The behavior of particles in a two-slit experiment can change significantly when we observe which slit they go through. This phenomenon is commonly known as the "observer effect" or "wavefunction collapse."
In a standard two-slit experiment, particles such as electrons or photons can exhibit wave-like properties and create an interference pattern on a screen behind the slits. This pattern arises due to the superposition of the particle's wave-like nature passing through both slits simultaneously.
However, when we introduce a measurement or observation to determine which slit the particle goes through, the wavefunction of the particle collapses. The collapse of the wavefunction means that the particle is forced into a particular state (either passing through one slit or the other) and loses its wave-like behavior. Consequently, the interference pattern disappears, and we observe a pattern consistent with particles behaving as individual entities.
The act of measurement or observation disrupts the delicate quantum state of the particle, causing it to "choose" a specific path rather than existing in a superposition of states. The exact mechanism and interpretation of this collapse are still debated in quantum mechanics and depend on the specific interpretation one adopts.
It is worth noting that the observer effect is not unique to the two-slit experiment but is a fundamental aspect of quantum mechanics. Observing or measuring a quantum system generally disturbs its state, leading to the collapse of the wavefunction and the emergence of classical-like behavior.