# Absolute Zero

Even when temperatures are below freezing, molecules in motion stay in motion. But at a certain temperature, all motion stops. A friend asks, “How close to absolute zero can scientists get?”

Absolute zero is the term for the lowest possible temperature –the temperature at which all motion within atoms stops. Temperature is a measure of heat flow, and you can’t have less than none — which is how much you’ve got, at absolute zero. Absolute zero is zero degrees on the Kelvin scale — that’s about minus 273 degrees Celsius, and minus 460 degrees Fahrenheit.

Scientists can cool things down with a process called “laser cooling” — which works by bouncing photons, or particles of light, off atoms. If you direct the photons against the motion of the atoms, the atoms will slow down. It’s like throwing ping-pong balls at an oncoming bowling ball. If you throw enough ping-pong balls, you can stop the bowling ball.

Or, to cool things down, scientists use what’s called “evaporative cooling.” This process works like the cooling of a cup of coffee. The hottest molecules escape, which leaves the rest with less energy. Scientists trap a gas of atoms and use magnets to remove the hottest atoms. But they still can’t get down to absolute zero — only to a few tens of billionths of a degree above it.

At temperatures a few tens of billionths of a degree above absolute zero, Bose Einstein condensation occurs. It’s a transition from one state of matter to another — something like water freezing into ice — except that it’s quantum mechanical in nature.

Dr. Hulet adds: We work at extremely low temperatures in order to enhance the quantum mechanical wave nature. We study phase transition phenomena, such as superconductivity or superfluity that occur when the quantum mechanical wavelength becomes larger than the average separation of particles. The wavelength of a particle is inversely related to its speed, which in turn is related to its temperature. These experiments help to better understand quantum mechanics and how quantum mechanics manifests itself in larger systems than just single particles, perhaps even macroscopic systems.

Dr. Randy Hulet Department of Physics Rice University Houston, TX

If you enjoyed this, you may be interested in the following website: Atom Cool! (Rice University)