This spooky season, IFLScience is taking you on a journey between science and superstition, to a dimension of imagination akin to The Twilight Zone. For the next several hundred words, we are going to ask you to believe in ghosts. We will build a case for how ghosts can work within the natural laws of the universe before the scariest of all branches of physics comes to crash our fun.

While physics is my domain, we need to raise a question that pertains to the world of biology: Why are ghosts in stories and folklore overwhelmingly human? And not just any human, modern humans – Homo sapiens. There is nothing in our biology or physiology that sets us apart for the task or that is utterly unique compared to other species.

Where are the Neanderthal ghosts roaming Europe? Why isn’t the ocean filled with the ghosts of dolphins and orcas that have unfinished business roaming around kelp forests? Shouldn’t poachers be haunted by the ghosts of great apes they had violently killed? Much to consider when you include biology. But, for now, our focus is physics.

Ghosts Vs The Forces of Nature

There are four fundamental forces of nature, based on our best understanding of physics: Gravity, electromagnetism, and strong and weak nuclear forces. Everything in reality interacts with these forces. Not all of them, but at least one. An interesting example is dark matter – the hypothetical substance that permeates space and outweighs the regular matter that makes us five to one. It is believed that it only interacts through gravity.

Now for our spooky spirits. We ought to have them interact with gravity at the very least, otherwise, there should be a helical residue of ghosts through space. Helical like a spiral staircase because the Earth goes around the Sun and the Sun moves forward with the whole Solar System. Our telescopes and our spacecraft have not picked up such a trail.

Another force that certainly matters to ghosts is electromagnetism. They are sometimes visible, although it is not clear if they emit or reflect light. They can touch objects and move them, so there’s a physicality to them that can only be expressed through electromagnetic interactions. And they are translucent – light changes as it passes through them.

This indicates a physical substance of some sort – which suggests that we might be dealing with one or both of the other two forces. For ghosts to be real, they need to play by the rules of the universe. The fundamental forces have put down some pretty stringent limits.

Ghosts Vs Quantum Mechanics

In the anthology A Random Walk in Science, D.A. Wright delights us with a fantastic essay titled A Theory of Ghosts. It starts from the assumption that ghosts can go through walls and doors but remain confined within the thick exterior walls of castles and old buildings – and that allows us to do some intriguing quantum mechanical calculations.

A standard exercise in quantum mechanics is a particle trapped inside a potential well. The particle has not got the energy to jump over the “walls” of this well so it remains trapped inside, although it has the ability, occasionally, to tunnel through the walls, if they are not too thick. That sounds quite apt for the concept of a ghost, doesn’t it? 

The conditions are so wonderfully stringent that Wright can calculate the mass of a ghost: around 10 trillion times less than a single electron. Basically, ghosts cannot be scary or lift you up because any fight back from you could easily accelerate them out of the Solar System.

So our ghosts are bound by the fundamental forces and they have a tiny mass, smaller than even that of a neutrino – aptly called the ghost particle. But maybe they are not just a thing, maybe they are an interaction… like a time crystal. They periodically form from interacting atoms, appear to us in visible light, and then disperse again until it is time for their next interaction. Wouldn’t that wrap everything up with a neat bow? Well, not quite…

Ghosts Vs Approximations

To make the science in the previous sections work, we had to cut corners and take creative liberties. If you start looking closely at every aspect of it, you will see just how flimsy our constructed physics of a ghost is.

For example, even if ghosts are immaterial – just magically made of light – they cannot pass through walls as long as we are seeing them. If they were made of radio waves or microwaves, sure that’s how Wi-Fi moves through a house, but we do not see Wi-Fi and we wouldn’t be seeing ghosts either.

The jokey tunneling example of quantum mechanics could not possibly work as long the ghosts were made of something. It would be like you were trying to tunnel through a wall. Yes, the possibility exists that as you run at the wall all your particles would align in a way that you can phase through said wall… but the probability is so close to zero that it is easier to bet on your broken nose. Even dramatically reducing the number of particles of the ghost would take enormous coordination to make it happen – and that means energy.

Ghosts Vs Thermodynamics

Either the well-tested laws of thermodynamics are real, or ghosts are. I am more inclined to disbelieve the former, than I am inclined to believe the latter.

Marcus Siddall

This is why we left the scariest branch of physics til last: thermodynamics. The laws of thermodynamics tell us how energy is transferred (never created nor destroyed) and how entropy tends to increase in an isolated system. We can’t even reach absolute zero or create order (reduce entropy) without wasting energy.

Basically, they explain to us that the universe is a big game: we can’t win (no energy out of nothing), we can’t draw (entropy increases), and we can’t even leave the table. Our physics-based ghosts have to face this impossible challenge.

“Either the well-tested laws of thermodynamics are real, or ghosts are. I am more inclined to disbelieve the former, than I am inclined to believe the latter,” scientist and educator Marcus Siddall told IFLScience.

If they exist in the universe, ghosts must obey the laws of thermodynamics. For a ghost to exist, it needs to draw energy from something and it will expend energy to float about, change shape, and pass through walls, so that means it must also release said energy. 

Not to be all Demi Moore about it, but ghosts are hot – if they have to be real. When supernatural TV shows have cold spots in infrared cameras, that’s a draft. A real ghost obeying the laws of physics needs to run hot. 

Since we are on the topic of tropes, what about flickering candles and such? The Spooky Science Sisters gave us a fantastic tidbit: infrasound – frequencies lower than a human can hear – can make candles flicker.

Thermodynamics being correct is the reason why we age, why it is easier to make a room messy than tidy it up, and why things break with time. And that’s way spookier and upsetting in the long run than any ghost.