In the double-slit experiment, when particles are not observed or measured, they behave as if they exhibit wave-like properties. The specific behavior depends on the nature of the particles and the experimental setup. I'll explain the general behavior using the example of electrons, but the principles apply to other quantum particles as well.
In the double-slit experiment, a beam of particles, such as electrons, is directed toward a barrier with two slits. Behind the barrier, a screen is placed to detect the particles' arrival. When the particles are not observed or measured individually, they exhibit an interference pattern on the screen, which is characteristic of wave behavior.
The particles pass through both slits simultaneously and interfere with themselves, creating regions of constructive and destructive interference. This results in an alternating pattern of bright and dark fringes on the screen where the particles are more likely to be detected or less likely to be detected, respectively. The interference pattern arises due to the superposition of the probability amplitudes associated with the different paths the particles can take.
This behavior is in contrast to the particle-like behavior observed when the particles are observed or measured. In that case, if one tries to determine through which slit each particle passes, the interference pattern disappears, and the particles behave as if they go through only one of the slits, resulting in a pattern of two separated bands on the screen.
This phenomenon, known as the wave-particle duality, demonstrates that quantum particles can exhibit both wave-like and particle-like behavior depending on the experimental context and whether or not they are observed or measured. It highlights the fundamental probabilistic nature of quantum mechanics and the importance of the observer's role in determining the observed behavior of particles.