Say you are trying to cool a banana to absolute zero. To get rid of all of the energy in the banana, you’ve got to stop its atoms from moving around; you have to put it in a box and cool it down. However, the box the banana is in is made of atoms, too. The box’s atoms are wiggling around, and they will bump the banana’s atoms and set them in motion again. Even if you get the banana to float in a perfect vacuum in the center of the box, you can’t stop the wiggling entirely, because dancing particles give off light. Light is constantly coming off of the box and striking the banana, getting the
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