Text 521, 89 rader
Skriven 2005-07-07 07:47:15 av Herman Trivilino (1:106/2000.7)
Ärende: PNU 736
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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@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@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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 * Origin: Big Bang (1:106/2000.7)