Let’s pretend — for argument’s sake — that ghosts are real. What, then, do you define as a ghost? If you believe paranormal activities can have a supernatural cause, then you’re basically saying you believe in the world of spirits and demons and a universe that is not limited to the laws of physics.

But let’s be real here: Ghost’s don’t exist. These apparitions aren’t manifestations of a spiritual world and can be explained by the natural laws that govern the universe.

Here are a few theories for what ghosts might be, based on some science.

Electromagnetism

Ghost hunters believe electromagnetic fields (EMFs) are the equivalent of a ghost autograph. EMFs could have an effect on the neural processes of the human brain, which is governed by electrical potentials that move back and forth. External EMFs might affect how those neural circuits operate and create the impression that we can sense supernatural activity in the outside world.

A Canadian neuroscientist named Michael Persinger tested out this theory in 2010 by subjecting participants to weak magnetic field patterns for about 15 to 30 minutes and found that people indeed would begin to feel like they could perceive a ghostly presence in the room. Of course, there are studies that offer a counter conclusion.

While this explanation suggests EMFs are actually just a manifestation of unusually large EMF activity on the brain, ghost hunters see EMFs as a sign of ghosts themselves. That’s the reason ghost hunters carry EMF meters around to gauge whether a haunted site is indeed haunted. The legitimacy of that work is very questionable, so take that as you will.

Really Dank Mold

Ghosts could just be a symptom of aberrant mold in the environment. One researcher at Clarkson University believes some molds can create feelings of anxiety and fear, engendering problems like acute dementia. Breathing those mold particles in could be a quick way to start feeling the ghostly chills.

This might make sense in a certain way — mold is typically found in homes and buildings that are not very well taken care of. You know, the kinds of dirty, abandoned places that may as well be plastered with a big banner that says “HAUNTED.”

Still, there’s not enough evidence to suggest mold is the case behind people seeing ghosts.

Infrasound

There are some frequencies of noise humans can’t actually hear, but that doesn’t mean the effects go unnoticed. Infrasound refers to the sound emitted at ultra-low frequencies. Although infrasound cannot be heard, the vibrations they cause have tangible effects — wind turbines and traffic noises can create residual infrasound that can induce feelings of disorientation and aberrations in blood pressure and heart rate that are in line with what happens when one is stressed or panicked … or the same frightful effects one might expect when they chance upon a ghost.

Quite a few papers have attempted to make the connection between infrasound and ghosts — and it seems clear thus far that low frequency vibrations cause some kind of physical effects on the body.

Drafts

Yes, a ghost could be caused simply by a rush of cold air swooshing through the halls of a house. A cold spot is often cited by ghost hunters as a sign a haunting spirit remains close by. And yet, scientists — and pretty much anyone who has ever lived in a house with a temperate climate — will probably chalk up cold spots to being caused by a draft that’s rushed in from the chimney or an open window.

In addition, even if a room is sealed shut, hot air will continue to rise, and cold air will still fall. If you happen to be in the room, you might feel a rush of that cold air. Maybe it’s a cold spot caused by a ghost — but much more likely it’s just a pocket of air moving through space.

Neel is a science and tech journalist from New York City, reporting on everything from brain-eating amoebas to space lasers used to zap debris out of orbit, for places like Popular Science and WIRED. He's addicted to black coffee, old pinball machines, and terrible dive bars.