[EAS]Quantum Gravitation

pjk pjk at design.eng.yale.edu
Thu Jan 17 02:07:24 EST 2002

Subject:   Quantum Gravitation

In a somewhat different vein I wanted to communicate my wonder about
much recent experimental work at the limits of quantum physics, this
perhaps most astonishing of all in recent months.  --PJK

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

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
     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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