Consider a particle with energy E in the inner region of a one-dimensional potential well V(x). ..... In classical mechanics, if E < V (the maximum height of the potential barrier), the particle remains in the well forever; if E > V , the particle escapes. In quantum mechanics, the situation is not so simple. The particle can escape even if its energy E is below the height of the barrier V , although the probability of escape is small unless E is close to V .