In quantum mechanics, the change in the position of a particle, such as an atom, after a measurement is performed is described by the process known as wavefunction collapse or the measurement postulate.
According to the measurement postulate, when a measurement is made on a quantum system, its wavefunction, which describes the probabilities of different possible states, collapses or "snaps" into one of the eigenstates corresponding to the measured observable. In the case of position measurement, the wavefunction collapses into an eigenstate that localizes the particle at a specific position.
The collapse of the wavefunction is a non-deterministic process, and the specific outcome of the measurement is probabilistic. The probability of obtaining a particular measurement outcome is related to the squared magnitude of the wavefunction associated with that outcome.
The act of measurement itself involves an interaction between the quantum system (the atom) and the measuring apparatus. This interaction disturbs the state of the system, causing the wavefunction collapse. The exact details of the measurement process and the interaction between the system and the measuring apparatus are beyond the scope of basic quantum mechanics and require a more detailed understanding of quantum measurement theory.
It is important to note that wavefunction collapse is a topic of ongoing debate and interpretation in quantum mechanics. There are different interpretations of quantum mechanics that offer alternative explanations or perspectives on the collapse process, such as the Copenhagen interpretation, many-worlds interpretation, and others. These interpretations provide different philosophical viewpoints on the nature of quantum measurement and the cause of the changes observed in the system after measurement.