mass-energy

like space -- time,mass -- energyand life and death.

It basically says that mass is concentrated energy,and mass and energy are exchangeable,like two currencies with a huge exchange rate.

Einstein had thought a little bit more about his theory,and realized it also implied that mass and energy,one apparently solid and the other supposedly ethereal,

E=mc^2 E equals m c squared.It basically says that mass is concentrated energy,and mass and energy are exchangeable,