A legitimate Question to the whole community of global physics:which c is the c in the equation of E=m*c*c?

which c is the c in the equation of E=mcc?

A legitimate Question to the whole community of global physics:

https://doi.org/10.5281/zenodo.20719277 向全世界的物理学社区提问：E=mc² 中的 c，到底是哪一个光速

I am posing a legitimate question that I believe demands the attention of the global physics community — a question so fundamental that it touches the logical foundation of modern physics itself.

The question is this:

When we write E = mc^2 , which physical definition of the speed of light does the symbol c refer to?

This is not a rhetorical question. It is a precise operational question that admits only three possible answers:

The defined speed of light ( c_{def} ), a human convention fixed by the SI system;
The coordinate speed of light ( c_{coord} ), which varies with the choice of coordinates in general relativity;
The locally measured speed of light ( c_{local} ), which is the actual physical speed measured by local rulers and clocks at the spacetime point in question.

For over a century, the physics community has used the symbol c without explicitly specifying which of these three distinct definitions applies in the mass-energy equation. The three definitions are not interchangeable — they belong to different logical layers: convention, coordinate artifact, and local physical quantity.

I am not posing a theoretical alternative. I am asking a question of clarification:

Which one is the c in E = mc^2 ?

Every physicist who writes this equation, regardless of their status, institution, or background, must be able to answer this question — because if the answer is ambiguous, then the equation itself is not fully defined.

I do not ask for publication, not a question to the journal. I ask for clarity. I raise this question to the entire global physics community, and I wait for an answer.

Respectfully,

Zhang Dongxiao
Independent Theoretical Physics Researcher