superselectionsääntöjen

Superselection rules are a set of principles in quantum mechanics that describe the conditions under which a quantum system can be considered to be in a well-defined state. These rules are particularly relevant in the context of quantum decoherence, where a quantum system interacts with its environment, leading to the apparent collapse of the wave function.

The concept of superselection rules was first introduced by Eugene Wigner and later developed by other physicists. These rules state that certain observables, known as superselection observables, cannot be simultaneously measured with arbitrary precision. This means that if a system is in a superselected state, it cannot evolve into a state that is not superselected.

Superselection rules are crucial for understanding the emergence of classical behavior from quantum mechanics. They provide a way to explain why, despite the fundamental indeterminacy of quantum mechanics, we observe a world that appears to be deterministic and classical.

There are different types of superselection rules, depending on the specific system and the observables being considered. For example, in the case of a spin-1/2 particle, the superselection rule might be that the spin cannot be simultaneously measured in both the x and y directions with arbitrary precision.

Superselection rules are also closely related to the concept of decoherence, which is the process by which quantum systems lose their quantum coherence and become classical. Decoherence can be understood as a consequence of superselection rules, as the interaction with the environment effectively imposes superselection rules on the system.

In summary, superselection rules are a fundamental concept in quantum mechanics that describe the conditions under which a quantum system can be considered to be in a well-defined state. They play a crucial role in understanding the emergence of classical behavior from quantum mechanics and are closely related to the concept of decoherence.