But doesn’t E=mc² teach us that mass and energy are somewhat interchangeable?
There are two ways to read that equation. Either you use the instant mass, that changes with the object’s speed and has an undefined result for anything moving at the speed of light (this is the one Einstein wrote in awe about), or you use the rest mass and that’s the half of the equation that doesn’t consider the object’s speed (this is the one everybody actually uses).
On the second case, the complete equation is E² = (mc²)² + (pc)² where p is the momentum.
There are two ways to read that equation. Either you use the instant mass, that changes with the object’s speed and has an undefined result for anything moving at the speed of light (this is the one Einstein wrote in awe about), or you use the rest mass and that’s the half of the equation that doesn’t consider the object’s speed (this is the one everybody actually uses).
On the second case, the complete equation is E² = (mc²)² + (pc)² where p is the momentum.