The observation or measurement in a double-slit experiment affects the outcome because of the inherent nature of quantum mechanics and the wave-particle duality of quantum objects.
In a double-slit experiment, a beam of particles, such as electrons or photons, is directed at a barrier with two slits. Behind the barrier, a screen records the pattern of particles that pass through the slits and arrive at different points on the screen.
When the particles are not observed or measured during their journey from the source to the screen, they exhibit an interference pattern on the screen. This pattern arises due to the wave-like nature of quantum objects. Each particle behaves as a wave that passes through both slits simultaneously, creating an interference pattern where waves interact constructively or destructively, leading to light and dark regions on the screen.
However, if one tries to determine which slit a particle passes through by introducing a measurement or observation, the interference pattern disappears. The act of observing or measuring the particles collapses their wavefunction, causing them to behave more like localized particles with definite positions. The act of measurement "forces" the particle to take a definite path through one of the slits, eliminating the interference effect that arises from the wave-like behavior.
This phenomenon is known as the "observer effect" or "quantum measurement collapse." The act of measurement disrupts the delicate superposition of states, collapsing it into a specific outcome. The process of observation introduces interaction between the quantum system (particles) and the measurement apparatus, leading to entanglement and decoherence, which destroy the interference pattern.
The exact mechanisms and interpretations of the observer effect are still topics of debate and ongoing research in quantum mechanics. Various interpretations, such as the Copenhagen interpretation, many-worlds interpretation, or consistent histories interpretation, provide different perspectives on the nature of observation and its impact on the results of quantum experiments.