Thousands of people in China saw what looked like a giant, floating city in early October, spurring conspiracy theories involving holograms or a window into a parallel universe.
While thoses theories were obviously false, there's actually some cool science that explains how people saw an entire city as a mirage floating in the sky.
It has to do with physics and the way our eyes perceive objects around us. When we look at something, our brain assumes the light coming from it traveled in a straight line. But this becomes an issue when the light travels through either air or liquid with different densities — when it hits the boundary between the two it bends, sending it off its straight course.
That's why things look like they're in a different spot when they're underwater — the boundary between the water and the air bends the light we are looking at and our brains get the wrong idea of where the object is.
The same thing happens when light travels through air with different densities. When warm air rises, an invisible barrier between it and the cooler air below sometimes forms that bends light. So when a beam of light travels through warm air then cold air, it shifts down because cold air is denser than warm air, making the light look like it came from above.
This phenomenon, called fata morgana, can make objects look like they're floating. As this helpful story in Wired explains, it's the same reason why sailors thought they were seeing the Flying Dutchman, the ghost ship that would appear just beyond the horizon and then suddenly vanish.
If conditions are just right, like they were that day in Jiangxi and Foshan, China, this effect can make entire cities look like they're floating.
The floating city mirage wasn't because of secret government tests, aliens, holograms, or parallel universes — just really cool science.
