[?]

Must massless particles be absolutely massless in the SM? Being a non-scientist I find it hard to grasp the concept of a massless particle. How does gravity interact with a truely massless particle?

[wisp]

Hi

This is wisp theory's view only.

Massless particles such as light are cause by wobbles moving at speed c through the ether. Being massless, they don't create holes in the ether and so are unaffected by gravity. But they can follow curvature in space.
The space around heavy objects can be thought of as "curved" due to density variation in the ether.

Anything with mass make a hole in the ether, and so is subject to gravitational effects.

[TimLong]

The mass of the photon was DEFINED as zero back in the early days of the 20th century because values less than approximately 10^-20 were considered to be of vanishingly small significance, and I think this is where the confusion started (where we went wrong). Now, calculations of the mass of the components of the photon structure (at the gammaray pair-formation threshold energy of 1.0216 MeV) using E = mc^2 give exactly the electron mass and positron mass. Energized by the interaction of the components of the photon, it becomes self-propagating with no necessity for an ether as the wave carrier. This interaction binds the positronic and negatronic particles creating the ether's Fresnel Dragging without the necessity of an ether. (The Fresnel Dragging Coefficient of ether theory is equivalent to the special relativistic velocity equation.)

http://www.PhotonStructure.com