Text 456, 85 rader
Skriven 2005-05-24 14:52:32 av Herman Trivilino (1:106/2000.7)
Ärende: PNU 732
===============
PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 732 May 24, 2005
by Phillip F. Schewe, Ben Stein
                                                                
THE FIRST DIRECT MEASUREMENT OF RECOIL MOMENTUM for single atoms struck by
light in an absorptive medium has been made by Gretchen Campbell, Dave
Pritchard, Wolfgang Ketterle and their colleagues at MIT.  Parcels of light,
photons, do not possess mass, but a beam of light does carry momentum.  In
general, when light strikes a mirror, the mirror will recoil ever so slightly,
and this recoil has previously been measured.  But what about a single photon
striking a single atom in a dilute gas?  The momentum of a photon equals
h/lambda, where h is Planck's constant and lambda is the wavelength of the
light in vacuum.  In a dispersive medium, a medium which can scatter or absorb
light, the index of refraction for the medium, n, comes into play: an object
absorbing the photon will recoil with a momentum equal to nh/lambda.  This is
what has been measured for the first time on an atomic basis.  The MIT team
used laser beams sent into a dilute gas; a beat note between recoiling atoms
and atoms at rest provided the momentum measurement of selected atoms.  The
fact that the recoil momentum should actually be proportional to the index of
refraction came as something of a surprise to the experimenters.  You might
expect that in isolated encounters, when an individual atom absorbs a single
photon, that the recoil of the atom should not depend on n.  That's because the
atoms in the sample---in this case a Bose-Einstein condensate of Rb atoms---is
extremely dilute, so dilute that each atom essentially resides in a vacuum. 
Nevertheless, the interaction of the light with all the atoms has to be taken
into account, even if the specific interaction being measured, in effect, is
that of  single atoms.  The atoms "sense" the presence of the others and act
collectively, and the extra factor, the index of refraction, is applicable
after all.  At several colloquia before audiences of physicists, Ketterle has
put the question: will the recoil be h/lambda or nh/lambda?  Generally the
opinion among these experts divides about 50/50.  So, on this basic question of
light traveling a medium, a physicist's intuition can be wrong, at least in
half the cases.  Ketterle believes that this new insight about what happens
when light penetrates a dispersive medium provides an important correction for
high-precision measurements using cold atoms.  (Campbell et al., Physical
Review Letters, 6 May 2005)

WATER'S CHEMICAL FORMULA MAY ALWAYS BE H2O, and not different on shorter
timescales, according to a new paper.  In earlier experiments, a research group
reported that neutrons and electrons interacting with room-temperature water
molecules for very brief times (0.1-1 femtoseconds) saw a ratio of hydrogen to
oxygen of roughly 1.5 to 1, suggesting a chemical formula of H1.5O for water at
short timescales (Update 648).  According to the data analysis of those
researchers, incoming neutrons scattered from at least 25% fewer hydrogen
nuclei (protons) than expected.  They proposed that quantum entanglement
between protons (hydrogen nuclei) on a sub-femtosecond timescale was causing
this anomalous scattering.
This result stimulated a flurry of theoretical and experimental activity,
including a new experiment at Rensselaer Polytechnic Institute in Upstate New
York that now disputes these earlier results.  The experimenters, coming from
Ben Gurion University and RPI (Raymond Moreh, morehr@rpi.edu), use
higher-energy neutrons which interact with pure liquid water, pure D2O, and
mixtures of the two liquids, on shorter timescales (0.001-0.01 femtoseconds)
than in the earlier experiments.  (Theorists had predicted that the shorter
timescales would lead to an even more pronounced scattering anomaly, since
quantum decoherence would have less time to spoil the proposed entanglement
between protons.)   However, the Ben Gurion-RPI team
did not detect an anomalous dropoff in n-p scattering.  They conclude that no
entanglement takes hold and water is accurately described as H2O, after all, at
these shorter timescales.  They cite several advantages of their experiment,
including the following: they looked at a single, simpler scattering signal
arising from the three nuclei of the water and D2O molecules (as opposed to the
separate neutron scattering signals for oxygen, hydrogen, and deuterium in the
earlier experiments); and their data did not require complicated processing,
leading to a much simpler data analysis than was necessary in the previous
work. Researchers from the earlier experiments contend that the new experiment
does not probe the timescales that they originally explored; the new team
counters that their data does address the original team's timescales. In
addition, Moreh and colleagues argue that one would have to shake many well
established notions in physics to explain the suggested scattering anomaly. 
(Moreh, Block, Danon, Neumann, Physical Review Letters, 13 May 2005.

PHYSICS NEWS UPDATE presently goes into summer recess for three weeks.

***********
PHYSICS NEWS UPDATE is a digest of physics news items arising from physics
meetings, physics journals, newspapers and magazines, and other news sources. 
It is provided free of charge as a way of broadly disseminating information
about physics and physicists. For that reason, you are free to post it, if you
like, where others can read it, providing only that you credit AIP.Physics News
Update appears approximately once a week.

---
 * Origin: Big Bang (1:106/2000.7)