In the double-slit experiment, the behavior of particles like photons or electrons can exhibit both wave-like and particle-like properties. When these particles are not observed, they can behave as waves and exhibit interference patterns, producing an interference pattern of light or dark fringes on a screen behind the slits. This is known as the wave-particle duality.
However, when an observing machine or a detector is introduced to determine which slit the particles pass through, the situation changes. The act of observation or measurement disturbs the system and causes the particles to behave differently. The interference pattern disappears, and the particles appear to behave more like individual particles, showing a pattern of two distinct bands corresponding to the two slits.
This change in behavior is not directly caused by the observing machine itself. Instead, it is a consequence of the interaction between the particles and the observing apparatus. The detection process involves interactions between the particles and the detector, which can alter the particle's momentum or position, disrupting the interference pattern.
In other words, the act of observation introduces additional interactions and disturbances that collapse the wave-like behavior of the particles into a localized particle-like state. This phenomenon is known as the "observer effect" or "measurement effect" in quantum mechanics. It demonstrates the delicate relationship between the observer and the observed system and highlights the role of measurement in determining the behavior of quantum particles.