > motion is described is entirely dependent on the coordinate system, > hence relative motion, but all inertial observers must see the same > trajectory. Precisely. Ken thinks that the priority of relative motion came with SR, and fails to realize that it is fundamental to classical mechanics. (From Galileo and Newton's First Law onwards.) To him, it is inconceivable that relative motion can occur without individual motion -- it's the only way he understands relative motion. His difficulty, then, is not with SR, but with classical mechanics. PD
From: kenseto on 25 Aug 2006 19:16 "YBM" <ybmess(a)nooos.fr> wrote in message news:44ef194f$0$19795$636a55ce(a)news.free.fr... > kenseto a ?crit : > > "YBM" <ybmess(a)nooos.fr> wrote in message > ... > >>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. > > Please define what a world line is without reference to a given frame. World line is a path of an object in space traces out by its absolute motion. Relative motion between two objects A and B is the vector difference of the vector components of A's absolute motion and the vector component of B's absolute motion along the line joining A and B. Ken Seto > > >>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. > >
From: kenseto on 25 Aug 2006 19:27 "Igor" <thoovler(a)excite.com> wrote in message news:1156525443.210500.226300(a)75g2000cwc.googlegroups.com... > > kenseto wrote: > > "Igor" <thoovler(a)excite.com> wrote in message > > news:1156435588.287603.157960(a)p79g2000cwp.googlegroups.com... > > > > > > kenseto wrote: > > > > "Igor" <thoovler(a)excite.com> wrote in message > > > > news:1156353371.333406.159360(a)i42g2000cwa.googlegroups.com... > > > > > > > > > > kenseto wrote: > > > > > > "Igor" <thoovler(a)excite.com> wrote in message > > > > > > news:1156349832.759078.175940(a)74g2000cwt.googlegroups.com... > > > > > > > > > > > > > > kenseto wrote: > > > > > > > > "Igor" <thoovler(a)excite.com> wrote in message > > > > > > > > news:1156265582.026355.324580(a)p79g2000cwp.googlegroups.com... > > > > > > > > > > The worldline is invariant. Motion is relative. That's all there is > > > > > to it. Beyond that, I don't really understand what you're even > > asking. > > > > > > > > > What I am asking is: what motion of an object that gives rise to it > > > > worldline? Is it the object's absolute motion (individual motion)? If > > not > > > > why not? > > > > > > > > Ken Seto > > > > > > Ask Galileo. This is not just an issue with SR. You might want to > > > reprase your question as "How can motion in space be relative and still > > > have a well-defined trajectory?" > > > > > Of course it is an issue with SR. SR claims that there is only relative > > motion and the fact that each object has its own world line refute this > > claim. The world line of an object is due to the individual motion of the > > object in space. > > But Galileo claimed the exact same thing, except with invariant time. > So it's not strictly a SR thing. Thus, the notion of relative motion > has been around since the 17th century. So how can you have relative > motion and still have a well-defined trajectory? SO???? Both Galileo and SR failed to recognize that world line is the result of absolute motion and relative motion between A abnd B is the vector components difference of the vector component of A's absolute motion and the vector component of B's absolute motion along the line joining A and B. > It's simple. How the > motion is described is entirely dependent on the coordinate system, > hence relative motion, but all inertial observers must see the same > trajectory. > But relative motion is derived from the absolute motions of the interacting objects. All inertial observers do not see the same world line for the same object. Ken Seto
From: YBM on 25 Aug 2006 21:48
kenseto a ?crit : > World line is a path of an object in space traces out by its absolute > motion. Whis this definition, which assumes absolute motion, it is indeed clear that worldline => absolute motion. Note that it is NOT the definition of worldline for the rest of us. > Relative motion between two objects A and B is the vector difference > of the vector components of A's absolute motion and the vector component of > B's absolute motion along the line joining A and B. For the rest of us relative motion does NOT need to refer to absolute motion to be defined. Could you provide at least one case where your definition of relative motion will give another result than the usual one ? |