I'm trying to understand the stress-energy tensor and I keep seeing the phrase, "momentum in the time direction is energy". I don't understand this. In the definitions of the momentum four-vector that I've found, the time component is the object's rest mass times the speed of light times gamma. Here's an example: http://scienceworld.wolfram.com/physics/MomentumFour-Vector.html This gives units of momentum, not of energy. What am I missing?