Re: one billionth degree above absolute zero, how did they measure it?
- From: Andy Resnick <andy.resnick@xxxxxxxxxxx>
- Date: Mon, 16 Apr 2007 09:08:57 -0500
John Smith wrote:
The lowest temperature achieved in the lab is one billionth degree
above absolute zero. How is this temperature measured and how
precise is the method? Any references would be greately appreciated.
That's a good question- and it shows that a common-sense idea about
'temperature' can't be extrapolated down to these values.
First, recall that thermodynamics and statistical mechanics has a
fundamental relationship between energy and temperature, encoded in
terms like exp(E/kT). One result is that temperature is actually a
logarithmic scale- getting to a temperature of 0 K requires infinite
energy. Also, the precision of the measurement isn't as amazing as it
may seem.
Another result is that 'temperature' has less to do with how hot or cold
something is, but how the energy in a collection of objects is
distributed. For example, take 100 atoms, each with 1 femto-erg of
energy. That's our common sense idea of atoms at about 7 K. Now, let's
elastically collide the atoms together somehow so that one atoms ends up
with nearly all of the 100 femto-ergs of energy, while the others have
nearly no energy. Then remove that single atom. The remaining atoms are
much 'colder', meaning they have much less energy.
I couldn't easily find any good references, but atom trapping
experiments, which are presumably what you read about, assign
temperature by measuring energy, specifically energy shifts by collision
with carefully tuned 'cooling' lasers. And these temperatures are of
very small numbers of atoms, not something macroscopic. The experiments
are also done in a vacuum, so air atoms at 300 K don't collide with and
heat up the cold atoms.
There's a lot on low-temp physics and methods on the web: magnetic
cooling, spin cooling, laser cooling, etc. that have nothing to do with
putting a sample in thermal equilibrium with, for example, liquid helium.
--
Andrew Resnick, Ph.D.
Department of Physiology and Biophysics
Case Western Reserve University
.
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- From: John Smith
- one billionth degree above absolute zero, how did they measure it?
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