The integral with respect to this quantity of a Lagrangian yields an action
The integral with respect to this quantity of a Lagrangian yields an action. The principle of detailed balance requires that dynamics be symmetric under the reversal of this variable, which is conjugate to energy. If a quantity's value is independent of this variable, the quantity is conserved. The SI unit of this variable is defined by a device that employs cesium. This quantity experiences "dilation" at relativistic speeds. The derivative of momentum with respect to this quantity is force. Entropy may explain why the so-called "arrow" of this phenomenon is unidirectional. For 10 points, name this dimension that is used to describe which things happen before or after other things. 
The integral with respect to this quantity of a Lagrangian yields an action.
time