[wruza]

Sorry for interrupting your discussion with maybe dumb question, but isn't gravitation defined by spacetime geometry rather than some sort of particle exchange? Do not particles always fly forward, with forward changing its meaning with time?

[wayn3]

"spacetime geometry" is a mathematical formulation of the physics that happens to correctly describe WHAT is happening.

it makes no prediction at all about WHY its happening. einsteins theory does not explain, and does not try to explain, why the gravitational field exists. it just tells us the effects of the gravitational field being around.

[wruza]

That's "why" thing is new to me. I read before that there is no why, only how. "Why do particles exist? Why is something fundamental like it is?" Are these questions to be answered in ST framework?

[lawpoop]

Not a physicist, but questions of "how" and "why" are complicated by their ambiguity in English.

In common English, "how" means the mechanics of a system, and "why" generally means for what purpose someone has undertaken an action. Think of "Why did you build a garage?" (for what purpose or to what end?) vs. "How did you build a garage?" (what exact method did you use).

To complicate things, for action by non-agents, "why" can also mean "how". Children might ask "Why is the sky blue?" but what they mean (or the answer they get) is "How is the sky blue".

So when it comes to physics, we generally say there is no "why" (teleology, or mental purpose of an agent), only "how" (observable physical mechanisms). As far as physics is concerned, there is no God who set up the universe in a particular way, because he liked things a certain way, or he wanted a certain outcome.

However, the ambiguity of "why" can still be re-purposed, if you will, in physics. It's not referring to the will of an agency, but instead to lower-order phenomena that give rise to higher-order phenomenal. Why does the Earth sky have to be blue instead of, say, red? We're asking about why the mechanics of Releigh scattering play out one way verses another.

[wayn3]

There can be relative WHYs. We will probably not be able to answer why particles exist. But we can tell you why gravity works the way it works.

I dont know what ST refers to.

[auggierose]

The whole point of general relativity is that there doesn't need to be a particle that does something, but that gravity is just an emergent property of space. So no, we don't know that a gravity particle has to exist.

[ylem]

The problem is that we have two formulations of physics. Basically, gravity and everything else. Theories such as QCD, QED, etc give rise to quantum mechanics, electricity and magnetism, etc. In these theories + and - charges for example feel an attraction to each other due to an exchange of "virtual" particles. General relativity is based on geometry as you say. Physicist would like to have one framework to describe everything--that's why it would be nice to find a graviton.

[drostie]

"String theory."