If a photon does the 4 things listed below, why do we say it doesn’t have mass? I guess I’m asking if there’s a clear definition of mass.
- follows geodesics in spacetime (classically: accelerated in a grav field),
- curves spacetime (classically: creates a gravitational field),
- has momentum / exerts pressure on things it impacts, and
- has inertial mass (put it in a mirrored box, now the box’s acceleration decreases per unit applied force).