Ectoplasm was made popular in the spiritualist movement in the early 1900s when physical psychic mediums produced material that was said to be the stuff spirits were made of. As intrigued as the concept was, it was found to be a marvelously elaborate hoax in which they utilized cloth, cotton, strings, and other materials to give the appearance of expelling it during a seance.
We've moved on nowadays to realize that there is no ectoplasm and that ghosts do not manifest themselves in such a physical form. We still persist in the concept that a soul can enter the human body and take command (possession). The assumption is that if ghosts can affect doors opening and closing and create odors and sounds, they can certainly penetrate a human body. I remain skeptical on that POV since if ghosts could command us, our deceased parents and grandparents would be taking command of our personal sinking ships. That said, however, there could be conditions in which one is able to affect a person for short bursts of time, just like they can open a door once during the night and that's all the force they get.
The question this leads to is pretty important to discuss; what are ghosts made of? We know that they are not "solid" as we are and yet they can manage to move objects and touch us. So, how do they perform this task?
It would appear that the logical conclusions is that whatever a ghost is (whether that's a prankster from another dimension, psychokinesis from the living, the soul of a dead one, or the past in a sporadic loop), it is not solid by our interpretation of solid and yet it does affect solid objects, so it must be something happening in a molecular level that, though we cannot see it, has an interaction with other particles in solid objects resulting in rocking chairs moving, doors slamming and people being touched.
Let's discuss one potential means by which ghosts launch into our world and mess with us: The Hutchison Effect. This Canadian engineer (John Hutchison) came up with the concept of extremely high levels of EMF utilized basically to affect objects around the machine. Whether this is genuine or not has been debated hotly and lots of intrigue about where he is and if the Canadian government was stealing his equipment, trying to discredit him, and stop his experiments or what. See this video and you decide -
Some of the supposed claims of the experiments are objects interacting and melding with other objects, levitation, new metal alloys being made, and poltergeist activity. Here is a site with more pics and info. If this truly happened and he did have such accomplishments, then it would seem comparable to stories of the Philadelphia Experiment.
Whether this was successful or not, an urban legend or the madness of a manic depressive with grandiosity, the concept is a sound one. There might be way that using natural forces in our world, we can affect objects. Just look at electricity. We learned to light bulbs and power appliances when we discovered it and learned to harness it. It was, however, always here in natural forms like static electricity and lighting power. So, it might be with the ghost world - we will learn its energy source and how to control it.
Ghosts might not be made of ectoplasm, but what their energy source is and their mechanism for reacting and affecting our world--is quite an exciting scientific frontier. I think our answer will lie in explanations by physicists and if we find out their makeup, we can find out their origin, if we find out their origin, we finally know what a ghost is and, better yet, how to have a back and forth with them with reliability.
I suppose the next question after what are they made of is, what the heck is the driving force behind them?