Function relativistic_current_kernel 

pub fn relativistic_current_kernel(
    em_tensor: &CausalTensor<f64>,
    _metric: &CausalTensor<f64>,
) -> Result<CausalTensor<f64>, PhysicsError>

Calculates current density $J^\mu$ compatible with curved spacetime. $$ J^\mu = \nabla_\nu F^{\mu\nu} $$ (Divergence of Electromagnetic Tensor).

Arguments

• em_tensor - Electromagnetic tensor $F^{\mu\nu}$ (Rank 2, contravariant).
• metric - Metric tensor $g_{\mu\nu}$ (Rank 2). Note: This kernel assumes em_tensor is already raised ($F^{\mu\nu}$). If input is $F_{\mu\nu}$, user must raise indices first. For the divergence $\nabla_\nu$, we need covariant derivative. In flat space, $\partial_\nu F^{\mu\nu}$. This kernel approximates $\partial_\nu F^{\mu\nu}$ via simple contraction if Christoffel symbols aren’t provided. For full GR, one needs connection coefficients. This implementation computes the Partial Divergence which is exact in locally inertial frames or Minkowski space. $$ J^\mu_{approx} = \partial_\nu F^{\mu\nu} $$

Returns

• Result<CausalTensor<f64>, PhysicsError> - Current density vector $J^\mu$.