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
pschewe@aip.org