I was watching a programme on equations the other night and Einstein's equation for the equivalence of mass and energy was described, E=mc2.

I understand that there is an equivalence between mass and energy and they can be converted.

However, my question is 'why does the equation depend on the speed of light and not just some arbitrary constant'?

Einstein knew that the constant was the square of speed of light well before practical experiment so there must be some logic to it.

Many thanks

According to Einstein's famous equation E = MC2, or Energy = Mass times the speed of light times the speed of light. This tells us that anything that has mass, such as matter, is comprised entirely of energy. Any physical object with mass is therefore simply a lump of energy existing in some stable form.

As I understand it, all forms of energy, including chemical energy, kinetic energy and potential energy are covered by Einstein's equation, meaning that fundamentally all forms of energy are the same basic 'stuff' (for want of a better word). My question is, therefore, what exactly is energy? I want to get to a fundamental understanding of energy, because it seems that such an understanding would help to explain much of reality.

I'm not asking for explanations about work done etc, I'm asking at the fundamental level what is energy?