- During the discussion of Michelson-Morley experiment, we have seen that Lorentz and Fitzgerald explain the null result by showing the contraction in space-and-time-dimensions. These are popularly known as 'Fitzgerald-Lorentz Contractions' and their values can be found by 'Lorentz Mathematical Equations'.

Einstein's theory of relativity also implies that when a system moves, there occurs a contraction in the space-and-time-dimensions. This means that the length of the moving body contracts in the direction of the motion and a clock placed in such a system moves slowly. The amount of these contractions depends upon the velocity of the system. The faster the system moves, the slower the clock becomes and the smaller the measuring rod becomes. And if we imagine that the speed of the moving system becomes equal to that of light, the clock placed in the system would stop and the measuring rod placed in the system would have no length at all. But actually this is impossible and hence, it is implied that the velocity of light cannot be reached in practice.

- Like space-and-time-dimensions, mass
^{[1]}of a body in motion is also affected by the motion. In classical physics, the mass of a body was regarded to remain constant. That is to say that if a body is at rest or in motion, its mass always remains the same But the theory of relativity shows that the mass of a body is a function of its velocity,^{[2]}and thus increases with the increase in the speed of the body. When the velocity of the body is negligible in comparison to that of light, the increase in the mass of the body is also negligible. But when the velocity of the body approaches the velocity of light, the increase in mass also becomes considerable; and if we imagine a body moving with the velocity of light, its mass would become 'infinite'. But this is impossible, and therefore, it follows that the velocity of any moving body cannot become equal or greater than that of light.

- In classical physics, matter and energy were regarded to be independent of each other, and hence, there were two independent laws of conservation-one the law of conservation of matter and the other that of conservation of energy. Matter was considered to be inactive, visible and possessing mass, while energy, in contrast to matter, was conceived to be active, invisible and massless.

Now, Einstein, as a consequence of the theory of relativity, showed that mass and energy were essentially the same thing. He argued that the increase in the mass of a body with the increase in its speed indicated that there should be a direct relation between mass and energy (one of whose forms is motion).

Advancing on this line of argument, Einstein established the 'principle of equivalence of mass and energy', and with the help of mathematics, discovered the epoch-making 'mass-energy equation'. This principle shows that mass and energy are mutually convertible under certain conditions. Thus, when a certain amount of matter is converted into energy, the energy obtained is equal to the product of mass and square of the velocity of light,^{[3]} and similarly when an amount of energy is converted into matter, the mass produced is given by the energy divided by the square of the velocity of light.

An illustration will clarify the relation: Suppose that one kilogram of coal is totally converted into energy; then the energy obtained would be equivalent to 25 billion kilo-watt-hour of electricity.

As a result of this new discovery, the two independent laws of conservation of mass, and that of energy, have been united to form a single law of conservation of mass and energy, enunciating that the sum total of energy and mass x c^{2} (where c=velocity of light) is constant for any system and cannot increase or decrease. Thus the mass-energy equivalence establishes the universal fact that mass and energy are essentially the forms of the same entity.

The chief merit of the mass-energy relation lies in the fact that it is experimentally verified for all types of energy. The most prominent verification is found in the discovery of the atom-bomb.^{[4]}

Though, popularly, mass and weight are regarded to connote the same meaning, in scientific definition, they devote different properties of matter. Mass is considered to be more fundamental than weight. It is a matter of observation that a force applied to a body produces an acceleration proportional to the force. The constant of proportionality is the mass of the body. Thus, the mass of a body is a measure of its 'inertia' or reluctance to change its motion. The greater the mass of a body is, the more is the force required to move it. It may be remarked here that usually, even in the scientific works, mass is regarded as the amount of matter and sometimes used as synonymous to matter.

mthen_{o}= mass of the body at rest,

m = mass of the body in motion,

v = the velocity of the body,

c = the velocity of the light,

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Mathematically, the equation connecting the two quantities in any such transformation is:

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where

c = velocity of light.

E = energy released, and

m = mass completely converted into energy.