---------- Forwarded message ----------
Date: Wed, 2 Feb 2000 09:40:38 -0500 (EST)
From: AIP listserver <[EMAIL PROTECTED]>
Subject: Update 469
PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 469 February 2, 2000 by Phillip F. Schewe and Ben Stein
DIGITAL ENTROPY. How much information does it take to control
something? By combining thermodynamics with information theory,
MIT researchers (contact Seth Lloyd, 617-252-1803, [EMAIL PROTECTED])
have determined the minimum amount of information one needs to bring
an unruly object under control, providing quantitative answers to such
subjects as taming chaos. From the perspective of thermodynamics,
controlling an object means reducing its disorder, or entropy. Lowering
the disorder of a hot gas, for example, decreases the number of possible
microscopic arrangements in the gas. This in turn removes some of the
uncertainty from the gas's detailed properties. According to information
theory, this reduced uncertainty is tantamount to increased information
about the gas. Applying this "digital entropy" perspective to the notion of
control, the researchers found that controlling an object becomes possible
when one acquires enough information about it (and then applies this
information to the object) to keep the uncertainties in its properties at
manageable levels. Chaotic systems are particularly hard to control
because they constantly manifest new amounts of uncertainty in their
properties. Perhaps there is no better everyday example of chaos than
steering a car: a tiny change in steering can quickly be amplified into a
huge change in course. For example, if a blindfolded driver initially
knows that her car is within two feet from a curb, tiny fluctuations in
steering can make this uncertainty 4 feet after one second, 8 feet after two
seconds, and so on. Only if the driver receives second-by-second
instructions for adjusting the steering to keep the uncertainty down to the
two-feet level does she have any hope of controlling it. If the driver
makes such steering adjustments only half as frequently, her car will go
out of control (crash into the curb) but it will take exactly twice the
amount of time than if no adjustments were made. (Touchette and Lloyd,
Physical Review Letters, 7 February 2000; Select Articles.)
THE MOST PROTON-RICH NUCLEUS,
[text deleted]
GUIDING NEUTRAL ATOMS AROUND CURVES
[text deleted]
____________________________________________________________________
The future is downloading. Can you hear the impact?
O[rphan] D[rift>]
Cyber Positive
The Armadillo Group ,::////;::-. James Choate
Austin, Tx /:'///// ``::>/|/ [EMAIL PROTECTED]
www.ssz.com .', |||| `/( e\ 512-451-7087
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