PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 736   July 6, 2005
by Phillip F. Schewe, Ben Stein
                        
A COULOMB EXPERIMENT FOR THE WEAK NUCLEAR FORCE.  Physicists at the SLAC
accelerator have measured, with much greater precision than ever before,
the variation in the weak nuclear force, one of the four known physical
forces, over an enormous size scale (a distance of more than ten proton
diameters) for so feeble a force.  Although the results were not surprising
(the weak force diminished with distance as expected) this new quantitative
study of the weak force helps to cement physicists' view of the sub-nuclear
world.   The SLAC work is, in effect, a 21st century analog of the landmark
18th experiments in which the intrinsic strength of the electromagnetic and
gravitational forces were measured (by Charles Coulomb and Henry Cavendish,
respectively) through careful observation of test objects causing a torsion
balance to swing around.
The weak force, in the modern way of thinking, is a cousin of the
electromagnetic (EM) force; both of them are considered as different
aspects of a single "electroweak" force.  The EM force is much
better known to physicists and to non-experts: it's responsible for all
electric, magnetic, and optical phenomena, and keeps atoms intact and holds
atoms together in all the molecular and crystalline forms which make up our
world.  Over sizes larger than the atom, the strength of the EM force is
prescribed by Coulomb's law, which states that the force between two
charged objects (say, two electrons)  is proportional to the charges of the
electrons and inversely to the square of the distance between them.  For
sub-atomic distances the Coulomb way of describing electron scattering gets
complicated because of vacuum polarization, a process which takes into
account the fact that at short distances an electron can longer be
portrayed as a lone pointlike particle; instead we must view it as
accompanied by a cloud of virtual particles sprouting out of the vacuum. 
These extra short-lived particles serve to redefine, or
"renormalize," the effective electron charge and along with it
the very nature of the EM force mediating the interaction with the other
electron.
    The weak force is an important force---responsible for some kinds of
radioactivity and for select fusion reactions vital to energy production
inside the sun---but is very different from the electromagnetic force and
generally operates only over the tight confines of the nucleus.  In this
realm, the weak force is right there along with the EM force, a
doppelganger that can often be ignored because it is so very weak.  But
physicists, in search of a fuller explanation of the universe, don't want
to ignore the weak force.  At SLAC they painstakingly extract weak effects
from the much larger EM effects involved when two electrons interact.  In
the case of their present experiment (E158), a powerful electron beam
scatters from electrons bound to hydrogen atoms in a stationary target.  By
using electrons that have been spin polarized---that is, the electron's
internal magnetism (or spin) has been oriented in a certain direction---the
weak force can be studied by looking for subtle asymmetries in the way
electrons with differing polarizations scatter from each other.
    One expects an intrinsic fall off in the weak force with the distance
between the electrons.  It should also fall off owing to the great mass
that the Z boson, unlike its EM counterpart, the massless photon. Finally,
the weak force weakens because the electron's "weak charge"
becomes increasingly shielded (just as the electron's electrical charge had
been) owing to a polarization of the vacuum---but this time with virtual
quarks, electrons, and W and Z bosons needing to be taken into account.  
Previously, the weak
charge has been well measured only at a fixed  distance scale, a small
fraction of the proton's diameter.  The SLAC result over longer distances
confirms the expected falloff.  According to E158 researcher Yury
Kolomensky (yury{at}physics.berkeley.edu), the result is precise enough to
rule out certain theories that  invoke new types of interactions, at least
at the energy scale of this experiment.  (Anthony et al., Physical Review
Letters, upcoming article; lab website,
http://www-project.slac.stanford.edu/e158)

WHY IS THE SKY BLUE, AND NOT VIOLET?  The hues that we see in the sky are
not only determined by the laws of physics, but are also colored by the
human visual system, shows a new paper in the American Journal of Physics. 
On a clear day when the sun is well above the horizon, the analysis
demonstrates, we perceive the complex spectrum of colors in the sky as a
mixture of white light and pure blue.  When sunlight enters the earth's
atmosphere, it scatters (ricochets) mainly from oxygen and nitrogen
molecules that make up most of our air.  What scatters the most is the
light with the shortest wavelengths, towards the blue end of the spectrum,
so more of that light will reach our eyes than other colors.  But according
to the 19th-century physics equations introduced by Lord Rayleigh, as well
as actual measurements, our eyes get hit with peak amounts of energy in
violet as well as blue.  So what is happening?
    Combining physics with quantitative data on the responsiveness of the
human visual system, Glenn Smith of Georgia Tech
(glenn.smith{at}ece.gatech.edu) points to the way in which our eye's three
different types of cones detect color.  As Smith shows, the sky's complex
multichromatic rainbow of colors tickles our eye's cones in the same way as
does a specific mixture of pure blue and white light.  This is similar to
how the human visual system will perceive the right mixture of pure red and
pure green as being equivalent to pure yellow.  The cones that allow us to
see color cannot identify the actual wavelengths that hit them, but if they
are stimulated by the right combination of wavelengths, then it will appear
the same to our eyes as a single pure color, or a mixture of a pure color
and white light.  (Smith, American Journal of Physics, July 2005)

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