Hot and cold are descriptive terms based on human definitions. Subjective, and abstract.
Humans would describe 120 degrees as a hot day.
The sun would giggle.
Myrtle Beach would call 35 degrees a bitter cold day.
Canadians would giggle.
"There's cold in the same sense that there's dark. You can't open the door to a room and have the dark spill out. Dark is the absence of light," says Dr. David Goldberg, a Drexel University physicist. "If you leave your door open when the air-conditioning is on, you're not letting the cold out. You're letting the heat in."
The idea that there is no cold is a fundamental building block in physics and thermodynamics. Kids study the concept in school, but it often fades into the haze of all the stuff they have to master to move on and up, when they're not praying for snow days so they can ditch class and homework entirely.
Tulasi Nandan Parashar, a post-doctoral researcher in physics and astronomy at the University of Delaware, explains the concept of heat by starting at the atomic level.
"Everything is made up of atoms – yourself, your desk, your belt, your cellphone," he says. "And atoms vibrate very fast. If they are free to move, they move around very fast. If they are in a solid state, they shake around in that position. There's random shaking of liquids, gasses and movement. It's a form of energy.
"The more shaking we have, the more energy is emitted and we call it hotter, or it has temperature."
Or heat.
There is a limit to that shaking, though: At absolute zero, at which point scientists believe all the motion of atoms ceases.
There is no cold. Only absence of heat