"We also knew that photons are affected by gravitational fields not because photons have mass, but because gravitational fields (in particular, strong gravitational fields) change the shape of space-time."Originally posted by: Woodchuck2000
Not strictly true - gravity can also affect particles with zero mass. The sun causes a measurable deflection in the light passing nearby from distant stars. Photons are expected by most physicists to have zero mass (zero non-rest mass) and therefore they shouldn't be affected if we use that definition of gravity.Originally posted by: Mrpilot007
I'm surprised no one has defined gravity for this discussion:
(physics) the force of attraction between all masses in the universe;
especially the attraction of the earth's mass for bodies near its surface;
"the more remote the body the less the gravity";
"the gravitation between two bodies is proportional to the product of their masses and inversely proportional to the square of the distance between them";
"gravitation cannot be held responsible for people falling in love"--Albert Einstein
http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/961102.html