Relative direction
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The most common relative directions are: left, right, forward, backward, up, and down.
No absolute direction corresponds to any of the relative directions. This is a consequence of the translational invariance of the laws of physics: nature, loosely speaking, behaves the same no matter what direction one moves. As demonstrated by the Michelson-Morley null result, there is no absolute inertial frame of reference.
There are definite relationships between the relative directions, however. Left and right, forward and backward, and up and down are three complementary pairs orthogonal to each other. If you climb a rope one meter up, you will have moved negative one meter down. Furthermore, you will not have moved left or right at all, nor forward or backward. One must mind the geometry of his environment when using relative direction to express motion, however. For example, if you continue walking forward until you have almost circumnavigated the Earth, you will expend much effort only to move backward slightly. This is because, on Earth, we use a spherical coordinate system wherein the direction vectors depend on position.
The right-hand rule is one common way to relate the three principal directions. (See below to decide which is your right hand.) For many years a fundamental question in physics was whether a left-hand rule would be equivalent. Many natural structures, including our own bodies, follow a certain handedness, but it was widely assumed that the nature did not distinguish the two possibilities. This changed with the discovery of CP violation in particle physics. If you magnetize a sample of cobalt-60 atoms so that they spin counterclockwise around some axis, the beta radiation resulting from their nuclear decay will be preferentially directed opposite that axis. Since counterclockwise may be defined in terms of up, forward, and right, this experiment unambiguously differentiates left from right using only natural elements: If they were reversed, or the atoms spun clockwise, the radiation would follow the spin axis instead of being opposite to it.
This definition of left and right is unwieldy. In practice, the meaning of relative direction words is conveyed through tradition, acculturation, education, and direct reference.
One common definition of up and down uses gravity and the planet Earth as a frame of reference. Since there is a very noticeable force of gravity acting between the Earth and any other nearby object, down is defined as that direction which an object moves in reference to the Earth when the object is left to fall freely. Up is then defined as the opposite direction of down. Another common definition uses a human body, standing upright, as a frame of reference. In that case, up is defined as the direction from feet to head, perpendicular to the surface of the Earth.
Forward and backward may be defined by referring to an object or person's motion. Forward is defined as the direction in which the object is moving. Backwards is then defined as the opposite of forward. Alternately, forward may be the direction pointed by the observer's nose, defining backward as the direction from the nose to the sagittal border in the observer's skull.
The terms left and right are derived from the fact that some 90% of the population use the hand of the same side of their body for writing and other manual tasks. Through cognate of the word right in the meaning correct or good, the side with the hand most used was defined as the right side. The word left comes from the Old English lyft, meaning weak. This dichotomy can also be seen in the words dextrous and sinister, from the Latin words for right and left.
In the vast majority of the population, the heart is slightly to the left of the body's centre line and the liver is to the right. Less visibly, the two hemispheres of the brain differ in function.
In nautical usage, the halves of a ship are designated port (left) and starboard (right), relative to a person looking forward (towards the bow of the ship). [Note that port and starboard, while defined by left and right, is not relative to anyone or anything on the ship. It is only relative to the ship itself.] However, Left and Right are used when giving orders to the helmsman (to ensure that they do not take conversation as an order to change direction).
[edit] Example
In this diagram, showing a road with right-hand traffic, the red car is to the left of the blue car. The blue car is, therefore, on the right-hand side. Should the blue car move backward, it would reach the position of the yellow car, causing an accident. For the red car to be where the green car is, it would have to move forward.
The lane containing the green and red cars is the left lane, the lane with the yellow and blue cars is the right lane.
