The Alan Sondheim Mail Archive

July 6, 2005

---------- Forwarded message ----------
Date: Wed, 6 Jul 2005 13:59:13 -0400
To: sondheim@PANIX.COM
Subject: Physics News Update 736

The American Institute of Physics Bulletin of Physics News
Number 736   July 6, 2005  by Phillip F. Schewe, Ben Stein

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 falloff 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 (, 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,

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
( 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)

PHYSICS NEWS UPDATE is a digest of physics news items arising
from physics meetings, physics journals, newspapers and
magazines, and other news sources.  It is provided free of charge
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Physics News Update appears approximately once a week.

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---------- Forwarded message ----------
Date: Wed, 6 Jul 2005 16:48:26 -0700 (PDT)
From: Gerald Jones <>
To: Projectory <>
Subject: fast forward from 1720 to the KRONOS PROJECTOR 2005

   *****  now Fast Forward -- from 1720
              to The Khronos Projector 2005

The following website offers demos of a new technology
to be shown at the upcoming SIGGRAPH in LA at the
beginning of Aug. The Khronos Projector, being
developed in Japan, by Alvaro Cassinelli & Masatoshi
Ishikawa is an interactive display technology where
the audience, by touching the projection screen, can
send parts of the image forward or backwards in time.
They also have video clips posted showing it in

as ever,


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