The comment was that Einstein didn't originally write that equation in that form, and (IIRC) there was a little more to what the symbols represented (evident from his commentary) than the 'simple' version suggests.
I asked Gemini, and this sounds like the very brief end-of-barbecue summary (which came after a discussion of how "I'm completely stuffed!" would be translated into French, Italian, and German - we never got onto the Serbian translation).
No, Albert Einstein did not write it that way initially. When he first introduced the concept of mass-energy equivalence in 1905, the famous equation $E = mc^2$ did not appear anywhere in his paper.
Instead, his original formulation and the evolution of how he wrote the formula follow a specific timeline:
1. The 1905 Original Formulation
In his late 1905 paper published in
Annalen der Physik, titled
"Does the Inertia of a Body Depend Upon Its Energy Content?", Einstein used different notation entirely.
He used
$L$ to represent energy (specifically in the form of light radiation) and
$V$ to represent the speed of light. He concluded that if a body loses an amount of energy $L$, its mass diminishes by $L / V^2$.
He famously stated the concept in prose rather than a standalone equation:
2. The 1906–1907 Shift
As Einstein and other physicists (like Max Planck) continued to develop special relativity, the notation began to shift. Einstein started using
$E$ for energy and
$c$ for the speed of light. However, he still preferred to express the relationship by solving for mass, often writing it as:
$$\Delta m = \frac{\Delta E}{c^2}$$
To Einstein, the core philosophical breakthrough was that
mass is a direct measure of a body's energy content—meaning mass is a secondary, dependent property.
3. When Did He Finally Write $E = mc^2$?
Einstein did eventually write the equation in its universally recognized form, but it took several years:
- In Manuscript (1912): The earliest known instance of Einstein writing the formula in its familiar form by his own hand appears in a 1912 manuscript on special relativity.
- In Public (1946): He wrote a famous essay for Science Illustrated titled "E = mc²: The Most Urgent Problem of Our Time," completely embracing the popular notation that the public had come to associate with his work following the development of the atomic bomb.
Why Did the Public Version Win?
The reason $E = mc^2$ became the pop-culture shorthand for physics boils down to aesthetic elegance. Solving for $E$ isolates the energy potential of matter on one side, and multiplying a tiny amount of mass ($m$) by the unfathomably large speed of light squared ($c^2$) perfectly illustrates the terrifyingly massive amount of energy locked inside everyday matter.
