# Energy and Mass

In the year 1905, the probably best known physicist of the last century, invents the best known formula:Albert Einstein (PDF-Datei, 178 KB) develops the Special Theory of Relativity with its consequence of the equivalence of energy and mass. The famous equation goes:

Here m stands for mass of a body and c for the speed of light. This formula had not been found as the result of a riddle as shown in the cartoon on the left.
(Although physicists do sometimes write down a new equation "by feeling" and only later find authentic arguments to prove their intuition.)

Another statement of the Special Theory of Relativity says that the mass of a body is composed of its rest mass mo and an additional part resulting from its motion. This is described by the following formula

# m = m0 / √(1 - (v/c)2)

This formula shows that a body that does not move (its velocity is v = 0) has only the rest mass mo. In case the speed of the body starts to approach the speed of light c, the term v/c becomes close to 1, and, consequently, m0 becomes divided by a steadily declining value in the radicand. Dividing mo by a very small number means that the mass m of the body grows with its increasing speed.

In the limiting case of v = c the mass would even become infinite! But mathematical laws forbid division by zero; so the physical conclusion is drawn: A particle with a finite rest mass (i.e. m0 > 0) cannot be accelerated up to the speed of light. A particle approaching the speed of light is not getting faster but heavier. This law guarantees that the energy invested into the acceleration cannot get lost and the basic law of conservation of energy is fulfilled.

Generation of Particles