The peculiar effect observed in the behavior of particles under observation is a fundamental aspect of quantum mechanics known as the "observer effect." This phenomenon is most famously illustrated by the double-slit experiment. In this experiment, particles such as electrons are fired at a barrier with two slits. When not observed, particles create an interference pattern on a detection screen, typical of wave-like behavior. However, when measurement devices are used to determine through which slit the particle passes, the interference pattern disappears, and the particles behave as if they are classical, passing through one slit or the other and not both.
This change in behavior under observation challenges classical intuitions, suggesting that quantum systems exist in a superposition of states until measured. The act of observation causes the system to 'collapse' into one of the possible definite states. This phenomenon is still a subject of interpretation within quantum mechanics, with various theories such as the Copenhagen interpretation positing that particles exist in a probability wave until measured. Another perspective, the many-worlds interpretation, suggests that all possible outcomes occur but in separate, branching universes.
Measurement in quantum mechanics involves more than just human observation; it entails any interaction that extracts information about a quantum system, thereby influencing its state. This challenges the classical notion of objective observational processes, suggesting that the observer is part of the system being observed, contributing to the ongoing complexity and intrigue of quantum mechanics.