Quantum theory has equally profound implications for an understanding of reality itself. It states in effect that what happens in the microscopic world is the result of measurement - that is, a quantum state does not have a definite character until it is measured (for example, a particle does not have a definite path until the path is calculated). This is because prior to the measurement being made, the quantum situation consists of a range of possibilities - it is indeterminate - and the measurement settles which of the possibilities counts as actual. This raises a vexing question: must there
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