if we paint rainbow's color on a circle top and rotate it,wecan see color white. that means our eyes have a time limit to distinguish different colors.
Use the formula:\Delta\vec{A}=\nabla(\nabla\bullet\vec{A})-\nabla\times(\nabla\times\vec{A}). You'll get three Laplace equation about P,Q,R.Assume \vec{A}=(P,Q,R).\Deltameans twice \nabla.
Maybe you'd better learn the Maxwell's equation first to see how light's speed comes from and why it travels slower in some media.Wikipedia is not science book. lol
It's hard to answer.Actually physics is to solve how it could be not why it is.You can keep asking what causes something to cause gravity.so maybe you can put forward a theory to explain it by yourself.