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For questions related to observers in relativity or quantum mechanics.

An observer in relativity defines a reference frame in which observations or measurements are made.

An observer in quantum mechanics is anything that makes a quantum measurement, but there is no consensus on a precise definition.

This tag is not normally used for reference to:

  • Astronomical, meteorological, or other observers where understanding the observer is not an essential part of the physics.
  • Instrumentation "observer" effects where experimental apparatus affects the quantity being measured, e.g. inserting a thermometer changes the temperature of system, or adding a multimeter changes a circuit.
  • Human observer biases that may affect scientific results.