I would place Ki and flat earth in the same category - based on belief and at risk of being soundly debunked.
Ki that is manifest as a result of having a unified mind and body is both real and demonstrable. And I'm not referring to the woo-woo I'll knock you down from 10 feet away nonsense. It's more of the, when you walk into a kokyu nage throw and it feels like nage just dropped a house on you despite being 50 pounds lighter than you and you feel like you've barely been touched, variety.
It's a shame that the charlatans get all the play and notoriety these days and that it has taken someone from outside the art to reintroduce Ki back into mainstream Aikido, renamed and repackaged though it may be.