Physics News Update -- Number 732, May 24, 2005 by Phil Schewe and Ben Stein
- From: Sam Wormley <swormley1@xxxxxxxxx>
- Date: Fri, 27 May 2005 14:06:17 GMT
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. (Campbell et 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@xxxxxxx), 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)
.
- Prev by Date: Re: QFT question
- Next by Date: Re: FTL Question
- Previous by thread: Dimensional Aspects of Rare, Fairly Frequent, Very Frequent Events
- Next by thread: Foundations of statistical physics
- Index(es):
Relevant Pages
|
