It can be shown that this definition of the heat flux vector also satisfies the second law of thermodynamics, ∇ ∇ ⋅ ⋅ ( q θ θ ) + ρ ρ ∂ ∂ s ∂ ∂ t = σ σ , where s is specific entropy and σ is entropy production. This mathematical model is inconsistent with special relativity: the Green function associated to the heat equation (also known as heat kernel) has support that extends outside the light-cone, leading to faster-than-light propagation of information. For example, consider a pulse of heat at the origin; then according to Fourier equation, it is felt (i.e. temperature changes) at any distant point, instantaneously. The speed of propagation of heat is faster than the speed of light in vacuum, which is inadmissible within the framework of relativity.