# World line

World line

In physics, the world line of an object is the unique path of that object as it travels through 4-dimensional spacetime.The concept of "world line" is distinguished from the concept of "orbit" or "trajectory" (such as an "orbit in space" or a "trajectory" of a truck on a road map) by the "time" dimension, and typically encompasses a large area of spacetime wherein perceptually straight paths are recalculated to show their (relatively) more absolute position states &mdash; to reveal the nature of special relativity or gravitational interactions.The idea of world lines originates in physics and was pioneered by Einstein. The term is now most often used in relativity theories (i.e., general relativity and special relativity).

However, world lines are a general way of representing the course of events. The use of it is not bound to any specific theory.Thus in general usage, a world line is the sequential path of personal human events (with "time and place" as dimensions) that marks the history of a person &mdash; perhaps starting at the time and place of one's birth until their death. The log book of a ship is a description of the ship's world line, as long as it contains a time tag attached to every position. The world line allows one to calculate the speed of the ship, given a measure of distance (a so-called metric) appropriate for the curved surface of the Earth.

Usage in physics

In physics, a world line of an object (approximated as a point in space, e.g., a particle or observer) is the sequence of spacetime events corresponding to the history of the object. A world line is a special type of curve in spacetime. Below an equivalent definition will be explained: A world line is a time-like curve in spacetime. Each point of a world line is an event that can be labeled with the time and the spatial position of the object at that time.

For example, the "orbit" of the Earth in space is approximately a circle, a three-dimensional (closed) curve in space: the Earth returns every year to the same point in space. However, it arrives there at a different (later) time. The "world line" of the Earth is helical in spacetime (a curve in a four-dimensional space) and does not return to the same point.

Spacetime is the collection of points called "events", together with a continuous and smooth coordinate system identifying the events. Each event can be labeled by four numbers: a time coordinate and three space coordinates; thus spacetime is a four-dimensional space. The mathematical term for spacetime is a four-dimensional manifold. The concept may be applied as well to a higher-dimensional space. For easy visualisations of four dimensions, two space coordinates are often suppressed. The event is then represented by a point in a two-dimensional spacetime, a plane usually plotted with the time coordinate, say $t$, upwards and the space coordinate, say $x$ horizontally.

A world line traces out the path of a single point in spacetime. A world sheet is the analogous two-dimensional surface traced out by a one-dimensional line (like a string) traveling through spacetime. The worldsheet of an open string (with loose ends) is a strip; that of a closed string (a loop) is a volume.

World lines as a tool to describe events

A one-dimensional "line" or "curve" can be represented by the coordinates as a function of one parameter. Each value of the parameter corresponds to a point in spacetime and varying the parameter traces out a line. So in mathematical terms a curve is defined by four coordinate functions $x^a\left( au\right),; a=0,1,2,3$ (where $x^\left\{0\right\}$ usually denotes the time coordinate) depending on one parameter $au$. A coordinate grid in spacetime is the set of curves one obtains if three out of four coordinate functions are set to a constant.

Sometimes, the term world line is loosely used for "any" curve in spacetime. This terminology causes confusions. More properly, a world line is a curve in spacetime which traces out the "(time) history" of a particle, observer or small object. One usually takes the proper time of an object or an observer as the curve parameter $au$ along the world line.

Trivial examples of spacetime curves

A curve that consists of a horizontal line segment (a line at constant coordinate time), may represent a rod in spacetime and would not be a world line in the proper sense. The parameter traces the length of the rod.

A line at constant space coordinate (a vertical line in the convention adopted above) may represent a particle at rest (or a stationary observer). A tilted line represents a particle with a constant coordinate speed (constant change in space coordinate with increasing time coordinate). The more the line is tilted from the vertical, the larger the speed.

Two world lines that start out separately and then intersect, signify a "collision" or "encounter." Two world lines starting at the same event in spacetime, each following its own path afterwards, may represent the decay of a particle in to twoothers or the emission of one particle by another.

World lines of a particle and an observer may be interconnected with the world line of a photon (the path of light) and form a diagram which depicts the emission of a photon by a particle which is subsequently observed by the observer (or absorbed by another particle).

Tangent vector to a world line, four-velocity

The four coordinate functions $x^a\left( au\right),; a=0,1,2,3$ defining a world line, are real functions of a real variable $au$ and can simply be differentiated in the usual calculus. Without the existence of a metric (this is important to realize) one can speak of the difference between a point $p$ on the curve at the parameter value $au_0$ and a point on the curve a little (parameter $au_0+Delta au$) farther away. In the limit $Delta au ightarrow 0$, this difference divided by $Delta au$ defines a vector, the tangent vector of the world line at the point $p$. It is a four-dimensional vector, defined in the point $p$. It is associated with the normal 3-dimensional velocity of the object (but it is not the same) and therefore called four-velocity $vec\left\{v\right\}$, or in components::$vec\left\{v\right\} = \left(v^0,v^1,v^2,v^3\right) = left\left( frac\left\{dx^0\right\}\left\{d au\right\};,frac\left\{dx^1\right\}\left\{d au\right\};, frac\left\{dx^2\right\}\left\{d au\right\};, frac\left\{dx^3\right\}\left\{d au\right\} ight\right)$

where the derivatives are taken at the point $p$, so at $au= au_0$.

All curves through point p have a tangent vector, not only world lines. The sum of two vectors is again a tangent vector to some other curve and the same holds for multiplying by a scalar. Therefore all tangent vectors in a point p span a linear space, called the tangent space at point p. For example, taking a 2-dimensional space, like the (curved) surface of the Earth, its tangent space at a specific point would be the flat approximation of the curved space.

Imagine a pendulum clock floating in space. We see in our mind in four stages of time; NOW, THEN, BEFORE, and THE PAST. Imagine the pendulum swinging and also the “Tick Tock” of the internal mechanism. Each swing from right to left represents a movement in space, and the period between a “Tick” to a “Tock” represents a period of time.

Now, if we image a wavy line between the different locations of the pendulum at the time intervals of: NOW, THEN, BEFORE and THE PAST. The line is a World line and is a representation of where the pendulum was in space-time at any point between the intervals. Time flows from The Past to Now.

World lines in special relativity

So far a worldline (and the concept of tangent vectors) is defined in spacetime even without a definition of a metric. We now discuss theories in which, in addition, a metric is defined.

The theory of special relativity puts some constraints on possible world lines. In special relativity the description of spacetime is limited to "special" coordinate systems that do not accelerate (and so do not rotate either), called inertial coordinate systems. In such coordinate systems, the speed of light is a constant. Spacetime now has a special type of metric imposed on it, the Lorentz metric and is called a Minkowski space, which allows for example a description of the path of light.

World lines of particles/objects at constant speed are called geodesics. In special relativity these are straight lines in Minkowski space.

Often the time units are chosen such that the speed of light is represented by lines at a fixed angle, usually at 45 degrees, forming a cone with the vertical (time) axis. In general, curves in spacetime with a given metric can be of three types:

* light-like curves, having at each point the speed of light. They form a cone in spacetime, dividing it into two parts. The cone is a three-dimensional hyperplane in spacetime, which appears as a line in drawings with two dimensions suppressed and as a cone in drawings with one spatial dimension suppressed.

* time-like curves, with a speed less than the speed of light. These curves must fall within a cone defined by light-like curves. In our definition above: world lines are time-like curves in spacetime.

* space-like curves falling outside the light cone. Such curves may describe, for example, the length of a physical object. The circumference of a cylinder and the length of a rod are space-like curves.

At a given event on a world line, spacetime (Minkowski space) is divided into three parts.

* The future of the given event is formed by all events that can be reached through time-like curves lying within the future light cone.
* The past of the given event is formed by all events that can influence the event (that is, which can be connected by world lines within the past light cone to the given event).
* The lightcone at the given event is formed by all events that can be connected through light rays with the event. When we observe the sky at night, we basically see only the past light cone within the entire spacetime.
* The present is the region between the two light cones. Points in an observer's present are inaccessible to her/him; only points in the past can send signals to the observer. In ordinary laboratory experience, using common units and methods of measurement, it may seem that we look at the present, "Now you see it, now you don't," but in fact there is always a delay time for light to propagate. For example, we see the Sun as it was about 8 minutes ago, not as it is "right now." Unlike Galilean/Newtonian theory, the present is thick; it is not a sheet but a volume.
* The present instant is defined for a given observer by a plane normal to her/his world line. It is the locus of simultaneous events, and is really three-dimensional, though it would be a plane in the diagram because we had to throw away one dimension to make an intelligible picture. Although the light cones are the same for all observers, different observers, with differing velocities but coincident at an event or point in the spacetime, have world lines that cross each other at an angle determined by their relative velocities, and thus the present instant is different for them. The fact that simultaneity depends on relative velocity caused problems for many scientists and laymen trying to accept relativity in the early days. The illustration with the light cones may make it appear that they cannot be at 45 degrees to two lines that intersect, but it is true and can be demonstrated with the Lorentz transformation. The geometry is Minkowskian, not Euclidean.

World lines in general relativity

The use of world lines in general relativity is basically the same as in special relativity, with the difference that spacetime can be curved. A metric exists and its dynamics are determined by the Einstein field equations and are dependent on the mass distribution in spacetime. Again the metric defines lightlike (null), spacelike and timelike curves. Also, in general relativity, world lines are timelike curves in spacetime, where timelike curves fall within the lightcone. However, a lightcone is not necessarily inclined at 45 degrees to the time axis. However, this is an artifact of the chosen coordinate system, and reflects the coordinate freedom (diffeomorphism invariance) of general relativity. Any timelike curve admits a comoving observer whose "time axis" corresponds to that curve, and, since no observer is privileged, we can always find a local coordinate system in which lightcones are inclined at 45 degrees to the time axis. See also for example Eddington-Finkelstein coordinates.

World lines of free-falling particles or objects (such as planets around the Sun or an astronaut in space) are called geodesics.

ee also

Some specific type of world lines:
*Geodesics
*Closed timelike curves
*Causal structure for a variety of different types of worldline.

External links

* [http://www.bbc.co.uk/dna/h2g2/A3086039 World lines] article on h2g2.

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