[Xcelerate]

> Mass and energy are exactly the same property

Definitely not. Everything couples to the stress-energy tensor, but not every type of particle has rest/invariant mass. No one really speaks in terms of relativistic mass anymore. (So a photon has energy but no rest mass.)

[psyc]

Relativistic mass is the one that is equivalent to energy, so if no one speaks of it, they'll perpetuate the legacy sloppiness I referred to.