In Return of the Jedi Obi-wan, as a Force ghost, sits down on a rock while talking to Luke, which implies that even though Force ghost are translucent, that they can interact with the physical world. Or was he just projecting the appearance of sitting, to seem more lifelike to Luke?
Just how much can a Force ghost interact with the physical environment? Can they pick up objects? Can they use the Force to manipulate objects or do the other things they could do with the Force when alive? Are those that see and hear them actually experiencing a physical presence by hearing sound and seeing reflected light, or is it all in their minds?
How much can a Force ghost interact with the physical world?
Added after a couple answers: Is there any evidence in the EU that Force ghosts interact with the physical world or anything to prove they cannot?