Physics News Update no. 573

The American Institute of Physics Bulletin of Physics News
Number 573 16 January 2002

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.).

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