Sorry for the period of silence (hectic couple weeks on my end!) but I really appreciate all the new replies and explanations! I have read all of it and it's fascinating, especially the descriptions of different theories.
A few further questions based on what has been stated:
Wait, does that mean they can smell everything around them as well? Ghosts often seem to haunt old and abandoned places, so I would imagine that were quite unpleasant, being able to smell all the damp and decay and so on!
Also, I thought physical things went through ghosts, and while I've not seen any real footage or anything of someone trying to touch a ghost, it's usually portrayed in media that the ghost isn't even aware of them being there. Surely if they could feel the energy, they'd feel the heat from our hands, or the disruption of air as our body travels through it! I heard recently on the news that the sound made by wind isn't actually made by the air itself, but by the air hitting small particles and the energy produced by that collision. If ghosts had improved sight/hearing etc, would they be able to detect things like that? How come they're not reacting to physical outputs all the time?
I'm interested in how you worded that, with "get to". What do you think decides whether they do or not?
So is this term "manifestation" a specific thing? Is there a definition of what counts as one? If you know what to look out for, is there a way to say for sure whether a manifestation has occurred or not?
Also, just one further point - as we approach Halloween, my village's local ghost legend is making the rounds again. I'm unsure whether I believe in him (though I have had something that could count as an experience...), but he supposedly rides a bicycle and you can hear his bell ringing. How does that factor in in terms of spirits being able to produce sounds? Obviously the bicycle hasn't died and become a ghost (silly to say, but it needs typing out for my point) so does the cyclist himself produce the bell ringing sound, almost like a kind of parrot, just replicating sounds he knew in life? Maybe without vocal chords any sound is possible? Would he even be aware he's doing it?