01-19-2002, 02:05 AM
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#1
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Veteran Member
Join Date: Jan 2001
Location: Median strip of DC beltway
Posts: 1,888
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Quantum Gravity
Interesting article I got from my brother:
Quote:
PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 573 January 16, 2002 by Phillip F. Schewe, Ben Stein,
and James Riordon
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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