Why A Grape Turns Into A Fireball In The Microwave

The web is full of videos of thoughtful people lighting things on fire. Here’s an all-time favorite: Cleave a grape in half, leaving a little skin connecting the two hemispheres. Blitz it in the microwave for five seconds. For one glorious moment, the grape halves will produce a fireball unfit for the confines of your home.

It’s something crowds will love, as long as you avoid melting your kitchen appliances. But it turns out, even after millions of YouTube views and probably tens thousands of seared microwaves, no one knew exactly why the fireball forms. Viral online explanations usually say that the grape halves act like an antenna, and they somehow direct microwaves onto the small patch of skin to ignite the first spark. But nobody had actually done the math to prove it. After several years of microwaving grape-shaped objects and simulating the microwaving of those objects, a trio of physicists in Canada may have finally figured it all out.

The physicists note that people have let themselves be sidetracked by the fireball: It’s “exciting and memorable” but “of secondary interest,” they write in a paper that appears currently in the Proceedings of the National Academy of Sciences. The fireball is merely a beautiful, hot blob of loose electrons and ions known as a plasma. The most fascinating science is actually in the steps leading up to the plasma, they say. The real question is how the grape got warm enough to produce the plasma in the first place.

Their conclusions? The grape is less like of an antenna and more similar to a trombone, though for microwaves instead of musical notes. When you play a trombone, you push vibrating air into it. The trombone will only maintain vibrations of a particular wavelength—the musical note you hear—depending on where your slide is positioned. Only certain wavelengths, known as standing waves, fit perfectly inside the trombone. As vibrating air of various wavelengths enter the trombone, the standing waves add constructively, while other wavelengths cancel each other out. In other words, the trombone amplifies the standing waves and mutes all others.

The grape, actually, is the ideal size for amplifying the microwaves that your kitchen machine puts off. The appliance sends microwaves into the two grape halves, where the waves bounce around and add up to focus the energy to a spot on the skin. Both grape halves happen to focus the energy to the same tiny point and that intense energy jostles the atoms and molecules at that spot, heating them up so much that they can no longer latch on to their electrons.

They also discovered that the shape of the two grape halves and the little bridge of skin connecting them is actually not important. You can make the fireball by microwaving two whole grapes sitting next to each other, for example. And it doesn’t even need to be a grape. “Anything grape-sized will work, if it’s watery enough,” says physicist Hamza Khattak, a graduate student at McMaster University who did a good bit of the microwaving. They demonstrated this point by making fireballs out of microwaved gooseberries, large blackberries, and even quail eggs.

Still, the case isn’t entirely closed. During all their microwaving, they observed that two grapes placed side by side repeatedly bump into each other, back and forth. They don’t know why that happens, and they’ll be studying that in the coming weeks, says Bianucci.

For those studies, they’ll probably stick with the plastic grape replicas, rather than the actual fruit. “That’s because they don’t go bad,” says Khattak. Maybe they could try raisins.

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