When something is cooled to absolute zero (Kelvin), do the electrons and other sub-atomic particles stop moving? Or does "absolute zero" only mean that movement stops at the molecular level (as opposed to the sub-atomic level)? Peter, Someplace, World

I've heard that at absolute zero molecular motion stops. But what happens to electrons, do they also stop? If they do, what prevents them from falling into the nucleus? William, Austin, Texas, USA

Absolute zero is zero degrees on the Kelvin thermometer scale; it corresponds to about -460 degrees Fahrenheit and -273 degrees Celsius.

Even space isn't that cold. The lingering afterglow of the big bang heats space to 3 degrees Kelvin, on average - some colder pockets exist. The Boomerang Nebula (at 1 degree K, 5000 light years away) is the coldest known natural spot in the universe.

We have artificially lowered the temperature of atoms on Earth to almost absolute zero. Atoms near absolute zero slow by orders of magnitude from their normal room-temperature speed. At room temperature, air molecules zip around at about 1800 kilometres an hour. At about 10 micro degrees Kelvin, Rubidium atoms move at only about 0.18 kilometres an hour - slower than a three-toed sloth, says physicist Luis Orozco of the University of Maryland.

But matter cannot reach absolute zero, because of the quantum nature of particles. This has to do with Heisenberg's uncertainty principle (we can never know exactly both a particle's speed and position; in fact, the more precisely we know its speed, the less precisely we know its position).

If an atom could reach absolute zero, its temperature would be precisely zero, which implies an exact speed of zero. But knowing the atom's speed exactly, means we know nothing at all about its position.

"There really is no physical description that allows for [an atom at]zero temperature" e-mails physicist Erik Ramberg of Fermilab. If an atom could attain absolute zero, its wave function would extend "across the universe," which means the atom is located nowhere. But that's an impossibility. When we try to probe the atom or electron to localize it, then we give it some velocity, and thus a non-zero temperature.

By the way, we can think of an atom either as a particle (a little billiard ball) or as a wave. As atoms come close to absolute zero, their waveforms spread out. A waveform as big as the universe may seem weird, but various research groups have cooled atoms to where their wave functions are as big as the inter-atomic distance. When that happens, all of the atoms at that temperature form one big "super-atom," says Mr. Ramberg. This is called a Bose-Einstein condensate.

In 2000, the Helsinki University of Technology lab in Finland, lowered the temperature of a few atoms even farther than the researchers in 1995 - to the coldest temperature yet reached - 0.0001 micro degrees K. But the atoms continued to vibrate.

Near absolute zero, electrons "continue to whiz around" inside atoms, says quantum physicist Christopher Foot of the University of Oxford. Moreover, even at absolute zero, atoms would not be completely stationary. They would "jiggle about," but would not have enough energy to change state. In musical terms, it's as if the atom cannot go from middle C to high C. It still vibrates, but cannot change its wave pattern. It's energy is at a minimum.

Further Reading: Non-quantum explanation of the unattainability of absolute zero by Christopher Foot, University of Oxford Ultracold atoms and absolute zero

NOVA Ultra-cold quantum matter group , Christopher Foot, University of Oxford

Bose-Einstein condensation - what is it? , University of Colorado at Boulder

What is absolute zero? Michigan State University

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