Re: Attraction vs repulsion - why does it depend on spin?

akalaniz_at_hotmail.com
Date: 02/28/05 

Date: 28 Feb 2005 11:59:32 -0800

An outline as to why spin is involved in determining whether a force
between two like charges is attractive or repulsive may be found in
part I of Zee's, "Quantum Field Theory in a nutshell." The full reason
lies in the abstract algebra (group theory) of special relativity and
SU(N) groups. Wigner published a paper on this in the 1930s.

AA

PS--Uncle Al, an experiment was conducted in France a few years back
showing that gravity is quantized. I suppose this pushes the
theoretical gravition a bit closer to reality. Here is the report:

QUANTUM GRAVITATIONAL STATES have been observed for the first time. An
experiment with ultracold neutrons shows that their vertical motion in
Earth's gravitational field come in discrete sizes. Quantum properties
such as the quantization of energies, wavelike dynamics including
interference, and an irreducible uncertainty in the simultaneous
measurement of position and momentum usually emerge only at the atomic
level or under special circumstances (e.g., low temperatures) wherein a
particle is trapped in a potential well by a controlling force.
Observing such properties in phenomena governed by the electromagnetic
or the weak and strong nuclear forces is common enough, but the
strength of gravity, many orders of magnitude weaker than the other
forces, has not previously been strong enough to enforce the kind of
confinement needed to make quantum reality manifest. Such an effect has
now been seen. Physicists at the Institute Laue-Langevin reactor in
Grenoble, France employ a beam of ultracold neutrons. Moving at a pace
of 8 m/sec (compared to 300 m/sec for an oxygen molecule at room
temperature), the neutrons are sent on a gently parabolic trajectory
through a baffle and onto a horizontal plate. Because the neutrons
bounce at such a grazing angle, the plate is essentially a mirror for
the neutrons, which are reflected back upwards until gravity saps their
ascent; then the neutrons start falling again, eventually to be
captured by a detector. In effect the neutrons are caught in a vertical
potential well: gravity pulls down, while atoms in the surface of the
mirror push up. The researchers report seeing a minimum (quantum)
energy of 1.4 picoelectron volts (1.4 x 10^-12 eV), which corresponds
to a vertical velocity of 1.7 cm/sec. A comparison of this energy level
to the minimum energy for an electron trapped inside a hydrogen atom,
-13.6 eV, demonstrates why this kind of detection has not been made
before. The experiment provides also preliminary evidence for higher
quantized motion states as well. In the horizontal direction there is
no confinement and therefore no quantum effect. (By the way,
neutron-interferometry experiments, in which neutron waves are split
apart, moved around separate paths, and then brought back together in
order to produce an interference pattern, have been influenced by
gravity, but these neutron waves were not quantum states owing to the
gravitational field. By contrast, the Laue-Langevin experiment is the
first to observe quantum states of matter (neutrons) in Earth's
gravitational field.) The next step is to use a more intense beam and
an enclosure mirrored on all sides (the energy resolution improves the
longer the neutrons spend in the device). An energy resolution as sharp
as 10^-18 eV is expected, which would allow one to test such basic
propositions as the equivalence principle, according to which the
neutron's gravitational mass (as measured by its free fall in gravity)
is the same as its inertial mass (as prescribed by Newton's second law,
F=ma, where F is a generic force and a the acceleration imparted).
(Nesvizhevsky et al., Nature, 17 Jan 2002.)

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