mass-energy equivalence

Science Dictionary
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 = mc2, 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 × 108 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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Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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WordNet
mass-energy equivalence

noun
(physics) the principle that a measured quantity of mass is equivalent (according to relativity theory) to a measured quantity of energy 
WordNet® 3.0, © 2006 by Princeton University.
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