Relativity, Mass Energy Equivalence and Nuclear Reactions

The result from the theory of relativity that Mass and Energy are different manifestations of the same physical entity and that it is possible to convert mass into energy finds an application in the processes of nuclear fission and nuclear fusion.

In the process of nuclear fission, a large unstable nucleus such as that of Uranium-235 decays into two smaller nuclei. Interestingly, the sum of masses of these two nuclei is smaller than the mass of the original larger nucleus. This "mass defect" is responsible for the release of a large amount of energy in this process of nuclear fission. The difference in mass, when multiplied with the square of the speed of light in vacuum (that is, c²), gives the amount of energy released in the process by the famous equation -

E = mc²

where, E is the energy released in the process,
m is the mass difference,
c is the speed of light in vacuum.

In the process of nuclear fusion, two small nuclei combine to form a larger nucleus again with the release of a large amount of energy. The sum of masses of the original nuclei is greater than the mass of the resulting nucleus and this "mass defect" is responsible for the release of energy in nuclear fusion. Again the same equation of mass-energy equivalence may be applied to find out the energy released.

Applications of Relativity