My Cat from Hell: What is the shortest measurable amount of time? You’d think the answer would involve advanced physics, something to do with some invisible force and nanoseconds. But no. Scientists now agree that the shortest measurable amount of time is the micro-instant between when a person first acquires a cat and when that person feels the need to use the phrase “bad kitty.” The list of cat misbehaviors — scratching, biting, yowling, spraying, furniture wrecking, drapery shredding, hairball disgorging — is so long that it’s amazing that the animals can find time to get the 20 hours of sleep they seem to need per day. Most cats, though, know how to balance their evil tendencies with general adorableness, so that their owners are too busy reaching for the video camera to notice the claw marks on their upholstery and the blood on their arms. That’s most cats. But the beasts in “My Cat From Hell,” a series that begins on Saturday on Animal Planet, are an entirely different story. These cats are the Regan MacNeils of the feline world: hissing, clawing, cursing, eyes aglow with demonic possession. But whereas Regan, the kid in “The Exorcist,” warranted a visit from a priest, these animals get a dude named Jackson Galaxy.