String theory is discredited in some circles, yet still endorsed in others. I think they need 8 dimensions to make it actually work. Again, I am not really knowledgeable and certainly not current on the debate.
Ok, in layman's terms, imagine a timeline along a string. Physical space consists of vectors attached to the string at a specific point, representing an instant of time. Think of time as displacement along the string. Now jumble the string up and drop it on the table, so that time touches where the cord of the string overlaps. The spatial vectors should be orthonormal, but what if you have the spatial vectors very slightly penetrating one another from non-adjacent times along the string. From your current reference plane, perhaps you'd see something from the space of that other time. Might you call that a ghost?
Probably a very poor explanation, only good for metaphysical quackery.
And, even worse, it causes all sorts of problems with conservation of energy.