**Split from here: http://www.thenakedscientists.com/forum/index.php?topic=7762.0**I'll answer this question from the perspective of my own non-standard model, so don't put any part of this answer on your homework, or you'll fail the course.

In Einstein's general relativity (GR), which is valid only in Minkowski space-time, it is said that light has no mass. That is a result of the definition of straight lines in space-time as being the path of light. Most people who are comfortable using the GR equations will tell you that there is no other kind of space, so the statement "light has no mass" is true in an absolute sense, not just in GR. I say, that's just plain scientific bigotry. Minkowski space-time is highly useful, but it is not the only mathematical analogy for representing the physical universe.

In old fashioned Euclidean space (where a straight line is the shortest distance, and where the internal angles of a triangle always add up to 180°) light does not follow straight lines in a gravity field. Instead, light changes direction as well as energy. That happens because light has mass, and like all masses, it is attracted to all other masses. What light doesn't have is rest mass, because light cannot be at rest in any reference frame; it must always move at c.

In my model, fundamental particles which can be at rest in their own reference frame consist of orbiting pairs or groups of photons, held in orbit at the speed of light around one another by the Higgs force. The rest mass of the particle is the sum of the masses of the orbiting photons in the reference frame whose origin is the center of the orbital paths. Larger particles form when fundamental particles orbit one another.

When photons are grabbed by the Higgs force, they fall into a deap potential well, which increases their energy and mass many fold. (This converts zero-point energy to rest mass.) All other forces are derived from the Higgs force, which results from exchange of momentum between regular energy and dark energy. Free photons are surrounded by a Higgs field; the Higgs field of orbiting photons is spun into a spiral pattern; the spiraling Higgs fields of particles interact resulting in the other forces.

In Euclidean space, accelerating a photon in a given reference frame is a matter of changing its direction and energy in that reference frame. (In Minkowski space-time, you can change a photon's energy, but not its direction; so you can't accelerate a photon.) Accelerating a fundamental particle with rest mass is a matter of accelerating the center of the orbital paths of the constituent photons, and thus translating those photons into a different reference frame. The complicate way to calculate the change of the particle's momentum would be to calculate the change of momentum for each orbiting photon, averaged over time. The result would be the same as the much simpler Newton's formula, f = ma.

As has already been mentioned in this discussion, f = ma only works for non-relativistic speeds. That is because the mass is not constant. Approaching the speed of light, additional force is needed to change the mass, and f = dp/dt is the formula to use (where dp is the incremental change of momentum, and dt is the increment of time). In Euclidean space, F = dp/dt works for both particles and photons. At relativistic speeds, dp = m•dv + v•dm. At non-relativistic speeds dm is practically zero, so dp = m•dv, and f = ma.

I could explain where the Higgs force comes from, but that would mean going more deaply into the nature of regular energy and dark energy and how they exchange momentum with one another. In other words, I'd have to explain my whole model. I have already done that in the New Theories section.