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.)
ACOUSTIC-DEPENDENT FRICTION. Studies of friction are
often carried out at modest relative speeds: the two moving
surfaces in question typically slide past each other at 1 cm/s.
However, researchers at UCLA (Anders Johansen, 310-825-2863)
wondered if new mechanisms might appear when surfaces slide
against each other at higher velocities, such as those associated
with friction between tectonic plates during earthquakes.
Observing the jerky "stick-slip" motion of a steel block riding on a
rotating steel table, the researchers carefully measured the friction
forces for relative velocities up to 0.35 m/s, by monitoring the
expansion and compression in a spring attached to the steel block.
At these high velocities, they noted that the significantly increased
production of sound waves (largely neglected in past analyses)
dissipates a large amount of energy, stealing away some of the
energy of motion required for two surfaces to slide past each other
and thereby amounting to an increase in friction. This suggests
that the generation of sound waves between two sliding fault
surfaces during an earthquake may provide a significant feedback
mechanism that mitigates a quake's effects, by converting energy
of motion (friction which might otherwise have caused fracturing
in the Earth) into sound energy. (Johansen and Sornette, Physical
Review Letters, 21 June 1999.)