The paper that weighed light
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On 27 September 1905, the journal Annalen der Physik received a three-page note from Albert Einstein with a title that does not look like a slogan: "Does the Inertia of a Body Depend Upon Its Energy Content?" Special relativity had already gone out that year, in June. This was the leftover. He asked whether a body that emits energy L loses mass.
The answer in the paper is yes: the mass drops by L/c². In later shorthand that became E = mc². The 1905 version is the bookkeeping version. If light leaves a system, the system is lighter. Light carries inertia. Mass is a name for energy that has not left yet.
It is easy to turn this into a black-hole poster. That is last week's story, not this one. GW150914 was spacetime ringing; the missing solar masses left as gravitational waves. This September paper is quieter. No merger, no chirp. A thought experiment about a body that radiates, and a demand that energy and inertia not keep two sets of books.
Einstein was 26, at the patent office in Bern. The paper is short enough to read on a lunch break and dense enough that "what does it really say" is still a cottage industry. The operational claim is simple: you cannot emit energy and keep the same inertial mass. Chemistry already knew that burning fuel "uses up" something. Relativity said the something has a weight, even when the something is light.
No Lab Pick under this note on purpose. The matching equation tee would sit next to Wednesday's Faraday/Maxwell shirt — two equation graphics in one week, the same stacking we just did with two black holes. The story can stand without a checkout button.
If you want a shirt that belongs to light as electromagnetism rather than light as E = mc², Wednesday is that email: Faraday's birthday, Maxwell's wave, a chalkboard and a bulb. This Monday note is the 1905 addendum. Mass depends on energy content. The universe has to balance. The paper is three pages. The argument is still running.