Number 568, December 7, 2001
by Phil Schewe, James Riordon, and Ben Stein
Ultrasound Scans are Audible to a Fetus
Ultrasound scans are audible to a fetus, researchers reported at this
week's meeting of the Acoustical Society
of America in Fort Lauderdale, Florida.
Ultrasound by definition is sound that lies beyond the range of human
hearing. So how can a fetus hear an ultrasound scan? As explained by
the researchers (Mostafa Fatemi, Mayo Foundation, Minnesota, fatemi.mostafa@mayo.edu),
traditional imaging systems produce ultrasound as sequences of short-duration,
high-energy bursts, called "pulse trains."
When the pulses enter the body, they tap internal organs at a regular
rate. When the ultrasound points at the head of the fetus, its sensitive
hearing structure gets vibrated at a rate equal to the number of pulses
per second. (Typically, several thousand pulses are transmitted per
second in a pulse train, a rate equal to several thousand Hertz.)
The fetus senses these vibrations as tones, equivalent to the high
notes of a piano. The sound can get loud--about the equivalent of 100-120
decibels of airborne sound, or the level of sound of an approaching
subway train.
Rather than being akin to a sound from the outside world, though, the
sensation is more like what you hear when your finger taps a spot close
to an ear--which is why it's inaudible to others, including the mother.
What's more, the sound is focused on a tiny, square-millimeter spot, and the
sound diminishes rapidly from that spot.
Fatemi stresses that their findings do not suggest that this sound
is harmful to a fetus. These studies can help explain physicians' observations
that a fetus moves vigorously when ultrasound is directed at its head.
They eliminate the notion that ultrasound is a passive observation technique,
but they may also inspire new ultrasound exams for testing normal fetal
function. (Paper
1pBB6 at meeting.)
Tracking DNA Motion with Picometer Accuracy
Scientists don't have to settle for averaged results when studying
tiny things with x rays. In x-ray diffraction, for example, a crystallized
sample with billions of molecules scatters the x rays into a characteristic
pattern of spots on a detector which is then decoded to yield lattice
structure information.
A team of Japanese scientists has developed a method, which they call
diffracted x-ray tracking (DXT), in which the bobbing Brownian motion
of single nanocrystallites in water are watched by tracking scattered
x rays; with this method one acquires information not about the position
but the rotary motion of single nanoparticles (Sasaki
et al., Physical Review E, September 2000).
Now the process has been extended to single DNA molecules, whose Brownian
motion can be tracked, for the first time, with a precision of picometers,
or 10-15 m (see figure).
The researchers will soon broaden their measurements of important biomolecules.
For example, they hope to observe the structural changes accompanying
the activation of ion channels in living cells. (Sasaki
et al., Physical Review Letters, 10 December 2001;
contact Yuji Sasaki, Japan Synchrotron Radiation Research Institute,
ycsasaki@spring8.or.jp, 81-791-58-0831)
Breaking a Quantum Symmetry on the Tabletop
A recurrent theme in art and science, the concept of symmetry has become
a powerful scientific tool for the analysis of physical systems. However,
under special circumstances, a "quantum anomaly" occurs: the
laws of quantum physics break a system's apparent symmetry.
After a long search, a research group (Horacio Camblong, University
of San Francisco, camblong@usfca.edu, and collaborators at Universidad
Nacional de La Plata, Argentina) has found a relatively simple example
of a quantum anomaly: the interaction of a polar molecule with an electron.
A polar molecule, despite being neutral, has a permanent separation
of electric charge--a dipole. This dipole produces an electric field,
which can capture electrons if it is strong enough.
Can such an arrangement exist as a stable ion, with its "extra"
electron? The researchers formulated the answer to this question in
the language of symmetry. In physics, symmetry means that a system,
such as the molecule-electron arrangement, behaves the same after you
perform a change to it, such as stretching the molecule to larger scales
and making appropriate adjustments to other variables in the system.
At first glance, the electron-molecule interaction exhibits a remarkable
scale invariance: the system "looks" the same when viewed
from different scales in space and time--at least in a classical physics
description which treats the molecule as a dipole and the electron as
a point of charge.
But this tidy picture breaks down with a proper treatment of the system,
as prescribed by quantum field theory. A quantum field theory treatment
requires the process of renormalization, which removes certain mathematical
infinities and inconsistencies from the quantum approach. This process
also makes the molecule's energy levels discrete or quantized rather
than continuous.
Examining the system this way, the researchers found that the scale
invariance broke down. In fact, a large body of existing evidence, both
experimental and numerical, supports their conclusion. While all other
known quantum anomalies occur at high energies (an example is chiral
symmetry in nuclear physics), the work suggests that quantum symmetry
breaking can occur at much lower energies, in the domain of interacting
electrons and molecules. (Camblong
et al., Physical Review Letters, 26 November 2001;
for a discussion of symmetry breaking in physics, with examples, see
paper by Barry Holstein
of University of Massachusetts at Amherst.)
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