In the context of the double-slit experiment, "observing" the particle refers to the act of measuring or detecting its position or momentum in order to determine which path it takes or which slit it passes through.
In the basic double-slit experiment, a beam of particles, such as electrons or photons, is directed toward a barrier with two narrow slits. Behind the slits, there is a screen where the particles are eventually detected.
When the particles pass through the double slits, they create an interference pattern on the screen, indicating their wave-like behavior. This pattern arises because the particles exhibit wave interference when the wave functions associated with each slit overlap and interact with each other.
However, if one tries to determine which slit a particle goes through by placing detectors or measuring devices at the slits, something interesting happens. The act of measuring the particle's path disrupts the interference pattern, and the particle behaves more like a classical particle, producing two distinct patterns on the screen, each corresponding to one of the slits.
This change in behavior is known as the collapse of the wave function or the observer effect. The act of measurement disturbs the quantum system, causing it to "choose" a definite state, either passing through one slit or the other. The interference pattern, which emerges when the particle is not observed, is lost when its path is determined.
Therefore, "observing" the particle in the double-slit experiment refers to performing a measurement that determines the particle's path or gathers information about its behavior. The act of observation disturbs the quantum system and leads to the disappearance of the interference pattern, revealing the particle-like behavior of the system.