By Roland Pease
BBC science correspondent
Physicists are celebrating the centenary of Albert Einstein's best known equation: E=mc˛.
To many, the equation remains a symbol of Einstein's genius
Published in the fourth of a series of papers that shook the foundations of physics in 1905, E=mc˛ is now linked with the power of the atom bomb.
No equation is anywhere near as recognisable as E=mc˛.
In 1905, it was final proof of the genius and imagination of a young German-born scientist who had yet to land a university post.
It seems so simple: three letters standing for energy, mass, and the speed of light, brought together with the tightness of a soundbite.
Yet what it encapsulates is still hard for scientists to grasp.
Einstein showed in a handful of lines that as you accelerate an object, it not only gets faster, it also gets heavier.
That in turn makes further pushing less fruitful so that eventually nothing can be accelerated beyond the speed of light.
The equation rounded out the theory of relativity he had started earlier in the year.
Einstein soon recognised through the equation that the energy released in radioactivity - a phenomenon hardly understood at the time - might lead to measurable changes in mass.
That idea was eventually to be translated into the physics of the atom bomb.