The double-slit experiment is a classic experiment in quantum mechanics that demonstrates the wave-particle duality of light and matter. When performed with light, it involves shining light through two narrow slits onto a screen, resulting in an interference pattern.
The difference in light behavior between when the experiment is being watched versus when it is not being watched is a concept commonly referred to as the observer effect or the measurement problem in quantum mechanics.
When the experiment is conducted without any measurement or observation, light behaves as a wave and produces an interference pattern on the screen. This pattern arises due to the constructive and destructive interference of the light waves passing through the two slits. The interference pattern suggests that light behaves as a wave and exhibits wave-like properties.
However, when the experiment is observed or measured, such as by placing detectors to determine which slit the photons pass through, something interesting happens. The presence of the detectors or any other means of observation disturbs the system and collapses the wave-like behavior into a particle-like behavior. This means that the interference pattern disappears, and the light appears to behave as individual particles or photons that hit the screen behind the slits, creating a pattern similar to two distinct bands.
The act of measurement or observation affects the behavior of the light by collapsing its wave function, which describes the probability distribution of the particle's properties. The specific process that causes this collapse is still a topic of debate and interpretation in quantum mechanics.
It's important to note that the observer effect is not limited to conscious observers. Any interaction or measurement that provides information about the particles' paths can lead to the collapse of the wave function. Even if the detection is done indirectly or if the information is available in principle but not accessed, the collapse still occurs.
In summary, when the double-slit experiment is observed or measured, the light behaves more like particles, and the interference pattern disappears. When it is not observed or measured, the light behaves as a wave, creating an interference pattern on the screen. The act of measurement or observation affects the behavior of light, illustrating the peculiar nature of quantum mechanics.