Watched Days

On 27 September 1905 a paper arrived at Annalen der Physik. It is the paper we credit with E=mc2. It never writes E=mc2.

It ends in a sentence, in German: “Gibt ein Koerper die Energie L in Form von Strahlung ab, so verkleinert sich seine Masse um L/V2.” A body that gives off energy L as radiation loses mass L/V2. V was Einstein's letter for the speed of light in 1905. The lowercase c came later.

And the last line of the paper is its own receipt, printed by the journal:

(Eingegangen 27. September 1905.)

Received. It signed for itself. The date nobody puts on the story is the one the paper kept.

The equation we quote is a later compression. Planck rewrote the relation in 1907; Einstein first used a lowercase c that year and moved to the familiar notation afterwards. The most quoted formula in physics began as a sentence, and the sentence was careful: it ties a change in mass to a change in energy, not the absolute equation.

Usually a retelling loses a date. This one lost the form. The clean version always arrives second, and it travels lighter.