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Gravity is a force, and it is assumed to be mediated by the graviton. However, we have not observed any of these (and it is not clear we will never be able to).

uh huh ... look like you've been busy ...

saves me the trouble of apologizing for a late reply since i havent for several days now ... so this graviton particle (is a particle? theoretically speaking?) is responsible for "the weak force" but it only exists in theory and even then it is not part of the standard model ? so where do you place it ? you have a 3d matrix of sorts to put these kind of elusive little buggers in ? in a sheet next to the standard model, well in another row or page so to speak ?

I wouldnt say never ... people would never fly and basically if you think about it if efforts were combined and a decent space station would be built (i'm not even speaking about an actual space elevator , it would be completely possible to have a hangar in orbit where ships could be built, or at least for the first stage kept so they don't need to use massive amounts of resources to escape the gravity well , in which way a supply line of sorts could be set up because ship starting from there would need only a little push to get started, which could be a supply line to the moon, or to mars or actually to the gas mines of further planets , all of that is technically possible RIGHT NOW, but its not being done, for political reasons and because competitive psychopaths at high levels need to "win" instead of advance, so seeing all that i wouldnt say never to the discovery of said graviton in our, your or my lifetime ... there's no reason why not, right ? only politics :)

There is a good reason explaining why gravity is mediated by the graviton. All the other forces (strong, weak and electromagnetic) are quantized. Therefore, it would be really weird not to quantize gravity, wouldn't it? And if one does so, one ends up with a graviton. We also know that its interactions are super super weak. For this reason, it is impossible to observe it. If you find a way and you succeed, I think you may get a Nobel prize!

The extra problem with gravity is that we have no idea about how to quantize it. This is incompatible with general relativity and we need to somehow extend our framework (quantum field theory) to be able to do so. This is today a very active field of research.