Mass-energy

An important year in scientific history is 1905, when Einstein changed the face of physics with his theories of special relativity, mass-energy equivalence and the photoelectric effect.

Scientists theorize that stars, planets and the molecules they contain represent less than five percent of the mass-energy content of the universe. The rest is dark energy and dark matter, which cannot be directly detected.

Whereas in classical physics mass and energy are distinct concepts, on relativistic scales, they become indistinguishable and are replaced by mass-energy.

In higher-energy physics, mass and energy eventually become meaningless as individual concepts; a particle's so-called "mass" is actually mass-energy and is often given equivalently in units of either mass (GeV/c²) or energy (GeV).

For equations normalized with Planck units, the quantities of mass and energy become numerically identical, revealing their true nature as mass-energy.

Mass-energy conversion occurs during nuclear fusion and fission.