Questions on world-line
in [Relativity]
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From: kenseto on 23 Aug 2006 12:20 "Mike" <eleatis(a)yahoo.gr> wrote in message news:1156342431.430984.214950(a)h48g2000cwc.googlegroups.com... > > kenseto wrote: > > "Mike" <eleatis(a)yahoo.gr> wrote in message > > news:1156277943.578929.7420(a)b28g2000cwb.googlegroups.com... > > > > > > 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. > > > > > > Not exactly but ok for starters. > > > > > > > > > > Questions: > > > > 1. Does this mean that the world-line of an object is the result of the > > > > individual motion of the object? > > > > > > You do not need the term "individual". nd you must add ...motion of the > > > object in spacetime (not quite but ok for starters) > > > > > > It seems you are setting up a straw man argument using the word > > > "individual". > > > > > > > 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? > > > > > > Here we go again. The straw man is here. SR does not "say" anything > > > about "individual" motion. The subject of individual motion is > > > traditionally a subject of metaphysics and fundational physics > > > questions not dealt by experimental physics. Despite that, your > > > question is meaningless anyway. The other name for Relativity is > > > "Theory of Absolutes", suggested by Planck but rejected by Einstein > > > because he thought, although it was the correct name, it was too late > > > to change it. > > > > > > You probably confuse absolute motion with individual motion. Absolute > > > motio is motion WRT an absolute media, such as an absolute spacetime. > > > Individual motion exists in both absolute and relational spacetimes. In > > > the former it can be measured WRT the absolute media. In the latter it > > > can only be measured WRT another body in motion. That you need another > > > body to measure the motion of a body does not negate individual motion. > > > Actually, "it takes two to tango". > > > > You did not answer my question: What is the motion of an object that causes > > it to have a world-line??? > > Why do you think anyone can answer your straw man question? Try you > local priest for a reference to prayers about Creation of the Universe > by God. > > "Causes" is not the subject of experimental physics. You have been told > that zillion times bozzo. Hey idiot.....a line trace out by an object in space-time requires some kind of motion. So what is that motion?
From: Igor on 23 Aug 2006 12:17 kenseto wrote: > "Igor" <thoovler(a)excite.com> wrote in message > news:1156265582.026355.324580(a)p79g2000cwp.googlegroups.com... > > > > kenseto wrote: > > > In SR the world line is the path of an object in space with the passage > of > > > time. > > > > Not quite. It's literally the path taken through spacetime. > > > > >Each object has its own world-line. > > > > Yes. > > > > > > > Questions: > > > 1. Does this mean that the world-line of an object is the result of the > > > individual motion of the object? > > > > Yes, through spacetime, but as opposed to what? > > > > > 2. SR says that there is no such thing as individual motion. > > > > Depends on what you mean by individual motion. If you mean absolute, > > then you're correct. > > > > >There is only > > > relative motion then how does an individual object have world-line? > > > > The world line is fixed in spacetime. It's invariant, so everyone > > agrees on the path. How you define the coordinate system, however, is > > entirely up to you. That's where relative motion comes in. > > > The question is: how does an object have world line? What make it trace out > a world line? The best analogy would be a trajectory in spacetime. What makes a body have a trajectory? In classical physics, all bodies have a unique trajectory, as opposed to QM, where their trajectories become smeared over many different paths.
From: Igor on 23 Aug 2006 12:21 Paul B. Andersen wrote: > Igor wrote: > > surrealistic-dream(a)hotmail.com wrote: > >> Not true. SR treats accelerations as absolute, but velocites and > >> positions as relative. > > > > Wrong. There is a Lorentz transformation for acceleration also. > > No, it's correct. > But the acceleration that is absolute is the proper acceleration, > that is the acceleration measured in the instant inertial > rest frame of the object. It is the acceleration that is > measured by an accelerometer. This acceleration is absolute, > that is independent of frames of reference. > The co-ordinate acceleration is however frame dependent. > > Example: > If you travel in space with a rocket with constant thrust, > you will have a constant proper acceleration. > (Assume the mass of of the ship is constant.) > Your accelerometer shows a constant acceleration of - say 1g. > This acceleration is absolute, independent of frames of reference. > > But your co-ordinate acceleration measured in some inertial frame > is NOT constant. As your speed in this frame of reference > (after a year or so) approaches c, your co-ordinate acceleration > approaches zero. > > >> With the exception of the worldline of a particle moving at light > >> speed, the worldline of a particle is a specific 'curve' (or piecewise > >> collection of curves and/or line segments) in a specific spacetime > >> diagram. This curve is generally timelike and not an invariant of a > >> Lorentz transformation. > > > > Wrong again. The worldline is invariant regardless of whether it is > > spacelike, timelike, or lightlike. > > An interval between two events is timelike if it is > possible for a massive object to be present at both events. > The word line of an object is its path through space-time. > Any interval between events on this world line must thus per > definition be time-like. > There is no such thing as a "space-like world line". > > Paul Maybe not the worldline, but ds can be spacelike. I think that was what I thinking of.
From: kenseto on 23 Aug 2006 12:30 "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... > > > > > > kenseto wrote: > > > > In SR the world line is the path of an object in space with the passage > > of > > > > time. > > > > > > Not quite. It's literally the path taken through spacetime. > > > > > > >Each object has its own world-line. > > > > > > Yes. > > > > > > > > > > Questions: > > > > 1. Does this mean that the world-line of an object is the result of the > > > > individual motion of the object? > > > > > > Yes, through spacetime, but as opposed to what? > > > > > > > 2. SR says that there is no such thing as individual motion. > > > > > > Depends on what you mean by individual motion. If you mean absolute, > > > then you're correct. > > > > > > >There is only > > > > relative motion then how does an individual object have world-line? > > > > > > The world line is fixed in spacetime. It's invariant, so everyone > > > agrees on the path. How you define the coordinate system, however, is > > > entirely up to you. That's where relative motion comes in. > > > > > The question is: how does an object have world line? What make it trace out > > a world line? > > The best analogy would be a trajectory in spacetime. What makes a body > have a trajectory? In classical physics, all bodies have a unique > trajectory, as opposed to QM, where their trajectories become smeared > over many different paths. > 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??
From: Greg Hansen on 23 Aug 2006 12:22
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. In Newtonian mechanics we plot position versus time and call it a trajectory. In relativistic mechanics we plot time versus position and call it a world-line. A world-line is just a trajectory with the graph rotated 90 degrees. And flip the time axis so it's positive to the right. > Questions: > 1. Does this mean that the world-line of an object is the result of the > individual motion of the object? A world-line is a trajectory turned on its side. It is not "caused" by the motion, it's a model of the motion. It is "caused" by the student drawing a line on a peice of paper that represents the motion. > 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? Individual motion and relative motion are not opposites. Pick a reference frame and plot time versus position. That's no different from drawing a trajectory by picking a reference frame and plotting position versus time. Newtonian mechanics only has relative motion, too. There's really nothing astonishing or exotic about worldlines, except for the orientation of the axes. |