What Is an Observer

An observer in quantum mechanics can mean a physical detector, an agent assigning probabilities, or a subsystem that stores a record. These roles sometimes belong to the same laboratory, but they are conceptually different. A foundational argument should identify which role is intended before drawing conclusions about observation.

Three roles that need not coincide

A detector couples to a system and produces a calibrated output. An agent uses preparation and record information to assign probabilities. A physical observer model includes some memory and response degrees of freedom in the quantum description. None of these definitions, by itself, makes consciousness a dynamical variable.

An automated apparatus can perform the first role for months before anyone reads its memory. Two agents with different records can make different conditional assignments to that same apparatus. A later coherent experiment can in principle include the memory as part of the measured system, provided the required isolation and control are physically available.

The Observer in Quantum Mechanics develops the operational details. This page supplies the vocabulary needed to avoid changing roles unnoticed.

Different information, different state assignments

Suppose a source chooses $|0\rangle$ or $|1\rangle$ fairly and stores the choice in a classical register. An agent who reads zero assigns $|0\rangle\langle0|$ to the prepared qubit. An agent who knows only the fair preparation assigns $I/2$. These assignments give different predictions conditional on different information:

$$p(Z=+1\mid\text{label }0)=1, \qquad p(Z=+1)=1/2.$$

No physical contradiction follows. Once the agents exchange the same records and use the same preparation model, the ordinary conditional predictions agree. Their initial disagreement did not demonstrate that a person created the state by thinking about it.

For entangled laboratories the relation between internal conditional and external global descriptions is subtler; Wigner’s Friend specifies that scenario rather than treating all state disagreements as equivalent.

Observer-independent laws and observer-relative information

A probability assignment can depend on available information while the law connecting states, measurements and probabilities is shared. Conversely, an interpretation can make certain physical facts relational, which is a stronger claim than ordinary ignorance. Relational Quantum Mechanics must therefore explain which facts are relative and how comparisons are represented.

The word observer should not erase the distinction between a mathematical conditioning operation and a physical interaction. Specify both when they matter. For example, a remote result can update an agent’s prediction without altering an unconditioned local marginal.

Exercise: identify the role

A paper says, “The observer was replaced by a hard drive; later the observer updated the state after reading it.” Identify the two uses of observer and rewrite the claim precisely.

Solution

The first use means record-bearing apparatus; the second means an agent using information. A precise version is: “An autonomous detector stored its outcome on a hard drive. An agent later conditioned predictions on the stored label.” Whether the registration involved objective collapse is an additional physical claim absent from this operational description.

References

• Everett, Hugh, III, “Relative State Formulation of Quantum Mechanics,” Reviews of Modern Physics 29, 454–462 (1957), doi:10.1103/RevModPhys.29.454.
• Rovelli, Carlo, “Relational quantum mechanics,” International Journal of Theoretical Physics 35, 1637–1678 (1996), doi:10.1007/BF02302261.