THE NOVEMBER 2004 ISSUE included "Holes in the Missile Shield," by Richard L. Garwin, a topic that attracted volleys of letters from all sides. David Caccia of Honokaa, Hawaii, found an additional hole in the shield: "If an enemy nation could produce only a few nuclear weapons, would it risk sending them on rockets, which have a considerable chance of malfunctioning? And even if the launch was successful, the country could expect retaliation. Wouldn't it rather transport a weapon to one of our cities in a shipping container, which would have a much better chance of reaching its target and also leave no trace of its sender after detonation?" But Taras Wolansky of Kerhonkson, N.Y., saw a hole in one ofthe arguments against a defense system: "The Soviets went to great lengths to prevent the [Reagan administration's] Strategic Defense Initiative. Perhaps they understood that to make use of those 'easy' countermea-sures, they would have to rebuild their entire ICBM arsenal every time the Americans tweaked their detectors." Other stimulating queries and observations on more topics follow.
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BLACK HOLE DATA CRUNCH I enjoyed "Black Hole Computers," by Seth Lloyd and Y. Jack Ng, but have two questions: When the radius of the space being measured in the sidebar "Computing Spacetime" doubles, wouldn't the maximum number of satellites allowed in it increase eightfold rather than double, allowing the same spacing of satellites without exceeding the critical density?
Also, what would be the effect of rel-ativistic time dilation on a particle (and its encoded information) from the perspective of a frame of reference outside a black hole? Would relativistic time dilation cause the particle and its information to appear to "freeze" on the surface of the event horizon from the perspective of an outside observer, thereby conserving the amount of information available to the universe outside the black hole?
Michael Sklar Huntington Woods, Mich.
LLOYD AND NG REPLY: To answer Sklar's first question, recall that we are talking about a system (of measuring apparatus consisting of the GPS satellites) on the verge of becoming a black hole. Thus, the maximum total energy is proportional to the radius ofthe region being mapped, not the volume of the region.
/4s for the second, this is an excellent point. In fact, from the perspective of an observer outside the black hole, a clock falling into the hole slows down and effectively stops just before it falls through the event horizon. So according to an outside observer, nothing ever falls into the black hole. The problem arises when one considers the perspective of a person falling into the black hole. From his vantage point, he falls through the horizon and hits the singularity infinite time.
Lloyd and Ng evidently are not mathematicians, for they imply that computation is mostly a physical process. For instance, they assert that a group of particles swapping their spin axes forms a fundamental "computer." Computation is a mathematical concept, however, not a mere physical one.
An electron spinning on its axis is not "information" any more than a rock falling and splintering into flint edges exhibits "knowledge" of toolmaking. An event's significance is purely subjective.
The article suggests that the whole universe, though observably finite, qualifies as the most prolific computer, if not the best ever. But this view quickly deteriorates into two self-referential paradoxes: First, if the entire universe is a computer, who (or what) is left to watch it run? It isn't enough to simply mandate quantum-mechanical event measurements in the absence of interpretation. Second, assuming the universe is computing its own evolution, as the authors
I Letters contend, does it have a finite lifetime or not? If it is infinite, then its self-computation won't get done; it never produces an answer (not even The Hitchhiker's Guide to the Galaxy's "42"). Hence, it does not qualify as a computation.
Simi Valley, Calif.
"What's in a Name?" by Christine Soares [Insights] contains a number of factual errors that must be corrected if your readership is ever to come to grips with the Linnaeus-PhyloCode debate. The International Code of Zoological Nomenclature was first published in 1960, not 250 years ago as the article states. This distinction is extremely important since the latter date suggests that the present Code (fourth edition, 1999) is considerably more antiquated than is the case.
Furthermore, proponents of the Lin-naean system these days are mostly full-fledged phylogenetic systematists. The crux of the issue here is the distinction between phylogenetic systematics ("Hennigian cladistics") and biological nomenclature—the naming of living organisms. The former is a specialized discipline, whereas the latter is firmly in the public domain, reaching into every area of biology, including biodiversity, conservation, agriculture, medicine and veterinary studies. All serious taxonomists adopt the phylogenetic framework for constructing classifications. The naming of animals, however, requires pragmatism if it is to be at all useful.
The idea proposed by Kevin de Queiroz that the PhyloCode can somehow coexist with the official ICZN Code is analogous to enforcing two sets of traffic laws simultaneously, one stating that we drive on the right, the other, on the left (the disastrous consequences of which are obvious). At the same time, we have long been aware that the current Code is in need of modification and have not hesitated to say so in public and in print. The Code is now available on the Web (www.iczn.org). The next stage is to make it user-friendly and to provide a system to facilitate the official naming of living organisms. Our position is that the present challenge is to describe the world's vanishing biodiversity with the tools we already have; the PhyloCode is a regrettable distraction.
Andrew Polaszek Executive Secretary, International Commission on Zoological Nomenclature James Carpenter American Museum of Natural History Quentin D. Wheeler Natural History Museum, London
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