A measurement is the process of interaction between the object of the measurement — what I want to know — and the instrument, the machine or observer that “reads”. In the classical way of thinking this interaction is one-way on the object: the instrument is disturbed (the needle moves, the film is exposed), but the measured quantity already existed, precise and independent, before and after the reading. Quantum mechanics breaks exactly this asymmetry.
Principle — What is a measurement?
In classical physics the measurement disturbs the instrument but leaves the object unchanged. In quantum mechanics, instead: the measured object itself is also modified by the measurement.
The measurement, in other words, does not photograph a pre-existing state: it helps to create it. Before interaction with the detector, many quantum properties simply do not have a defined value; the value emerges — and remains imprinted on the object — only at the instant of the measurement.
Giancarlo's cat
A picturesque way of saying the same thing: “The cat, after I’ve photographed it, is no longer the same.” In classical physics the cat remains the cat — the photo is just an imprint on the film. In quantum physics the cat itself has changed: its wave function has collapsed into a new state, dependent on the measurement carried out. The measurement does not passively reveal what was there before: it creates it.
(Not the famous Schrödinger’s cat, which is a different example: here the cat is only a didactic metaphor for the feedback of measurement.)
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Topics: Fisica quantistica
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