How could you have ghosts in a story without an afterlife? Several ideas pop into mind, but I’m leaning toward a psychically powerful character who projects manifestations of people. The dead are dead and are not manifesting, but this person is making them appear real. Probably through the strength of their emotions.
From personal experience, I know that the loss of a loved one can put a person in a state of greatly heightened emotions, as if you’re super stimulated and yet super depressed at the same time, a state which can last for months, and during which feelings, dreams, premonitions, are all exacerbated, and seem so real…
This state fades eventually, but suppose someone had an exceptional capacity to attain this state, even without personal loss, and suppose they could manipulate electromagnetic forces to manifest their thoughts and feelings and that this would appear as a person to others…
Another suggestion would be a hologram in the image of a person, programmed to speak and react like them, like the doctor in Star Trek. We’ve seen this so many times in movies, it almost seems like it should already exist!
A story I read a long time ago suggested that all objects record every last thing that happens around them, so that you could rewind the “tape” of your father’s favorite armchair, for example, and hear and see him again, sitting in that chair, watching the ball game. This would not be a ghost, but an apparition nonetheless.
Another way to summon “ghosts” would be to expose people to hallucinatory substances after having created a suggestible mood. 183
The ghosts aren’t really there, but the people would see or hear them and believe that they were truly in the presence of their loved one. In your future world, the specific mix and dosage of substances would have been precisely calibrated for a reliable effect. Scientists have found that low frequency sound has an effect on people akin to “meeting a ghost”, that is feeling chills, sorrow, or fear and revulsion. This could be part of the “recipe”.
Scientific explanations of ghosts include exposure to mold, which causes delirium, dementia and irrational fears, and to carbon monoxide, which in its early stages, before poisoning you, causes auditory hallucinations. It is true that most haunted houses are old and moldy… 978