Photons and and equivalent number of atoms on gravity

[ensabah6]

due to e=mc2

would massless photons and an equivalent number of mass-energy atoms have the same effects on gravity?

Does mass play any special role in gravity or only energy?

 

[Ich]

would massless photons and an equivalent number of mass-energy atoms have the same effects on gravity?

No, the photons would gravitate more due to their pressure.
For homogeneous, isotropic sources the "equivalent gravitational mass" is $\rho + 3 p$.

Does mass play any special role in gravity or only energy?

Neither do. You need the full http://en.wikipedia.org/wiki/Stress-energy_tensor" as a source term.