by ahan abhati

Created on: April 29, 2010   Last Updated: May 01, 2010

The discovery of the equivalence of mass and energy by Einstein affected our ideas of physics and other branches of human thought.

For over two centuries the equations of motion developed by Newton were believed to describe the nature correctly. But, in 1905, Einstein discovered an error in these laws of dynamics. Newton had assumed that the mass is always constant, same all the time; but Einstein discovered that the mass of a body increases with velocity and he corrected Newton’s second law of motion by introducing a correction factor to the mass of a body.

In Newtonian mechanics, if a constant force acts on a body for a long time, the speed of the body keeps on increasing until it goes greater than the speed of light. However, in relativistic mechanics, the speed does not increase, but the momentum increases continually because the mass of a body is increasing. This is the relativistic change of mass.

Let us take an example of further consequence of relativistic change of mass. Consider the motion of molecules in a small tank of gas. When a gas is heated, the speed of molecules increases, and therefore the mass is also increases. This is the reason why the gas becomes heavier.  When the molecules are moving that fast their mass are enormous, but their speed cannot exceed the speed of light, because light always travels faster. We can say that the increase in mass is proportional to the increase in temperature. We can also say that increase in mass of all the body of gas is proportional to the increase in kinetic energy.  

This observation led Einstein to suggest that the mass and energy are equivalent. He interpreted that the total energy of body consists of the kinetic energy and intrinsic energy known as “rest energy”. When a body is at rest, it has rest energy. Then we apply a force to the body, which starts it moving and gives it kinetic energy; since the energy has increased, the mass has increased. So long as the force continues, the energy and the mass both continue to increase.

In our daily experience, we do not feel changes in mass, because most of time we cannot generate much energy from a given amount of a material. The theory of equivalence of mass and energy is beautifully verified by experiments in which matter is annihilated- converted totally to energy.

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