Editing Relativistic wave equations (section)

== Non-linear equations ==
{{further|Non-linear differential equation}}

There are equations which have solutions that do not satisfy the superposition principle.

=== Nonlinear gauge fields ===
* [[Yang–Mills theory|Yang–Mills equation]]: describes a non-abelian gauge field
* [[Yang–Mills–Higgs equations]]: describes a non-abelian gauge field coupled with a massive spin-0 particle

=== Spin 2 ===
* [[Einstein field equations]]: describe interaction of matter with the [[gravitational field]] (massless spin-2 field): <math display="block">R_{\mu \nu} - \frac{1}{2} g_{\mu \nu}\,R + g_{\mu \nu} \Lambda = \frac{8 \pi G}{c^4} T_{\mu \nu}</math> The solution is a [[metric tensor|metric]] [[tensor field]], rather than a wave function.