However, for those who don’t want to set their kitchen on fire, there is good news. For one, it’s not too hard to satisfy your curiosity on YouTube — but more importantly, after decades of speculation, a team of researchers has finally figured out the physics behind this mind-bending phenomenon, so you don’t have to (please don’t). Your kitchen—and your landlord—will thank you.
Here’s the deal. In most online iterations, an intrepid citizen scientist will cut the grapefruit in half, leaving only a thin, connective bridge of skin, and place the divided fruit on top. After a few seconds, the center of the crushed grape will begin to emit fiery, amorphous little sparks that whiz through your microwave. Voila: DIY plumes of piping hot plasma (as an aside, this is probably the point where the reaction should stop).
This plasma, of course, is not the plasma of blood, but is a state of matter (as in solid, liquid, gas, plasma) that is gas-like, but consists of charged, or ionized, atoms whose electrons have been stripped from their positively charged nuclei. The result is a swarm of subatomic particles that collide and collide with each other, often emitting rapid blobs of light and heat that can resemble molten fire.
Plasma is found naturally in lightning, Earth’s ionosphere, and the Sun’s corona, but can also be generated artificially by exposing gas to extreme temperatures or electromagnetic fields – basically, anything that can impart enough energy to a gas to knock electrons out of its atoms.
So what’s the business of plasma coming out of raw grapes?
This question has troubled physicist Aaron Slepkov of Trent University in Canada for two decades. Slepkov first observed this phenomenon in 1995 while surfing a website called “Fun with Grapes”. But while videos and blog posts of microwavable plasma were abundant, it seemed there was no rigorous, scientific explanation for the physics behind the triviality. All these years later, when Slepkov started his own research group, he and his trainees, including study author Hamza Khattak, decided to test some of the theories. The scorched fruit of their labor is published in the magazine today PNAS.
One myth was immediately busted: the torn grapes were not a necessary component of the fire; In fact, this phenomenon was not grape-specific at all. Sparks kept flying well with whole grapes as well as gooseberries, especially buxom blueberries, and even self-contained pearls of brine—as long as there were two of them, and they were touching.
The key, it seems, is concentrating the energy present in the microwave into a very small spot – the point of contact between the objects in question. In your garden-variety microwave oven, the wavelength of the microwave is approximately 12.5 cm. But nearby grapes (which are filled with water that can absorb said microwaves) can concentrate the energy into an area where the two spheres touch, which is no more than a few millimeters wide. This creates a very strong, very condensed electric field at their interface – a pocket of gunpowder powerful enough to liberate negatively charged electrons from, say, the salts naturally present in grapes and other fruits. And the results are explosive.
However, a single grape cannot do this on its own. In these cases, the energy is concentrated only in the center of the grape. But if a willing dance partner joins in, the “hotspot” in each grape moves toward the other until both are in sync in a blaze of glory.
Ultimately, says study author Pablo Bianucci, a physicist at Concordia University in Canada, microwaving your way to a plasma is actually a very flexible feat, as long as you’re mindful of the size. With microwaves of this wavelength, the diameter of normal grapes is quite ideal. Scaling up anything much larger than a grape – such as a tomato – won’t concentrate the energy into a tight enough space (for this, you’ll need to scale up the wavelength as well). Conversely, reducing the size would prevent the sphere from absorbing enough energy initially.
“This really shows that there is an explanation for everything,” says Lydia Kisley, a physicist and nanoscience expert at Case Western University, who was not involved in the study. “Physics can be used and applied to everyday phenomena. All these principles that were developed with pencil and paper can actually be applied to something you throw in your microwave.”
And beyond piecing together the physics behind parlor tricks, the results could have implications for broader studies on plasma and light, says Julie Bitten, a biophysicist and chemist at the University of Michigan who was not involved in the study. One example is nanophotonics, or the study of light at the nanometer scale – another example in which wavelengths are condensed into extremely small spaces. Nanophotonics can usually only be seen with expensive microscopes. But the grape-microwave combination offers a way to tinker with these phenomena on a large scale with affordable everyday equipment.
Bianucci says that replicating these effects with visible light will require some changes. But it is a logical and exciting next step.
In the meantime, it looks like we finally have some answers to the mysteries behind the rampant outbreak of these particular grapes. However, it’s worth noting that the results didn’t necessarily come easy: The path to publication was littered with casualties – including a series of fruits of varying sizes and a dozen or more microwaves, each given a name to honor their sacrifices in the name of science (among the fallen microwaves were George I, George II, Jesus, Albert, and Thomas). One thing hasn’t changed: plasma is a fickle and dangerous beast that cannot be underestimated.
Even Bianucci is hesitant to try it at home. “I’m waiting until my microwave actually goes off,” he says.
“With plasma in the picture, you have to be careful not to blow a hole in the top of your microwave,” says Khattak. “I mean you can Give it a try, but I wouldn’t recommend it.”
For the record, not even Nova.
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