In the double-slit experiment, when a sensor or detector is placed after the slits to observe which path the particle takes, it disrupts the interference pattern and the wave-like behavior of the particles. This is known as the "observer effect" or "wave function collapse."
When no detector is present, particles such as electrons or photons can exhibit wave-like behavior and create an interference pattern on the screen behind the double slits. This pattern arises from the superposition of the particle passing through both slits simultaneously and interfering with itself.
However, when a detector is introduced to determine which slit the particle passes through, it interacts with the particle and "collapses" the wave function. The act of measurement or observation forces the particle to behave more like a classical particle with definite path information. Consequently, the interference pattern disappears, and the particle behaves more like a particle going through one of the slits.
This outcome arises from the inherent nature of quantum mechanics, where the act of measurement disrupts the delicate superposition and entanglement of quantum systems. The measurement process causes the system to "choose" a definite state, erasing the interference pattern.
If the sensor or detector is placed after the slits but not actively observed, the interference pattern will still be disrupted. Even if the information about the particle's path is not explicitly extracted or recorded, the potential for measurement is present, and it will collapse the wave function. The exact mechanism of wave function collapse and the nature of measurement in quantum mechanics are topics of ongoing scientific inquiry and philosophical debate.
It's important to note that the outcome described above is based on the standard interpretation of quantum mechanics. Various alternative interpretations, such as the many-worlds interpretation, attempt to address the behavior of the wave function and the role of measurement in different ways, but they remain subjects of theoretical discussion and interpretation.