November 21, 1905 – Albert Einsteins’s paper, “Does the Inertia of a Body Depend Upon Its Energy Content?”, is published in the journal Annalen der Physik. This paper reveals the relationship between energy and mass. This leads to the mass-energy equivalence formula, E = mc².
E is the energy of a physical system, m is the mass of the system, and c is the speed of light in a vacuum (about 3×108 m/s). In words, energy equals mass multiplied by the square of the speed of light. Because the speed of light is a very large number in everyday units, the formula implies that any small amount of matter contains a very large amount of energy. Some of this energy may be released as heat and light by chemical or nuclear transformations. This also serves to convert units of mass to units of energy, no matter what system of measurement units is used.
Many countries have issued postage stamps commemorating Albert Einstein. In fact collection of these stamps forms the basis for a philatelist’s topical or thematic collection. Most of the stamps issued show a familiar portrait of Einstein with his disheveled hair and work-weary eyes. However, a few, including these two from Ireland and Germany, show the scientist with his famous formula: