A negative particle experiences a reversed force by the way its time axes are arranged and when aligned with that of a positive particle, points its \(x\) axis at the negative \(x\) direction of the positive particle. The energy density \(\psi\), a scalar, is the same type on both positive and negative particle.
Notice the abrupt change in gradient on the factor \(e^u(e^{2u}-1)^{1/2}\) at the minimum \(x=x_z\). This is the reason for the \(-i\) factor when \(x\to x_z^{-}\), which switches the direction of \(v\) to be along a radial line \(x\).
Quantum Tunneling! But only in one direction.
