I have been reading about four momentum. There are four component vectors, three spatial, momentum, components and a time, energy, component. They each have a direction. I understand direction for the momentum components, being in the direction of the respective spatial components of the velocity of an object. I understand that as a component vector the energy has direction. However, I do not understand how to interpret the direction of the energy component and I have not seen an explanation. Does it mean that energy is in the direction of time and if so, is that meant to be so obvious that no one seems to explain it? Alternatively, is it just a mathematical attribute that works and has no particular physical association?