In Walter Greiner book about relativistic quantum mechanics, he writes the uncertainty relations for 4-position and 4-momentum in a neat way as:
$$[p^\mu, x^\nu] = i\hbar \eta^{\mu\nu}{\bf 1}$$
with Minkowski sign convention $(+,-,-,-)$. This gives manifestly all the uncertainty relations we want. But here, time is just a coordinate, so how do we interpret the commutation relation between time and energy?
