We have two rovers on Mars and you can get a heart transplant–but until recently we didn’t know how our knuckles make that annoying sound when we crack them. Way back in the 1940s it was thought that the knuckle-cracking action formed a bubble in the joint fluid–and this swift formation of the bubble created the sound. But in the 1970s, scientists (and we’re not saying they were smoking anything, despite the timing) revisited the bedeviling knuckle crack and decided it wasn’t the bubble’s formation that made the sound: it was the bubble collapsing. Like a balloon popping.
They all at least agreed on the bubble–and that was it for a while. Fortunately there are Canadians with curiosity not so easily slaked by cavalier cracking theories, however bubbly. Greg Kawchuck at the University of Alberta and his team took a colleague who’s an inveterate knuckle-cracker and put his hand in an MRI scanner. Voila! It’s the formation, not the collapse, of the bubble that makes the sound. So once again people and ideas from the 1940s trump those of the 1970s. Tom Brokaw, author of The Greatest Generation
