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"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
by InclinedPlane 8y ago
"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 8y ago> "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 8y agoThe "source" of gravity is the stress-energy tensor. "Mass" (rest-energy) is a simplification that is useful for approximations and conceptualizations.
- pdonis 8y ago> "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).
- pdonis 8y ago> 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.