E=mc^2 , please explain how this relates to thermal energy or similar ?sorry to intrude but we do not measure energy . We measure joules etc . Energy is generalised term that we use when not being specific . There is many types of energy . We actually measure output and give it a unit name .I agree that there are, as I said, many forms of energy. However, Joules is the SI named unit for the quantity of energy. Energy is what we measure, in whatever form it happens to be in eg heat energy, mass energy, etc. Yes, we can also use the term as a general, nonspecific quantity.
Doesn't E=mc^2 actually imply the inverse square of light which the energy magnitude is proportional to the area?