If you had a perfectly reflective impervious box, put in 500g of matter and 500g of antimatter, and shook it, the box would then contain 1000g of gamma ray phot

If you had a perfectly reflective impervious box, put in 500g of matter and 500g of antimatter, and shook it, the box would then contain 1000g of gamma ray photons. Same gravitational and inertial mass as 1000g of inert matter. (Due to gravitational blueshifting, the photons hit the bottom of the box slightly harder than the top. This is not a halfbaked science blooper; your local physicist will tell you this is actually correct.) QM says photons have zero 'rest mass', but that's not the same thing as actual zero mass.