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Even though I’m familiar with quantum mechanics, I’m unfamiliar with general relativity, so I hope for an explanation simple enough that I can understand.

As far as I know the following three (contradictory?) statements are correct, how do they coexist?

  1. Light is described as a reservoir of photons, which are massless.

  2. Gravitational forces are proportional to mass, which would mean massless particles should not participate in gravitational interactions.

  3. Light bends due to gravitational pull.

My questions are:

  1. If photons are massless why do they interact with mass?
  2. My understanding is that it has to do with curvature of spacetime. Does it mean that slower massless particles, which should be less susceptible to relativity, don’t interact with mass?
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Light is described as a reservoir of photons, which are massless.

"Light" is Electromagnetic radiation and is made up of packets of energy called photons. They are indeed massless.

Gravitational forces are proportional to mass, which would mean massless particles should not participate in gravitational interactions.

Gravitational fields are a result of either mass or energy (mass being a form of energy).

Photons follow paths which are of a very particular form and this is best described using general relativity, which is maths heavy. The technical description is that "massless particles like the photon instead follow null geodesics" which I suspect is next to meaningless to you (and really requires some basic relativity to understand). The linked Q&A does try and explain it.

Light bends due to gravitational pull.

As these paths can be curved in the presence of a gravitational field, light can, in effect, bend.

Does it mean that slower massless particles, which should be less susceptible to relativity, don’t interact with mass?

Massless particles (in mainstream physics) all travel at the speed of light. The two things are linked, so if you find something traveling at the speed of light it is also massless, and if you find something that's massless, you know it's going to travel at the speed of light.

Any discovery that contradicts that would be a shock to physics.

So "slower massless particles" don't (as far as we know) exist.

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  • $\begingroup$ I read your answer again, and I find that I still miss something. Your explanation tells me that given the null geodesics defined trajectories of massless particles, and given the curvature of these (due to time dilation in response to field?) light bends. However, on a more fundamental level, isn't it surprising that masses particles interact with the field to begin with? What forces the photons to follow null geodesics? $\endgroup$
    – Yair M
    Sep 3 '18 at 6:18
  • $\begingroup$ The curvature is due to the distortion of space time by mass and/or energy. For photons, time dilation is meaningless. The null geodesic and why it describes a photon's path is explained here. In GR any mass or energy will distort spacetime and it's the spacetime (not the objects in it) that are being affected. An object in the distorted space time of some other mass or energy "just happens to be there", but the spacetime would be affected even if they were not. $\endgroup$
    – StephenG
    Sep 3 '18 at 6:34

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