On: Black holes ruled out as universe’s missing dark m...

[InclinedPlane]

"Mass" is just another name for "rest-energy".

A single photon cannot have mass because it has zero energy in the reference frame where it is at rest (zero net momentum). But a pair of non-parallel photons can have rest-mass, and indeed the mass of the photons inside a star is considerable, for example.

[pdonis → InclinedPlane]

> A single photon cannot have mass because it has zero energy in the reference frame where it is at rest (zero net momentum)

I didn't catch this before, but it is not correct. There is no such thing as a frame in which a photon is at rest.

[pdonis → InclinedPlane]

> "Mass" is just another name for "rest-energy".

Not if you're talking about what the source of gravity (spacetime curvature) is. Massless particles still have energy and still cause spacetime curvature.

[InclinedPlane → pdonis]

The "source" of gravity is the stress-energy tensor. "Mass" (rest-energy) is a simplification that is useful for approximations and conceptualizations.

[pdonis → InclinedPlane]

> "Mass" (rest-energy) is a simplification that is useful for approximations and conceptualizations.

Yes, but not in a context where radiation is present, since radiation can have energy but has no rest mass (rest energy).