The behavior of particles in the double-slit experiment can indeed seem counterintuitive, but it is based on the principles of quantum mechanics. When the experiment is set up with an electron source and a barrier with two slits, and the electrons are not being observed or detected as they pass through the slits, they exhibit a wave-like behavior and interfere with each other, creating an interference pattern on the screen.
However, when we introduce an observation or measurement apparatus to determine which slit the electron passes through, the situation changes. The act of measurement interferes with the wave-like behavior of the electron, collapsing its wavefunction into a particular state. This collapse means that the electron is forced to behave more like a particle and can be detected as having passed through one specific slit, rather than exhibiting interference.
In other words, by introducing an observation, we disrupt the superposition of states where the electron can pass through both slits simultaneously. This phenomenon is known as the "observer effect" or "wavefunction collapse." The act of measurement disturbs the system being observed, altering its behavior.
The common sense we typically develop based on classical physics, which deals with macroscopic objects, does not directly apply to the quantum realm. Quantum mechanics operates on a different set of rules, where particles can exhibit both wave-like and particle-like behavior depending on the circumstances.
It's important to note that the double-slit experiment is often discussed in the context of understanding the fundamental nature of particles and the wave-particle duality in quantum mechanics. The behavior of particles at the quantum level can challenge our intuition based on everyday experiences, but it is the subject of ongoing scientific investigation and study.