When detectors are placed at the double slits in the famous double-slit experiment, the behavior of light or other quantum particles can change depending on how the experiment is set up.
In the basic double-slit experiment without detectors, when a beam of light or particles such as electrons or photons pass through the slits, they exhibit an interference pattern on the screen behind the slits. This pattern arises due to the wave-like nature of particles. Each particle behaves as a wave, passing through both slits simultaneously and interfering with itself, creating regions of constructive and destructive interference on the screen.
However, when detectors are placed at the slits to observe which slit each particle goes through, the behavior of the particles can change. The act of measurement or observation collapses the particle's wave function, forcing it to behave more like a classical particle rather than a wave. In this case, the interference pattern disappears, and the particles start to behave as if they are going through one slit or the other, producing a pattern similar to two separate beams of particles.
The introduction of the detectors disrupts the delicate quantum interference that is responsible for the interference pattern. The measurement process "forces" the particles to behave as individual particles with definite paths rather than exhibiting wave-like behavior. This effect is often referred to as the "collapse of the wave function" or "quantum decoherence."
It's important to note that the mere presence of the detectors or the act of measurement itself is what causes the change in behavior. The particles' behavior is influenced by the interaction with the detectors, which disturbs the quantum system.
This experiment highlights the wave-particle duality of quantum particles and the role of observation or measurement in determining their behavior. Without the detectors, the particles exhibit wave-like behavior with interference patterns, and with the detectors, they behave more like classical particles with definite paths through one slit or the other.