The collapse of the wave function when observed is a fundamental aspect of quantum mechanics, and it is described by a mathematical rule known as the measurement postulate. However, the underlying mechanism or explanation for why this collapse occurs is still a subject of debate and interpretation within the field of quantum mechanics.
The measurement postulate states that when a quantum system, such as a particle, is measured, its wave function instantaneously "collapses" or "reduces" to one of the possible measurement outcomes. This collapse corresponds to the system transitioning from a superposition of states to a single well-defined state.
There are several interpretations of quantum mechanics that offer different explanations for the collapse of the wave function. Here are a few of the prominent interpretations:
Copenhagen interpretation: This interpretation, proposed by Niels Bohr and Werner Heisenberg, is one of the most well-known and widely accepted interpretations. It suggests that the wave function collapse is a fundamental and irreducible aspect of the measurement process. According to this view, the act of measurement interacts with the quantum system in a way that forces it into a specific state. The collapse is regarded as an intrinsic property of the quantum formalism and is not further explained or derived.
Many-Worlds interpretation: In contrast to the Copenhagen interpretation, the Many-Worlds interpretation, proposed by Hugh Everett, does not involve a collapse of the wave function. Instead, it suggests that the wave function evolution is deterministic and continuous. According to this interpretation, when a measurement occurs, the universe splits into multiple branches, each corresponding to a different measurement outcome. In each branch, the observer perceives a different measurement result, but the wave function does not collapse.
Decoherence theory: Decoherence theory provides an explanation for the apparent collapse of the wave function by considering the interaction between the quantum system and its environment. According to this view, the system and the measuring apparatus become entangled through their interactions, leading to the appearance of a collapse. The entanglement with the environment causes the system's superposition to become "decoherent" or effectively suppressed, making one measurement outcome dominate while others become negligible.
It's worth noting that these interpretations are philosophical interpretations of quantum mechanics and do not have definitive empirical evidence supporting one over the others. The collapse of the wave function remains a subject of ongoing research, and different interpretations offer various perspectives and philosophical frameworks to understand this phenomenon.