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An orb is an alleged ghost, spirit energy, or multi-dimensional being invisible to the naked eye but visible as a ball of light on film. Though some people believe orbs are life-forms that travel in groups, they are most likely due to reflections caused by flash or other naturalistic light sources caught on film.
One orb enthusiast claims that "the orb is the basic energy pattern of the spirit world," though how he knows this is unclear.
Another thinks "the orb is the energy being transferred from a source (i.e. powerlines, heat energy, batteries, people, etc.) to the spirit so they can manifest." This explains why they are round, he says, because that is what one would expect from the laws of physics. He fails to mention which laws he has in mind, however.
The photo of the orbs above, taken with a flash through a car window on a rainy night, shows that orbs sometimes look suspiciously similar to reflections of light. Sometimes they might be insects, lint, or dust (Radford). Distinguishing orbs that are ghosts from those that are not is on par with trying to distinguish which of two people playing tennis had his soul removed last night by the soul stealer.
books and articles
The Riddle of the Orbs by Philip Carr (Fortean Times)