That’s essentially a “god of the gaps” argument, i.e. if we cannot demonstrate it scientifically, therefore it must be God, or ghosts, or the Great Bacterial Collective Intelligence. But, in any case, turn that question around: do we have good reason to scientifically exclude the possibility of ghosts? And the answer there is a very strong ‘yes’.

Ryan North has a lot of Dinosaur Comics exploring concepts around ghosts, but the one that sticks in my mind is the one in which T-Rex muses about finding out what makes a poltergeist angry, triggering its ire constantly, and connecting the object(s) it manipulates to a generator in order to get infinite free energy.

Because, the physical world that we know and inhabit works on energy. For a ghost to interact with our world, it would simply have to inject energy into it. Sound, light, heat, et cetera, it’s energy. There’s no way around it. And we have laws of physics, like conservation of energy, which we very, very, very thoroughly tested at the scale, energy level, and relativistic velocities (that is, our human environment) at which ghosts would interact. In our natural world, we’d have to see macroscopic effects without causes, and energy entering or leaving the system. We’d be able to measure it, but we have not. E = mv², and the two sides of the equation balance, always.

More prosaically, another Dinosaur Comics strip posits that ghosts must be blind because they’re invisible. Invisibility means that all light passes through them, but if it doesn’t strike whatever ghosts use for photoreceptors, they’d by needs be blind. If their eyes did intercept light so that they were able to see, then if a ghost was watching you in a bright room, you’d at least see the faint shadows of its retinas. (Creepy!) In short, we don’t have to make any claims about the supernatural to say that if ghosts, or other supernatural phenomenon, interact with our natural world, we’d have to be able to see and measure the effect beyond subjective reports. However, we don’t, and there really just aren’t any gaps in the physics for ghosts to reside in.

As for the book, well, we all live inside these meat-based processors that are not exactly reliable in interpreting sensory input, or making narrative sense of it, and are well-known to just fabricate experiences and memories out of the ether when the sensory input is absent, scrambled, or just not interesting enough. It seems to me that the strongest likelihood is that brains did what brains habitually do (i.e. come up with fantastical stories), and that our theory of physics is pretty decent, since it has enabled us to create all sorts of technology.