Did you know that Einstein's most famous equation didn't actually originate with Einstein?
We're all taught this simple equation even as children and we're told that Einstein did it.
But he didn't in fact, Einstein never published a single paper about it.
Where does it come from?
Well as it turns out, Einstein's paper on Mass / Energy equivalency never featured that equation.
What Einstein actually said is that Mass = Energy/The square of the speed of light.
M=E/C2
These are similar, but mean different things.
The meaning under Einstein's formulation is that as you add energy to a system it gains mass, because mass is a property of energy.
When you say E=MC2 you are describing how much energy is pent up as "rest mass",should you decide to release that energy.
The net effect is that the other energy in the system is ignored. Thus it feels counter intuitive when we go to measure an object at rest and we discover that it weighs less than it's constituent atoms. Yet things like a wound up clock on an express train will weigh more than their atoms.
So when we say E=MC2 it's because we like to think in terms of matter, which under quantum field theory is a form of potential energy, but when we do this, it causes us to forget the simple truth.
Energy is what actually exists. It exists in various forms and mass is just another one of it's properties.
