I got a laugh out of this video of a so-called ghost swing in Rhode Island. The explanation, almost certainly, is that the 'ghost' swing has a much larger surface area than the other two swings, and is just catching more of the wind. Notice that the middle swing is also moving, but the camera moves too fast to determine if the right most swing is also moving. In the video it's claimed that there is no wind, but the overcast sky suggests there is some amount of wind. If the date was known, the wind speed for that day and location could be determined. http://turnto10.com/news/local/warwick-ghost-swing-video-goes-viral-07-11-2016 Having lived nearby on Cape Cod for decades, I know this region is very windy, in fact one of the windiest locations in the nation. Pedaling to work on my bike for many years, partly alongside the ocean, I constantly had to battle the prevailing Southwest wind, but got a nice boost on the way home.