Causality of Linear Phase Filter
For a generic value of the group delay $\alpha$, the filter has a 2-sided infinite impulse, and is therefore not causal.

Causal linear phase systems are (almost) always FIR systems with any of the 4 types of symmetry.

In these cases, the group delay is multiple of 1/2 (0, 1/2, 1, 3/2, etc.).

You can have causal linear phase IIR filters, but these are not of finite-order (cannot be implemented with a difference equation), so they are of little use in practice.