Where does displacement collide?

In physics, displacement is the change in an object's position: a vector that points in a straight line from where the object started to where it ended, with a size and a direction, whatever path it actually took. It is not the same as the distance travelled, which just adds up the length of the whole route. The word carries a second, older meaning in the physics of fluids: when a body is placed in water it pushes aside, or displaces, a volume of fluid, and by Archimedes' principle the weight of that displaced fluid equals the upward buoyant force on the body. Halliday, Resnick, and Walker's Fundamentals of physics treats both ideas, the position vector in its chapters on motion and the displaced fluid in its chapter on buoyancy. The one word covers a straight-line change of place and a pushed-aside volume of liquid, linked only loosely by the picture of something moved out of its original spot.[1]

Unit

A change in position, in metres, with a direction

Recovery means

If the object returns to where it began, its net displacement is zero, however far it travelled

Measured by

Found by subtracting the start position from the end position; for fluids, by the volume of liquid pushed aside

In psychoanalysis, displacement is a defence mechanism: a feeling or impulse aimed at a threatening target is redirected onto a safer, less risky one, so that anger at a boss is taken out on a family member instead. Sigmund Freud first described the same movement in the working of dreams, where the emotional weight attached to an important idea slides onto a trivial one and disguises what the dream is really about; he called this the displacement of the dream-work in The interpretation of dreams. Anna Freud later placed displacement among the ego's regular defences in The ego and the mechanisms of defence. Here displacement is not a movement in physical space but a redirection of mental energy from one object to another. The thing that moves is an impulse or a feeling, and what it moves between are ideas or people, not points on a coordinate grid.[2][3]

Unit

A mental impulse redirected from one target to another

Recovery means

Not applicable

Measured by

Read from clinical material, such as dreams and slips, rather than measured on a scale

In linguistics, displacement is one of the design features that Charles Hockett proposed to set human language apart from most animal signalling. It is the ability to talk about things that are not present in the here and now: events in the past or the future, objects somewhere else, and things that are hypothetical or do not exist at all. A bee's dance can point to a patch of food, but human speakers can refer to last year's harvest or a city they have never seen. Hockett laid out this feature, alongside others such as duality and productivity, in The origin of speech. Here displacement names a property of a communication system, the freedom of its signs from the immediate situation, and has nothing to do with moving an object or redirecting a feeling.[4]

Unit

A property of a language: whether its signs can refer beyond the here and now

Recovery means

Not applicable

Measured by

Judged by whether a communication system can refer to absent or hypothetical things, not by a number

In geology and seismology, displacement is the relative movement of the rock on the two sides of a fault: when a fault slips, the blocks on either side shift past each other, and the offset between once-matching features records how far and in what direction. It is measured along the fault, and it can be horizontal, vertical, or a mix of both, building up over many earthquakes or jumping in a single one. Wells and Coppersmith gathered records from earthquakes worldwide to relate the surface displacement in an event to its magnitude and rupture length, reported in the Bulletin of the Seismological Society of America. Here displacement is a lasting offset of solid rock across a break, not a reversible change of position, a redirected impulse, or a feature of language. The word marks how far the ground on one side of a fault has moved relative to the other.[5]

Unit

The offset of rock across a fault, in metres

Recovery means

Largely permanent: the rock stays offset after slip, though later movement can add to or partly reverse it

Measured by

Measured from offset geological or landscape features across the fault, and from earthquake records

In economics, displacement is the loss of jobs or workers as a technology takes over tasks people used to do. Daron Acemoglu and Pascual Restrepo built this into a model of automation in which new machines have a displacement effect, pushing workers out of the tasks the machines now perform, set against countervailing forces that can create work elsewhere. In their study Robots and jobs, they estimated that adding industrial robots to local US labour markets lowered employment and wages in those markets. Here displacement names people moved out of paid work by a change in production, counted in jobs and wages, not a vector, a defence, or an offset of rock. It shares with the other meanings only the bare idea of something pushed out of the place it held.[6]

Unit

Workers or jobs pushed out of employment by technology, in jobs and wages

Recovery means

Displaced workers may be re-employed elsewhere, but often at lower pay or not at all

Measured by

Estimated from changes in employment and wages when a technology such as robots is introduced