Although the uncertainty principle is probably the most well-known example of a fundamental limitation of measurement precision in quantum mechanics, it is not the only one. In fact, every physical system is characterized by a number of variables that do not change their values as the system evolves over time; such variables are called conserved quantities and they are said to obey a conservation law. The fact that some quantities cannot change their values suggests that there might be restrictions on the possible ways in which a measurement device can interact with a quantum object and extract information from it.

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