In quantum tunneling, the evanescent wave is not considered to be a part of the particle itself, but rather an associated phenomenon that arises during the tunneling process. When a particle encounters a potential barrier that it does not have enough energy to overcome classically, it can still penetrate through the barrier due to its wave-like nature in quantum mechanics.
During the tunneling process, a wave function associated with the particle extends into the classically forbidden region beyond the barrier. In this region, the wave function exponentially decays with distance, forming what is known as an evanescent wave. This wave does not represent a physical propagation of the particle itself but rather a mathematical description of the particle's behavior in the tunneling process.
The evanescent wave does not carry energy or momentum across the barrier; instead, it represents the probability amplitude of finding the particle in the forbidden region. Once the particle has tunneled through the barrier and reached the other side, its wave function can resume propagating as a regular wave.
It is important to note that the concept of the evanescent wave is specific to quantum tunneling and is not limited to particles. It can also arise in other wave phenomena, such as electromagnetic waves, when they encounter interfaces or obstacles.