Causality of regular wave equations in an external field

For many years, attempts have been made to describe higher spin (s greater than or equal to 3/2) particles using relativistic wave equations. The results have been largely unacceptable because such equations exhibit casualties (have solutions which propagate faster than the speed of light) when coupled to some external field. Here, the method of characteristics to prove the causality of a large class of wave equations are used. These regular wave equations can be used to describe particles of arbitrarily high spin and remain causal when coupled to an arbitrary external field