## E = mc^{2} Meaning

At the beginning of the 20th century, the notion of mass underwent a radical revision. Mass lost its **absoluteness**. One of the striking results of **Einstein’s theory of relativity** is that **mass and energy are equivalent and convertible** one into the other. **Equivalence** of the mass and energy is described by Einstein’s famous formula **E = mc ^{2}**. In words,

**energy**equals

**mass**multiplied by the

**speed of light squared**. Because the speed of light is a very large number, the formula implies that any small amount of matter contains a very large amount of energy. The mass of an object was seen to be equivalent to energy, to be interconvertible with energy, and to increase significantly at exceedingly high speeds near that of light. The

**total energy**of an object was understood to comprise its

**rest mass**as well as its

**increase of mass**caused by

**increase in kinetic energy**.

**In special theory of relativity** certain types of **matter may be created or destroyed**, but in all of these processes, the mass and energy associated with such matter **remains unchanged in quantity**. It was found the **rest mass an atomic nucleus is measurably smaller than the sum of the rest masses of its constituent protons, neutrons and electrons**. Mass was no longer considered unchangeable in the closed system. The difference is a measure of the nuclear binding energy which holds the nucleus together. According to the Einstein relationship (**E = mc ^{2}**) this binding energy is proportional to this mass difference and it is known as the

**mass defect**.

**E=mc ^{2}** represents the new conservation principle – the conservation of mass-energy.

, where m is the small amount of mass and c is the speed of light.

What that means? If the nuclear energy is generated (splitting atoms, nuclear fusion), a small amount of mass (saved in the **nuclear binding energy**) transforms into the pure energy (such as kinetic energy, thermal energy, or radiant energy).

**Example:**

The energy equivalent of one gram (1/1000 of a kilogram) of mass is equivalent to:

**89.9 terajoules****25.0 million kilowatt-hours (≈ 25 GW·h)****21.5 billion kilocalories (≈ 21 Tcal)****85.2 billion BTUs**

or to the energy released by combustion of the following:

**21.5 kilotons of TNT-equivalent energy (≈ 21 kt)****568,000 US gallons of automotive gasoline**

Any time energy is generated, the process can be evaluated from an **E = mc ^{2}** perspective.

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