What's the science behind Gru's freeze ray in Despicable Me? Is such a device possible? Ask Science investigates.
Water, Water Everywhere
Humidity is the measurement of how much water vapor there is in the air. Most of the time when you hear about humidity in a weather report, the meteorologist is referring to relative humidity. Relative humidity tells you how much water vapor is in the air compared to how much water vapor could be in the air. So when you hear on the news that the humidity is 90%, that doesn't mean that 90% of the air is water vapor. It means that water vapor has filled up 90% of the space that water vapor could take up in the air. Just how much water vapor the air can hold depends on the temperature. The hotter it is, the more water vapor it can hold.
If our freeze ray does in fact operate by freezing the water vapor around a person, then it would have the notable drawback of only working well on days with high relative humidity.
If You Can't Take The Heat...
So how exactly would you go about freezing the water vapor around someone? While many of us think of freezing as an excess of cold, it is better to think of it as a lack of heat. There is no such thing as cold energy. If we want to freeze something, we want to lower its temperature by removing heat energy.
In your freezer, for example, a cold refrigerant is passed through tubing inside the freezer. Heat energy leaves the food and enters the colder refrigerant. Since the food is losing heat energy, it cools down.
So one idea is that the freeze ray is shooting some kind of super-cold refrigerant around the person, which causes heat energy to pass from the water vapor into the refrigerant, resulting in the water vapor freezing. Unfortunately this super-cold refrigerant would likely also cause harm to the target.
So despite extensive scientific analysis, I was unable to find a way to turn the freeze ray depicted in the film into reality. This is perhaps why nobody has yet invented a portable freeze ray (as of the time of this writing).
See also: Do the Minions Speak a Real Language?
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