E = mc^{2} is the world’s most famous equation—a mathematical formula with the power to transcend the barriers of language and culture. Matching its popularity is its deceiving complexity. Its symbols, although easily recognized, embody concepts contrary to the way things *seem* to be.

On Sept. 27, 1905, Albert Einstein‘s paper “Does the Inertia of a Body Depend Upon Its Energy Content?” was published. It was the last of four papers he submitted that year to the journal *Annalen der Physik*. The first explained the photoelectric effect, the second offered experimental proof of the existence of atoms, and the third introduced the theory of special relativity. The last and final paper of the series introduced m = E/c^{2}, which was later streamlined to its now instantly recognizable form.

That year, 1905, remains one of the most significant in the history of physics. Before Einstein, entities such as time and space and mass and energy were separate. But by bringing these then seemingly unrelated elements together, first in the concept of space-time and immediately thereafter in the equation E = mc^{2}, Einstein completed his theory of special relativity. Special relativity is perhaps one of the least intuitive theories ever conceived in the history of science, yet it is central to physics.

In E = mc^{2} Einstein concluded that mass (m) and kinetic energy (E) are equal, since the speed of light(c^{2}) is constant. In other words, mass can be changed into energy, and energy can be changed into mass. The former process is demonstrated by the production of nuclear energy—particles are smashed and their energy is captured. The latter process, the conversion of energy into mass, is demonstrated by the process of particle acceleration, in which low-mass particles zipping through a device collide to form larger particles.

The inclusion of the speed of light in Einstein’s equation was based on the principles of classicalmechanics and electromagnetic radiation, the latter of which is pure energy. Electromagnetic radiation is constant—it always travels at the speed of light, or 186,000 miles/sec (300,000 km/sec).

E = mc^{2} was a product of special relativity, which itself seemed enigmatic to Einstein’s colleagues and a complete mystery to just about everyone else. According to Britannica’s relativity article:

“Special relativity is limited to objects that are moving at constant speed in a straight line, which is called inertial motion. Beginning with the behaviour of light (and all other electromagnetic radiation), the theory of special relativity draws conclusions that are contrary to everyday experience but fully confirmed by experiments. Special relativity revealed that the speed of light is a limit that can be approached but not reached by any material object; it is the origin of the most famous equation in science, E = mc

^{2}; and it has led to other tantalizing outcomes, such as the twin paradox.”

“In developing special relativity, Einstein began by accepting what experiment and his own thinking showed to be the true behaviour of light, even when this contradicted classical physics or the usual perceptions about the world.”

A 2005 Britannica special report titled “Celebrating the Centennial of Einstein’s ‘Miraculous Year’,” written by University of Chicago, Loyola physicist John J. Dykla, states:

“Einstein’s work was not immediately accepted, and the idea of light quanta, in particular, was considered so radical that few physicists immediately adopted it…In time, however, the idea of quanta, or discrete units, in many physical properties came to pervade physics and formed the basis of the field of quantum mechanics. By the end of the 1920s, quantum mechanics and its implications for basic atomic structure and interactions had been established, and today they inform the way scientists and nonscientists alike think about matter.”

Einstein’s publications of 1905 revealed connections in natural phenomena where others saw nothing. His remarkable insights forever associated his name with genius. Yet, given the counter-intuitive nature of his equation, it is sometimes surprising to consider how widely recognized it has become. The notation E = mc^{2} is visually simple and elegantly meaningful. It is capable of powering the production of energy and of causing great devastation, a conflicted dimension that physicists have had to confront ever since.