The double-slit experiment is a classic experiment in quantum mechanics that demonstrates the wave-particle duality of particles, such as electrons or photons. In this experiment, a beam of particles is directed towards a barrier with two narrow slits, and behind the barrier is a screen that records the pattern of particle impacts.
When particles are fired one at a time, they can exhibit an interference pattern on the screen, suggesting wave-like behavior. This means that the particles can interfere with themselves, creating regions of constructive and destructive interference. However, when the particles are observed or measured to determine which slit they pass through, the interference pattern disappears, and the particles behave more like individual particles with no interference.
The disappearance of the interference pattern when the particles are observed is known as the collapse of the wave function. The wave function describes the probabilistic behavior of a particle before it is observed. When a measurement is made, the wave function collapses into a specific state corresponding to the measurement outcome. In the case of the double-slit experiment, the act of observing which slit the particle passes through collapses the wave function and destroys the interference pattern.
To address your question about how scientists are sure that it is the act of observation that introduces the collapse, and not some other interference, it's important to note that extensive experimental evidence supports this conclusion. The double-slit experiment has been repeated and verified by numerous researchers using different particles (such as electrons, photons, and even larger molecules), different apparatus setups, and various methods of observation.
When observations are made to determine the slit through which a particle passes, the measurement process inevitably disturbs the particle in some way. For example, shining a light or using detectors to determine the particle's path can impart momentum to the particle, altering its behavior. This disturbance destroys the delicate interference pattern and results in particle-like behavior.
The collapse of the wave function and the influence of observation have been studied and confirmed through numerous experiments and are a fundamental aspect of quantum mechanics. It is an accepted interpretation based on the consistency of experimental results. However, it's worth noting that there are different interpretations of quantum mechanics, such as the Copenhagen interpretation, which views observation as a fundamental factor in the collapse of the wave function, and other interpretations that propose alternative explanations for the observed phenomena.