Questions on world-line
in [Relativity]
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From: Dirk Van de moortel on 23 Aug 2006 09:46 "Paul B. Andersen" <paul.b.andersen(a)hiadeletethis.no> wrote in message news:echlqg$7t6$1(a)dolly.uninett.no... > 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". I'd prefer to say that there is no such thing as "an object with a space-like worldline". A light signal clearly has a light-like worldline, and a shadow or the spot of a laser can have a space-like worldline. That is of course, unless you prefer to call these lines lightlines and spotlines :-) Dirk Vdm
From: surrealistic-dream on 23 Aug 2006 10:04 kenseto wrote: > <surrealistic-dream(a)hotmail.com> wrote in message > news:1156267045.611522.11170(a)h48g2000cwc.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. > > > 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. > > > > Not true. SR treats accelerations as absolute, but velocites and > > positions as relative. > > > > > There is only > > > relative motion then how does an individual object have world-line? > > > > > > Ken Seto > > > > 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. In other words, the worldline in one spacetime > > map according to one inertial frame will not be the same worldline in a > > spacetime diagram in another inertial frame, though some properties are > > preserved under a Lorentz transformation (e.g., straight lines are > > mapped into straight lines; lightlike lines are mapped to themselves). > > Thus, generally but not always, the attributes of the curve in the > > spacetime diagram DEPEND on the relative motion of the object to the > > inertial reference frame. > > > > For an example of how the diagram (set of worldlines) changes when the > > frame of reference changes, see > > You did not answer my question: What is the motion of an object that cause > it to have a world-line??? > > Ken Seto Both spacetime and worldlines are freely created concepts used as physical models that have no real existence.
From: Dirk Van de moortel on 23 Aug 2006 10:05 <surrealistic-dream(a)hotmail.com> wrote in message news:1156341879.581940.69490(a)p79g2000cwp.googlegroups.com... > > kenseto wrote: >> <surrealistic-dream(a)hotmail.com> wrote in message >> news:1156267045.611522.11170(a)h48g2000cwc.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. >> > > 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. >> > >> > Not true. SR treats accelerations as absolute, but velocites and >> > positions as relative. >> > >> > > There is only >> > > relative motion then how does an individual object have world-line? >> > > >> > > Ken Seto >> > >> > 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. In other words, the worldline in one spacetime >> > map according to one inertial frame will not be the same worldline in a >> > spacetime diagram in another inertial frame, though some properties are >> > preserved under a Lorentz transformation (e.g., straight lines are >> > mapped into straight lines; lightlike lines are mapped to themselves). >> > Thus, generally but not always, the attributes of the curve in the >> > spacetime diagram DEPEND on the relative motion of the object to the >> > inertial reference frame. >> > >> > For an example of how the diagram (set of worldlines) changes when the >> > frame of reference changes, see >> >> You did not answer my question: What is the motion of an object that cause >> it to have a world-line??? >> >> Ken Seto > > Both spacetime and worldlines are freely created concepts used as > physical models that have no real existence. Never mind, he doesn't even read the answers. I always wonder why he bothers to ask questions in the first place. Dirk Vdm
From: Mike on 23 Aug 2006 10:13 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. Mike > > Ken Seto
From: Sorcerer on 23 Aug 2006 10:37
"Paul B. Andersen" <paul.b.andersen(a)hiadeletethis.no> wrote in message news:echlqg$7t6$1(a)dolly.uninett.no... | 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. The cuckoo transformation is not correct. http://www.androcles01.pwp.blueyonder.co.uk/Rocket/Rocket.htm You are stupid. Androcles |