Some of your may remember the delightful 1990 movie, Ghost in which Patrick Swayze, killed early on, manages to communicate with medium Whoopi Goldberg, and through her and his lover Demi Moore to arrange a proper punishment for his killer. As a physicist, it was necessary that I suspend belief (like the rest of you) in order to enjoy the picture. But also as a physicist I got to thinking about the place of ghosts in the natural order of things.

The primary rule by which nature operates is that one which states that for every action there is an equal and opposite reaction. We encounter this rule all the time. Here all the time is meant literally, as in continuously. Nothing we can do in this universe can be done unless it is done against something else, and that something else instantly does the very same thing against us.

Four examples:

(1) You're sitting in a chair. Your body pushes down and perhaps sideways against the chair. The chair pushes up and sideways against you. Everything stays put.

(2) You're walking along the floor. Your feet push down and backward (!) against the floor. The floor in turn pushes up and forward against you, keeping you level and propelling you forward. (Should you wonder about the ability of an inanimate floor to push you forward, consider what happens if the floor is exceedingly slippery -- when you attempt to walk forward, nothing happens. You are unable to push back against the floor and the floor is unable to push forward against you. You stay put.)

(3) An airplane flies through the air. Some part of the plane pulls or pushes against the air -- a propeller pulling air through itself and throwing the air backward, or a turbojet sucking air into a tube and ejecting it backward. In either case, the air pushes the plane forward because the plane has pushed the air backward. Fortunately the air also holds the plane up, pushing upward on the plane as the plane pushes downward on the air.

(4) A rocket ship accelerates in outer space. It ejects rocket fuel backward, pushing against the fuel in the process. In turn the fuel pushes the ship forward.

Now what about ghosts? The time is 3 a.m. and you have been awakened from a deep sleep in an antique canopied bed in a mansion reputed to be haunted.. Opening your eyes, you see an iridescent glow near the window and the curtains are swaying. The window and shutters are closed. Nothing from outside can account for what you see. Clearly a ghost.

The curtains moved. So something must have pushed against them, and the curtains in reaction must have pushed back. Against what? The "what" had to be material in order to be pushed against. It had to possess inertia both to push and be pushed. But ghosts (by hypothesis) are immaterial. Were you to tell me of such an experience, I would be very skeptical, and blame the moving curtains on a mouse. The glow I would account for by light seeping in through cracks in the shutter or perhaps your vivid imagination.

I would argue that the question of ghosts comes down to definition. If they are truly immaterial, by definition they cannot interact with our world. Conversely, if they interact with our world, they cannot be immaterial.

One argument you might be bold enough to venture. You say, "Okay bright and shining physics teacher, how do you know your physics is right? How do you know that action and reaction must always be equal? Perhaps ghosts operate by other laws, if any." I don't know, of course. Accepting nature as nature reveals herself, consistently and immutably throughout all our experience, is a matter of faith in one's perception of the universe. A ghost that enters our world and interacts with it, sending out light and causing objects to move, must cope at least with the rules that govern material objects on our side of reality. The ghost, in other words, must have "materialized."

One final point: If ghosts are not material creatures, how do they manage to follow the particular locale they haunt as that locale moves through space? Consider, for example, a haunted house. That house is fixed to an earth which rotates on its axis once every twenty-four hours. The earth revolves around the sun once a year. The sun moves through the Milky Way (our home galaxy). The Milky Way revolves (admittedly slowly) about its center. It is also moving relative to the galaxies in the Local Cluster of galaxies. And the Local Cluster is moving relative to other clusters.

The allegedly haunted house stays fixed on the earth because it is a material object, subject to all the natural laws which keep it from wandering away from its position. But what could drag a ghost along with the highly varying motion of the earth? If indeed there is some unknown influence acting on the ghost enabling it to keep track of where its house is, then that influence must constitute an interaction between ghost and house, giving the ghost a material property, something ghosts are not supposed to have.

Perhaps there are other universes in which ghosts reside and perhaps those universes do interact with ours in ways that are consistent with unknown and unobserved rules. My faith, built on experience, says no.