What is the reason for gravity?

We want to make sure any amateur reading the thread gets legitimate physics answers instead of speculative ones. If someone reads an answer and gets the idea that gravitons might be photons, that's undesirable.

 

We know that gravitons have to have certain properties; timo has explained some of the issues: Because the stress-energy tensor is rank-2, the graviton must be spin-2, and photons are spin-1. And we already have photons interacting with matter via the electromagnetic force, and the gravitational force is different. The interaction must be different.

I don't understand what stress-energy tensor ranking is. Does that have to do with the rate at which the field weakens as distance from the center increases? I also don't understand how spin is measured and what its mechanics are. How does a photon spinning affect its behavior? The relationship between photons and electromagnetic force is a reason that makes sense to me, though, unless some connection between that force and gravity could be uncovered.

I see why you wouldn't want to mislead someone who is attempting to learn orthodox theoretical developments regarding gravitons or anything else. My concern with any orthodox learning is that people substitute rote memorization of facts and propositions for critical understanding. Likewise, people sometimes rehearse the trains of thought and equations of established scientists and come to fetishize those instead of engaging them in a way that enhances their own ability to think creatively about the material. A critical discussion questioning an orthodox fact or proposition often results in a more thorough understanding than a rehearsed explanation, although that can be beneficial as well.