The double-slit experiment is a classic experiment in quantum mechanics that demonstrates the wave-particle duality of quantum particles, such as electrons or photons (particles of light). It consists of a barrier with two narrow slits, a source of particles, and a screen or detector to observe the particles.
When particles are sent through the double-slit apparatus one at a time, they behave as if they can pass through both slits simultaneously and interfere with themselves. This results in an interference pattern on the screen, with alternating bright and dark regions where the particles are more likely to be detected or less likely to be detected, respectively. This interference pattern is characteristic of wave-like behavior.
However, if the particles are observed or measured to determine which slit they pass through, the interference pattern disappears, and the particles behave as individual particles. The act of measurement or observation collapses the particles' wavefunction, causing them to behave more like classical particles with well-defined paths. The interference pattern is lost because the knowledge of which slit the particle went through eliminates the possibility of interference.
This phenomenon, known as the observer effect or wavefunction collapse, is a fundamental aspect of quantum mechanics. It implies that the act of observation or measurement influences the behavior of quantum particles. Until observed, quantum particles can exist in multiple states or locations simultaneously, described by a wavefunction that encompasses all possible outcomes. Observation "selects" a particular outcome and collapses the wavefunction into a single state.
It's important to note that this behavior is specific to the quantum realm and does not apply to macroscopic objects in our everyday experience. Classical objects behave in a purely particle-like manner and are not subject to the same wave-particle duality or observer effect as observed in quantum systems.