What happens at absolute zero?
The cold, hard truth of life at -460 degrees
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 the average. 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 lowered temperatures of atoms artificially 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 1,100 mph
(1800 km/hr). At about 10 micro degrees Kelvin, Rubidium atoms move at
about 0.11 mph (0.18 km/hr) — 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. Why this is so 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" emails 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 cool to near absolute zero, their waveforms do, indeed, spread out.
A waveform as big as the universe seems "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
1995, researchers from JILA reported cooling rubidium atoms to less than 0.17
micro degrees K. Reaching almost absolute zero caused the individual atoms to
condense into a "superatom" behaving as a single entity. The graphic shows
three-dimensional successive snapshots in time in which the atoms cooled and condensed.
The false colors appearing white and light blue
show atoms at lowest velocities and coldest temperatures. Image courtesy of NIST and the University of
Colorado at Boulder.
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, since they were not at (the impossible) absolute zero.
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
(Answered April 13, 2008)
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