>Adios Xeno

From: Spudboy100@aol.com
Date: Tue Jun 06 2000 - 00:54:56 MDT

Source: Weizmann Institute (http://www.weizmann.ac.il/)
Date: Posted 6/5/2000

Zeno's Quantum Paradox Reversed: Watching A Flying Arrow Increase Its Speed

Is motion an illusion? Can"glimpses" freeze radioactive decay?
For over 2,500 years, scientists and philosophers have been grappling with
Zeno of Elea's famous paradox. More recently, scientists believed that the
counterpart of this paradox, known as the quantum Zeno paradox, is realizable
in the microscopic world governed by quantum physics. Now, scientists from
the Weizmann Institute of Science have shown that in most cases, the quantum
Zeno paradox should not take place. An article describing the calculations
that lead to this surprising conclusion appears in today's Nature. The
article is also surveyed in the journal's News and Views section.

The Greek philosopher Zeno, who lived in the 5th Century B.C., decades before
Socrates, dedicated his life's work to showing the logical paradoxes inherent
to the idea of indefinite divisibility in space and time (i.e., that every
line is composed of an infinite number of points). One of these paradoxes is
known as the arrow paradox: if the motion of a flying arrow is divided ad
infinitum, then during each of these infinitesimal moments, the arrow is at
rest. The sum of an infinity of zeroes remains zero, and therefore the arrow
cannot move. One can imagine how someone giving a flying arrow quick,
repeated glimpses, can actually freeze it in place. Zeno inferred from this
that movement cannot happen. Indeed, he was a true follower of Parmenides,
his teacher and mentor, who advocated that any change in nature is but an

This philosophical view was rejected by Aristotle, as well as by scientists
and philosophers of the 19th century, who resolved Zeno's paradox by showing
that non-zero velocity can exist in the limit of infinitesimal divisions of a
trajectory. However, the paradox was bolstered in the 1960s by the physicist
Leonid A. Khalfin, working in the former USSR, and by physicists E.C.G.
Sudarshan and Baidyanath Misra, working in the USA during the 1970s. Using
quantum theory, they concluded that if an "observer" makes repeated quick
observations of a microscopic object undergoing changes in time, it is highly
probable that the object will indeed stop changing. The frequent observations
divide the trajectory along which the object evolves into infinitesimal
segments in which there is no change. In other words, in the quantum world an
observer can freeze the evolution of an object, in accordance with Zeno's

Skeptics who doubted those calculations must have been genuinely surprised
when, in 1990, Colorado University physicist John Wineland proved in an
experiment that "freezing glimpses" do work in the real world (or at least in
a "simple" world with only two energy levels). Ever since, physicists have
been struggling to understand the implications of the experiment. Can the
Zeno paradox, for example,"glimpse-freeze" radioactive nuclear decay, thus
stopping radiation? The prevailing answer during the past 30 years has been
that such a freeze should be possible, provided the successive observations
are made frequently enough.

Prof. Gershon Kurizki and Dr. Abraham Kofman of the Weizmann Institute of
Science have recently shown that, for better or for worse, such "freezing"
does not take place in reality, and decay cannot actually be stopped by
"bombarding" the system with glimpses. According to their calculations, the
ability to "freeze" changes through quick glimpses depends on the ratio
between the memory time of the decay, and the time interval between
successive observations. Every process of decay has a memory time. In the
case of radioactive decay, for instance, this is the period in which the
radiation has not yet escaped from the atom, allowing the system to
"remember" its state prior to the decay. The memory time in a radiative decay
process of an excited atom (an atom occupying an unstable energy level) is
less than one billionth of a billionth of a second. To "freeze" this decay,
the observations would have to be at intervals of much less than a billionth
of a billionth of a second.

However, a sequence of observations so close in time would cause the
appearance of new particles, changing the system completely and destroying
it, and thus the question of stopping the decay would become meaningless. On
the other hand, if the time interval between observations is longer than the
decay's memory time, the rate of decay and radiation is actually increased.
Not only does Zeno's paradox not take effect in such a case but there is
actually an opposite effect: the "anti-Zeno effect".

Prof. Kurizki: "In other words, if we make the analogy between an object
undergoing changes in time, for example a decaying nucleus or an excited
atom, and Zeno's moving arrow, the arrow will increase its speed as the rate
of the 'glimpses' increases. The surprising conclusion of this research is
that the anti-Zeno effect (i.e., the increase of decay through frequent
observations) can occur in all processes of decay, while the original Zeno
effect, which would slow down and even stop decay, requires conditions that
only rarely exist in such processes."

Professor Gershon Kurizki is the holder of the George W. Dunne Chair of
Chemical Physics.


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