In quantum mechanics, particles are described by quantum fields, and these fields can exist in superpositions of different states. The question of which state is "real" or observed depends on the process of measurement or interaction with the system.
When a measurement is made on a quantum system, the system's state "collapses" into one of the possible eigenstates of the observable being measured. This process is described by the mathematical formalism of quantum mechanics known as wavefunction collapse or projection postulate.
The specific outcome of a measurement is probabilistic and governed by the wavefunction of the system. The probabilities of different outcomes are determined by the amplitudes of the superposed states. The more probable outcomes are more likely to be observed.
It's important to note that the act of measurement itself introduces an interaction between the quantum system and the measuring apparatus, which can disrupt the superposition and cause the system to "choose" a definite state. This is often referred to as the measurement problem in quantum mechanics and is still a topic of debate and ongoing research.
In summary, the question of which state is "real" is determined through the process of measurement or interaction, which causes the system to collapse into one of its possible states according to the probabilities dictated by the quantum formalism.