Where does observer collide?

In relativity, an observer is the frame of reference from which positions, times, and motions are measured, so what one observer records as simultaneous, or as a given length, can differ for another observer in relative motion. Einstein's 1905 Zur Elektrodynamik bewegter Körper (On the Electrodynamics of Moving Bodies) made the observer's frame constitutive of measurement, showing that intervals of time and length are read off relative to the observer's own clocks and rulers. Here the observer is an abstract coordinate viewpoint carrying its own clocks, not a person estimating a machine's hidden state, a researcher living among the people studied, or a software object waiting for notice of a change.[1]

Unit

One frame of reference, with its own synchronised clocks and measuring rods

Recovery means

Not applicable

Measured by

Realised through the clocks and rulers a given frame uses to assign coordinates to events

In ethnographic fieldwork, the participant observer is a researcher who joins the daily life of a community while systematically recording what happens, so that taking part becomes the instrument and the account depends on who is watching and from what position. Malinowski's 1922 Argonauts of the Western Pacific established this immersive fieldwork among the Trobriand Islanders, and Whyte's 1943 Street Corner Society became a seminal application in an urban neighbourhood. This observer is a situated human researcher gathering meaning from inside a group, not a coordinate frame in physics, a state-estimating computation in control engineering, or a subscriber object in software.[2][3]

Unit

One researcher embedded in a setting over a stretch of fieldwork

Recovery means

Not applicable

Measured by

Recorded through field notes, interviews, and reflexive accounts of the researcher's own position in the group

In control engineering, a state observer, or estimator, is an auxiliary dynamical system that reconstructs the unmeasured internal state of a plant from its known inputs and its measured outputs. Luenberger's 1964 Observing the state of a linear system introduced the deterministic observer for linear systems, developed further in his 1971 An introduction to observers, while the stochastic counterpart is the Kalman filter. This observer is a computation that estimates the hidden variables of a machine, not a viewpoint that defines measurement in physics, a fieldworker embedded in a community, or a design idiom for notifying software components.[4][5]

Unit

One estimator producing a running estimate of the plant's state vector

Recovery means

The estimate converges to the true state as the observer's error dynamics are made stable

Measured by

Assessed by how fast and how closely the estimated state converges to the true state

In object-oriented software design, the observer pattern defines a one-to-many dependency in which a subject keeps a list of dependent objects, its observers, and notifies them automatically whenever its own state changes. Gamma, Helm, Johnson, and Vlissides's 1994 Design Patterns: Elements of Reusable Object-Oriented Software catalogued it as a standard way to keep views and data in step without tight coupling between them. Here an observer is a subscriber object registered to receive change notifications, not a relativistic frame, a state-estimating filter, or a human researcher living among those being studied.[6]

Unit

One subject holding a set of registered observer objects

Recovery means

Not applicable

Measured by

Seen in code through the register and notify interface and the observers updated on each state change