Number 226, May 15, 1995 by Phillip F. Schewe and Ben Stein
MOLECULE INTERFEROMETRY , the splitting and recombining of molecule waves,
has now been performed by David Pritchard and his colleagues at MIT, enabling
new types of measurements on molecules. Like all forms of matter, a molecule
can be thought of as a quantum wave spread out in space rather than as
a particle having a definite location. But a molecule's wave nature is
hard to detect because its relatively large mass gives it a high momentum
even at low velocity. Equivalently, the molecule's "wavelength"
is small, at least 10,000 times smaller than that of visible light. Fortunately
new nanotechniques have permitted the building of an obstacle course for
waves on a size scale that allows one to do for molecule waves what one
normally does only for light waves, such as measuring an index of refraction.
For example, with their interferometer (employing tiny nanofabricated gratings),
the MIT researchers have conducted refraction measurements of diatomic
sodium molecule waves passing through gas samples. The interferometer splits
the molecule wave (wavelength 0.11 angstroms) into separate wavelets. Some
of the molecule wave traverses a sample of neon gas and later recombines
with another part of the wave to form an interference pattern which yields
the index of refraction. The measurements suggest that the neon exerts
a relatively large long-range force on the sodium molecule. (M.S. Chapman
et al, 12 June 95 article in Physical Review Letters; Pritchard's work
on atomic interferometry (Update 209) appeared in the 13 February 1995
Physical Review Letters.)
IS SPACE DISCONTINUOUS? Several theories attempt to combine quantum
mechanics, which reigns supreme in the microcosmic world of atoms, and
gravity, which governs the macrocosmos of planets and galaxies. One quantum
gravity theory, the superstring model, holds that all substance in the
universe consists of the ceaseless quantum interactions of tiny strings.
In this theory nothing is said of the background space in which the strings
move. In another theory, one introduced by Oxford scientist Roger Penrose,
space itself is quantized into discrete volumes (each consisting of a spinning
loop) with a characteristic size about the same as that for superstrings,
10**-35 m. Although these theories do not make predictions that will be
tested in the lab anytime soon (the energy needed to explore so small a
size domain is greater than accelerators can muster) theoretical progress
continues. For example, Lee Smolin of Penn State and Carlo Rovelli of the
University of Pittsburgh have extended the work of Penrose and others.
Without making any assumptions about the nature of space, they discover
that space is indeed lumpy. Furthermore, they are able to calculate the
range, or "spectrum," of allowable lump sizes. In other words,
just as quantum mechanics obliges atoms to exist in only certain energy
states, so the combination of quantum mechanics and gravity (at least in
this particular theory) results in space itself being quantized. (Nuclear
Physics B, 29 May 1995.)
HELIUM WAS FIRST DISCOVERED 100 YEARS AGO . Most helium in the universe
is cosmic (having been formed in the aftermath of the big bang) but terrestrial
helium is mostly a byproduct of radioactive decays from the Earth's interior,
whence it percolates into natural gas pockets. Once allowed to escape into
the atmosphere, helium is now captured and used in commercial applications
(e.g., as a coolant for superconductors). Helium usage has increased in
the past few years by as much as 5 to 10% per year. (New Scientist, 8 April.)
|
