MELISSA BLOCK, host:

One hundred years ago today, September 27th, 1905, a physics paper was dropped into the mail addressed to a German journal called Annalen der Physik.

MICHELE NORRIS, host:

The paper was penned by a 26-year-old Swiss patent clerk with dark, curly hair years before he sprouted that signature nest of white hair. The paper was just three pages long, and it introduced what would arguably become the most famous string of letters and numbers in the world.

BLOCK: E=MC squared.

(Soundbite of vintage broadcast)

Mr. ALBERT EINSTEIN: E=MC square, in which energy is part equal to mass, multiplied by the square of the velocity of light. So a very small amount of mass may be converted into a very large amount of energy and vice versa.

BLOCK: As Albert Einstein says, vice versa; mass may converted into energy and back again.

Mr. JOHN RIGDEN (Washington University, St. Louis): It is vice versa. And Einstein--what he showed was that, in fact, these two things are the same.

BLOCK: That's John Rigden, a physicist at Washington University in St. Louis. He's the author of "Einstein 1905," a book on Einstein's so-called miraculous year. In 1905, Einstein published five major papers.

Mr. RIGDEN: We would have no understanding of the big bang without E=MC squared because in the early seconds, minutes of the big bang, energy and mass were being exchanged just indiscriminately, going back and forth between mass and energy, energy and mass.

NORRIS: E=MC squared also explains the power behind the atomic bomb, and it's all due to that number C squared.

Mr. RIGDEN: A small amount of mass produces enough energy, because of that C squared, to destroy a city.

BLOCK: So happy 100th birthday to the physics paper so enticingly titled Does the Inertia of a Body Depend on Its Energy Content? And happy centennial to the simple and famous equation.

Mr. RIGDEN: Everybody knows the equation. I had to run to pick up some eyeglasses, and so as I picked them up, I said, `You know, this is the hundredth anniversary of a famous equation.' And the person behind the counter said, `What's that?' And I said, `E='--and then he said, `Oh, MC squared.'

BLOCK: That's physicist John Rigden, the author of "Einstein 1905: The Standard of Greatness."

NORRIS: If you want to know more about Einstein's miraculous year, go to our Web site, npr.org.

BLOCK: You're listening to ALL THINGS CONSIDERED from NPR News. Transcript provided by NPR, Copyright NPR.