Maximizers in Lipschitz spacetimes are either timelike or null

We prove that causal maximizers in $C^{0,1}$ spacetimes are either timelike or null. This question was posed in [17] since bubbling regions in $C^{0,\alpha }$ spacetimes ($\alpha <1$) can produce causal maximizers that contain a segment which is timelike and a segment which is null, cf. [3]. While $C^{0,1}$ spacetimes do not produce bubbling regions, the causal character of maximizers for spacetimes with regularity at least $C^{0,1}$ but less than $C^{1,1}$ was unknown until now. As an application we show that timelike geodesically complete spacetimes are $C^{0,1}$-inextendible.