The most common relative directions are left, right, forward(s), backward(s), 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 pairs of complementary directions, each pair orthogonal to both of the others. Relative directions are also known as egocentric coordinates.[1]
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Many human cultures use relative directions for reference. However, some do not. The Australian Aboriginal people the Guugu Yimithirr for instance, in their language have no words denoting the egocentric directions, but instead exclusively refer to cardinal directions, even when describing small-scale spaces. For instance, if they wanted someone to move over on the car seat to make room, they might say "move a bit to the east". To tell someone where exactly they left something in their house, they might say, "I left it on the southern edge of the western table." Or they might warn a person to "look out for that big ant just north of your foot". Other peoples "from Polynesia to Mexico and from Namibia to Bali" similarly have predominantly "geographic languages".[1] American Sign Language makes heavy use of geographical direction through absolute orientation. When speaking Mongolian, speakers will typically use the words for "front", "back", "left", and "right" to mean "south", "north", "east", and "west", respectively.
Bow, aft, port, and starboard are nautical terms that convey an impersonal relative direction in the context of the moving frame of persons aboard a ship.
If someone climbs a rope one meter up, they will have moved negative one meter down. Furthermore, they will not have moved left or right at all, nor forward or backward. One must mind the geometry of the environment when using relative direction to express motion, however. For example, if someone continues walking forward until they have almost circumnavigated the Earth, they will expend much effort only to move backward slightly. This is because Earth is (near-)spherical.
The right-hand rule is one common way to relate the three principal directions. For many years a fundamental question in physics was whether a left-hand rule would be equivalent. Many natural structures, including human bodies, follow a certain handedness, but it was widely assumed that nature did not distinguish the two possibilities. This changed with the discovery of parity violations in particle physics. If a sample of cobalt-60 atoms is magnetized so that they spin counterclockwise around some axis, the beta radiation resulting from their nuclear decay will be preferentially directed opposite that axis. Since counter-clockwise 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.
Since definitions of left and right based on the geometry of the natural environment are 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 allowed 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. In most cases, up is a directionally oriented position generally opposite to that of the pull of gravity.
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. Backward is then defined as the opposite direction to forward. Alternatively, '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. With respect to a ship 'forward' would indicate the relative position of any object lying in the direction the ship is pointing. Objects on ships can be named from their location, for example 'Forward Fire Pump' or 'Aft Fire Pump'.
The terms left and right are derived from the fact that 70-90% of the human population use the right hand 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.
Given significant distance from the equator, one can figure which hand is which using a clock, a compass, and the sun. Facing the sun, in the northern hemisphere, before noon, the compass points to the left hand. After noon, it points to the right. The opposite is true of the southern hemisphere. In most people, the heart is to the left of the body's center line and the liver is to the right.
Left-right confusion is the difficulty some people have in distinguishing the difference between the directions left and right. According to research by John R. Clarke (Drexel University) it affects about 15% of the population. These people can usually normally perform daily activities such as driving according to signs and navigating according to a map, but will often take a wrong turn when told to turn left or right and may have difficulties performing actions that require precise understanding of directional commands, such as ballroom dancing.[2][3][4][5]