Physics News Update #573: Quantum Gravitational States; Extrasolar Planets
Quantum Gravitational States
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.)
Looking at Extrasolar Planets By Direct Observation
Looking at extrasolar planets by direct observation will be possible
soon, says UC-Berkeley astronomer Ray Jayawardhana. Because a star is
so much brighter than any planet (viewed from outside our solar system,
Jupiter would be only one billionth as bright as the sun), the presence
of extrasolar worlds around distant stars has so far been inferred only
indirectly, by the slight distortion imparted to the star’s spectrum.
But with new adaptive optics technology—which, with computer-controlled
flexing of secondary mirrored surfaces, can partly undo the fuzzy distortions
of incoming light introduced by atmospheric air currents overhead-attached
to the largest optical telescopes, such as the 8.1-m-diameter Gemini
North and the 10-m Keck telescopes, the prospect of gaining the needed
clarity for seeing planets has improved greatly.
At last week’s meeting of the American
Astronomical Society in Washington, DC, Jayawardhana reported an
example of the new, sharper viewing: a picture taken with Gemini showing
not yet a planet exactly but a planet in the making near the star MBM12,
some 900 light years away. This protoplanetary disk (see NOAO
press release) is the first such disk imaged for a four-star system
and the first edge-on disk discovered with the help of adaptive optics.
Furthermore, this star is still quite young and the disk itself only
an estimated 2 million years along on its planet-building mission.
It is young star systems like this that offer hope of seeing planets
directly since the star-to-planet brightness ratio might be only as
little as 100,000. With the higher angular resolution available (80
milli-arcseconds for the case of this disk, which lies at a distance
of only 150 AU from the star) from adaptive optics coupled with large
ground-based telescopes Jayawardhana believes planets, and not just
disks, can be spotted in the next few years. Indeed he referred to some
planetary candidates already glimpsed but not yet subjected to the full
battery of tests needed for planetary designation-such as observing
the planet candidate co-move with its star and recording a spectrum
consonant with planets (methane, water, etc.).
