Number 732, May 24, 2005
by Phil Schewe and Ben Stein
The First Direct Measurement of Recoil Momentum
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. (Campbellet al., Physical Review Letters, 6 May 2005)
Water's Chemical Formula May Always Be H20
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)
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