Sakurai"s Comment Anti-particle in Klein-Gordon equation: Reversing momentum and Moving backward in time

Question

In JJ Sakurai, p.492-493,

he made Comments on Anti-particle in Klein-Gordon equation. By analyzing the current EOM, he said that charge current density is the same for all particles, (regardless positive and negative energy, or positive and negative charged) which is contradicting.

Sakurai proposed two resolutions

(1) Reversing momentum,

or

(2) negative energy particle moving backward in time.

Could someone explain the arguments more cleanly? It does not seem that his resolutions are very precise.

I think he proposed two different ways to resolve the problems.

So please explain (1) and (2) separately. Thanks~

This post imported from StackExchange Physics at 2020-10-30 22:43 (UTC), posted by SE-user annie marie heart