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Physics, Astronomy & Cosmology / How do worldlines and timelines differ?
« on: 18/12/2018 15:39:01 »
I've had a shot at answering this and would appreciate comments.
A timeline is a list of events in chronological order. Obviously, the events are “real” in the sense that they can be experienced sequentially in our 3+1 dimensional Universe, but the timeline, itself, has no independent existence, and is simply a useful contrivance for the measurement and recording of change. Consequently, a point on a timeline has no intrinsic spatial dimensions, and may be expressed as zero-dimensional in time
A worldline is the conceptual path that an object takes in 4-dimensional spacetime. It is distinguished from, for example, a trajectory by the essential inclusion of time as a dimension. Thus, the trajectory of an arrow, from bow to target, may be expressed without reference to the time involved in that journey. The same journey, expressed as a worldline, would include time, and would, therefore, denote the relative speed of the arrow. Graphically, it would be shown as an inclined line within a light cone. This cone would be bounded by lines showing the path that a single flash of light, from a specific source, would trace, travelling at “c” through space.
Unlike a timeline, a worldline always has an object, physical or conceptual, that gives it meaning. Each of us has a worldline, which we follow from conception to death and arguably beyond. Thus, it can be reasoned that a worldline is a physical entity, because, in this case, it has a physical object that is present throughout its history.
A timeline is a list of events in chronological order. Obviously, the events are “real” in the sense that they can be experienced sequentially in our 3+1 dimensional Universe, but the timeline, itself, has no independent existence, and is simply a useful contrivance for the measurement and recording of change. Consequently, a point on a timeline has no intrinsic spatial dimensions, and may be expressed as zero-dimensional in time
A worldline is the conceptual path that an object takes in 4-dimensional spacetime. It is distinguished from, for example, a trajectory by the essential inclusion of time as a dimension. Thus, the trajectory of an arrow, from bow to target, may be expressed without reference to the time involved in that journey. The same journey, expressed as a worldline, would include time, and would, therefore, denote the relative speed of the arrow. Graphically, it would be shown as an inclined line within a light cone. This cone would be bounded by lines showing the path that a single flash of light, from a specific source, would trace, travelling at “c” through space.
Unlike a timeline, a worldline always has an object, physical or conceptual, that gives it meaning. Each of us has a worldline, which we follow from conception to death and arguably beyond. Thus, it can be reasoned that a worldline is a physical entity, because, in this case, it has a physical object that is present throughout its history.