The relativistic formulation of electromagnetism simplifies Maxwell's equations by unifying the fields and sources into four-vectors and tensors, making their consistency with Special Relativity manifest. The electromagnetic field tensor $F^{\mu \nu}$ is a rank-2 anti-symmetric tensor that combines the six components of the electric field (E) and magnetic field (B) into a single object. Similarly, the four-current density $K^\nu$ is a four-vector that unifies the scalar charge density ( $\rho$ ) and the vector current density ( $J$ ). This entire covariant framework is naturally expressed using the four-dimensional spacetime coordinate $x^\mu=\left(c t, x^1, x^2, x^3\right)$.

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1. How does the four-dimensional spacetime coordinate relate to the relativistic formulation of Maxwell's equations?
2. What physical quantities are embedded in the four-vector (four-current density)?
3. What physical quantities are embedded in the electromagnetic field tensor? </aside>