TITLE: Photons, Gravity and Higgs field
QUESTION [0 upvotes]: (a) A photon's path bends towards a large mass, so we say it has gravitational attraction (or that the large mass bends spacetime so that the photon's path curves towards it). But if the photon feels the effect of gravity, why doesn't the photon accelerate or decelerate when moving towards or away from a large mass? Why feel the gravitational effect only perpendicular to its direction of motion and not parallel to it? By comparison, any other massive particle would feel the same effect of gravitation in both directions - parallel and perpendicular to its direction of motion, and even while at rest. The large mass has curved spacetime the same way, both for the massive particle and the photon moving towards or away from the large mass. Why then would the photon and the massive particle have different behaviors following the same space-time curvature in moving towards or away from the large mass?
We can say that as the photon passes close to a heavy object,it has an "acceleration" towards the heavy mass and perpendicular to its original direction of motion because it starts to have a velocity towards the heavy mass where previously it had none.
(b) We say that the Higgs field gives particles their inertia (i.e. mass), and that it is not responsible for gravitational effect or bending of spacetime because even massless particles feel gravitational effect. Then, is it just a coincidence that the amount of effect or bending of spacetime is directly proportional to the inertia of the particle as given by the Higgs field, or is there any reason for it? Why would both gravity and inertia be proportional to the same quantity of mass as produced in a particle by the Higgs field?
(c) Why doesn't the Higgs field give massless particles any mass? What property of massless partilces makes them immune to the Higgs mechanism?

REPLY [4 votes]: I would consider asking the other two questions separately. Answering your first question:
There is, in fact, a gravitational effect on a photon parallel to its direction of travel. In fact, at its core, it's the same effect that gravity has on any object, massive or not. For any object, including photons, gravitation, being modeled as a force, changes an object's momentum. A photon's momentum increases as it travels toward a massive object, and its momentum decreases as it goes away. This means that its energy increases and decreases in the same manner, and its wavelength changes in the opposite sense. This phenomenon is known as gravitational redshift. The fact that a photon's momentum isn't related to its speed doesn't prevent gravity from changing its momentum.
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