The behavior of particles can change when observed due to the phenomenon known as wave-particle duality, which is a fundamental aspect of quantum mechanics. According to wave-particle duality, particles can exhibit both wave-like and particle-like characteristics. The specific behavior of a particle is described by its wave function, which represents the probability distribution of finding the particle in different states.
When a particle is not observed, its wave function evolves according to a mathematical equation called the Schrödinger equation, which describes how the wave function changes over time. The wave function is said to be in a superposition of states, meaning the particle exists in multiple states simultaneously. This superposition allows particles to exhibit wave-like behavior, such as interference and diffraction.
However, when a measurement or observation is made on the particle, the wave function "collapses" into a single well-defined state. The act of measurement forces the particle to be observed in a particular state or value. The outcome of the measurement is probabilistic, with the probability of obtaining a specific result given by the squared magnitude of the particle's wave function at that state. This is known as the Born rule.
The nature of quantum entanglement is closely related to the behavior of particles in quantum mechanics. Quantum entanglement refers to a phenomenon where two or more particles become correlated in such a way that the quantum state of one particle cannot be described independently of the other particles' states, even if they are separated by vast distances.
When particles become entangled, their quantum states are intrinsically linked. This means that the measurement of one particle instantaneously affects the state of the other particle, regardless of the distance between them. This phenomenon is often described as "spooky action at a distance," as it challenges our intuitive understanding of causality and locality.
Quantum entanglement is a fundamental concept in quantum information and plays a crucial role in various applications, such as quantum computing, quantum cryptography, and quantum teleportation. It has been experimentally verified through a series of sophisticated experiments, including the famous Bell's theorem experiments, which provide strong evidence for the non-local nature of entangled particles.
The exact mechanism behind quantum entanglement is still an area of active research and debate. Various interpretations and mathematical formalisms within quantum mechanics, such as the Copenhagen interpretation, many-worlds interpretation, and the formalism of quantum field theory, offer different perspectives on the nature and implications of quantum entanglement.