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{at}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.

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