[Physics] What are the problems in trying to interpret the Klein-Gordon equation as a single particle equation

Question

What is the problem if we try to interpret KG equation as a single-particle equation? Also, I wish to know whether the born interpretation of wavefunction is applicable in relativistic quantum mechanics.

Answer 1

If you try to construct the probablilty current for the KG equation, its zero component $j^0$ is not positive definite, even though you want to interpret it as a probability.