RETROSPECTIVE

PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 583 April 1, 2002 by Phillip F. Schewe, Ben Stein, and James Riordon

"SCIENCE AND ULTIMATE REALITY," a meeting about forefront theoretical and experimental physics, was held at Princeton 15-18 March in honor of John Wheeler's 90th birthday and his many contributions to quantum mechanics, cosmology, and information science. Such a meeting is especially timely because these fields have enjoyed a burst of fruitful research in recent years. New experiments demonstrating nonlocality, the idea that an event in one place can affect an event at another place more quickly than it would take a light pulse to pass from the one place to the other, and the pursuit of robust systems which could perform extended "quantum computing," have energized the study of quantum reality. In the celestial realm the advent of automated redshift surveys of the galaxies and compilation of sharp maps of the cosmic microwave background are making possible an era of "high precision cosmology."

The Princeton meeting served up an impressive menu of hot topics and notable speakers (http://www.metanexus.net/ultimate_reality/agenda.htm). Examples include the subject of decoherence (Wojciech Zurek, Los Alamos), the process by which a quantum system (one whose whereabouts and movements can only be described in terms of likelihood, using a complex wave function) converts to a classical system (with definite observable coordinates) by subtle but often swift interactions with the surrounding environment; the many-worlds interpretation of quantum mechanics (Bryce DeWitt, Texas), according to which a quantum system does not suffer a "collapse of probability"¯rather the universe itself continues to bifurcate into multiple versions corresponding to the many possible histories available to the quantum system as it moves through space-time; the entanglement of ions in an atom trap (i.e., putting them into a special quantum state in which properties of the participating particles, such as spin or movement, are correlated) for the purpose of forming logic gates for a future quantum computer (Chris Monroe, Michigan).

Several speakers addressed the persistent problem of bringing quantum mechanics and general relativity into a single framework. Prominent issues here include the fate of information supposedly lost inside black holes (Juan Maldacena, Institute for Advanced Study); comparisons of string theory with the rival quantum loop gravity theory, which holds that space is not a mere platform for interactions but is itself a sort of dynamical thing; how gravity behaves in extra dimensions (Lisa Randall, Harvard); and the effort to detect gravity waves. Raymond Chiao(UC Berkeley) described an experiment in which he will try to convert electromagnetic waves into controlled gravitational waves inside a device in which a circuit is poised to go from a normally conducting state into a superconducting state. Using a second such device he hopes to convert gravity radiation back into electromagnetic radiation. Robert Laughlin (Stanford), who won the Nobel Prize for his studies of how patterns emerge in two-dimensional electron gases by way of the quantum hall effect, spoke about how general relativity might "emerge" at the edge of a black hole (for background see the online paper arXiv:gr-qu/0012094).

One purpose of the meeting was to promote freewheeling debate on all of the above issues, including the role of human consciousness in the measurement process. Young scientists were especially encouraged to engage in this debate, for which scholarships were given for attending the meeting. In fact a Young Researchers Competition was held for papers on quantum reality. The joint winners, from among 64 entries, were Raphael Bousso from UC Santa Barbara and Fotini Markopoulou-Kalamara from the University of Waterloo in Canada.

At the heart of the meeting was the keynote speech by the always interesting Anton Zeilinger (Vienna), who paid tribute to John Wheeler's many physics insights. One of those ideas was a proposal for a "delayed choice" experiment in which the dissipation of wavelike interference effects brought about by the experimenter's efforts to determine which of several possible paths a particle took in going toward a detector might be avoided by delaying the observation of the path until the particle (or wave) had made its mark. Zeilinger has carried out just such an experiment with entangled photons in a setup he referred to as a "Heisenberg microscope."

Zeilinger mentioned another of his recent experiments, one in which carbon-70 molecules, in wavelike form, passed through a series of slits to form an interference pattern. The C-70 molecules, however, were produced in an oven at 900 K, and this warm birth imparted a diversity of vibrations to the molecule, prompting it to shed an average of four or five photons on its way through the apparatus. Why did this communication between the molecule wave and its environment not result in decoherence and loss of interference effects? Answer: the "size" of the photons was much larger than slit spacing or the deBroglie (quantum) wavelength of the molecule itself, and so the photons did not betray any "which-path" information. Apparently a quantum system doesn't decohere if useful information is not being passed along.

Zeilinger holds that quantum reality needn't seem so weird if only students were exposed to the subject at an earlier stage. After all, we teach youngsters that the Earth goes around the sun and not vice versa, even though the sun seems to "rise" each morning. Could early instruction in wave mechanics reduce schoolkids' (and adults') alienation from "quantum weirdness"? Zeilinger thought that the time to start was in kindergarten. He said someday he wanted to devise a game with slits and counters which would show what happens when you turn interference off and on. He hadn't thought of the details for the game but he knew there would be no math, no equations, just demonstration.

Phillip F. Schewe
American Institute of Physics
One Physics Ellipse
College Park, MD 20740
301-209-3092, fax 301-209-0846
[email protected]