<HTML><DATE>January 16, 2002 </DATE>by Phil Schewe, James
Riordon, and Ben Stein</span>
<p class="headline"><PNUZONE1>Quantum Gravitational States</PNUZONE1>
<blockquote>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 (<i>e.g.</i>, 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.
<p>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.
<p>The researchers report seeing a minimum (quantum) energy of 1.4 picoelectron
volts (1.4 x 10<sup>-12</sup> 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.]
<p>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<sup>-18</sup> 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 <i>et al.</i>, <i><a href="[
www.nature.com">Nature<];,
17 Jan 2002.)</blockquote>
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