In the double-slit experiment, the observation or measurement of photons does not directly "turn" them from a wave into a particle. Rather, the act of observation or measurement reveals the wave-particle duality of photons, allowing us to see their behavior as either wave-like or particle-like.
In the double-slit experiment, a beam of photons is directed at a barrier with two narrow slits. On the other side of the barrier, a screen is placed to detect where the photons land. When the experiment is set up without any observation, the photons exhibit an interference pattern on the screen, which is characteristic of their wave-like behavior. This pattern indicates that the photons pass through both slits simultaneously and interfere with each other, resulting in areas of constructive and destructive interference.
However, when detectors are placed at the slits to determine which slit each photon passes through, something interesting happens. The act of measurement introduces an interaction between the photons and the measurement apparatus. This interaction disturbs the system and leads to the collapse of the wavefunction, causing the photons to behave more like classical particles. As a result, the interference pattern on the screen disappears, and the photons now exhibit a particle-like behavior, producing two distinct bands on the screen corresponding to the slits.
The key factor in this change of behavior is the interaction between the quantum system (photons) and the measurement apparatus (detectors). The measurement process disrupts the superposition of states and collapses the wavefunction into a specific outcome. This collapse reveals the position information of the photons, which is incompatible with the wave-like behavior required for interference.
It's important to note that the precise mechanism by which observation causes the collapse of the wavefunction is still a subject of debate and interpretation in quantum mechanics. The Copenhagen interpretation suggests that observation by a conscious observer plays a fundamental role in the collapse. Other interpretations, such as the many-worlds interpretation or the decoherence approach, provide alternative explanations without invoking conscious observation.
Overall, the act of observation in the double-slit experiment doesn't directly change the nature of photons from a wave to a particle. Rather, it reveals the wave-particle duality and demonstrates how the measurement process can influence the behavior of quantum systems.