In quantum mechanics, the concept of a system being in a definite state is closely related to the measurement process. When a measurement is performed on a quantum system, its wavefunction collapses into one of the possible eigenstates of the measured observable. This collapse of the wavefunction is often referred to as "reduction of the wavepacket" or "wavefunction collapse."
The question of how we can be sure that a quantum system is in a definite state raises fundamental philosophical and interpretational debates within quantum mechanics. Different interpretations provide different perspectives on the nature of measurement and the collapse of the wavefunction. Let's explore a few interpretations briefly:
Copenhagen interpretation: The Copenhagen interpretation, proposed by Niels Bohr and his colleagues, is one of the earliest and most widely known interpretations of quantum mechanics. According to this interpretation, the act of measurement inherently involves an interaction between the quantum system and the measurement apparatus, causing the wavefunction to collapse. It does not provide a deeper explanation of the measurement process but emphasizes the probabilistic nature of outcomes.
Many-worlds interpretation: The many-worlds interpretation, proposed by Hugh Everett, offers a different perspective. It suggests that when a measurement occurs, rather than collapsing into a single outcome, the wavefunction of the system and the measurement apparatus continue to evolve and branch into multiple parallel universes, each corresponding to a different possible outcome. In this interpretation, the idea of a definite state is reinterpreted as the observer's subjective experience of being in a particular branch of the multiverse.
Consistent Histories interpretation: The Consistent Histories interpretation, developed by Robert Griffiths and Murray Gell-Mann, focuses on describing the history of a quantum system. It suggests that a quantum system can be described by a consistent set of histories that do not lead to logical contradictions. In this interpretation, a definite state arises when the set of histories associated with that state is consistent.
It's important to note that these interpretations are theoretical frameworks and do not offer direct experimental evidence. The question of how we can be sure that a quantum system is in a definite state ultimately depends on the interpretation one adopts. Different interpretations may have different philosophical implications but can yield equivalent predictions for the outcomes of measurements.
In practice, experimental verification of the state of a quantum system often involves performing repeated measurements and statistical analysis. By making a series of measurements on identically prepared quantum systems, the probabilities associated with different outcomes can be determined. These experimental results can provide strong evidence for the probabilistic nature of quantum systems and their behavior in certain states.