Well, Wheelers Delayed Choice Experiment will explain it most efficiently in simple terms.
I will simplify it even further.
Imagine a photon traversing the universe in some wordline. However according to quantum mechanics, this photon does not travel one wordline alone, in fact, it takes...
Do you know in principle how chirality enters the Dirac equation? How is your covariant notation?
I'll take you through a small derivation of the dirac equation, a famous one.
\partial_{t} \psi + \alpha^i \partial^i \psi = \beta m \psi
Move everything to the left hand side...