Ok, I was thinking about this the day. Let me know how far off I am on this. As we go smaller and smaller on physical scales we find that 'solid' objects are actually comprised of 'atoms' and 'empty space' <--- loose definition, the relationship of which is in proportion to the density. Like 'zooming' into foam. Now with that I concluded that actual physical 'contact' between any 'structure' is impossible. It's the binding and repulsive forces that control the enmeshment of two objects or the ability to 'pass' through. So when two objects collide the interaction of them is dependent upon the 'change' in those states of the 'forces'. extrapolating out then, the ability to 'walk thru walls' is dependent upon being able to 'alter' the atomic forces and not the 'physical' element. And to me this also explain the 'gravitational' potential of an object based upon it's cumulative quantum binding forces. higher density objects would have more interactions and thus proportionally more 'force' than one with low density.