Editing Conservation of energy (section)

==Special relativity==
With the discovery of special relativity by [[Henri Poincaré]] and [[Albert Einstein]], the energy was proposed to be a component of an [[four-momentum|energy-momentum 4-vector]]. Each of the four components (one of energy and three of momentum) of this vector is separately conserved across time, in any closed system, as seen from any given [[inertial reference frame]]. Also conserved is the vector length ([[Minkowski space|Minkowski norm]]), which is the [[rest mass]] for single particles, and the [[invariant mass]] for systems of particles (where momenta and energy are separately summed before the length is calculated).

The relativistic energy of a single [[mass]]ive particle contains a term related to its rest mass in addition to its kinetic energy of motion. In the limit of zero kinetic energy (or equivalently in the [[rest frame]]) of a massive particle, or else in the [[center of momentum frame]] for objects or systems which retain kinetic energy, the [[total energy]] of a particle or object (including internal kinetic energy in systems) is proportional to the rest mass or invariant mass, as described by the equation <math>E=mc^2</math>.

Thus, the rule of [[Mass in special relativity|''conservation of energy'' over time in special relativity]] continues to hold, so long as the [[frame of reference|reference frame]] of the observer is unchanged. This applies to the total energy of systems, although different observers disagree as to the energy value. Also conserved, and invariant to all observers, is the invariant mass, which is the minimal system mass and energy that can be seen by any observer, and which is defined by the [[energy–momentum relation]].