When I first saw this video I thought it was fake. Perhaps an April Fool's joke. But, not only is it real, it is a phenomenon that's been known for hundreds of years. Why am I just hearing about it?

(there's also a full version in which you can hear the tones: https://www.youtube.com/watch?v=1yaqUI4b974)

So, what are we seeing here?

This demonstration is by a prolific YouTube user who goes by the handle brusspup. I've been enjoying his amazing visual illusions for a few years - and I'm not the only one! His videos have wracked up tens of millions of views.

But this one isn't an illusion. Rather, it's a clever way to reveal patterns not normally visible to our senses. And it traces back to the 18th - and even the 17th - century and a somewhat obscure scientist.

Ernst Chladni was a German-born Hungarian physicist and musician who did pioneering work in acoustics and also in the study of meteorites.

In fact, in 1794, he was the first to publish the outlandish idea that meteorites were extraterrestrial in origin, a proposal for which he was ridiculed. At the time they were thought to be of volcanic origin.  But we all know who got the last laugh on that one.  He was vindicated within ten years - within his lifetime - when a dramatic meteor shower left hard-to-dispute meteoritic evidence all over a French town.

But the phenomenon seen in the video is the one for which Chladni is perhaps best known. It is a technique to reveal the complex patterns of vibration in a rigid surface.

From Wikipedia...

"A plate or membrane vibrating at resonance is divided into regions vibrating in opposite directions, bounded by lines of zero vibration called nodal lines... Chladni's technique... consisted of drawing a bow over a piece of metal whose surface was lightly covered with sand. The plate was bowed until it reached resonance, when the vibration causes the sand to move and concentrate along the nodal lines where the surface is still, outlining the nodal lines."

Modern versions of the demonstration tend to use modern equipment such as loudspeakers and signal generators with adjustable frequency. In the video, as the frequency is altered we are able to see how the patterns in the plate assume various intricate shapes. The sand is pushed away from the areas of vibration and gathers in the places where the surface remains motionless (the nodal lines).

The beautiful patterns that emerge are now called Chladni figures, although Chladni was actually building on earlier experiments performed by Robert Hooke, who, in 1680, observed these nodal patterns in vibrating glass plates.

Thus, Chladni, like so many scientists, was "standing on the shoulders of giants."  And, in his case, not just any giant, but the very giant to whom Isaac Newton was writing when he famously explained how he was able to see further than others who had come before him.

UPDATE: If you're interested in the physics, be sure to check out a very in-depth explanation at Skulls in the Stars, which includes a bunch of original images of wonderful patterns and also relates the subject to lasers and quantum mechanics. Recommended!