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First Principles of PhysicsLimits & Uncertainty · Rule 33

Limits & Uncertainty

Rule 33Chapter V

The observer is part of the experiment

Why this rule exists

Measurement is a physical interaction, so extracting information always disturbs the thing measured — there is no frictionless way to look. In quantum mechanics this is sharpened: a measurement selects an outcome from a superposition, and the choice of what to measure determines which properties even have definite values. The observer's apparatus is part of the system's dynamics, not an outside spectator.

First Principles of Physics

In practice

Always state what is being measured and how, because the result depends on both. Account for back-action: probing a delicate system perturbs it, so estimate the disturbance and design measurements that keep it below your needed precision. In quantum contexts, specify the measurement basis before asking what value a property has.

When it doesn't apply

For macroscopic systems the disturbance is usually negligible and the observer can be idealized away. 'Observer' means a physical measuring interaction, not a conscious mind; decoherence from any environment plays the same role without anyone watching.

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