Why Doesn't a "Reverse Microwave" for Cooling Food Exist?

Most kids are full of questions: Why is the sky blue? Why do we have eyebrows? Why can't we feel the planet spinning? Your grownup curiosities probably became more advanced, but if you're like us, a few of those questions from childhood never got answered. Like this one: Why isn't there a "reverse" microwave for cooling food? The answer is more complicated than you might think.

Sizzling Hot Entropy

It's easy to heat food in a microwave. When you pop in a bag of frozen veggies and press the start button, the microwave sends a specific frequency of radio waves into the food to excite the water molecules within. The activity, or energy level, of a group of molecules is essentially a measure of its heat: the more excited the molecules, the hotter they are.

But there's no frequency of radio waves that can calm molecules down to make them colder. Radio waves are a type of electromagnetic radiation: an umbrella term that includes visible light, infrared, and X-rays, all of which are a form of energy. Energy excites molecules, and excited molecules are hotter.

But excited molecules aren't just hotter — they're also in a state of higher entropy. Entropy is basically a scientific measurement of disorder, and according to that old chestnut known as the second law of thermodynamics, any process in a closed system progresses toward increasing disorder. That's why it's so much easier to heat food up than it is to cool it down: You can't reduce entropy, and cold things are at a state of lower entropy than hot things.

When you put a lukewarm ice tray in your freezer, for instance, heat flows from the water to the colder air of the freezer. That may sound like decreasing entropy, but not if you take the entire fridge into account: It's using a ton of energy to take heat out of the things inside and transfer it into the surrounding air (feel how warm the back of your fridge is!). That's an overall increase in entropy. Meanwhile, the cold air is a poor conductor, meaning it doesn't do a very good job at removing heat from the water. That's why you have to wait for hours before you have solid ice cubes.

I'll Get You Next Time, Physics!

There are certain materials that can cool quickly, but they don't lend themselves to eating. A gas cools by expansion, which is why a freshly sprayed aerosol can feels so cold. But gas isn't all that filling, compared to solids or liquids. (Our all-helium diet is failing us, but we sound hilarious.)

There are also other cooling methods: Conduction can make heat flow from food onto some colder surface, like oysters served on ice, and convection transfers heat from one place to another in a fluid, which is how you can thaw frozen meat under running water.

But barring some dangerously cold substance like liquid nitrogen, nothing can instantly cool food the way a microwave can instantly heat it. When it comes to popsicles freezing and beer chilling, you'll just have to wait. (Though there are shortcuts to cooling your beer ...)

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For more answers to deceptively complex science questions, check out "Ask a Science Teacher: 250 Answers to Questions You've Always Had About How Everyday Stuff Really Works" by Larry Scheckel. We handpick reading recommendations we think you may like. If you choose to make a purchase, Curiosity will get a share of the sale.

Written by Ashley Hamer July 19, 2017

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