'nothing, not even light'

[Papewaio]

Thanks Pape. You turn your back for two decades and suddenly your degree is out of date. Sounds as if I was a bit short with DJ as well. My apologies. Saying it is ok to regard photons as having mass sounds generous in view of Pape's links. Mind you I think that it takes a theortical physicist to say that a photon has momentum, inertia and experiences gravity but does not have any mass. This probably explains why Stephen Hawking uses mass to mean relativistic mass in his popular works. Just goes to show are never to old to learn something new.

As for the second bit about the hammer + feather analogy you are correct, the mass of the orbiting object cancels out when calculating the accelaration of it.