c represents here the speed of light in vacuum. According to Relativity, this is a universal constant (c = 299 792 458 Km/s). The key idea leading to eq. (1) is that, unlike the mass of Newtonian physics (which is independent of the relative state of motion and plays the role of a constant of proportionality connecting force and acceleration), the relativistic mass increases the faster a body moves. Suppose we know that a body has mass m0 when it is at rest in a given inertial reference frame. Then, according to Relativity, when the body starts moving at velocity v, its mass will be modified as follows:
This equation forbids a body to be accelerated faster than light, since, when the velocity v approaches c, the inertial mass m, which represents the resistance to acceleration, tends towards infinity. In other words, the amount of energy required to further increase the velocity becomes infinite. At ordinary velocities (v<<c), however, the classical formula for the variation of kinetic energy with velocity is recovered (see the section on origins) and deviations from Newtonian dynamics remain undetectable. However, because of the huge factor cē in eq. (1), any body, even if microscopic and at rest, possesses an enormous amount of energy that can be extracted in certain circumstances (see implications).
One fundamental consequence of equation (1) is that energy can be converted into mass and vice versa. Thus, for example, one electron and its anti-particle (the positron) can be created in the process of pair production from pure radiant energy (γ rays, i.e. highly energetic photons). Conversely, when coming close to each other, electrons and positrons annihilate, originating a γ photon (see experimental evidence). Antiparticles have the same mass and spin as their corresponding particle, but opposite electric charge . They were first predicted by Paul Dirac, on the grounds of symmetry considerations (see origins).