0.8768
femtometers
established
estimate of
proton’s radius
0.84184
femtometers
new measure
of proton’s
radius
Smaller proton could be a big deal
subatomic particle may not be as large as theory dictates
By Rachel Ehrenberg
Nothing is immune to downsizing in
tough economic times — not even subatomic particles. New experiments suggest that the proton’s radius is about
4 percent smaller than had been thought.
It could just be a mistake. But if confirmed, the finding could have enormous
implications, scientists say.
“If this result holds up, there’s something drastically wrong,” says physicist
Jeff Flowers of the National Physical
Laboratory in England. “That opens the
door for a major advancement in theory.”
It could be that there’s a problem with
quantum electrodynamics, or QED, a
theory that incorporates Einstein’s special relativity into quantum mechanics
to describe how light and matter interact.
In the ne w work, reported in the July 8
Nature, a team led by Randolf Pohl of
the Max Planck Institute of Quantum
Optics in Garching, Germany, created
an exotic form of hydrogen in which a
muon replaces the atom’s lone electron.
Muons have the same charge as electrons
but are about 200 times heavier, so they
orbit much closer to the proton at the
atom’s center. This coziness enhances
the muon’s interaction with the proton,
so researchers can probe it in more detail
than is possible with ordinary hydrogen.
New laser measurements suggest the
proton is smaller than previously thought.
gas, hoping to bump the muon to a higher
energy level. Measuring the gap between
the muon’s lower and higher energy levels would allow the team to calculate the
proton’s radius.
Yet after years of trying, the team still
wasn’t having any luck. The laser had
been tuned so that it could measure the
proton’s radius if it fell within 0.87 to
0.91 femtometers, in line with QED. But
by tuning the laser to work with a smaller
proton, the team finally saw results. The
new measure of the proton’s radius: just
over 0.84184 femtometers ( 10-15 meters).
“There was no signal till the last three
weeks before the experiment would have
been stopped,” says study coauthor Aldo
Antognini of the Paul Scherrer Institute
in Villigen, Switzerland. “It was like in a
Hollywood movie where everything goes
bad till five minutes before the end.”
The new proton radius puzzles physi-
cists because it is more precise than pre-
vious measurements but well outside
their range. “Presumably somebody
made a mistake,” says Pohl. “But every-
body’s convinced that nobody made a
mistake, so it’s really intriguing.”
Getting control
over turbulence
new model helps predict
fluid flow near boundaries
With just a single measurement, a new
model may deftly describe turbulent fluid
flows near an airplane wing, ship hull or
cloud. If it proves successful, the model
may lead to more efficient airplanes and
more accurate weather forecasts.
Fluid dynamicist Alexander Smits of
Princeton University calls the new model
“a very significant advance” that opens
up a new way of thinking about chaotic,
energy-sapping turbulence.
far beyond a bumpy plane ride. Fluid
flowing past a body — whether air blowing
by a fuselage or water streaming across
Michael Phelps’ swimming suit — contorts and twists as it bounces off an edge
and interferes with incoming flows,
creating chaotic patterns. Airliners
squander up to half of their fuel just overcoming the turbulence within a foot or so
of the aircraft, and turbulent patterns in
the lower 100 meters of the atmosphere
confound weather predictions.
Physicists and engineers have had
a good grip on the basic behaviors of
fluids since the mid-1800s, but have
been baffled by the complexity of flows
near a boundary.
“We don’t really have a handle on
the physics,” says study coauthor Ivan
Marusic of the University of Melbourne
in Australia. “So even though the problem
is over a hundred years old, we still really
haven’t had a major breakthrough.”
In the new study, published July 9 in
Science, the researchers measured forces
in a giant wind tunnel and found a tight
link bet ween small-scale turbulence near
a wall and large, smoother patterns of
airflow farther away. In particular, newly
identified flow patterns called super-
structures had a big effect on turbulence
near the wall. These smooth flow patterns
away from the wall change the turbulence
right next to the wall in predictable ways,
an association that allowed Marusic and
colleagues to write a mathematical for-
mula relating the two.
