 ....  Thanks to Albert Einstein's revelation that energy and mass are related, determining exactly how much energy is in that unit can let scientists define mass in terms of Planck's constant - a value that should hold up across space and time - rather than relying on an inconstant metal cylinder....  Thanks to Albert Einstein's revelation that energy and mass are related, determining exactly how much energy is in that unit can let scientists define mass in terms of Planck's constant  -  a value that should hold up across space and time  -  rather than relying on an inconstant metal cylinder....  