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I was working with Dirac's equation:

$$ ( i \hbar \gamma^\mu \partial_\mu - m ) \psi = 0, $$

and I found that if you work with a function that depends on the momentum, $$ \psi ( \mathbf{p} ), $$ you obtain:

$$ ( i \gamma \cdot \mathbf{p} + m ) \psi ( \mathbf{p} ) = 0. $$

The problem is that I can't figure out how did the imaginary number not disappear in the last equation. I tried to work with $$ p_\mu \rightarrow i \hbar \partial_\mu , $$

and I obtained the following:

$$ ( \gamma \cdot \mathbf{p} + m ) \psi ( \mathbf{p} ) = 0. $$

Help?

$$ --- $$

The $$ \gamma $$ are the Gell-Mann marices.

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# Little issue with relativistic quantum mechanics

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