noun

*E = mc*

^{2}, where

*E*represents energy,

*m*the equivalent mass, and

*c*the speed of light.

noun

The physical principle that a measured quantity of energy is equivalent to a measured quantity of mass. The equivalence is expressed by Einstein's equation *E = mc* ^{2}, where *E* represents energy, *m* the equivalent mass, and *c* the speed of light.

THE AMERICAN HERITAGE® DICTIONARY OF THE ENGLISH LANGUAGE, FIFTH EDITION by the Editors of the American Heritage Dictionaries. Copyright © 2016, 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

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"mass-energy equivalence." YourDictionary, n.d. Web. 14 January 2019. <https://www.yourdictionary.com/mass-energy-equivalence>.

**APA Style**

mass-energy equivalence. (n.d.). Retrieved January 14th, 2019, from https://www.yourdictionary.com/mass-energy-equivalence

An equation derived from Einstein's theory of Special Relativity expressing the relationship between the mass and energy of objects with mass. The equation is *E = mc*^{2}, where *E* is the energy of the object in joules, *m* is its **relativistic mass** in kilograms, and *c* is the speed of light (approximately 3 × 10^{8} meters per second). Mass-energy equivalence entails that the total mass of a system may change, although the total energy and momentum remain constant; for example, the collision of an electron and a proton annihilates the mass of both particles, but creates energy in the form of photons. The discovery of mass-energy equivalence was essential to the development of theories of atomic **fission** and **fusion** reactions.

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**MLA Style**

"mass-energy equivalence." YourDictionary, n.d. Web. 14 January 2019. <https://www.yourdictionary.com/mass-energy-equivalence>.

**APA Style**

mass-energy equivalence. (n.d.). Retrieved January 14th, 2019, from https://www.yourdictionary.com/mass-energy-equivalence

- mass-energy equivalence
- mass extinction
- mass-extinction
- mass-extinctions