# Solution to Klein-Gordon equation always valid?

We know that there is a relativistic version of Schrodinger equation called Klein-Gordon equation. However, it has some problems and due to these problems, there is Dirac equation that handles these problems.

So, the question is, if there is a solution that is allowed by Klein-Gordon, but not by Dirac, can this solution be considered valid?

Also, can Dirac equation be used for spin-0 particles?

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see arxiv.org/abs/quant-ph/0307059 for a solution to the problems of the KG-equation – Christoph Oct 8 '12 at 11:41

The converse of course doesn't hold. The most basic reason is that the Klein-Gordon equation should really act on scalars, a single bosonic field, while the minimum number of components for the $d=4$ Dirac equation is four (and they should be fermionic fields). So a general (or generic) valid solution to the Klein-Gordon equation is a valid solution to the Klein-Gordon equation (this much is a tautology, but you were asking about it), but it is not a solution to the Dirac equation.