The double-slit experiment is a well-known quantum mechanical experiment that demonstrates the wave-particle duality of particles, such as electrons or photons. In the experiment, when particles are sent through two closely spaced slits and allowed to hit a screen, an interference pattern emerges if the particles are not observed. This 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 more like individual particles, creating a pattern of two distinct bands behind the slits.
The key point to understand here is that observation or measurement in quantum mechanics generally refers to the act of interacting with the system in a way that extracts information about its properties. This interaction can disturb the system and cause it to behave differently from when it is unobserved. In the case of the double-slit experiment, the act of measuring or observing which slit the particles pass through introduces interactions that disturb the particles' wave-like behavior and collapses their wavefunctions, leading to the disappearance of the interference pattern.
While it is true that we cannot directly observe the unobserved outcome of the double-slit experiment, we can indirectly infer the results through the detection of the particle's impact on the screen. By analyzing the pattern of particle impacts on the screen, we can determine whether an interference pattern, characteristic of wave-like behavior, has emerged or not.
Moreover, the predictions of quantum mechanics have been extensively tested and confirmed through a wide range of experiments and observations. These experiments provide consistent evidence for the existence of quantum phenomena and the different outcomes of the double-slit experiment when observed versus unobserved.
In summary, while the act of observation or measurement in quantum mechanics can alter the behavior of a system, the effects of these interactions have been extensively studied and confirmed through experimental evidence. The observation of interference patterns or their absence in the double-slit experiment and other similar experiments supports the understanding that particles exhibit wave-like behavior when unobserved and particle-like behavior when observed.