Scientists finally explain why microwaved grapes emit glorious bursts of plasma

The mystery of the glorious fireball emitted by microwaved grapes (featured in my novel Little Brother) has been resolved, thanks to a paper in the Proceedings of the National Academy of Sciences in which Trent University researchers Hamza Khattak and Aaron Slepkov explain how they destroyed a dozen microwaves before figuring out that the grapes were just the right size and had enough humidity to set up standing waves that amplify the microwaves — and anything roughly grape-sized will do the same.

The paper is offline at both PNAS and Sci-Hub, which is weird, but there's good coverage of it at Ars and Wired.

"Previous explanations leaned on the idea that the grape was acting as an antenna and that an electrical current was being generated across the 'skin bridge' holding the two halves to a grape together," said co-author Pablo Bianucci of Concordia University in Montreal, who did the computer simulations for the study. It's that current, conventional wisdom goes, that generates the plasma.

These new experiments show that's not quite right. The skin bridge isn't necessary for the effect to occur.

Rather, "Our interpretation is that the plasma is generated by an electromagnetic 'hot spot' that is a purely (microwave) bulk effect," said Bianucci. "The grapes have the right refractive index and size to 'trap' microwaves, and putting two of them close together leads to the generation of this hot spot between them."