In the double-slit experiment, the paradox of wave-particle duality arises, which involves the behavior of particles like electrons or photons that exhibit both wave-like and particle-like characteristics. The experiment demonstrates that when a beam of particles passes through two closely spaced slits, an interference pattern emerges on a screen behind the slits, suggesting a wave-like nature. However, when attempts are made to determine which path a particle takes, such as by placing detectors at the slits, the interference pattern disappears, indicating a particle-like behavior.
The act of "observing" or measuring the particles in the double-slit experiment refers to acquiring information about their path. Any interaction with the particles, whether it's through the use of detectors or other means, can disturb the system and cause the wave function to collapse, resulting in the disappearance of the interference pattern. This is known as the observer effect or measurement problem in quantum mechanics.
However, it's important to note that the mere presence of a measuring device or recording apparatus does not automatically cause the wave function collapse. The collapse occurs when the information about the particle's path becomes available to an observer or becomes entangled with the environment. Simply recording evidence of the wave-like behavior, such as capturing the interference pattern on a photographic plate or detecting the pattern on a screen, does not involve acquiring information about individual particle paths. Therefore, it does not lead to the collapse of the wave function.
The key point is that in the double-slit experiment, the interference pattern is a statistical result obtained by repeating the experiment numerous times and observing the overall pattern. Each individual particle behaves like a wave and passes through both slits, interfering with itself, to create the pattern. The wave-like behavior is evident when we observe the cumulative effect of many particles, rather than tracking the paths of individual particles.
In summary, recording evidence of the wave-like behavior by observing the interference pattern does not cause the collapse of the wave function because it does not involve acquiring information about individual particle paths, which is what disrupts the interference pattern in the first place.