H.H. posted this to the vort list on ZPE motion in a BCE;
PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 433 June 15, 1999 by Phillip F. Schewe and Ben Stein
ZERO-POINT MOTION IN A BOSE-EINSTEIN CONDENSATE
has been quantitatively measured for the first time, allowing
researchers, in effect, to study matter at a temperature of absolute
zero.
According to quantum mechanics, objects cooled to absolute zero do not
freeze to a complete standstill; instead they jiggle around by some
minimum amount.
MIT researchers (Wolfgang Ketterle, 617-253-6815) measured such
"zero-point motion" in a sodium BEC, a collection of gas atoms that are
collectively in the lowest possible energy state (Update 233).
According to Ketterle, "the condensate has no entropy and behaves like
matter at absolute zero." The MIT physicists measured the motion (or
lack thereof) by taking advantage of the fact that atoms absorb light at
slightly lower (higher) frequencies if they are moving away from
(towards) the light.
To determine these Doppler shifts (100 billion times smaller than those
of moving galaxies), the researchers used a technique known as Bragg
scattering. In this technique, atoms absorb photons at one energy from
a laser beam and are stimulated by a second laser to emit a photon at
another energy which can be shifted upward or downward depending on the
atoms' motion towards or away from the lasers.
Measuring the range in energies of the emitted photons allowed the
researchers to determine the range of momentum values in the condensate.
Multiplying this measured momentum spread (delta p) by the size of the
condensate (delta x) gave an answer of approximately h-bar (Planck's
constant divided by 2 pi)--the minimum value allowed by Heisenberg's
uncertainty relation and quantum physics.
While earlier BECs surely harvested this zero-point motion, previous
measurements of BEC momentum spreads were done with exploding
condensates having energies hundreds of times larger than the zero-point
energy. (J. Stenger et al., Physical Review Letters, 7 June 1999.)
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